Module 8

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Module 8

• Central Nervous System

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• The Nervous System is split up into two sections
  
• The central nervous system (CNS) and
• The peripheral nervous system (PNS).
• In this module, we want to concentrate on the
  CNS. In the next module, we will focus on the
  PNS.

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• Your brain is probably the most interesting organ
  in your body. It weighs about three pounds and is
  composed of bundles and bundles of neurons that
  are connected to one another in a vast network.
  The figure below gives you an idea of what all of
  this looks like.

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• This photo is a false-color electron microscope
  image of neurons in a culture.
• Notice how the neurons link to one another.
• This means that they are association neurons.
  That's what your brain is full of association
  neurons linked to each other in a great network.
  This culture is a reasonable representation of
  what that network looks like.

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• In the average adult brain, there are something
  like 100 billion neurons.
• Since these neurons are association neurons, they
  must link to one another.
• Most neurologists indicate that there are
  trillions and trillions of links between neurons.
  
• In addition, the brain processes information
  significantly more quickly than does the most
  sophisticated computer.
• Neurologists, together with computer scientists,
  have estimated that the brain can perform in 100
  computational steps what it would take the most
  sophisticated computer nearly one billion steps
  to accomplish!

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• The brain, then, is a complex network of neurons.
  
• Think about what these neurons do. When they are
  at rest, they are actively transporting Na and
  K to maintain the resting potential.
• Then, when they need to pass a signal on to
  another neuron, they send action potentials down
  their axons.
• They also receive signals from other neurons and
  process them. As you might expect, this takes a
  lot of energy.
• Thus, your brain needs a steady supply of oxygen
  and glucose so that it can manufacture the
  enormous amount of ATP it needs. So, even though
  the average adult's brain makes up only about 3
  of the body's weight, it uses 20 of the body's
  blood supply!

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• If the blood supply is cut off from the neurons,
  the lysosomes in the cell body burst and the
  neurons are destroyed.
• Neurons cannot reproduce. Once you have lost a
  neuron, it is gone forever.
• Thus, this condition, called hypoxia (hi pok'
  see uh), is very damaging to the brain.
• The brain not only needs oxygen, it needs
  glucose. In fact, glucose is the brain's only
  fuel. As the body starves, other cells can use
  other nutrients to make ATP. However, neurons in
  the brain can only use glucose.
• If the glucose level in your blood drops enough,
  you have a condition called hypoglycemia. When
  this happens, you can't think clearly. Your brain
  fogs, because the neurons simply cannot produce
  the ATP they need to send and receive signals.

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Stroke

• A stroke is a medical condition that has profound
  effects on the brain.
• There are two basic types of strokes
• In an ischemic stroke, a blood clot cuts off the
  blood supply to the brain.
• In a hemorrhagic stroke, a blood vessel in the
  brain bursts.
• Which do you think is the more deadly of the two?
  Actually, it's the second one. Hemorrhagic
  strokes most often are fatal. Why? Remember the
  blood-brain barrier. The blood brain barrier
  exists to shield the neurons from the chemicals
  in the blood which are toxic to them. When a
  blood vessel bursts, the neurons are bathed in
  these chemicals. This kills large numbers of
  neurons!

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• Ischemic strokes are damaging as well.
• Indeed, if an ischemic stroke goes on long
  enough, it can be deadly.
• There are drugs which have been developed
  specifically to deal with them. These drugs are
  designed to get rid of the blood clot and restore
  the blood supply to the brain. As a result, they
  are often called clot-busting drugs. These
  drugs must be administered within three hours of
  the symptoms of a stroke to be effective,
  however, because the brain cannot survive hypoxia
  for too long.
• You don't mess around and hope it will go away.
  If someone has the symptoms of a stroke (loss of
  motor function or sensation, collapse-especially
  on just one side) you should get that person to a
  hospital immediately. The consequences of an
  untreated stroke are too severe to risk!

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• In addition to the glucose, your brain does need
  other kinds of nutrition.
• It needs B vitamins.
• It also needs the right kind of fat. It needs fat
  (called essential fat) not just for the
  neurons, but even more for the neuroglia.
• The neuroglia use these fats to make myelin. The
  brain, then, has very high nutritional needs, and
  our brains function better when we feed them
  well.
• Fruits, vegetables, and other whole foods are the
  best brain food, as they contain the nutrients
  that the brain needs.

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Gray and White Matter

• Before we discuss brain anatomy, however, there
  are two definitions that you need to learn gray
  matter and white matter.Gray matter
  Collections of nerve cell bodies and their
  associated neurogliaWhite matter Bundles of
  parallel axons and their sheaths
• Gray matter in the CNS may be grouped in
  structures called nuclei.
• White matter in the CNS may be grouped in
  tracts.
• A bundle of nerve cell bodies in the PNS is
  called a ganglion, and bundles of axons in the
  PNS are called nerves.

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• Figure 8.2 shows you two magnetic resonance
  images of the brain. Magnetic resonance imaging
  (MRI) is an incredibly useful tool that chemists
  developed to determine the arrangements of atoms
  in molecules. It was later adapted in conjunction
  with our amazing computational technology to
  provide beautiful images of a living person's
  interior.

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• On the left-hand side of the figure, you see the
  image of the brain's exterior. You don't see much
  detail, because the brain is mostly covered with
  the folded tissue of the cerebrum.
• The figure on the right shows a slice down the
  center of the brain. This particular slice is
  called a midsagittal section, because it splits
  the brain into equal left and right halves.

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• The brainstem can be separated into three
  sections the pons, the medulla oblongata, and
  the midbrain. The medulla oblongata (often just
  called the medulla) is the site of
  decussation.

