The%20Nervous%20System

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The Nervous System

• Chapter 13

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Introduction

• Communication system of an animals body.
• Study is called Neurology
• Composed of two main divisions
• Central Nervous System (CNS)
• Composed of brain and spinal cord
• Peripheral Nervous System (PNS)
• Consist of nerves that come off of the CNS and
  innervate rest of body
• Function of nervous system activities
• Sensory functions
• Integrating functions
• Motor functions

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Neurons

• Basic functional unit of the nervous system.
• Have high oxygen requirement
• Can not reproduce but in some cases can
  regenerate.
• Neuroglia or glial cells- supporting cells to
  neurons
• Structurally and functionally support and protect
  the neurons
• Are more numerous than neurons
• Do not transmit impulses

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Neuron composition

• Soma (perikaryon) central cell body
• Dendrites receive stimuli or impulses from
  other nuerons and transmit info to soma.
• Sensory components feel things like heat and cold
• Very short and branched
• Axons Conduct nerve impulses away from the cell
  body to another neuron. Have single, long body
  with branches at end.
• Secondary cells are called effector cells because
  they do something when stimulated.
• Covered in myelin.
• White matter, specialized glial cells called
  oligodendroctytes in brain and spinal cord and
  Schwann cells in nerves outside brain and spinal
  cord.

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• Between the adjacent glial cells there are small
  gaps in meylin sheath called Nodes of Ranvier.
• Gaps help to increase speed of impulse along the
  axons.

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• http//www.youtube.com/watch?vi-NgGKSNiNw

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Organization of Nervous System

• 1. Anatomical
• Central nervous system (CNS)
• Brain and spinal cord
• Peripheral nervous system (PNS)
• Extends outward from the central axis toward the
  periphery of the body
• Cranial nerves originate directly from the brain
• Spinal nerves emerge from the spinal cord

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Organization of Nervous System

• 2. Direction of Impulses
• Afferent nerves - conduct impulses TOWARD CNS
• Also called sensory nerves - conduct sensations
  from sensory receptors in skin and other
  locations to CNS
• Efferent nerves - conduct impulses AWAY from CNS
• Also called motor nerves - cause skeletal muscle
  contraction/movement
• Some nerve fibers are sensory (optic), motor
  (oculomotor), or both (glossopharyngeal)

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Organization of Nervous System

• 3. Function Autonomic vs. Somatic
• Somatic nervous system
• actions under conscious, or voluntary, control
• Autonomic nervous system
• controls and coordinates automatic functions
• Example slowing of the heart rate in response to
  an increased blood pressure
• Sympathetic Division (fight or flight)
• Parasympathetic Division (feed or breed)

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Neuron Function Depolarization and Repolarization

• Resting state - when neuron is not stimulated
• But, not truly resting still working to
  maintain resting state
• Sodium-Potassium Pump proteins in the neurons
  cell membrane pump sodium ions out and potassium
  ions into the cell
• Sodium cant diffuse through membrane on its own
  which creates a higher concentration of sodium
  ions outside and higher concentration of
  potassium inside
• Positive charges outside and negative charges
  inside create a polarized membrane

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Neuron Function

• Resting membrane potential - difference in
  electrical charge across neuronal membrane
• Due to differences in distribution of positive
  and negative charges from sodium, potassium,
  proteins, and other charged ions on either side
  of neuronal membrane
• Resting membrane potential is a negative number
  indicating the negative charge inside the cell
• Sodium-potassium pump maintains this negative
  charge

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Depolarization

• Steps
• Neuron receives stimulus/impulse
• Na channel opens in neuron cell membrane
• Na flow into cell by passive diffusion
• Down concentration gradient
• Electrical gradient (opposites attract)

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Depolarization Continued

• During depolarization, inside of neuron goes from
  negative to a net positive charge due to inflow
  of Na ions
• ACTION POTENTIAL significant change in
  electrical charge from a negative to positive

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Repolarization

• Steps
• Na channels close
• K channels open
• K diffuses out of cell
• Down electrical and concentration gradients, just
  like sodium
• Resting potential (charge) restored
• Cell is REPOLARIZED sodium and potassium once
  again on opposite sides
• But, Na is inside, K outside (switched places)

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AFTER Repolarization

• Na/K pump moves ions back to their original
  sides
• Resting state restored

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Threshold Stimulus

• Stimulus must be sufficient to make the neuron
  respond and cause complete depolarization weak
  stimuli do not cause depolarization
• All-or-nothing principle - neuron depolarizes
  to its maximum strength or not at all
• Strong stimuli cause depolarization and sodium
  channels open which causes adjacent channels to
  open wave of depolarization
• Conduction of action potential spreading wave of
  opening Na channels in sufficient numbers to
  allow Na influx and depolarization
• Nerve impulse wave of depolarizationconduction
  of AP

