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Mammalian Physiology
Nervous System
Peripheral and Central
UNLV
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UNIVERSITY OF NEVADA LAS VEGAS
PHYSIOLOGY, Chapter 6
Berne, Levy, Koeppen, Stanton
Objectives
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Describe the organization of the nervous system
Describe the central nervous system
Discuss the different cell types in the nervous system
Describe characteristics of axons
Describe neuronal pools
Discuss the peripheral nervous system
– Sensory receptors
– Somatic motor nerves
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Basic Nervous System Functions
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Sensory Input – provides the central nervous system with information
about the internal and external environment
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Integration - CNS takes all the incoming information, interprets it, then
selects an appropriate response
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Motor Output - executes the central nervous system commands to
effect the appropriate physical response
Organization of the Nervous System
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Central Nervous System (CNS)
– Brain and spinal cord
– Integration and command center
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Peripheral Nervous System (PNS)
– Neurons outside the CNS
– Paired spinal and cranial nerves
– Sensory division
• Afferent fibers transmit
impulses from receptors to
CNS
– Motor division
• Efferent fibers transmit
impulses from CNS to effector
organs
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Organization of the Nervous System
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Central Nervous System
CNS is comprised of brain, brain
stem, and spinal cord
Important structures include:
-Medulla – cardiovascular &
respiratory control
-Cerebellum – motor control, motor
learning
-Hypothalamus – autonomic and
endocrine control
-Basal ganglia – motor control
-Cerebral cortex – sensory
perception, cognition, learning &
memory, voluntary movement
-Spinal cord – sensory input,
reflexes, somatic and autonomic
motor output
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CNS Environment
Local environment is controlled by
-blood-brain barrier
-buffering of neuroglia (astrocytes)
-exchange between CSF and brain ECS
Blood-brain barrier limits
movement large molecules
(proteins) and charged ions
from the blood into the brain
(Capillary endothelial cells
of CNS have tight junctions)
CSF has lower concentration of K+, glucose ,
and protein, but higher concentration of Na+
and Cl- than does blood (Table 6-5)
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Regions of the Brain and Spinal Cord
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White matter – dense collections of myelinated fibers
Gray matter – mostly soma and unmyelinated fibers
Sensory neurons enter via the dorsal root
Motor neurons exit via the ventral root
Histology of Nerve Tissue
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The two principal cell types of the
nervous system are:
– Neurons – excitable cells that
transmit electrical signals
– Supporting cells – cells that surround
and wrap neurons
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The supporting cells (neuroglia or
glial cells):
– Provide a supportive scaffolding for
neurons
– Segregate and insulate neurons
– Guide young neurons to the proper
connections
– Promote health and growth
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Neuroglia: Astrocytes
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Most abundant, versatile, and highly branched glial cells
They cling to neurons and their synaptic endings, and cover
capillaries
Functionally, they:
– Support and brace neurons (glial filaments in cytoplasm)
– Anchor neurons to their nutrient supplies (capillaries & pia matter)
– Control the chemical environment (take-up K+ & neurotransmitters)
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Neuroglia: Microglia
• Small, ovoid cells with spiny processes
– Phagocytes that monitor the health of neurons
– Remove cellular debris when CNS is damaged
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Neuroglia: Ependymal Cells
• Range in shape from squamous to columnar
– Line the central cavities of the brain and spinal column
– Form the epithelium that separates CNS from cerebral spinal
fluid in the ventricles
– Lie between the brain extracellular space and theCSF
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Neuroglia: Oligodendrocytes
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Branched cells that wrap CNS nerve fibers – produce myelin
sheath for neurons in the CNS
One oligodendrocyte myelinates many neurons
CNS version of Schwann cells
Neurons (Nerve Cells)
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Structural units of the nervous system
– Composed of a body, axon, and dendrites
– Long-lived, amitotic (non-divisible), and have a high metabolic rate
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Their plasma membrane functions in:
– Electrical signaling
– Cell-to-cell signaling during development
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Neurons (Nerve Cells)
Basic Elements
-Soma (cell body)
-Dendrites
-Axon
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Development of Neurons
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The nervous system originates from the neural tube and neural
crest
The neural tube becomes the CNS
There is a three-phase process of differentiation:
– Proliferation of cells needed for development
– Migration – cells become amitotic and move externally
– Differentiation into neuroblasts
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Axonal Growth
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Guided by:
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NCAM
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Scaffold laid down by older neurons
Orienting glial fibers
Release of nerve growth factor by astrocytes
Neurotropins released by other neurons
Repulsion guiding molecules
Attractants released by target cells
N-CAM – nerve cell adhesion molecule
Important in establishing neural pathways