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Medulla Oblongata

• Decussation A crossing overThis word comes
  from the Latin word decussatus, which means to
  form an X.What is the point of decussation?
  Have heard that the left side of our body is
  controlled by the right half of the brain and
  vice versa? Well, it turns out that it's true.
  The right half of the brain does, indeed, control
  the left side of the body.
• Most of this is the result of decussation, which
  takes place in the medulla. The nerves actually
  cross over each other at this point, sending
  signals from the right side of the brain to the
  left side of the body, and sending signals from
  the left side of the PNS to the right side of the
  brain. Now, not all crossing over occurs in the
  medulla. Sometimes, crossing over happens right
  at the spinal cord. That is not the norm,
  however. Most of the decussation occurs at the
  medulla.The medulla not only contains the point
  of decussation, but it also contains discrete
  clumps of gray matter called nuclei. These nuclei
  act as control centers for the body's vital
  functions.

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• Vital functions Those functions of the body
  necessary for life on a short-term basisWhat
  are the vital functions of the body?
• Well, one thing you can think of right away is
  the heart. If the heart stops beating, you die
  very quickly.
• Thus, the medulla contains nuclei which control
  the heart rate.
• Breathing is obviously a vital function. The
  medulla, therefore, also sets the respiration
  rate.
• The medulla also contains the vasomotor area,
  which controls the dilation or constriction of
  blood vessels throughout the body. This is an
  important blood pressure control mechanism. In
  addition, nuclei in the medulla also control
  reflexes like swallowing and vomiting.

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• Why can a sudden hard blow to the head be fatal?
• One reason is that swelling of the brain caused
  by a blow to the head can force the medulla to be
  pushed through the foramen magnum, which is the
  hole in the bottom of the skull through which
  the spinal cord and brain are connected.
• This will damage the medulla, and since the
  medulla controls the body's vital functions, that
  damage can be fatal.

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Pons

• Just superior to the medulla is the pons. The
  term pons means bridge. This is a good
  description of one of its functions it forms a
  bridge between the medulla and the upper
  brainstem. It also has several nuclei that relay
  messages between the cerebrum and the cerebellum.
  Interestingly enough, the pons also contains
  nuclei which are involved in respiration. They
  work in conjunction with the respiration nuclei
  in the medulla, but their precise functions are
  unknown. There is evidence to suggest that they
  help the body make the switch from breathing in
  (inspiration) to breathing out (expiration).

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Midbrain

• Just superior to the pons is the midbrain. The
  name describes its position well. It is in the
  middle between lower and upper parts of the
  brain. It has four nuclei that form mounds on the
  surface of the midbrain. Two of those nuclei are
  involved in the sense of hearing. The other two
  are involved in the sense of sight. In addition,
  of course, there are many nerve pathways which
  connect the brainstem to the upper parts of the
  brain.

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• Although the brainstem can be split into the
  three sections we mentioned above, there are some
  functions of the brainstem that are not isolated
  into one of these three regions.
• For example, there are several nuclei distributed
  throughout the brainstem which are called the
  reticular formation. These nuclei receive
  information from various afferent nerves,
  especially those of the face. These nuclei and
  their interconnections are called the reticular
  activation system, which plays a major role in
  determining the cycle of sleeping and waking.

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• Moving on up, we find the diencephalon. If you
  take that word apart, you can better understand
  its meaning. The last part of the word comes from
  cephalic, which refers to the head. The di
  comes from dia, which can mean in between.
  Thus, the diencephalon is the between brain.
• It exists between the brainstem and the cerebrum.
  The major components of the diencephalon are the
  thalamus and the hypothalamus .

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Thalamus

• The thalamus is an important relay station.
• Axons come in, and they synapse with neurons in
  the thalamus.
• Those neurons then send information back out of
  the thalamus to the cerebrum or other parts of
  the brain. A large fraction of the incoming axons
  come from sensory nerves, especially those
  related to touch.
• Apparently, some crude interpretation of the
  sensory information takes place in the thalamus,
  but it is not clear exactly what that
  interpretation is.
• In addition to being a sensory relay station, the
  thalamus affects mood and body movements,
  especially those related to strong emotions such
  as fear and anger. It also serves as a relay
  station for motor neurons between the cerebrum
  and the lower parts of the body.

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Hypothalamus

• The hypothalamus is a tiny area that is inferior
  to the thalamus and superior to the pituitary
  gland.
• The hypothalamus is very important to
  psychologists, since it is involved in many
  emotional and mood functions.
• These functions of the hypothalamus, however, are
  poorly-understood.
• One hypothalamus function that we understand
  fairly well is that it regulates the pituitary
  gland.
• The pituitary gland is often called the master
  endocrine gland because it secretes many
  hormones which affect such diverse functions as
  metabolism, reproduction, urine production, and
  response to stress.

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• In addition to control of the pituitary gland,
  the hypothalamus is also involved in controlling
  the autonomic nervous system.
• The autonomic nervous system controls smooth
  muscle, cardiac muscle, and glands. It is also
  involved with body temperature.
• Research of a colony of Rhesus monkeys had a
  device implanted into the hypothalamus. The
  device could heat or cool the hypothalamus
  directly.
• The monkey could be made to shiver by just
  cooling the hypothalamus.
• Alternatively, the monkey could be made to pant
  (Rhesus monkeys don't sweat, they pant) by just
  heating up its hypothalamus.
• These responses would occur even though the
  room's temperature was maintained as normal.
  Thus, the hypothalamus clearly influences body
  temperature.

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Cerebrum

• The cerebrum is obviously the biggest part of the
  human brain.
• It deals with what are often called
  higher-level brain functions.
• These include interpreting the signals sent from
  the various receptors in the body, reasoning, and
  memory.

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• The outer surface of the cerebrum, called the
  cortex, is composed of gray matter that is
  folded.
• The folds, called gyri, increase the surface area
  of the cortex.
• The grooves between the gyri are called sulci.
• The general pattern of gyri and sulci is similar
  in all brains, although there are some minor
  variations from person to person. Indeed, there
  are even variations from one hemisphere to
  another in the same brain.

