Chapter 17: Nervous System

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Chapter 17 Nervous System
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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
  service 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.
• Motor neurons take information from the CNS to
  muscles or glands.

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Types of neurons
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Myelin Sheath

• Long axons are covered by a protective myelin
  sheath formed 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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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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Resting Potential

• When an axon is not conducting a nerve impulse,
  the inside of an axon is negative (-65mV)
  compared to the outside this is the resting
  potential.
• A sodium-potassium pump in the membrane actively
  transports Na out of the axon and K into the
  axon to establish resting potential.
• The membrane is more permeable to K and much of
  the resting potential is due to the excess of K
  outside of the neuron.

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Resting potential
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Action Potential

• An action potential is a rapid change in polarity
  as the nerve impulse occurs.
• The action potential occurs if a stimulus causes
  the membrane to depolarize past threshold.
• An intense stimulus causes many firings (reaching
  action potential) in an axon a weak stimulus may
  cause only a few.
• The action potential requires two types of gated
  channel proteins one each for Na and K.

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• Sodium Gates Open
• The gates of sodium channels open first and Na
  flows into the axon.
• The membrane potential depolarizes to 40 MV.
• Potassium Gates Open
• The gates of potassium channels open next and K
  flows to the outside of the axon.
• The membrane potential repolarizes to 65 MV.

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Action potential
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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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Transmission Across a Synapse

• The tip of an axon forms an axon bulb that is
  close to a dendrite or cell body of another
  neuron this region of close proximity 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, gated
  channels for calcium 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 cleft.
• 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 Integration

• 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.
• Integration is the summing up of these excitatory
  and inhibitory signals.

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Integration
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Neurotransmitter Molecules

• Out of 25, two well-known neurotransmitters are
  acetylcholine (ACh) and norepinephrine (NE).
• Neurotranmitters 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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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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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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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 Brain

• The brain has four cavities called ventricles.
• The cerebrum has two lateral ventricles, the
  diencephalon has the third ventricle, and the
  brain stem and cerebellum have the fourth
  ventricle.

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The human brain
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The Cerebrum

• The cerebrum or telencephalon has two cerebral
  hemispheres connected by the corpus callosum.
• Learning, memory, language and speech take place
  in the cerebrum.
• Sulci divide each hemisphere into lobes including
  the frontal, parietal, occipital, and temporal
  lobes.

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Cerebral hemispheres
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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.
• The primary visual area is at the back occipital
  lobe.
• The temporal lobe has the primary auditory area.

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• The parietal lobe provides taste sensation.
• All have adjacent association areas that
  integrate signals the prefrontal area is an
  important association area for appropriate
  behavior.
• White matter consists mostly of long myelinated
  axons forming tracts these cross over so the
  left side of the brain handles right side
  information.
• Basal nuclei are masses of gray matter deep
  within the white matter integrate motor commands.

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The lobes of a cerebral hemisphere
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The Diencephalon

• The hypothalamus and thalamus are in the
  diencephalon that encircles the third ventricle.
  
• The hypothalamus controls homeostasis and the
  pituitary gland, and the thalamus receives all
  sensory input except smell and integrates it and
  sends it to the cerebrum.
• The pineal gland is also located here and
  secretes melatonin that may regulate our daily
  rhythms.

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The Cerebellum

• The cerebellum receives sensory input from eyes,
  ears, joints and muscles and receives motor input
  from the cerebral cortex.
• It integrates this information to maintain
  posture and balance.
• The cerebellum is involved in learning of new
  motor skills, such as playing the piano.
• A thin layer of gray matter covers the white
  matter.

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The Brain Stem

• The brain stem contains the medulla oblongata,
  pons, and midbrain.
• The medulla oblongata and pons have centers for
  vital functions such as breathing, heartbeat, and
  vasoconstriction.
• The medulla also coordinates swallowing and some
  other automatic reactions.
• The midbrain acts as a relay station between the
  cerebrum and spinal cord or cerebellum.

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The Reticular Formation

• The reticular formation is a complex network of
  nuclei and fibers that extend the length of the
  brain stem.
• One portion of the reticular formation, called
  the reticular activating system, arouses the
  cerebrum via the thalamus causing alertness.
• An inactive reticular activating system results
  in sleep.

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The reticular activating system
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The Limbic System and Higher Mental Functions

• Limbic System
• The limbic system is involved in our emotions and
  higher mental functions.
• The limbic system is a complex network of tracts
  and nuclei involving cerebral lobes, basal nuclei
  and the diencephalon.
• Two structures, the hippocampus and amygdala are
  essential for learning and memory.

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The limbic system
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Higher Mental Functions

• Animal research, MRI, and PET scans allow
  researchers to study the functioning of the
  brain.
• Memory and Learning
• Memory is the ability to hold a thought in mind
  or recall events from the past.
• Learning takes place when we retain and utilize
  past memories.

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• Short-term memory involves activity in the
  prefrontal area.
• Long-term memory includes semantic memory
  (numbers, words, etc.) and episodic memory
  (persons, events, etc.).
• Skill memory involves ability to ride a bike, for
  example, and involves all motor areas of the
  cerebrum below the level of consciousness.

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Long-term Memory Storage and Retrieval

• Our long-term memories are stored in bits and
  pieces throughout the sensory association areas
  of the cerebral cortex.
• The hippocampus is a bridge between sensory
  association areas and the prefrontal area where
  memories are utilized.
• The amygdala associates danger with sensory
  stimuli.

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Long-term memory circuits
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Long-Term Potentiation

• Long-term potentiation is increased response at
  synapses within the hippocampus and is essential
  to long-term memory.
• However, a postsynaptic neuron in the hippocampus
  can become too excited and then die.
• Excitotoxicity, a form of cell death, is due to
  the neurotransmitter glutamate rushing in too
  quickly.

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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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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, and mixed
  nerves have both types of fibers.
• 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.

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The Reflex Arc

• Involuntary reflexes allow us to respond rapidly
  to external stimuli.
• In reflexes, sensory receptors generate nerve
  impulses carried to interneurons in the spinal
  cord.
• Next, interneurons signal motor neurons which
  conduct nerve impulses to a skeletal muscle that
  contracts, giving the response to the stimulus.
• Pain is not felt until the brain receives nerve
  impulses.

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A spinal nerve reflex arc
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Autonomic System

• The autonomic system of the PNS regulates the
  activity of cardiac and smooth muscle and glands.
  
• The system is divided into sympathetic and
  parasympathetic divisions that
• Function automatically and involuntarily
• Innervate all internal organs and
• Use two neurons and one ganglion.

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Sympathetic Division

• The sympathetic division is associated with
  responses that occur during times of stress,
  including fight or flight reactions.
• The postganglionic axon releases mainly
  norepinephrine which acts similar to adrenaline,
  the hormone from the adrenal medulla.

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Parasympathetic Division

• The parasympathetic system is associated with
  responses that occur during times of relaxation
  and promotes housekeeper activities.
• The postganglionic neurotransmitter used by the
  parasympathetic division is acetylcholine.

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Autonomic nervous system
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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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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.