Chapter 11: Nervous System

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Chapter 11 Nervous System
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• Function of the Nervous System
• To coordinate the actions of your body
• To ensure effective behaviour
• To maintain the internal environment within safe
  limits (homeostasis)
• Messages are relayed throughout the body via
  electrochemical messages from the brain or
  through chemical messengers hormones (hormones
  require more time than nervous transmission but
  are long lasting)
• There are more nerve cells in the body than there
  are visible stars in the Milky Way!
• 1 cm3 of brain tissue houses several million
  neurons with each connecting with several
  thousand others

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Nervous Tissue

• The nervous system is divided into a central
  nervous system (CNS), consisting of the brain and
  spinal cord, and a peripheral nervous system
  (PNS), consisting of nerves carrying sensory and
  motor information between the CNS and muscles and
  glands.
• Both systems have two types of cells neurons
  that transmit impulses and neuroglial cells that
  support neurons.

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Organization of the nervous system
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Neuron Structure

• Neurons are composed of dendrites that receive
  signals, a cell body with a nucleus, and an axon
  that conducts a nerve impulse away.
• Sensory neurons take information from sensory
  receptors to the CNS.
• Interneurons occur within the CNS and integrate
  input (nonmyelinated).
• Motor neurons take information from the CNS to
  muscles or glands.

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Types of neurons
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• dendrites receive information (either from
  receptor cells or other nerve cells), conducting
  towards the cell body (200 dendrites/cell body)
• cell body location of the nucleus, high
  metabolic rate (so contains mitochondria)
• axon may be 1m long, very thin, conducts the
  impulse towards other neurons or effectors,
  starts at axon hillock, the smaller the neuronal
  diameter, the faster the neuronal transmission

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• nodes of Ranvier the unmyelinated sections of a
  myelinated neuron, impulses jump between the
  nodes of Ranvier
• neurilemma a thin layer encompassing neurons in
  the peripheral nervous system, promoting their
  regeneration

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• Schwann cell responsible for the myelin
  synthesis, type of glial cell (supporting and
  nourishing cell found in the nervous system)
• Axon Bulb either at a synaptic bulb or end
  plate to muscle, contains neurotransmitter

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Myelin Sheath

• Myelination covers long axons with a protective
  myelin sheath (made by neuroglial cells called
  Schwann cells).
• The sheath contains lipid myelin which gives
  nerve fibers their white, glistening appearance.
• The sheath is interrupted by gaps called nodes of
  Ranvier.
• Multiple sclerosis is a disease of the myelin
  sheath.

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Myelin sheath
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• FYI
• Nerves are generally comprised of many neurons
  together (like fibre optic cable)
• Myelinated neurons in the brain are termed white
  matter (the myelin makes them look white)
• White matter may regenerate after injury, whereas
  grey matter (unprotected) will not

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The Nerve Impulse

• The nervous system uses the nerve impulse to
  convey information.
• The nature of a nerve impulse has been studied by
  using excised axons and a voltmeter called an
  oscilloscope.
• Voltage (in millivolts, mV) measures the
  electrical potential difference between the
  inside and outside of the axon.

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Membrane Polarization (Resting Potential)

• When an axon is not conducting a nerve impulse,
  the inside of an axon is negative (-70mV)
  compared to the outside(40mV) this is the
  resting potential.
• To establish the 70mV potential in the cell
• Na is actively pumped out of the cell
• K is actively pumped into the cell

Sodium pump
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Membrane Polarization (Resting Potential)

• Na and K diffuse down the concentration
  gradient, but K diffuses faster due to an
  increased number of ion channels (gates) open to
  K ions
• Since there is a net loss of positive ions to the
  outside of the cell, -70 mV is established inside
  the neuron
• There are also large negative proteins inside the
  neuron that contribute to the negative charge

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Resting potential
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Membrane Depolarization

