AP Biology Chapter 48

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AP Biology Chapter 48

• Nervous Systems

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Nervous Systems

• Nervous systems perform the three overlapping
  functions of
• Sensory input
• Integration
• Motor output
• Networks of neurons with intricate connections
  form nervous systems

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Sensory Input

• Sensory receptors collect information about the
  physical world outside and inside the body.
• Examples
• Light detecting cells of eyes
• Pressure sensors in skin
• Pain receptors in skin
• Hair cells in the ear

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Integration

• Integration is the process by which the sensory
  information is interpreted and associated with
  appropriate responses by the body.
• Integration is mostly carried out by the Central
  Nervous System (CNS).
• The CNS includes the brain and spinal cord in
  vertebrates.

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Motor Output

• Motor output is the conduction of signals from
  the integration center, the CNS, to effector
  cells.
• Effector cells are muscle or gland cells that
  carry out the bodys response to the stimulus.
• The signals are conducted by nerves ropelike
  bundles of extensions of neurons tightly wrapped
  in connective tissue.
• These nerves are part of the peripheral nervous
  system (PNS) or the nervous system of the rest of
  the body.

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Neuron Structure

• A neuron has
• A cell body that contains the nucleus and other
  organelles
• Dendrites that are short, highly branched
  processes that receive information from other
  cells and carry this information as an electrical
  signal toward the cell body.
• An axon, usually longer than a dendrite, convey
  outgoing messages from one neuron to the next.

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Axons

• Some axons, like the ones that connect your
  spinal cord to your foot, can be over a meter
  long.
• The conical region where the axon joins the cell
  body is called the axon hillock this region
  plays a role in the transmission and integration
  of nerve signals.
• Many axons are enclosed in a myelin sheath
  insulating layer that will be discussed in detail
  later in this PPT.

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• Synaptic terminals are the specialized endings
  that relay signals from the neuron to other cells
  by releasing neurotransmitters.
• Neurotransmitters are the chemical messengers of
  the nervous system.
• The synapse is the region between the synaptic
  terminal of the axon of one cell and the target
  cell.
• The transmitting cell is called a presynaptic
  cell and the target cell is a postsynaptic cell.

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The Reflex Arc

• A reflex is a response to a stimulus that acts to
  return the body to homeostasis.
• This may be subconscious as in the regulation of
  blood sugar by the pancreatic hormones, may be
  somewhat noticeable as in shivering in response
  to a drop in body temperature or may be quite
  obvious as in stepping on a nail and immediately
  withdrawing your foot.

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• A reflex arc refers to the neural pathway that a
  nerve impulse follows. The reflex arc typically
  consists of five components
• 1. The sensory neuron receives information. (i.e.
  foot steps on nail)
• 2. The sensory (afferent) neuron conducts nerve
  impulses along an afferent pathway towards the
  central nervous system (CNS).
• 3. The integration center consists of one or more
  synapses in the CNS.
• 4. A motor (efferent) neuron conducts a nerve
  impulse along an efferent pathway from the
  integration center to an effector.
• 5. An effector responds to the efferent impulses
  by contracting (if the effector is a muscle
  fiber) or secreting a product (if the effector is
  a gland). (i.e. you move your foot)

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Other Cells of the Nervous System

• Interneurons are cells that are go betweens
  between sensory receptors and effector cells to
  organize or integrate the most appropriate
  response.
• A ganglion is a cluster of nerve cell bodies,
  often of similar function, located in the PNS.
• Similar clusters in the vertebrate brain are
  called nuclei.
• Glia are support cells (more on these to follow)

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Types of Nerve Circuits

• There are three basic patterns of nerve circuits.
• One takes information from a single source to
  several parts of the brain.
• A second, information from presynaptic neurons
  converges at a single postsynaptic neuron.
• In the third, information flows in a circular
  path, from one neuron to others and then back to
  the source.

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Glial Cells

• Several types in brain and spinal cord
• Astrocytes in the mature CNS, these provide
  structural and metabolic support for neurons.
• They also induce the formation of tight junctions
  between the cells lining the capillaries of the
  brain.
• This creates the blood-brain barrier which
  restricts the passage of most substances into the
  brain, allowing the extracellular chemical
  environment of the CNS to be tightly controlled.