Without N-CAM, neural function is impaired
Found in the membrane of the growth cone
Nerve Cell Body (Soma)
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Contains the nucleus and a nucleolus
Is the major biosynthetic center
Is the focal point for the outgrowth of neuronal processes
Has no centrioles (hence its amitotic nature)
Has well-developed Nissl bodies (rough ER)
Contains an axon hillock – cone-shaped area from which axons
arise
Dendrites of Motor Neurons
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Short, tapering, and diffusely branched processes
Extensions of neuronal cell body
They are the receptive, or input, regions of the neuron
Electrical signals are conveyed as graded potentials (not action
potentials) (calcium spikes)
Account for 90+% of surface area
Axons
• Structure
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Slender processes of uniform diameter arising from the hillock
Long axons are called nerve fibers
Normally there is only one unbranched axon per neuron
Axonal terminal – branched terminus of an axon
Lack rough endoplasmic reticulum, free ribosomes, Golgi
apparatus
• Function
– Generate and transmit action potentials
– Secrete neurotransmitters from the axonal terminals
– Axonal transport
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Axonal Transport
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Distribution of membrane and cytoplasmic components from soma
to points along the axon (especially to axon terminus)
Energy supplied by glucose
Fast axonal transport
– Membrane-bound organelles and mitochondria
– Synaptic vesicles
– 400 mm/day
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Slow axonal transport
– Cytoplasmic prioteins
– 1 mm/day
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Axonal Transport
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Transport facilitated by microtubules
– Organelles attach to microtubules
– Movement triggered by calcium
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Microtubule motor proteins are required for transport
– Kinesin and Dynein
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Axonal transport is bidirectional
– Anterograde axonal transport (soma to axonal terminals)
• Kinesin – replenishment of synaptic vesicles and enzymes responsible for
neurotransmitter synthesis
– Retrograde axonal transport (axonal terminals to soma)
• Dynesin – return of synaptic vesicles to soma for lysosomal degradation
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Myelin Sheath
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Whitish, fatty (protein-lipoid),
segmented sheath around most
long axons
It functions to:
– Protect the axon
– Electrically insulate fibers from
one another
– Increase the speed of nerve
impulse transmission
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Myelin Sheath and Neurilemma
Formation
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Formed by Schwann cells in the PNS
A Schwann cell:
– Envelopes an axon in a trough
– Encloses the axon with its plasma
membrane
– Has concentric layers of membrane that
make up the myelin sheath
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Neurilemma – remaining nucleus and
cytoplasm of a Schwann cell
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Nerve Fiber Classification
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Nerve fibers are classified
according to:
– Diameter
– Degree of myelination
– Speed of conduction
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Functional:
– Sensory (afferent) — transmit
impulses toward the CNS
– Motor (efferent) — carry impulses
away from the CNS
– Interneurons (association neurons)
— shuttle signals through CNS
pathways
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Synaptic Transmission
Neurons communicate across
synapses using
neurotransmitters
–Released from presynaptic
membrane
–Binds to receptor on post
synaptic membrane
–Acetylcholine is
neurotransmitter in PNS
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Types of Synapses
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Axodendritic – synapses between the axon of one neuron and
the dendrite of another
Axosomatic – synapses between the axon of one neuron and
the soma of another
Other types of synapses include:
– Axoaxonic (axon to axon)
– Dendrodendritic (dendrite to dendrite)
– Dendrosomatic (dendrites to soma)
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Synaptic Transmission
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Excitatory postsynaptic potentials (EPSP)
– Cause depolarization which may or may not reach threshold [↑
Na+ permeability]
– Temporal summation: summing several EPSPs from one
presynaptic neuron
– Spatial summation: summing EPSPs from several different
presynaptic neurons
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Inhibitory postsynpatic potentials (IPSP)
– Cause hyperpolarization [↑ Cl- permeability, ↑ K+ permeability]
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Termination of Synaptic Transmission
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Neurotransmitter bound to a postsynaptic neuron:
– Produces a continuous postsynaptic effect
– Blocks reception of additional “messages”
– Must be removed from its receptor
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Removal of neurotransmitters occurs when they:
– Are degraded by enzymes (ie. Acetylcholinesterase)
– Are reabsorbed by astrocytes or the presynaptic terminals
– Diffuse from the synaptic cleft
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Synaptic Delay
– Neurotransmitter must be released, diffuse across the synapse,
and bind to receptors
– Synaptic delay – time needed to do this (0.3-5.0 ms)
– Synaptic delay is the rate-limiting step of neural transmission
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Neural Integration: Neuronal Pools
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Functional groups of neurons that:
– Integrate incoming information
– Forward the processed information to its appropriate destination
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Serial Processing
– Input travels along one pathway to a specific destination
– Works in an all-or-none manner
– Example: spinal reflexes