• When the nerve cell is excited, the membrane
  DEPOLARIZES (Action Potential)
• The membranes polarity changes
• Na channels open, Na rushes in, K gates
  close
• The positive ions flowing in causes a charge
  reversal to 40 mV inside the neuron
• (gated channel proteins)

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Action potential
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Membrane Repolarization

• Once the charge becomes positive, the Na gates
  close, K gates open, eventually restoring the
  charge inside the neuron to 70 mV (but the Na
  excess is inside and K excess is outside!)
• The Na/K Pump restores the ion concentrations
  inside and outside the cell

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Membrane Repolarization

• During the repolarization, the nerve cannot be
  reactivated this is called the refractory
  period (1 to 10 ms) and is a recovery time for
  the neuron
• The pump requires ATP in order to operate

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The Na/K Pump

• To be ready for another action potential, the
  membrane re-establishes the proper concentration
  gradient for sodium and potassium
• Three sodium ions are actively transported across
  the membrane and to the ECM
• Two potassium ions are then carried across to the
  cytoplasm

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Movement of the Action Potential

• The action in the neuron adjacent to an area of
  resting membrane causes that area to depolarize,
  moving the action potential along (due to
  attraction of opposite charges)
• Since the area from which the action potential
  came is still in recovery, the action potential
  will only move in one direction

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Propagation of an Action Potential

• The action potential travels the length of an
  axon, with each portion of the axon undergoing
  depolarization then repolarization.
• A refractory period ensures that the action
  potential will not move backwards.
• In myelinated fibers, the action potential only
  occurs at the nodes of Ranvier.
• This jumping from node-to-node is called
  saltatory conduction.

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Fig. 48-13
Schwann cell
Depolarized region (node of Ranvier)
Cell body
Myelin sheath
Axon
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The All-or-None Response (Threshold Potential)

• All neurons provide an all-or-none response
• - in response to a stimulus, they either
  activate (fire) and provide a certain level of
  response, or dont fire at all
• A neuron will only fire if it is stimulated with
  an intensity of at least threshold level
• Every action potential for a neuron is identical
  in strength and duration (regardless of how much
  beyond threshold the stimulus is)

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Threshold Potential

• All neurons differ in their threshold level
• To inform the brain of the intensity of a
  stimulus
• - the frequency of firing is increased (not
  speed, which is constant for each neuron)
• - the number of neurons that respond to that
  level of stimulus can increase (neurons may have
  different threshold)

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Transmission Across a Synapse

• The junction between neurons or neurons
  effectors is called the synapse.
• Transmission of a nerve impulse takes place when
  a neurotransmitter molecule stored in synaptic
  vesicles in the axon bulb is released into a
  synaptic cleft between the axon and the receiving
  neuron.

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• When a nerve impulse reaches an axon bulb,
  calcium channels open and Ca2 flow into the
  bulb.
• This sudden rise in Ca2 causes synaptic vesicles
  to move and merge with the presynaptic membrane,
  releasing their neurotransmitter molecules into
  the synapse
• The binding of the neurotransmitter to receptors
  in the postsynaptic membrane causes either
  excitation or inhibition.

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Synapse structure and function
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Synaptic Summation

• Many synapses per single neuron is not uncommon.
• Excitatory signals have a depolarizing effect,
  and inhibitory signals have a hyperpolarizing
  effect on the post- synaptic membrane.
• Summation is the summing up of these excitatory
  and inhibitory signals.

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Summation
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Summation
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Neurotransmitter Molecules

• Out of 25, two well-known neurotransmitters are
  acetylcholine (ACh) and norepinephrine (NE).
• Neurotransmitters that have done their job are
  removed from the cleft the enzyme
  acetylcholinesterase (AChE) breaks down
  acetylcholine.
• Neurotransmitter molecules are removed from the
  cleft by enzymatic breakdown or by reabsorption,
  thus preventing continuous stimulation or
  inhibition.