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• Oligodendrocytes in the CNS and Schwann cells in
  the PNS are glia that form insulating myelin
  sheaths around the axons of many neurons.
• The myelin sheaths are formed when
  oligodendrocytes or Schwann cells grow around
  axons such that their plasma membranes form
  concentric layers.
• These membranes are mostly lipids, which are poor
  electrical conductors this insulates the axon.

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Nerve Signaling

• All living cells have an electrical charge
  difference across their plasma membranes
  membrane potential.
• Neurons have this unequal distribution of ions
  and electrical charges between the two sides of
  the membrane.
• The outside of the membrane has a positive
  charge, the inside has a negative charge.
• The membrane potential of an unstimulated neuron
  is called the resting potential and is measured
  in millivolts.

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• Passage of ions across the cell membrane passes
  the electrical charge along the cell. The voltage
  potential is -65mV (millivolts) of a cell at rest
  (resting potential).
• Resting potential results from differences
  between sodium and potassium positively charged
  ions and negatively charged ions in the
  cytoplasm.
• Sodium ions are more concentrated outside the
  membrane, while potassium ions are more
  concentrated inside the membrane.

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• This imbalance is maintained by the active
  transport of ions to reset the membrane known as
  the sodium-potassium pump.
• The sodium-potassium pump maintains this unequal
  concentration by actively transporting ions
  against their concentration gradients.
• If the polarity is changed, then an action
  potential (the nerve impulse) response occurs and
  results in propagation of the nerve impulse along
  the membrane.
• An action potential is a temporary reversal of
  the electrical potential along the membrane for a
  few milliseconds.

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• Sodium gates and potassium gates open in the
  membrane to allow their respective ions to cross.
  
• Sodium and potassium ions reverse positions by
  passing through membrane protein channel gates
  that can be opened or closed to control ion
  passage.
• Sodium crosses first.
• At the height of the membrane potential reversal,
  potassium channels open to allow potassium ions
  to pass to the outside of the membrane

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• Potassium crosses second, resulting in changed
  ionic distributions, which must be reset by the
  continuously running sodium-potassium pump.
• Eventually enough potassium ions pass to the
  outside to restore the membrane charges to those
  of the original resting potential.
• The cell begins then to pump the ions back to
  their original sides of the membrane.

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• The action potential begins at one spot on the
  membrane, but spreads to adjacent areas of the
  membrane, propagating the message along the
  length of the cell membrane.
• After passage of the action potential, there is a
  brief period, the refractory period, during which
  the membrane cannot be stimulated.
• This prevents the message from being transmitted
  backward along the membrane.

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Steps in an Action Potential

• At rest the outside of the membrane is more
  positive than the inside.
• Sodium moves inside the cell causing an action
  potential, the influx of positive sodium ions
  makes the inside of the membrane more positive
  than the outside.
• Potassium ions flow out of the cell, restoring
  the resting potential net charges.
• Sodium ions are pumped out of the cell and
  potassium ions are pumped into the cell,
  restoring the original distribution of ions.

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Nerve Impulses

• All cells have membrane potential, but only
  certain kinds have the ability to generate large
  changes in their membrane potentials.
• These cells (neurons and muscles) are called
  excitable cells.
• The membrane potential of an excitable cell in a
  resting (unexcited) state is called the resting
  potential.

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Ion Channels

• Gated-ion channels open or close in response to a
  stimulus.
• Chemically-gated ion channels open or close in
  response to a chemical such as a neurotransmitter
• Voltage-gated ion channels respond to a change in
  membrane potential.
• Gated channels only allow one type of ion to pass.

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The Action Potential A Recap of Events

• A nerve impulse is an electrical charge that
  travels down the cell membrane of a neurons
  dendrite and/or axon through the action of the
  Na-K pump.
• Remember, the inside of a neurons cell membrane
  is negatively-charged while the outside is
  positively-charged.
• When sodium and potassium ions change places,
  this reverses the inner and outer charges causing
  the nerve impulse to travel down the membrane.
• A nerve impulse is all-or-none it either goes
  or not, and theres no halfway.
• However, a neuron needs a threshold stimulus, the
  minimum level of stimulus needed, to trigger the
  Na-K pump to go and the impulse to travel.
• A neuron cannot immediately fire again it needs
  time for the sodium and potassium to return to
  their places and everything to return to normal.
  This time is called the refractory period.