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• The cortex is not very thick, but it contains 75
  of our neuron cell bodies.
• Underneath the cortex lies white matter and then
  clumps of gray matter called nuclei.
• The cerebrum is an important part of the brain
  which deserves more attention.

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Cerebellum

• The last major division of the brain is called
  the cerebellum.
• Most of the functions controlled by the
  cerebellum are ones that occur without conscious
  thought.
• Based on our current understanding, however, the
  cerebellum is mostly considered the control
  center for subconscious motor functions.
• These are the functions of voluntary (skeletal)
  muscles that we perform without really thinking
  about them.

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• For example, consider muscle tone. As discussed
  in Module 5, our skeletal muscles are usually at
  least partially recruited. Thus, there is some
  contraction in virtually every skeletal muscle in
  the body. That's what we call muscle tone, and it
  is controlled in part by the cerebellum.
  Another thing controlled by the cerebellum is
  our equilibrium. Think about the physics behind
  standing up.
• You don't think it's hard, because you don't have
  to think about it. It is controlled by the
  cerebellum without any thought on your part.
  Unconsciously, however, we are maintaining the
  proper amount of muscle contraction and adjusting
  the contraction to maintain our balance. The
  cerebellum assists with that.

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• The cerebellum also assists with the sequencing
  of muscle contractions.
• Let's just imagine what has to happen to grab a
  coin and put it in your pocket. First, your
  forearm has to extend. Then, your fingers must
  flex in just the right way so as to get the coin
  in your grasp. Then, your forearm has to flex to
  bring the coin back to your body. Then, your arm
  has to move laterally to get to your pocket. Then
  your forearm has to extend again to get into the
  pocket. Finally, your fingers must extend to
  release the coin. All of this has to happen with
  precise timing in the sequence, or you don't get
  the coin in your pocket.
• This precise sequencing is controlled by the
  cerebellum. If a person's cerebellum is not
  working properly, he or she will lack
  coordination because the muscle contraction
  sequencing is not working as it should.

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• The cerebellum is also involved in muscle preset.
  
• What do we mean by this? Well, suppose you open
  your refrigerator and see a paper carton of milk.
  We are not talking about a clear, plastic carton.
  We are talking about a carton that you cannot see
  through. Thus, you don't know how much milk is in
  there. You go to pick it up, and it almost flies
  away because unconsciously, you thought it was
  full. However, the carton was nearly empty, so
  the force you used to pick it up was far too
  great. Why do things like that happen? When you
  are about to perform a task, you often
  unconsciously preset your muscles to a certain
  strength of contraction.
• You do this because in the past, you've learned
  about the amount of effort needed to complete the
  task. For example, in the situation we just
  discussed, you have learned how heavy a milk
  carton is when it's full, and you know the amount
  of strength necessary to lift the full carton. As
  a result, you preset your muscles for that
  strength. If the milk carton is not full, you get
  surprised, because you exert way too much force
  in lifting the carton. That's muscle preset, a
  predetermined correct amount of contraction to
  perform a task. Your cerebellum is a big part of
  that process.

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• Dampening is another situation that the
  cerebellum takes care of.
• When you walk quickly or run, your upper limbs
  should swing around wildly, since they are very
  similar to pendulums hanging down at your sides.
• That usually doesn't happen, however, because
  your cerebellum sends inhibitory signals to the
  motor neurons that control your arms. As a
  result, there is more or less an even swinging of
  the upper limbs back and forth.
• That inhibition is called dampening, and it is
  controlled by the cerebellum.

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Corpus Collosum

• The last structure we want to mention is the
  corpus(kor' pus) callosum (kuh loh' sum). The
  cerebrum is split into two halves, as you will
  see in the next section. Those halves must
  communicate with one another. This communication
  is accomplished through several connections
  between the hemispheres. The corpus callosum is
  the largest of these connections.

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The Cerebrum

• Divided into two hemisphere by the longitudinal
  fissure.
• Each of these hemispheres can be divided into
  lobes, and those lobes are named for the skull
  bones which lie directly above them.
• The temporal lobe is the easiest one to spot. It
  is separated from the rest of the brain by a
  lateral fissure. This lobe is involved in the
  senses of hearing and smell and also plays a role
  in memory. It is also thought that the temporal
  lobe plays a role in abstract thought.
• The frontal lobe is also relatively easy to find.
  The central sulcus intersects with the lateral
  fissure, forming the boundary of the frontal
  lobe. This lobe is involved in motor function and
  smell. It is also involved in your mood,
  especially in generating feelings of aggression.
• The occipital lobe is not easy to detect, as it
  has no sulcus or fissure as a boundary. However,
  its function is very clear. It receives and
  integrates your visual sensory information.
• The parietal lobe is involved in receiving all of
  the sensory information we have not discussed so
  far. Thus, it handles all sensory information
  except that of smell, vision, and hearing.
  Although the division between the parietal lobe
  and the frontal lobe is easy to see, the division
  between the parietal lobe and the occipital lobe
  is not at all distinct.

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Primary Somatic Sensory Area

• Primary somatic sensory area, also called the
  primary somatic sensory cortex. When discussing
  the cerebral cortex, the words area and
  cortex are often used interchangeably. The term
  somatic refers to the body as a whole. Thus,
  this area of the cerebrum receives generalized
  sensory input from all over the body.
• Located mostly on a single gyrus. That gyrus is
  just posterior to the central sulcus, so it is
  called the postcentral gyrus. Notice that the
  primary somatic sensory area does not take up the
  entire gyrus. It does, however take up a good
  portion.
• The postcentral gyrus is the physical location of
  this functional area on the brain. The functional
  area is the primary somatic sensory area. Thus,
  the functional area tells you what is done in
  that region of the brain. The physical location
  tells you where it is on the brain.

