Nervous System - Peripheral and Central
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Transcript of Nervous System - Peripheral and Central
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Mammalian Physiology
Nervous System
Peripheral and Central
PHYSIOLOGY, Chapter 6
Berne, Levy, Koeppen, Stanton
UNLVUNIVERSITY OF NEVADA LAS VEGAS
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Objectives
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
Sensory Input provides the central nervous system with informationabout the internal and external environment
Integration - CNS takes all the incoming information, interprets it, thenselects an appropriate response
Motor Output - executes the central nervous system commands toeffect the appropriate physical response
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Organization of the Nervous System
Central Nervous System (CNS) Brain and spinal cord
Integration and command center Peripheral Nervous System (PNS)
Neurons outside the CNS
Paired spinal and cranial nerves
Sensory division Afferent fibers transmitimpulses from receptors toCNS
Motor division
Efferent fibers transmitimpulses from CNS to effectororgans
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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
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
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Histology of Nerve Tissue
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
The supporting cells (neuroglia orglial 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
Most abundant, versatile, and highly branched glial cells
They cling to neurons and their synaptic endings, and covercapillaries
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
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
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Neurons (Nerve Cells)
Structural units of the nervous system
Composed of a body, axon, and dendrites
Long-lived, amitotic (non-divisible), and have a high metabolic rate
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
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
Guided by:
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
NCAM
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
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Nerve Cell Body (Soma) 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 axonsarise
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Dendrites of Motor Neurons
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
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Axons
Structure
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, Golgiapparatus
Function
Generate and transmit action potentials
Secrete neurotransmitters from the axonal terminals
Axonal transport
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Axonal Transport
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
Slow axonal transport
Cytoplasmic prioteins
1 mm/day
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Axonal Transport
Transport facilitated by microtubules
Organelles attach to microtubules
Movement triggered by calcium
Microtubule motor proteins are required for transport
Kinesin and Dynein 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 and Neurilemma
Formation 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 thatmake up the myelin sheath
Neurilemma remaining nucleus and
cytoplasm of a Schwann cell
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Nerve Fiber Classification
Nerve fibers are classifiedaccording to:
Diameter
Degree of myelination
Speed of conduction
Functional:
Sensory (afferent) transmitimpulses toward the CNS
Motor (efferent) carry impulsesaway from the CNS
Interneurons (association neurons)
shuttle signals through CNSpathways
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Synaptic Transmission
Neurons communicate across
synapses usingneurotransmitters
Released from presynaptic
membrane
Binds to receptor on postsynaptic membrane
Acetylcholine is
neurotransmitter in PNS
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Types of Synapses
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
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
Inhibitory postsynpatic potentials (IPSP)
Cause hyperpolarization [ Cl- permeability, K+ permeability]
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Termination of Synaptic Transmission
Neurotransmitter bound to a postsynaptic neuron:
Produces a continuous postsynaptic effect
Blocks reception of additional messages
Must be removed from its receptor
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
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
Functional groups of neurons that:
Integrate incoming information
Forward the processed information to its appropriate destination Serial Processing
Input travels along one pathway to a specific destination
Works in an all-or-none manner
Example: spinal reflexes
Parallel Processing
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
O i ti f N l P l i
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Organization of a Neuronal Pool in
the CNSEach input fiber divides numerous
times providing innumerable terminal
fibrils to synapse with the cell bodies(dendrites) of the neurons in the pool
Input
Output
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Neuronal Pools
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
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
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
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
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Types of Circuits in Neuronal Pools
Parallel after-discharge incoming neurons stimulate several
neurons in parallel arrays which stimulate a common output cell
complex neural functions such as calculations
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Peripheral Nervous System
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
Motor (efferent) division Transmits impulses from the CNS to
effector organs
Somatic nervous system
Conscious control of skeletal muscles
Autonomic nervous system (ANS)
Two divisions sympathetic and
parasympathetic
Regulates smooth muscle, cardiac
muscle, and glands
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Sensory (Afferent) Receptors
Special
Vision, hearing, taste, smell, balance
Superficial Touch, pressure, vibration, tickle, heat, cold, pain, itch
Deep
Position, kinesthesia, deep pressure, deep pain
Visceral
Hunger, nausea, distension, visceral pain
Classification
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Sensory Transduction
Response of a sensory receptor
to a stimulus
Chemoreceptor
Mechanocreceptor
Photoreceptor
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Sensory Coding
Stimulus intensity
Mean frequency of discharge (temporal summation)
Number of receptors activated (spatial summation)
Stimulus frequency
Intervals between discharges
Pattern of nerve impulses
Adaptation Accommodation to stimulus (slow or rapid)
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Neural IntegrationIntegration: 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 toincreasing stimulus
intensity
Adaptation signalstops when stimulus
becomes constant
Different pattern ofdischarge
S C di
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Sensory Coding
Pattern of discharge synchronized with stimulus frequency
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Somatic Motor Neurons
- motor neuron efferent extrafusal muscle fibers - voluntary
control
- efferent motor neuron intrafusal muscle fibers - musclespindle proprioception
Motor unit -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)
Skeletal Muscle Fiber Types
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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 byneural input pattern
-Slow-twitch = tonic innervation
pattern
-Fast-twitch = phasic
innervation pattern
Fiber type also determined by
trophic nerve substances
(axonal flow)
Motor Unit Recruitment
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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
Si P i i l f R it t
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Size Principle for Recruitment
In response to stretch, small motor
neurons recruited firstWhen stretch is released, large
motor neurons are deactivated first
Large motor neurons are moresusceptible to inhibition