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Graded Potentials

• Hyperpolarization is an increase in the voltage
  across the membrane due to a stimulus.
• Depolarizatin is a reduction I the voltage across
  the membrane.
• These voltage changes are called graded
  potentials because the magnitude of the change
  (either hyperpolarization or depolarization)
  depends on the strength of the stimulus.
• A larger stimulus opens more channels and a small
  stimulus opens less.

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Nerve Impulse Propagation

• The sodium channels in the neuronal membrane are
  opened in response to a small depolarization of
  the membrane potential.
• So when an action potential depolarizes the
  membrane, the leading edge activates other
  adjacent sodium channels.
• This leads to another spike of depolarization the
  leading edge of which activates more adjacent
  sodium channels ... etc.
• Thus a wave of depolarization spreads from the
  point of initiation.

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• If this were all there was to it, then the action
  potential would propagate in all directions along
  an axon.
• But action potentials move in one direction.
• This is achieved because the sodium channels have
  a refractory period following activation, during
  which they cannot open again. This ensures that
  the action potential is propagated in a specific
  direction along the axon.

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• The speed of action potential propagation is
  usually directly related to the size of the axon.
  Big axons result in fast transmission rates. For
  example, the squid has an axon nearly 1 mm in
  diameter that initiates a rapid escape reflex.
• A different means of speeding the propagation of
  action potentials has evolved in vertebrates.
• The presence of the myelin sheath, an insulation
  layer around the axon, works better for fast
  action potentials in vertebrates.

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Saltatory Conduction Speeding the Action
Potential

• The voltage-gated ion channels that produce the
  action potential are concentrated in the nodes of
  Ranvier, small gaps between successive Schwann
  cells along the axon.
• Also, extracellular fluid is in contact with the
  axon membrane only at the nodes, so that the flow
  of ions between the inside and outside of the
  axon can only occur in these regions.

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• So, the action potential can only propagate
  itself at the nodes of Ranvier.
• The action potential jumps from node to node,
  stimulating depolarization and a new action
  potential at each one along the way.

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Multiple Sclerosis

• Without the myelin sheath, we cannot function.
  This is demonstrated by the devastating effects
  of Multiple Sclerosis, a demyelinating disease
  that affects bundles of axons in the brain,
  spinal cord and optic nerve, leading to lack of
  co-ordination and muscle control as well as
  difficulties with speech and vision.

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Neuron Communication

• Neurons communicate at structures called synapses
  in a process called synaptic transmission.
• The synapse consists of the two neurons, one of
  which is sending information to the other. The
  sending neuron is known as the pre-synaptic
  neuron (i.e. before the synapse) while the
  receiving neuron is known as the post-synaptic
  neuron (i.e. after the synapse).

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• Now, although the flow of information around the
  brain is achieved by electrical activity,
  communication between neurons is a chemical
  process.
• When an action potential reaches a synapse, pores
  in the cell membrane are opened allowing an
  influx of calcium ions into the pre-synaptic
  terminal.
• This causes a small 'packet' of a chemical
  neurotransmitter to be released into a small gap
  between the two cells, known as the synaptic
  cleft.

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• The neurotransmitter diffuses across the synaptic
  cleft and interacts with receptors that are
  embedded in the post-synaptic membrane.
• These receptors are ion channels that allow
  certain types of ions to pass through a pore
  within their structure.
• The pore is opened following interaction with the
  neurotransmitter allowing an influx of ions into
  the post-synaptic terminal, which is propagated
  along the dendrite towards the soma.
• http//highered.mcgraw-hill.com/sites/0072495855/s
  tudent_view0/chapter14/animation__transmission_acr
  oss_a_synapse.html Animation

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http//catalog.nucleusinc.com/generateexhibit.php?
ID2728
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Neural Integration Occurs at the Cellular Level

• Neurotransmission can be either excitatory, i.e.
  it increases the possibility of the post-synaptic
  neuron firing an action potential, or inhibitory.
  