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Primary Somatic Sensory Area

• What does the primary somatic sensory area do
  with the sensory input it receives?
• Brain has little sections that cover every part
  of the body.
• There is a section in the primary somatic sensory
  area to which axons from the receptors in your
  elbow go. There is another section in this area
  that receives signals from your tongue. This goes
  on and on. In other words, there is a little area
  on the primary somatic sensory area that
  corresponds to pretty much every part of your
  body.
• Thus, when a signal comes in to the primary
  somatic sensory area, the brain knows where the
  signal came from. Thus, we say that this area
  localizes the sensations that come from the body.

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Primary Somatic Sensory Area

• Neurologists actually can map the primary somatic
  sensory area and tell you exactly where the
  elbow section is and exactly where the toe
  section is.
• In the 1950's there were a lot of experiments
  conducted where surgeons would open up a person's
  skull to expose the brain. Then, while the
  patient was conscious, they would lightly shock
  areas of the primary somatic sensory area to
  create action potentials.
• Then, they would ask the patient what he or she
  felt. If the patient felt a tingling in his or
  her elbow, then the surgeon would know that the
  elbow section of the primary somatic sensory
  area had been stimulated. If the patient felt a
  tingling in the big toe, then the surgeon would
  know that the big toe section of the primary
  somatic sensory area had been stimulated.

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Somatic Sensory Association Area

• The somatic sensory association area is the area
  of the cerebrum determines the nature of the
  sensation.
• For example, if a person pokes you with his or
  her finger, it is unpleasant. If the person
  touches you with his or her finger to get your
  attention, it is not as unpleasant as a poke.
• Your somatic sensory association area makes these
  distinctions for you, so that when you turn
  around, you know whether or not to be annoyed at
  the person who was touching you.
• Also, the somatic sensory association area has a
  sensory memory to which it refers. Thus, if you
  have experienced the sensation before, your
  somatic sensory association area will put it in
  the proper context.

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Visual Cortex

• The Visual Cortex is located in the occipital
  lobe.
• This region receives the action potentials from
  the optic nerves that are attached to the eyes.
• In this region of the cerebrum, the action
  potentials are interpreted to give the basics of
  vision shape, color, and size.
• Action potentials then pass from the visual
  cortex to the visual association area. Much like
  the somatic sensory association area, the visual
  association area compares this image to past
  experience so as to give you context. This is how
  we recognize a face.
• We get the basic size, color, and shape from the
  visual cortex, but then the information passes to
  the visual association area where it is connected
  to our past experience. At that point,
  recognition occurs, and you identify the person
  you are looking at. As soon as we're born, we
  start filling that visual association area with
  recognition and meaning so that even a young baby
  soon recognizes his mom's face as well as objects
  of interest such as a bottle or a rattle.

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Primary Auditory Area

• The primary auditory area on the temporal lobe is
  responsible for the basics of sound volume and
  pitch.
• If you stimulate this area of the brain in such
  an experiment, the patient will hear various
  pitches or volumes but will not be able to relate
  them to anything.
• When you actually hear something, however, the
  signals also pass to the auditory association
  area which puts the signals into historical
  context for you. Thus, you can then recognize the
  melody of a song or the fact that what you hear
  is the ringing of a phone

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Wernickes Area

• The auditory association area also enables us to
  make sense out of speech. However, there is more
  to it than that.
• Another area of the cortex plays a crucial role
  here Wernicke's area.
• Auditory signals first hit the primary auditory
  area in order for us to hear volume and pitch.
• Then, the signals move to the auditory
  association area which put those sounds into
  historical context.
• If the historical context indicates that the
  sounds are speech, they are sent to Wernicke's
  area where the speech is then comprehended. Thus,
  these three areas must work together so that you
  can actually understand the words which reach
  your ears.

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• The taste area interprets taste.
• Not surprisingly, the olfactory area interprets
  the signals from the nose in order to give us our
  sense of smell. This area is not shown in the
  figure, because it is on the inferior surface of
  the frontal lobe.

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Primary Motor Cortex

• How does the cerebrum control motor neurons?
• In the primary motor area, which can also be
  called the primary motor cortex.
• The primary somatic sensory area is localized
  mostly to one gyrus on each side.
• This gyrus is anterior to the central sulcus, so
  we call it the precentral gyrus.
• Just like the primary somatic sensory area, this
  area has been tested in experiments, and we know
  that it has regions which correspond to motor
  neurons which control specific areas of muscles.
• There is a knee area, a hip area, a tongue
  area, etc.
• It controls our basic skeletal movements. The
  actions of making a fist, extending the forearm,
  or flexing the neck are the result of signals
  sent from here.

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Primary Motor Cortex

• Although the signals that cause our basic
  skeletal muscle movements come from the primary
  motor area, they do not necessarily originate
  from there.
• When we need to make skilled muscle movements for
  actions like typing, handwriting, jumping rope,
  etc., the sequence of action potentials needed
  for this fine motor movement is actually worked
  out ahead of time by the premotor area.
• As its name implies, this area of the brain does
  all of the preparatory work, deciding what action
  potentials must be sent, how quickly, and in what
  order. Impulses are then generated in this area
  and sent to the primary motor area. The primary
  motor area then follows the instructions and
  causes those movements to take place.

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Brocas Area

• Broca's area is a subsection of the pre-motor
  area.
• Broca's area works out the detailed sequencing
  that needs to take place to carry out the finest
  of muscle movements - those related to speech.
• Remember Wernicke's area? It comes into play
  here. When you want to say something, Wernicke's
  area determines what words will convey the
  meaning that you wish to impart. This generates
  action potentials which are then sent to Broca's
  area.
• Broca's area then works out the precise sequence
  of muscle movements needed to produce the words
  that Wernicke's area has decided to use. Then,
  Broca's area sends action potentials to the
  primary motor area, which causes the muscle
  movements worked out by Broca's area. The result
  intelligent speech.