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Refractory Period

• Time when neuron is not sensitive to a stimulus
• Cell is still in depolarization/early
  repolarization
• Absolute refractory period
• during Na influx and early K outflow
• No amount of stimulus can elicit another AP
• Relative refractory period
• during end of repolarization period
• Strong stimulus may elicit response

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Saltatory Conduction

• Saltatory means to leap
• Rapid means of conducting an AP (more rapid than
  in unmyelinated axons)
• Depolarization in myelinated axons can only take
  place at nodes of Ranvier

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Local Anesthetics

• Work by blocking the propagation of the action
  potential.
• Blocks Na channels, so no net positive charge
  flows into cell, so no threshold is achieved.
• Example Lidocaine

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Synaptic Transmission

• Synapse - junction between two neurons or a
  neuron and target cell
• Synaptic cleft - gap between adjacent
  neurons/effector cell
• Presynaptic neuron - neuron bringing the
  depolarization wave to the synapse
• Releases neurotransmitter
• Postsynaptic neuron - contains receptors for the
  neurotransmitter

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Synaptic Transmission

• Axon - structure on presynaptic neuron
• Terminal bouton (axon terminal) - slightly
  enlarged bulb at end of axon
• Vesicles contain neurotransmitter (a chemical)
• When depolarization wave reaches axon terminal,
  calcium channels open and cause vesicles to fuse
  with cellular membrane and release
  neurotransmitter into the synapse

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Synaptic Transmission

• Neurotransmitters diffuse across synaptic cleft
  toward postsynaptic membrane
• Receptors on postsynaptic membrane bind
  neurotransmitter
• Receptors are VERY specific for each
  neurotransmitter
• (similar to a lock and key)

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Types of Neurotransmitters

• Excitatory neurotransmitters
• Usually cause an influx of Na postsynaptic
  membrane moves toward threshold (more positive)
• Inhibitory neurotransmitters
• Move the charge of postsynaptic cell farther away
  from threshold (more negative)
• May open K channels/Cl- channels

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Examples of Neurotransmitters

• Acetylcholine
• Can be excitatory or inhibitory depending on its
  location in the body
• Catecholamines
• Norepinephrine/epinephrine - associated with
  "fight or flight" reactions of sympathetic
  nervous system
• Dopamine - involved with autonomic functions and
  muscle control

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Recycling the Neurotransmitter

• Acetylcholinesterase
• found on postsynaptic membrane breaks down
  acetylcholine
• Remember this??
• Monoamine oxidase (MAO)
• breaks down norepinephrine
• Catechol-O-methyl transferase (COMT)
• breaks down norepinephrine that is not reabsorbed

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Central Nervous System

• Brain
• Cerebrum
• Cerebellum
• Diencephalon
• Brain stem
• Spinal Cord

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Cerebrum

• Gray matter
• cerebral cortex outer layer of the brain
• White matter
• fibers beneath cortex and corpus callosum
  (fibers that connect the two halves of cerebral
  cortex)
• Area of brain responsible for higher-order
  behaviors (learning, intelligence, awareness,
  etc.)

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Cerebrum Terminology

• Gyri (gyrus)
• folds (rises) in cerebral hemispheres
• Sulci (sulcus)
• shallow grooves (remember interventricular
  sulci?)
• Divides the cerebral hemispheres into lobes
• Fissures
• deep grooves separating the gyri
• Longitudinal fissure prominent groove
• that divides cerebrum into right and left
• cerebral hemispheres

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Cerebellum

• Located just caudal to cerebrum
• Area of brain responsible for coordinated
  movement, balance, posture, and complex reflexes

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Diencephalon

• Passageway between brain stem and cerebrum
• Structures associated with diencephalon
• Thalamus, hypothalamus, and pituitary gland

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Brain Stem

• Connection between brain and spinal cord
  contains
• Medulla oblongata
• Pons
• Midbrain
• Area of brain responsible for basic
  (subconscious, autonomic) functions of the body
• Heart Rate
• Breathing, coughing, sneezing,
• Blood Pressure
• Many of the cranial nerves originate from this
  area of brain

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Meninges

• Connective tissue layers that surround brain and
  spinal cord
• Contain blood vessels, fluid, and fat
• Supply nutrients and oxygen to the superficial
  tissues of the brain and spinal cord
• Provide some cushioning and distribution of
  nutrients for CNS.