• In this case, the inhibitory signal reduces the
  likelihood of an action potential being generated
  following excitation.
• So how does inhibition work?

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• We have seen that the action potential is
  propagated by the leading edge of a
  depolarization wave activating sodium channels
  further down the axon.
• We have also seen that the activation of these
  sodium channels is achieved by a small
  depolarization of the neuronal membrane.
• But what would happen if the membrane potential
  was stabilized?

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• The depolarization inside the neuronal axon would
  dissipate and the action potential would not be
  able to propagate any further - i.e. it would be
  inhibited.
• The stabilization of the membrane potential is
  achieved by an influx of negatively charged
  chloride ions that is unaffected by the
  depolarization wave coming down the axon.
• Formerly, this is equivalent to an efflux of
  positively charged sodium ions. Thus it is like
  punching a hole in a hose so that water will leak
  out through the puncture and not get to the
  sprinkler!

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• The same neurotransmitter can produce different
  effects on different types of cells.
• The versatility of the neurotransmitter depends
  on the receptors present on the postsynaptic
  cells and the receptors mode of action.

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Neurotransmitters

• Acetylcholine
• Acetylcholine, in vertebrates, can be excitatory
  or inhibitory.
• It has excitatory effects on skeletal muscle
  cells, but has an inhibitory effect on cardiac
  muscle (causing a reduction in the strength and
  rate of cardiac cell muscle contraction).

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Biogenic Amines

• Biogenic amines are neurotransmitters derived
  from amino acids.
• One group, catecholamines, consists of the
  neurotransmitters produced from tyrosine
  epinephrine, norepinephrine and dopamine.
• Seratonin, another biogenic amine, is synthesized
  from tryptophan.

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• Biogenic amines most commonly affect biochemical
  processes within the postsynaptic cell. In many
  instances, they trigger signal-transduction
  pathways that affect the activities of enzymes.
• Dopamine and serotonin in the brain, affect
  sleep, mood, attention, and learning.
• A lack of dopamine is associated with Parkinsons
  disease and too much is linked to schizophrenia.

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Other Chemical Neurotransmitters

• Four amino acids function as neurotransmitters
• Gamma aminobutyric acid (GABA)
• Glycine
• Glutamate
• Aspartate
• Some neuropeptides also serve as
  neurotransmitters (i.e. substance P mediates our
  perception of pain. Endorphins, also
  neuropeptides, decrease pain perception.

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Gaseous Signals of the NS

• Nitric oxide and carbon monoxide are used as
  local regulators.
• These gases are not stored, but rather,
  synthesized on demand.
• The gases diffuse into neighboring cells and
  stimulate a response.

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Nervous Systems

• Billions of years ago, prokaryotes could detect
  changes in their environment and respond in ways
  that enhanced their survival.
• Evolution of this sensing and responding behavior
  provided the multicellular organisms with a
  mechanism for communication between cells of the
  body.
• By the Cambrian explosion, 600 mya, systems of
  nerve cells had evolved to essentially their
  modern forms.

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• While nerve cell function is pretty uniform
  throughout the animal kingdom, nervous system
  arrangements are very diverse.
• It is how these cells are networked that
  distinguishes the levels of complexity among
  animal nervous systems.
• Some animals have no nervous systems sponges
  have no nerve cells.
• Cnidarians have nerve nets.
• Cephalization, the clustering of nerve cells to
  form a brain near the anterior end of the animal,
  led to more complexity.

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• The first real or true central nervous system
  is seen in planarians these have a small brains
  with a longitudinal nerve cord.
• In annelids and insects, a more complicated
  brain, a ventral nerve cord, and segmentally
  arranged ganglia control behavior.
• In sessile or slow-moving mollusks, little or no
  cephalization is found and these organisms have
  simple sensory organs.
• Squid and Octopuses (cephalopod mollusks) have a
  large brains, eyes, and giant axons.

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Invertebrate Nervous Systems
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Vertebrate Nervous Systems

• In all vertebrates, the brain and spinal cord
  make up the CNS and everything else in the
  nervous system is part of the PNS.
• The vertebrate CNS is derived from the dorsal
  hollow nerve cord of the embryo.
• The central canal of the spinal cord is
  continuous with the fluid filled spaces,
  ventricles, of the brain. These cavities are
  filled with cerebrospinal fluid.