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Prefrontal Area

• The prefrontal area is a large part of the
  cerebrum.
• It is dedicated to our ability to reason, to
  think things through, and our ability to foresee
  what's going to happen. It is also dedicated to
  our motivation.
• We might say that the many aspects of our
  personality are controlled in the prefrontal
  area.
• Prefrontal lobotomy was a surgical procedure used
  to control violent or disturbed people by just
  removing that part of the brain. Did it work?
• Fortunately, there are effective drugs these days
  for treating disturbances in behavior.

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• The brain is split into two hemispheres.
• As we mentioned before, the left side of the
  brain receives sensory information and controls
  the motor functions of the right side of the
  body.
• That's because of decussation, which occurs in
  the medulla oblongata and certain other parts of
  the CNS.
• The two hemispheres share much of this
  information, however. They do this through
  structures called commissures (kom' ih shyourz).

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Commissures

• Commissures - Connections of nerve fibers which
  allow the two hemispheres of the brain to
  communicate with one anotherThe largest of the
  commissures is the corpus callosum.

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• Although the two hemispheres of the brain are
  very similar, there are differences between them.
  
• Although most of the functional areas of the
  cerebrum are in both hemispheres of the brain,
  that is not always the case.
• Broca's area, for example, is almost always on
  the left hemisphere and not on the right
  hemisphere at all.
• As a result, the left hemisphere of the brain is
  more involved in speech than is the right
  hemisphere. In addition, the left hemisphere
  seems to be more active in mathematical
  reasoning.
• The right hemisphere, on the other hand, tends to
  dominate in determining spatial relationships,
  music, and face recognition.

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Other Important Structures in the Brain
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Basal Nuceli

• The two basal nuclei in the cerebrum, the
  lentiform nucleus and the caudate nucleus, are
  called the corpus striatum.
• They are the largest of the basal nuclei.
• The other two nuclei are not technically in the
  cerebrum. The subthalamic nucleus is in the
  diencephalon, and the substantia nigra is in the
  midbrain.

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• What do these nuclei do?
• They are involved in the planning, initiation,
  maintenance, and termination of motor activity.
• They also maintain muscle tone and are involved
  in the adjustments necessary to maintain posture.
  
• When everything works well, we're unaware of our
  basal ganglia. When there is damage there, there
  may be muscle tremors or other dysfunctions of
  the muscles and coordination.

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• Notice that there are two structures labeled in
  the figure which are not a part of the basal
  nuclei.
• First, notice the thalamus.
• The other structure in the figure is the
  amygdaloid nucleus. This nucleus is a part of the
  limbic system.

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The Limbic System

• The limbic system is a series of nuclei and
  commissures that run through the cerebrum and the
  diencephalon.
• The limbic system influences mood, emotion, fear,
  and the senses of pain and pleasure. The limbic
  system also seems to play a vital role in
  survival instincts. The desire for food, water,
  and reproduction seem to be centered in the
  limbic system.

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• The amygdaloid nucleus is involved in memory.
• There is a structure in the temporal lobe called
  the hippocampus (hip oh kam' pus). This
  structure is shaped like a sea horse, and it
  takes care of a significant part of your memory.
• For example, the memory of a person's name is
  stored in the hippocampus.
• The amygdaloid nucleus, however, is involved in
  remembering the emotions that you relate to that
  person. When you see a person and think of his or
  her name, then, the name comes from the
  hippocampus, while the emotions that you attach
  to that person (friendship, love, anger, etc.)
  come from the amygdaloid nucleus.
• This interaction between the limbic system, which
  is heavily involved in mood and emotion, and the
  hippocampus, which is heavily involved in memory,
  has led some to think that your emotions and your
  mood serve as gates in the brain. These gates
  determine what you remember on a long-term
  basis.

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Ventricles

• One other very important set of structures in the
  brain are the cavities, which are referred to as
  ventricles .
• When the CNS develops in a fetus, it begins as a
  hollow tube. That hollow tube grows with the
  fetus, changing shape.
• The top of the tube develops lobes that
  eventually form cavities in each of the
  hemispheres of the brain.
• The bottom of the tube expands and forms cavities
  in the diencephalon as well as in the midbrain.
  We call these the ventricles of the brain.

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Ventricles

• What is the function of these ventricles?
• They are the sites which produce cerebrospinal
  fluid, a fluid that cushions the brain and also
  provides a few nutrients to the brain.
• The ventricles are lined with ependymal cells.
• The ependymal cells produce and secrete the
  cerebrospinal fluid into the ventricles.
• The ependymal cells need an ample supply of
  nutrients and oxygen. Thus, there are blood
  vessels which supply them.
• In addition, there are other specialized cells
  which support the ependymal cells in their
  function. The ependymal cells, their support
  cells, and the blood vessels which supply them
  with oxygen and nutrients are collectively called
  choroid plexuses.
• The term choroid means lacey, because the
  tissue has a lacey look to it. The production of
  cerebrospinal fluid is an intricate procedure
  which is not fully understood at this time.

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Ventricles
62
Ventricles

• Notice in the figure that the largest ventricles
  are the lateral ventricles.
• There is one in each hemisphere of the brain, so
  they are called the left lateral ventricle and
  the right lateral ventricle.
• About 80 of the cerebrospinal fluid is produced
  in one of those two ventricles.
• The remainder is produced in the other two
  ventricles, which are called the third ventricle
  and the fourth ventricle. The third ventricle is
  in the diencephalon, while the fourth ventricle
  is in the midbrain. Notice that below the fourth
  ventricle, the cavity continues and becomes the
  central canal of the spinal cord.