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Meninges

• Three layers
• Dura mater tough, fibrous
• Arachnoid - delicate, spiderweb-like
• Pia mater - very thin lies directly on surface
  of brain and spinal cord
• Cerebrospinal Fluidin subarachnoid space

(Contains CSF)
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Cerebrospinal Fluid

• Fluid between Arachnoid and Pia mater and in
  canals and ventricles inside brain and central
  canal of spinal cord
• Provides cushioning function
• May play role in regulation of autonomic
  functions such as respiration and vomiting

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Blood-Brain Barrier

• Separates the capillaries in the brain from the
  nervous tissue
• Capillary walls in the brain have no
  fenestrations covered by cell membranes of glial
  cells
• Prevents many drugs, proteins, ions, and other
  molecules from readily passing from the blood
  into the brain

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Cranial Nerves

• 12 nerve pairs in PNS that originate directly
  from brain
• Numbered in Roman numerals from I through XII (1
  through 12)
• Each nerve may contain axons of motor neurons,
  axons of sensory neurons, or combinations of both

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Spinal Cord

• Medulla central part of spinal cord
• Composed of gray matter
• A lot of nerve processing occurs here (not just
  in brain)
• Central canal center of medulla
• Cortex outer part of spinal cord
• White matter
• Myelinated and unmyelinated nerve fibers
• Surrounds the gray matter

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Spinal Cord Roots

• Dorsal and Ventral Nerve Roots emerge as Spinal
  Nerves from between each pair of adjacent
  vertebrae
• Dorsal nerve roots contain sensory fibers
• Ventral nerve roots contain motor fibers
• Spinal nerve is a mix of sensory and motor neurons

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Autonomic Nervous System

• Controls automatic functions at subconscious
  level
• Sympathetic nervous system - nerves emerge from
  thoracic and lumbar vertebral regions
  (thoracolumbar system)
• Parasympathetic nervous system - nerves emerge
  from brain and sacrum (cranial-sacral)

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Neurotransmitters and Receptors

• Sympathetic nervous system
• Primary neurotransmitternorepinephrine
• Adrenergic neurons - neurons that release
  norepinephrine
• Epinephrine/norepinephrine also released from
  adrenal medulla
• To elicit an effect, effector organ must contain
  receptor for epinephrine/norepinephrine

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Neurotransmitters and Receptors

• Smooth/Cardiac muscles may constrict or dilate in
  response to epi/norepi
• 1. a1- adrenergic receptors cause
  vasoconstriction of skin, GI tract, and kidney
  dont need to digest, make urine, or bleed
  profusely
• 2. ß1-adrenergic receptors - increase heart rate
  and force of contraction
• 3. ß2-adrenergic receptors - cause
  bronchodilation (relaxation)

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Neurotransmitters and Receptors

• Parasympathetic nervous system
• 1º neurotransmitteracetylcholine
• Cholinergic neurons - release acetylcholine
• 2 types
• Nicotinic receptors
• Muscarinic receptors

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Reflexes

• Somatic reflexes
• involve contraction of skeletal muscles
• Autonomic reflexes
• regulate smooth muscle, cardiac muscle, and
  endocrine glands
• Contralateral reflex
• starts on one side of body and travels to
  opposite side
• Ipsilateral reflex
• stimulus and response are on same side of body

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Reflex Arc

• 5 Components
• Sensory receptoractivated by stimulus
• Sensory neurontransports Action Potential to
  gray matter of spinal cord or brain stem (cranial
  n.) and synapses with other neurons
• Interneuronsensory info integrated with info
  from other sensory neurons
• Motor Neuronresponse is sent out via motor
  neuron
• Target organeffector cell

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Stretch Reflex (tap knee)

• Monosynaptic (1) reflex arc ipsilateral reflex
• Involves 1 sensory neuron and 1 motor neuron
• Signals also sent to
• Antagonistic m (inhibitory)
• Cerebellum/Cerebrum

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Withdrawal Reflex (toe pinch)

• Also called flexor reflex ipsilateral reflex
• Several interneuron synapses
• Several segments of spinal cord
• Results in
• Contraction of muscles
• Before cerebrum is aware
• Inhibition of antagonist m

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Crossed Extensor Reflex

• Contralateral reflex
• Withdrawal reflex initiated
• afferent sensory neuron synapses with
  interneurons
• Causes contraction of opposite extensor muscles

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CNS Moderation of Reflexes

• Upper CNS (brain) normally produces an inhibitory
  effect on the reflex arcs (muffled effect)
• With injury, intact reflex arcs caudal to spinal
  cord trauma become hyperreflexive
• Trauma to a portion of the reflex arc results in
  either hyporeflexive or absent reflexes