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• FYI
• most synapses involve more than just 2 neurons
  (or neuron/effectors)
• neurotransmitters move only by diffusion, so
  synaptic transmission is MUCH slower than axonal
  transmission.
• insecticides interfere with enzymes that break
  down neurotransmitters causing their hearts to
  remain contracted,
• whereas LSD and other hallucinogens are believed
  to bind to the receptor sites for
  neurotransmitters

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• Lidocaine, an anesthetic works by stabilizing the
  neuronal membrane so it cant depolarize
• Endorphins and enkephalins are natural
  painkillers produced in the CNS, blocking the
  pain transmitter that usually attaches to the
  injured organ allowing the perception of pain
• opiates (heroin, codeine, morphine) block the
  production of the pain transmitter. Since they
  act to decrease the production of natural
  painkillers, the amount of opiate taken must be
  increased or at least maintained to maintain the
  effect

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• Valium and other depressants are believed to
  enhance the action of inhibitory synapses
• Alcohol acts to increase the polarization of the
  membrane, increasing the threshold
• Since many neurons will connect to a postsynaptic
  neuron, it is the summation of the effects of the
  presynaptic neurons that determine whether or not
  the postsynaptic neuron or effector will
  depolarize

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• Neural Circuits includes neuronal and synaptic
  transmission
• There are two types of neural circuits
• complicated neural circuits, involving conscious
  thought
• reflex arcs without brain coordination
• often unconscious, involuntary and faster than
  when thought is required (why are these useful?)

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• Nervous Control (in general)
• Stimulus?Receptor?SensoryNeuron?Interneuron?
• Brain?Interneuron?MotorNeuron?Effector?Response
• Reflex Arc (see diagram the reflex arc)
• Stimulus?Receptor?SensoryNeuron?Interneuron
• (spinal cord)?MotorNeuron?Effector?Response
• When the response is made at the spinal cord
  level (information does not have to go to the
  brain to be processed), the response is quick
  (and always correct given the circumstances)
• Reflexes protect the body from injury

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The Central Nervous System

• The central nervous system (CNS) consists of the
  spinal cord and brain.
• Both are protected by bone, wrapped in protective
  membranes called meninges, and surrounded and
  cushioned with cerebrospinal fluid that is
  produced in the ventricles of the brain.

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• The ventricles are interconnecting cavities that
  produce and serve as a reservoir for
  cerebrospinal fluid.
• The CNS receives and integrates sensory input and
  formulates motor output.
• Gray matter contains cell bodies and short,
  nonmyelinated fibers white matter contains
  myelinated axons that run in tracts.

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The Brain

• consumes more oxygen and glucose than any other
  part of the body
• meninges outer layers (protection) dura
  mater, arachnoid and pia mater
• cerebrospinal fluid between the inner, middle
  meninges central canal of s.cord, carries
  nutrients, acts as a shock absorber, relays waste
  by diffusion fac. diffusion, flows within
  ventricles four spaces in the brain

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The human brain
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Fig. 49-15
Frontal lobe
Parietal lobe
Somatosensory cortex
Motor cortex
Somatosensory association area
Speech
Frontal association area
Taste
Reading
Speech
Hearing
Visual association area
Smell
Auditory association area
Vision
Temporal lobe
Occipital lobe
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Fig. 49-17
Max
Hearing words
Seeing words
Min
Generating words
Speaking words
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Fig. 49-1
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The Cerebral Cortex

• The cerebral cortex is a thin, highly convoluted
  outer layer of gray matter covering both
  hemispheres.
• The primary motor area is in the frontal lobe
  this commands skeletal muscle.
• The primary somatosensory area is dorsal to the
  central sulcus or groove.