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Ventricles

• These cavities are all connected to one another.
• The two lateral ventricles are connected to the
  third ventricle by two foramina (plural of
  foramen) called the interventricular foramina.
• The third ventricle is connected to the fourth
  ventricle by the cerebral aqueduct.
• These connections allow cerebrospinal fluid to
  flow freely from the lateral ventricles all of
  the way down to the fourth ventricle and then
  onto the surface of the brain or into the spinal
  cord.

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Protection of the Brain
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• The protection of the brain begins with the
  skull.
• Below the skull, however, three specific layers
  of tissue, collectively called the meninges,
  surround and protect the brain as well.
• The meninges layer right beneath the skull is
  called the dura mater. It is firmly attached to
  the periosteum of the skull so that they form one
  layer.
• The dura mater extends down into the major
  fissures of the brain.
• Where the dura mater extends into the fissures,
  tube-like openings occur. These are called dural
  sinuses, and they collect the blood which returns
  from the brain as well as cerebrospinal fluid as
  discussed in detail below. The sinuses empty into
  the blood vessels which leave the skull.

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• Below the dura mater we find the next layer of
  meninges, the arachnoid mater.
• The arachnoid mater gets its name from class
  arachnida, which contains the spiders.
• It is given this name because the tissue
  resembles webbing.
• The space between the dura mater and the
  arachnoid mater is called the subdural space. It
  is filled with a small amount of fluid that
  moistens the tissue.

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• Below the arachnoid mater, we find the last
  meningeal layer, the pia mater.
• The word pia means affectionate.
• The pia mater is so named because it binds
  tightly (affectionately) to the brain itself.
• The pia mater cannot be separated from the brain
  because of this connection.
• The space in between the arachnoid layer and the
  pia mater is the subarachnoid space, which
  contains weblike strands of arachnoid mater and
  is filled with cerebrospinal fluid.

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Meningitis

• This is a potentially life-threatening situation
  which can be caused by viruses or bacteria.
• Meningitis is an inflammation of the meninges.
• Since the meninges are so important in the
  protection of the brain and spinal cord, an
  inflammation of these tissues can be quite
  dangerous.
• Pus formed as a result of the inflammation can
  accumulate in the subarachnoid space, blocking
  the flow of cerebrospinal fluid. This can lead to
  paralysis, coma, and even death.

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• When the ependymal cells of the choroid plexus
  make the CSF, it enters the ventricle.
• If the CSF is made in the lateral ventricles, it
  flows through the interventricular foramina to
  the third ventricle and then down through the
  cerebral aqueduct to the fourth ventricle.
• There are a few foramina in the fourth ventricle
  (the median foramen is shown in the figure) which
  then allow the CSF to leave the ventricles and
  enter the subarachnoid space.
• Notice in the drawing on the right how there are
  weblike structures in the subarachnoid space.
• Those are composed of arachnoid mater. All of the
  tendrils you see in the subarachnoid space are
  extensions of the arachnoid mater.
• The CSF then flows up and around the brain or
  down the spinal cord, as shown by the white
  arrows.

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• Now you might wonder what causes the pressure
  that pushes the CSF out of the ventricles and
  into the subarachnoid space.
• The major cause of this pressure is simply the
  fact that more CSF is being made by the choroid
  plexuses.
• Thus, the CSF that has already been made has to
  leave the ventricles in order to make room for
  the new CSF that is being made.
• The ciliated ependymal cells help facilitate the
  flow, but the impetus for the flow comes from the
  pressure of new CSF being made. This is why a
  blockage of CSF flow can be deadly. Since new CSF
  gets made constantly, the pressure would just
  build and build until the pressure on the brain
  destroyed neurons.

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• Why does new CSF have to be made?
• Well, the CSF that is flowing through the
  subarachnoid space eventually reaches arachnoid
  granulations, which dump the CSF into a dural
  sinus, which is a large vein whose walls are made
  of dura.
• In addition to the CSF, blood that has already
  supplied the brain tissue with nutrients and
  oxygen also gets dumped into these dural sinuses.
  The blood and the CSF that is mixed in with it
  then is picked up by veins which carry them back
  to the heart.

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The Spinal Cord

• The spinal cord is an integral part of the CNS.
• It is the communications link between the PNS and
  the brain. Without it, the brain would not
  receive a lot of sensory information and could
  not control much of the body.
• The spinal cord is, in fact, continuous with the
  brain, and you can think of is as the brain's
  ponytail.

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• The spinal cord is encased in the vertebral
  column.
• However, it is not quite as long as the vertebral
  column, reaching only to the second lumbar
  vertebra.
• There are 31 pairs of spinal nerves that exit the
  spinal cord and travel through the vertebral
  column through the intervertebral foramina.

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• The spinal cord gets smaller and smaller in
  diameter as it goes from the brain to the lower
  back.
• However, there are two areas of exception.
• The cervical enlargement is an area where the
  spinal cord's diameter increases so that the
  nerves of the upper limbs can enter and exit the
  cord.
• The lumbar enlargement is a region of increased
  diameter where the nerves of the lower limbs
  enter and exit the spinal cord.
• Just below the lumbar enlargement, the spinal
  cord tapers to a conelike structure called the
  conus medullaris.
• This structure and the many nerves that attach to
  it are often called the cauda equina because
  together, they look like a horse's tail.

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• On the left side of the figure, is a drawing of a
  cross section of the spinal cord.
• The gray portion in the center is gray matter,
  while the lighter portion is composed of white
  matter.
• This white matter is organized into three basic
  columns the dorsal column, the lateral column,
  and the ventral column.

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• Since the term dorsal means back and
  ventral means belly, the dorsal column is the
  column on the back side of the spinal cord, while
  the ventral column is on the belly side of the
  spinal cord.
• The term lateral refers to the side, so the
  lateral column is on the side of the spinal cord.
  