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Parallel Processing
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Input travels along several pathways
Pathways are integrated in different CNS systems
One stimulus promotes numerous responses
Example: a smell may remind one of the odor and associated
experiences
Organization of a Neuronal Pool in
the CNS
Each input fiber divides numerous
times providing innumerable terminal
fibrils to synapse with the cell bodies
(dendrites) of the neurons in the pool
Input
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Output
Neuronal Pools
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Simple neuronal pool
– Input fiber – presynaptic fiber
– Discharge zone – neurons most closely associated with the
incoming fiber
– Facilitated zone – neurons farther away from incoming fiber
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Types of Circuits in Neuronal Pools
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Divergent – one incoming fiber stimulates ever increasing
number of fibers
– Within a pathway to amplify the signal
– Into multiple tracts to send the signals to separate areas
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Types of Circuits in Neuronal Pools
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Convergent – opposite of divergent circuits, resulting in either
strong stimulation or inhibition
Convergence of signals
– Multiple inputs from a single neuron
– Inputs from multiple neurons
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Types of Circuits in Neuronal Pools
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Reverberating circuit – chain of neurons containing collateral
synapses with previous neurons in the chain making a positive
feedback loop – continuous output signal - control of rhythmic
activities such as sleep-wake cycle, breathing, walking etc
Types of Circuits in Neuronal Pools
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Parallel after-discharge – incoming neurons stimulate several
neurons in parallel arrays which stimulate a common output cell
– complex neural functions such as calculations
Peripheral Nervous System
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Sensory (afferent) division
– Sensory afferent fibers – carry
impulses from skin, skeletal muscles,
and joints to the brain
– Visceral afferent fibers – transmit
impulses from visceral organs to the
brain
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Motor (efferent) division
– Transmits impulses from the CNS to
effector organs
– Somatic nervous system
• Conscious control of skeletal muscles
– Autonomic nervous system (ANS)
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• Two divisions – sympathetic and
parasympathetic
• Regulates smooth muscle, cardiac
muscle, and glands
Sensory (Afferent) Receptors
Classification
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Special
– Vision, hearing, taste, smell, balance
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Superficial
– Touch, pressure, vibration, tickle, heat, cold, pain, itch
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Deep
– Position, kinesthesia, deep pressure, deep pain
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Visceral
– Hunger, nausea, distension, visceral pain
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Sensory Transduction
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Response of a sensory receptor
to a stimulus
– Chemoreceptor
– Mechanocreceptor
– Photoreceptor
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Sensory Coding
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Stimulus intensity
– Mean frequency of discharge (temporal summation)
– Number of receptors activated (spatial summation)
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Stimulus frequency
– Intervals between discharges
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Pattern of nerve impulses
Adaptation
– Accommodation to stimulus (slow or rapid)
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Neural Integration
Integration: summation of information coming into the neuron.
Spatial summation – summation of information coming into different places on the
neuron.
Temporal summation – summation of information coming into the neuron with time.
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Sensory Coding
Increasing frequency
of discharge in
response to
increasing stimulus
intensity
Adaptation – signal
stops when stimulus
becomes constant
Different pattern of
discharge
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Sensory Coding
Pattern of discharge synchronized with stimulus frequency
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Somatic Motor Neurons
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ά- motor neuron – efferent – extrafusal muscle fibers - voluntary
control
γ- efferent motor neuron – intrafusal muscle fibers - muscle
spindle – proprioception
Motor unit – ά-motor neuron, axon, and all the muscle fibers it
innervates
All the muscle fibers in a motor unit are the same type (I, IIa, IIb)
Muscle fibers contract on an all or none basis – each fiber
contracts fully when stimulated
Force increases incrementally by
– Recruitment (activating additional motor units)
– Summation (increasing frequency of stimulation)
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Skeletal Muscle Fiber Types
Fiber types classified by:
-Speed of contraction
-Energy producing pathways
-Fatigue resistance
-Fiber diameter
Fiber type determined by
neural input pattern
-Slow-twitch = tonic innervation
pattern
-Fast-twitch = phasic
innervation pattern
Fiber type also determined by
trophic nerve substances
(axonal flow)
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Motor Unit Recruitment
Size Principle
Weaker motor units recruited first,
those with smallest diameter axons:
type I → type IIa → type IIb
Type I – 0 to 50% maximum force
Type IIa – 20% to 100% max force
Type IIb – 80% to 100% max force
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Size Principle for Recruitment
In response to stretch, small motor
neurons recruited first
When stretch is released, large
motor neurons are deactivated first
Large motor neurons are more
susceptible to inhibition
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