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• Forebrain (cerebrum)
• contains two hemispheres for coordinating sensory
  and motor information speech,
• reasoning, memory, personality, which may be
  located on one side only
• the outer layer is called the cerebral cortex
  (only 1 mm thick), deeply folded into fissures(to
  increase surface area)

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Cerebral hemispheres
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Forebrain Continued

• - the two hemispheres are connected by the corpus
  
• callosum allowing info to be shared between the
• hemispheres (a collection of nerve fibres)
  which are
• sometimes severed to control epilepsy leading
  to
• interesting results
• - the cerebrum can be subdivided into 4 lobes
• Frontal (walking, speech, intellect,
  personality),
• temporal (hearing,vision, memory,
  interpretation),
• parietal (interpreting sensory info receptors,
  long term memory) and
• occipital (vision) lobes
• Brocas area - a part of the left hemisphere
  usually where
• speech centre is located

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The lobes of a cerebral hemisphere
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Forebrain Continued

• thalamus- below cerebrum, coordinates and
  interprets sensory info
• hypothalamus below the thalamus, related to
  pituitary,
• connects endocrine to the nervous system,
  receives sensory info, instincts, temperature
  control (ANS)
• pituitary gland influenced by the hypthalamus,
  part of the endocrine system (master gland)
• pineal gland part of the endocrine system
  melatonin production

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• midbrain - less developed in humans than the
  forebrain, 4 spheres relay centre for some eye
  and ear reflexes
• Hindbrain - located behind the midbrain, connects
  brain to spinal cord
• contains cerebellum (coordinates movement,
  balance, muscle tone), The cerebellum is involved
  in learning of new motor skills, such as playing
  the piano.
• pons (relay station between cerebellum areas, and
  cerebellum medulla)
• medulla oblongata (connection between peripheral
  and CNS, involuntary movements heart rate,
  breathing (ANS), crossover of control)

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• FYI
• much brain research takes place during brain
  surgery after people have strokes
• epileptics also provide insight into brain
  differentiation when they undergo severing of the
  corpus callosum to relieve extremely serious
  seizures
• although the brain must control the entire body,
  the volume of brain allocated to each part of the
  body is not proportional to that body parts size
  the face and hands account for the majority of
  the motor cortexs attention

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Fig. 49-16
Parietal lobe
Frontal lobe
Upper arm
Shoulder
Trunk
Head
Knee
Leg
Neck
Trunk
Hip
Elbow
Forearm
Hip
Elbow
Wrist
Forearm
Hand
Hand
Fingers
Fingers
Thumb
Thumb
Eye
Neck
Nose
Brow
Face
Eye
Lips
Genitals
Toes
Face
Teeth Gums Jaw
Lips
Jaw
Tongue
Tongue
Pharynx
Primary motor cortex
Primary somatosensory cortex
Abdominal organs
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Language and Speech

• Language and speech are dependent upon Brocas
  area (a motor speech area) and Wernickes area (a
  sensory speech area) that are involved in
  communication.
• These two areas are located only in the left
  hemisphere the left hemisphere functions in
  language in general and not just in speech.

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Language and speech
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Organization of the nervous system
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The Spinal Cord

• The spinal cord extends from the base of the
  brain through the vertebral canal.
• Structure of the Spinal Cord
• A central canal holds cerebrospinal fluid.
• Gray matter of the spinal cord forms an H and
  contains interneurons and portions of sensory and
  motor neurons.
• White matter consists of ascending tracts taking
  sensory information to the brain and descending
  tracts carrying motor information from the brain.

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• ventral root (towards front of body) carries
  motor neuron messages to muscles
• dorsal root (towards back) carries sensory neuron
  messages from the body

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Spinal cord
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Functions of the Spinal Cord

• The spinal cord is the center for many reflex
  arcs.
• It also sends sensory information to the brain
  and receives motor output from the brain,
  extending communication from the brain to the
  peripheral nerves for both control of voluntary
  skeletal muscles and involuntary internal organs.
  
• Severing the spinal cord produces paralysis.