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• Cerebrospinal fluid is formed in the brain by
  filtration of the blood.
• This fluid circulates through the central canal
  and ventricles to convey nutrients, hormones, and
  white blood cells across the blood-brain barrier.
• Its most important function is as a shock
  absorber for the brain.
• Also protecting the brain are the meninges,
  layers of connective tissues.

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• The mammalian brain has fluid circulating between
  two layers of meninges for extra protection.
• The axons of the CNS are located in well-defined
  bundles, whose myelin sheaths make them look
  whitish. This white matter is distinguished from
  the gray matter, which is mostly dendrites,
  unmyelinated axons, and clusters of nerve-cell
  bodies (nuclei).

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Peripheral Nervous System
Motor (Efferent Division)
Sensory (Afferent Division)
Sensing external environment
Sensing internal environment
Autonomic Nervous System
Somatic Nervous System
Parasympathetic Division
Sympathetic Division
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PNS

• The human peripheral system is composed of two
  types of nerves based on location
• Spinal nerves (31 pairs) connect with the spinal
  cord and innervate most areas of the body.
• Cranial nerves (12 pairs) connect vital organs
  directly to the brain.
• The PNS can be divided into Sensory and Motor
  divisions.

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Sensory and Motor Divisions

• The sensory division is made up of the sensory,
  or afferent (incoming), neurons that convey
  information to the CNS from sensory receptors
  that monitor the external and internal
  environment.
• The motor division is made of the motor, or
  efferent (outgoing), neurons that convey signals
  from the CNS to effector cells. The motor
  division is divided into the somatic and
  autonomic divisions.

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• Spinal and cranial nerves can also be classified
  on the basis of function
• The somatic nerves relay sensory information from
  receptors in the skin and muscles and motor
  commands to skeletal muscles (voluntary control).
• The autonomic nerves sends signals to and from
  smooth muscles, internal organs (visceral
  functions) cardiac muscle, and glands
  (involuntary control).

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• There are two types of autonomic nerves the
  parasympathetic and sympathetic nerves
• Parasympathetic nerves tend to slow down body
  activity when the body is not under stress.
• They originate in the brain and the sacral region
  of the spinal cord.
• Their ganglia are in walls of organs.
• They promote housekeeping responses, such as
  digestion.

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• Parasympathetic
• Sympathetic nerves increase overall body activity
  during times of stress, excitement, or danger.
• They also call on the hormone epinephrine to
  increase the "fight-flight" response.
• They originate in the thoracic and lumbar regions
  of the spinal cord.
• Their ganglia are near the spinal cord.

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The Vertebrate Brain

•  The brain develops from a hollow neural tube.
• Forebrain, midbrain, and hindbrain form from
  three successive regions of tube.
• THE HINDBRAIN
• The hindbrain is the region where spinal cord and
  brain join. It has three parts
• Medulla oblongata
• Cerebellum
• Pons

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• The medulla oblongata has influence over
  respiration, blood circulation, motor response
  coordination, and sleep / wake responses.
• The cerebellum acts as a reflex center for
  maintaining posture and coordinating limbs.
• The pons ("bridge") possesses bands of axons that
  pass between brain centers.

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• THE MIDBRAIN
• The midbrain lies between the hindbrain and
  forebrain.
• The midbrain originally coordinated reflex
  responses to visual input.
• The roof of the midbrain, the tectum, still
  integrates visual and auditory signals in
  vertebrates such as amphibians and reptiles.
• In mammals it is now mostly a pathway switching
  center.

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• THE FOREBRAIN
• The forebrain has undergone the greatest
  evolution. It is composed of four regions
• Olfactory lobes
• Cerebrum
• Thalamus
• Hypothalamus and pituitary gland
• The large olfactory lobes dominated early
  vertebrate forebrains.

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• The cerebrum integrates sensory input and
  selected motor responses.
• The thalamus (below the cerebrum) relays and
  coordinates sensory signals.
• The hypothalamus monitors internal organs and
  influences responses to thirst, hunger, and sex.
• The reticular formation is an ancient mesh of
  interneurons that extends from the uppermost part
  of the spinal cord, through the brain stem, and
  into the cerebral cortex.

