nervous system - peripheral and central

8/13/2019 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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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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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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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

nervous system - peripheral and central

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