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The Peripheral Nervous System

• The peripheral nervous system (PNS) contains
  nerves (bundles of axons) and ganglia (cell
  bodies).
• Sensory nerves carry information to the CNS,
  motor nerves carry information away
• Humans have 12 pairs of cranial nerves and 31
  pairs of spinal nerves.

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Nerve structure
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Cranial nerves
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• The dorsal root of a spinal nerve contains
  sensory fibers that conduct sensory impulses from
  sensory receptors toward the spinal cord.
• Dorsal root ganglia near the spinal cord contain
  the cell bodies of sensory neurons.
• The ventral root of a spinal nerve contains motor
  fibers that conduct impulses away from the spinal
  cord to effectors.

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Spinal nerves
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Somatic System

• The somatic system serves the skin, skeletal
  muscles, and tendons.
• The brain is always involved in voluntary muscle
  actions but somatic system reflexes are automatic
  and may not require involvement of the brain.
• nerves running to skeletal muscle system (under
  voluntary control)
• motor neurons ? voluntary effectors (skeletal
  muscle)
• control exists in the cerebrum cerebellum
  (coordination)

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Homeostasis and the Autonomic Nervous System

• All autonomic nerves are motor nerves that
  regulate the organs of the body without conscious
  control involuntary
• Control exists in the medulla
• Effectors are smooth muscle (digestive system),
  cardiac muscle (heart) and glands (exocrine
  endocrine)
• Responsible for maintaining homeostasis during
  times of rest and during emergencies

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• Consists of two parts
• Sympathetic
• prepares the body for stress, including fight or
  flight response
• short preganglionic nerve (Ach), long
  postganglionic nerve (NEp)
• originate in the thoracic vertebrae (ribs) or
  lumbar vertebrae (small of back)
• Parasympathetic
• restores normal balance times of relaxation
• long preganglionic nerve (Ach), short
  postganglionic nerve (ACh)
• originate in the brain (cranial nerves) or the
  spinal cord

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Fig. 49-8
Parasympathetic division
Sympathetic division
Action on target organs
Action on target organs
Dilates pupil of eye
Constricts pupil of eye
Inhibits salivary gland secretion
Stimulates salivary gland secretion
Sympathetic ganglia
Constricts bronchi in lungs
Relaxes bronchi in lungs
Cervical
Slows heart
Accelerates heart
Stimulates activity of stomach and intestines
Inhibits activity of stomach and intestines
Thoracic
Inhibits activity of pancreas
Stimulates activity of pancreas
Stimulates glucose release from liver inhibits
gallbladder
Stimulates gallbladder
Lumbar
Stimulates adrenal medulla
Promotes emptying of bladder
Inhibits emptying of bladder
Sacral
Promotes ejaculation and vaginal contractions
Promotes erection of genitals
Synapse
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Autonomic nervous system
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Disorders Associated With the Nervous System

• Parkinsons Disease inadequate production of
  dopamine in the brain causes involuntary muscle
  contractions and tremors can be partially
  alleviated with L-dopa (synthetic dopamine)

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• Alzheimers Disease decrease in CNS levels of
  acetylcholine
• Multiple Sclerosis degeneration of the Myelin
  sheath Many symptoms, partial paralysis, double
  vision,speech problems
• Amyotrophic lateral sclerosis (Lou Gehrig's
  disease (ALS) genetic disease causing motor
  neurons to die muscle control is lost, increased
  salivation, cramping, twitching

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• Epilepsy brain injury or lack of oxygen to the
  brain Seizures grand mal or petit mal
  transient loss of muscle control
• Spinal Cord Injuries through injury or disease,
  the spinal neurons are damaged, Results in loss
  of motor control -degree of which depends on
  where the damage occurred

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• Hydrocephalus water on the brain excess
  cerebrospinal fluid in the brain Increased
  pressure may lead to brain damage
• Cerebral Palsy Usually caused by oxygen
  deficiency before/during birth, reduced muscle
  coordination (cerebral damage)

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Drug Abuse

• Stimulants increase excitation, and depressants
  decrease excitation either can lead to physical
  dependence.
• Each type of drug has been found to either
  promote or prevent the action of a particular
  neurotransmitter.
• Medications that counter drug effects work by
  affecting the release, reception, or breakdown of
  dopamine, a neurotransmitter responsible for mood.