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The Reticular System

• The reticular formation is a set of
  interconnected nuclei that are located throughout
  the brain stem. It has two parts.
• The ascending reticular formation is also called
  the reticular activating system (RAS). It is
  responsible for the sleep-wake cycle, thus
  mediating various levels of alertness.
• The descending reticular formation is involved in
  posture and equilibrium as well as autonomic
  nervous system activity.

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The Cerebrum

• The cerebrum or cortex is the largest part of the
  human brain. It is associated with higher brain
  function such as thought and action.
• The cerebral cortex is divided into four
  sections, called "lobes"
• the frontal lobe
• parietal lobe
• occipital lobe
• temporal lobe.

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Lobe Function

• Frontal Lobe- associated with reasoning,
  planning, parts of speech, movement, emotions,
  and problem solving
• Parietal Lobe- associated with movement,
  orientation, recognition, perception of stimuli
• Occipital Lobe- associated with visual processing
• Temporal Lobe- associated with perception and
  recognition of auditory stimuli, memory, and
  speech

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• Note that the cerebral cortex is highly wrinkled.
  Essentially this makes the brain more efficient,
  because it can increase the surface area of the
  brain and the amount of neurons within it.
• A deep furrow divides the cerebrum into two
  halves, known as the left and right hemispheres.
  The two hemispheres look mostly symmetrical yet
  it has been shown that each side functions
  slightly different than the other.

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• Sometimes the right hemisphere is associated with
  creativity and the left hemispheres is associated
  with logic abilities.
• The corpus callosum is a bundle of axons which
  connects these two hemispheres.
• Nerve cells make up the gray surface of the
  cerebrum which is a little thicker than your
  thumb. White nerve fibers underneath carry
  signals between the nerve cells and other parts
  of the brain and body.

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• The neocortex occupies the bulk of the cerebrum.
• This is a six-layered structure of the cerebral
  cortex which is only found in mammals.
• It is thought that the neocortex is a recently
  evolved structure, and is associated with
  "higher" information processing by more fully
  evolved animals (such as humans, primates,
  dolphins, etc).

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Functional Cortical Areas

• There are two functional cortical areas of the
  cerebrum. The primary motor cortex and the
  primary somatosensory cortex form the boundary
  between the frontal and parietal lobes.
• The motor cortex functions in sending commands to
  the skeletal muscles and the somatosensory
  receives and partially integrates signals from
  touch, pain, pressure, and temperature receptors.

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Brain Hemispheres

• The left hemisphere is most adept at language,
  math, logic operations, and the processing of
  serial sequences of information. It has a bias
  for the detailed, speed-optimized activities
  required for skeletal motor control and
  processing of fine visual and auditory details.

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• The right hemisphere is stronger at pattern
  recognition, face recognition, spatial relations,
  nonverbal ideation, emotional processing in
  general, and parallel processing of many kinds of
  information.
• Understanding and generating the stress and
  intonation patterns of speech that convey its
  emotional content emphasizes right-hemisphere
  function, as does music.

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• The right hemisphere appears to specialize in
  perception of the relationship between images and
  the whole context in which they occur, whereas
  the left is better at focused perception.
• While working with their hands, most right-handed
  people us the left hand (right hemisphere) for
  context or holding and use the right hand (left
  hemisphere) for fine detailed movement.

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References

• http//csm.jmu.edu/biology/danie2jc/reflex.htm
• http//www.emc.maricopa.edu/faculty/farabee/BIOBK/
  BioBookNERV.html
• http//biology.clc.uc.edu/Courses/bio105/nervous.h
  tm
• http//www.bris.ac.uk/synaptic/public/basics_ch1_2
  .html
• http//www.csuchico.edu/pmccaffrey/syllabi/CMSD2
  0320/362unit6.html
• http//trc.ucdavis.edu/biosci10v/bis10v/week10/08b
  rain.html
• http//www.psychology-issues.com/Brain-anatomy.htm
  l
• http//serendip.brynmawr.edu/bb/kinser/Structure1.
  htmlcerebrum
• Campbell Biology, 6e