• These columns are further divided into fasciculi,
  which are also called nerve tracts. Axons which
  carry action potentials to or from the brain are
  grouped together so that axons which carry the
  same type of information are found in the same
  fasiculus.

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• The central gray matter in the spinal cord is
  organized into horns.
• Axons from the sensory neurons synapse with
  association neurons in the dorsal horn.
• The cell bodies of the motor neurons are found in
  the ventral horn, while the cell bodies of the
  autonomic neurons are found in the lateral
  horns.

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• On each side of the spinal cord, we find dorsal
  roots and ventral roots.
• The dorsal root enters the spinal cord at the
  posterior horn, and it carries afferent signals
  from the sensory receptors to the spinal cord.
  Each dorsal root also contains a ganglion, which
  is made up of the cell bodies of afferent
  neurons. These axons travel from the receptors to
  the spinal cord, and the cell body hangs off of
  the axon.
• The dorsal root ganglia are where these cell
  bodies are gathered.
• The ventral root leaves the spinal cord at the
  ventral horn, and it carries efferent action
  potentials from the spinal cord to the effectors.
  The dorsal and ventral roots come together to
  form a spinal nerve.

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• On the right side of the picture,highlights the
  meninges of the spinal cord.
• The spinal cord needs to be protected just like
  the brain, and it is continous with the brain.
• Thus, it has the three meningeal layers just like
  the brain has. The spinal cord is covered with
  pia mater. The space between the pia mater and
  the arachnoid mater (the subarachnoid space) is
  filled with CSF, just as it is in the brain.
  There is also a subdural space between the
  arachnoid mater and the dura mater.

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• There is a difference between the meninges in the
  spinal cord and the meninges of the brain.
• In the brain, the dura mater is attached to the
  periosteum of the skull. However, in the spinal
  cord, the dura mater is not connected to the bone
  of the vertebral column.
• There is a space, the epidural space, between the
  vertebral column and the dura mater. This space
  contains spinal nerves, blood vessels, connective
  tissue, and fat. If you ever receive an epidural
  anesthetic to relieve pain, the anesthetic is
  injected into this space, and it deactivates the
  spinal nerves so that the afferent pain signals
  do not reach your brain.

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The Reflex Arc

• When you think about what controls your muscles,
  the brain immediately comes to mind.
• However, it is important to realize that the
  brain is not always involved in the control of
  your muscles. Sometimes, your spinal cord is.
• Think about the situation where you touch a hot
  stove and immediately pull your hand away from
  it. The speed at which you do this is surprising.
  In fact, it is not uncommon for someone to react
  by saying, Wow, I pulled my hand away before I
  even knew what I was doing. In fact, that's
  exactly right. In a situation like this, your
  hand reacts before your brain knows what
  happened. How is that possible? It is possible
  because the nervous system has been elegantly
  designed with a system we call the reflex arc.

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• When you touch something hot, the pain receptors
  (part of the PNS) in your skin are stimulated.
• As a result, they send an afferent message to the
  spinal cord (part of the CNS).
• The spinal cord integrates that information and
  makes a judgment as to what to do. It sends an
  outgoing message to flexor muscles in your arm,
  telling them to contract.
• This results in your hand pulling away from the
  stove. Realize that this happens without the
  brain needing to be involved.

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• The left side of the figure is a cross section of
  the spinal cord.
• Notice the afferent neuron. Where is its cell
  body? It is in the dorsal root ganglion. It
  synapses with an association neuron in the dorsal
  horn.
• Finally, notice the motor neuron. Its cell body
  is in the ventral horn.

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• The reflex arc begins with sensory information.
  The sensory information from the afferent PNS
  nerve is sent into the spinal cord. In the
  figure, the afferent nerve involved in the reflex
  arc is a pain receptor in the skin.The afferent
  message reaches the spinal cord and is sent to an
  association neuron that directs the message to an
  efferent neuron. The efferent neuron then sends
  the message to an effector in order to generate a
  response. In this case, the effector is a flexor
  muscle, which causes your forearm to flex.

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• The afferent nerve sends a signal to the spinal
  cord, and then an association neuron in the
  spinal cord sends a signal to an effector.
• This allows us to respond to stimuli quickly.
• However, if that were the end of the story, we
  would never learn from the experience.
• Once again, think about the situation in which
  your hand touches a hot stove.
• The reflex arc allows you to pull your hand away
  quickly. However, if your brain never learns
  about this experience, you wouldn't know to avoid
  doing it in the future. Thus, in order to keep
  you from doing the same thing again, your brain
  has to be informed about the experience.

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• Although our effectors can be controlled by
  reflex arcs, that can't be all there is.
• After all, we not only need to move our arms by
  reflex, but we must also move them as a result of
  conscious thought.
• Thus, our brains must be able to send signals to
  the effectors as well. So, in addition to the
  reflex arc, we need to have signals sent from the
  afferent nerves to the brain. In addition, the
  brain must be able to send signals down the
  efferent nerves to control effectors like muscles
  and glands.

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• Look at the circle labeled diverging circuit.
• What does this circuit accomplish?
• Well, as the afferent message travels into the
  spinal cord, the message diverges to two
  different neurons.

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• One of the neurons is the association neuron that
  stimulates the reflex.
• The other neuron is one that sends a message to
  the brain. What does this circuit do?
• It allows the afferent signal to not only
  initiate the reflex but also go to the brain.
  That way, the reflex allows you to react quickly,
  but the message to the brain helps you learn from
  the experience.

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• Now look at the circle labeled converging
  circuit.
• In this circuit, two neurons both send signals to
  the same effector neuron.