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Drug actions at a synapse
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• A drug can affect a neurotransmitter in these
  ways
• cause leakage out of a synaptic vesicle into the
  axon bulb
• prevent release of the neurotransmitter into the
  synaptic cleft
• promote release of the neurotransmitter into the
  synaptic cleft
• prevent reuptake by the presynaptic membrane
• block the enzyme that causes breakdown of the
  neurotransmitter or
• bind to a receptor, mimicking the action or
  preventing the uptake of a neurotransmitter.

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Drug use
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Alcohol

• Alcohol may affect the inhibiting transmitter
  GABA or glutamate, an excitatory
  neurotransmitter.
• Alcohol is primarily metabolized in liver and
  heavy doses can cause liver scar tissue and
  cirrhosis.
• Alcohol is an energy source but it lacks
  nutrients needed for health.
• Cirrhosis of the liver and fetal alcohol syndrome
  are serious conditions associated with alcohol
  intake.

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Nicotine

• Nicotine is an alkaloid derived from tobacco.
• In the CNS, nicotine causes neurons to release
  dopamine in the PNS, nicotine mimics the
  activity of acetylcholine and increases heart
  rate, blood pressure, and digestive tract
  mobility.
• Nicotine induces both physiological and
  psychological dependence.

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Cocaine

• Cocaine is an alkaloid derived from the shrub
  Erythroxylum cocoa, often sold as potent extract
  termed crack.
• Cocaine prevents uptake of dopamine by the
  presynaptic membrane, is highly likely to cause
  physical dependence, and requires higher doses to
  overcome tolerance.
• This makes overdosing is a real possibility
  overdosing can cause seizures and cardiac arrest.
  

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Heroin

• Derived from morphine, heroin is an alkaloid of
  opium.
• Use of heroin causes euphoria.
• Heroin alleviates pain by binding to receptors
  meant for the bodys own pain killers which are
  the endorphins.
• Tolerance rapidly develops and withdrawal
  symptoms are severe.

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Marijuana

• Marijuana is obtained from the plant Cannabis
  sativa that contains a resin rich in THC
  (tetrahydrocannabinol).
• Effects include psychosis and delirium and
  regular use can lead to dependence.
• Long-term marijuana use may lead to brain
  impairment, and a fetal cannabis syndrome has
  been reported.

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Chapter Summary

• The nervous system consists of two types of
  cells neurons and mesoglia.
• Neurons are specialized to carry nerve impulses.
• A nerve impulse is an electrochemical change that
  travels along the length of a neuron fiber.
• Transmission of signals between neurons is
  dependent on neurotransmitter molecules.

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• The central nervous system is made up of the
  spinal cord and the brain.
• The parts of the brain are specialized for
  particular functions.
• The cerebral cortex contains motor areas, sensory
  areas, and association areas that are in
  communication with each other.
• The cerebellum is responsible for maintaining
  posture the brainstem houses reflexes for
  homeostasis.

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• The reticular formation contains fibers that
  arouse the brain when active and account for
  sleep when they are inactive.
• The limbic system contains specialized areas that
  are involved in higher mental functions and
  emotional responses.
• Long-term memory depends upon association areas
  that are in contact with the limbic system.

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• There are particular areas in the left hemisphere
  that are involved in language and speech.
• The peripheral nervous system contains nerves
  that conduct nerve impulses toward and away from
  the central nervous system.
• The autonomic nervous system has sympathetic and
  parasympathetic divisions with counteracting
  activities.
• Use of psychoactive drugs such as alcohol,
  nicotine, marijuana, cocaine, and heroin is
  detrimental to the body.