Download The Nervous System

The Nervous System
Chapter 13
Introduction
• Communication system of an animal’s 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
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
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.
• 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.
• http://www.youtube.com/watch?v=iNgGKSNiNw
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
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)
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)
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 neuron’s
cell membrane pump sodium ions out and potassium
ions into the cell
• Sodium can’t 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
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
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)
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
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)
AFTER Repolarization
• Na+/K+ pump moves ions back to
their original sides
• Resting state restored
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 depolarization=conduction of AP
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
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
http://www.blackwellpublishing.com/matthews/actionp.html
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
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
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
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)
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
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
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
Central Nervous System
• Brain
• Cerebrum
• Cerebellum
• Diencephalon
• Brain stem
• Spinal Cord
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.)
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
Cerebellum
• Located just caudal to cerebrum
• Area of brain responsible for coordinated movement,
balance, posture, and complex reflexes
Diencephalon
• Passageway between brain stem and cerebrum
• Structures associated with diencephalon:
• Thalamus, hypothalamus, and pituitary gland
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
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.
Meninges
• Three layers:
1. Dura mater – tough, fibrous
2. Arachnoid - delicate, spiderweb-like
3. Pia mater - very thin; lies directly on surface of brain and
spinal cord
Cerebrospinal Fluid—in subarachnoid space
(Contains CSF)
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
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
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
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
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
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)
Neurotransmitters and Receptors
• Sympathetic nervous system
• Primary neurotransmitter—norepinephrine
• 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
Neurotransmitters and Receptors
• Smooth/Cardiac muscles may constrict or dilate in
response to epi/norepi
1. α1- adrenergic receptors – cause vasoconstriction of skin, GI
tract, and kidney [don’t need to digest, make urine, or bleed
profusely}
2. β1-adrenergic receptors - increase heart rate and force of
contraction
3. β2-adrenergic receptors - cause bronchodilation (relaxation)
Neurotransmitters and Receptors
• Parasympathetic nervous system
• 1º neurotransmitter—acetylcholine
• Cholinergic neurons - release acetylcholine
• 2 types:
• Nicotinic receptors
• Muscarinic receptors
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
Reflex Arc
5 Components:
• Sensory receptor—activated by stimulus
• Sensory neuron—transports Action Potential to gray matter of spinal cord or
brain stem (cranial n.) and synapses with other neurons
•
•
•
Interneuron—sensory info integrated with info from other sensory neurons
Motor Neuron—response is sent out via motor neuron
Target organ—effector cell
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
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
Crossed Extensor Reflex
• Contralateral reflex
• Withdrawal reflex initiated
• afferent sensory neuron synapses
with interneurons
• Causes contraction of opposite
extensor muscles
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