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• The first neuron is the association neuron in the
  reflex arc. The other is a neuron coming from the
  brain. What does this accomplish?
• It allows the effector (in this case, a flexor
  muscle) to be controlled by both the reflex arc
  and the brain. Thus, your forearm will flex
  quickly in reaction to pain, but it will also
  flex in reaction to a conscious thought.

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Ascending and Descending Pathways in the Spinal
Cord

• Obviously, reflex arcs enable very important
  functions of the spinal cord.
• However, the main function of the spinal cord is
  to serve as a conduit for messages from the brain
  to the PNS and from the PNS back to the brain.
• The are two specific pathways that facilitate
  this process
• A motor pathway
• A sensory pathway.

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• Motor pathways are descending pathways. Why?
• The motor nerves carry signals from the CNS to
  the muscles. The decision to move a muscle starts
  in the brain and then travels down the spinal
  cord to the muscle.
• Thus, this information must descend from the
  brain to the muscles.
• On the other hand, sensory pathways are ascending
  pathways, because the action potentials begin in
  the receptors of the PNS and then travel up the
  spinal cord to the brain.

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• When we initiate a movement, the signals must
  originate in the primary motor cortex, which is
  on the pre-central gyrus.
• Because the left side of the brain controls the
  right side of the body, a decision to clench your
  right fist will start in the primary motor cortex
  on the left side of the brain.
• The goal, of course, is to get that signal down
  to the muscles that will actually clench the
  fist.

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• How does that happen?
• Well, it depends on the type of motion required.
• This kind of motion is considered fine muscle
  movement, so it will take a direct pathway from
  the brain to the muscles.
• It only takes two sets of motor neurons to do
  this job, so there's a very close connection
  between what we voluntarily choose to initiate in
  the brain and how it is played out at the
  skeletal muscle.

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• The neurons in the motor cortex, called the upper
  neurons, send action potentials down through the
  midbrain, through the pons, and to the medulla.
• At the medulla, the axons travel through a
  structure called the medullary pyramid.
• In the pyramid, approximately 80 of the axons
  cross over. Remember, this is called decussation.
  
• To represent this, one of the two axons in the
  figure crosses from the left to the right.
• The other axon does not cross over at the
  medulla. That's because it will cross over later
  in the spinal cord.
• This represents what happens to about 20 of the
  axons in the motor pathway.

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• Once past the medulla, the axons travel to the
  portion of the spinal cord which contains the
  spinal nerves that will contain the efferent
  nerves which go to the muscles that clench the
  fist.
• When the axon that crossed over in the medulla
  reaches the spinal cord, there is usually an
  association neuron there.
• The upper motor axon synapses with this
  association neuron.
• The association neuron then synapses with a lower
  motor neuron, which will carry the signals to the
  neuromuscular junction of the muscle.

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• The other axon (the one that did not cross over)
  crosses from the left side of the spinal cord to
  the right side.
• There, it also synapses with an association
  neuron, and the association neuron synapses with
  another lower motor neuron.
• That motor neuron then carries the signal to
  another neuromuscular junction.

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• Look at what we have in this pathway.
• Upper motor neurons which originate in the motor
  cortex send their axons all the way down the
  spinal cord.
• Those axons then cross over to the other side of
  the body, either at the medulla or in the spinal
  cord.
• They then synapse with association neurons which,
  in turn, synapse with lower motor neurons which
  send the action potentials to the muscles of
  interest.

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• This represents a very direct route from the
  motor cortex to the muscle.
• There are only two synapses for each neuron.
• First, there is a synapse with an association
  neuron and second, that association neuron
  synapses with a lower motor neuron. The
  particular pathway in this example is called the
  lateral corticospinal tract. The word lateral
  refers to the fact that it travels down the side
  of the spinal cord. The word corticospinal
  refers to the fact that the route is direct from
  the cerebral cortex, down the spinal cord, and to
  the muscle.

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• There are many other pathways that motor neurons
  can take down the spinal cord.
• These pathways have more synapses and are
  therefore called indirect motor pathways.
• What is the purpose of indirect pathways? Why
  aren't all motor pathways direct?
• Remember, each synapse allows for the regulation
  of the information carried in action potentials.
  Thus, indirect pathways carry those signals which
  need a significant amount of regulation.

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• As we mentioned before, sensory pathways are
  ascending pathways, because they send signals
  from the receptors to the brain.
• Like motor pathways, there are many different
  sensory pathways. The particular pathway we want
  to use as an example is the anterior
  spinothalamic tract.
• The word anterior means that this bundle of
  neurons is located in the anterior part of the
  spinal cord. The word spinothalamic refers to
  the fact that the signals travel from the spinal
  cord to the thalamus.
• This tract carries the signals that come from
  light touch sensations, tickle sensations, and
  itching sensations.

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• Since this is an ascending pathway, it begins at
  the bottom of the figure and works its way up to
  the somatic sensory cortex.
• The pathway begins with receptors which are very
  superficial in the skin. A very light touch will
  activate them, and action potentials will be
  produced in the primary neuron.
• In this illustration, the axon diverges, and each
  end synapses with an association neuron.
• Each association neuron then synapses with a
  secondary neuron. The axons cross over at the
  spinal cord and travel up the anterior
  spinothalamic tract.

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• The secondary neurons travel up to the thalamus
  and synapse with tertiary neurons.
• This is why we say there is a certain crude
  interpretation of sensory information in the
  thalamus.
• The thalamus gives us some awareness of touch
  because of the synapse there.
• To really interpret the signals, however, they
  must be sent on to the somatic sensory area,
  which is where the tertiary neurons carry the
  signals.

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• Notice that there are more synapses in this
  pathway than in the descending motor pathway.
  This should make sense.
• Sensory information must be regulated.
• If the conscious centers of the brain were to
  receive all of the sensory information gathered
  by our sensory receptors, we could not handle it.
  
• Thus, this information must be regulated. As a
  result, there are more synapses in the tract.