NERVOUS SYSTEMHONORS ANATOMY & PHYSIOLOGY

Nervous Tissue & Homeostasis

excitable characteristic of nervous tissue allows for generation of nerve impulses (action potentials) that provide communication & regulation of most body tissue.

together with endocrine system: responsible for maintaining homeostasis

Differences in Nervous & Endocrine Control of Homeostasis

NERVOUS ENDOCRINE

rapid responderaction potentials

slow, prolonged response

releases hormones

Structures of the Nervous System

total mass of 2 kg (~3% of total body mass)

SkullSpinal CordSpinal NervesCranial NervesGangliaEnteric PlexusSpecial Senses & other Sensory Receptors

Major Structures of the Nervous System

Functions of the Nervous System

3 basic functions:1. Sensory2. Integrative3. Motor

Sensory Function

sensory receptors detect internal & external stimuli

sensory (afferent) neurons carry this sensory information to spinal cord & brain thru cranial & spinal nerves

Integrative Function

integrate: processnervous system takes information from

sensory neurons & processes that information, analyzes it, stores some of it & makes decisions for appropriate responses

served by interneurons (connect 1 neuron to another neuron

Perception:◦ conscious awareness of sensory stimuli◦occurs in brain

Motor Function

served by motor (efferent) neuronscarry info from brain/spinal cord

effectors (muscle or gland) thru cranial or spinal nerves

results in muscles contraction or gland secreting

Quick Quiz

What terms are given to neurons that carry input spinal cord & brain?

What terms are given to neurons that carry output out of the brain & spinal cord?

Organization of the Nervous System

Histology of the Nerrvous System

2 cell types1. Neurons2. Neuroglia

Neurons

nerve cells that possess electrical excitability:◦ability to respond to a stimulus & convert it into an action potential

◦stimulus: any change in environment that is strong enough to initiate an action potential

Direction Action Potential Travels

Action Potential

electrical signal that propagates along surface of neurolema (membrane)◦begins & travels due to movement of ions between interstitial fluid & inside of neuron thru specific ion channels

◦once begun it travels rapidly @ constant strength

Parts of a Neuron

Parts of Neuron: Cell Body

contains nucleus, cytoplasm, typical organelles,

+ Nissl bodies clusters of RER◦make materials for:

growth of neuron regenerate damaged axons in PNS

Nerve Fiber

general term for any neuronal process or extension that emerges from cell body

most neurons have 2:1. Dendrites2. Axons

Dendrites

“little trees”input portion of neuronusually, short, tapering, highly branchedtheir cytoplasm contains Nissl bodies,

mitochondria

Axon

propagates action potentials ◦another neuron◦muscle fiber◦gland cell

Parts of an Axon

joins cell body @ cone-shaped elevation: axon hillock

part of axon closest to hillock = initial segment

jct of axon hillock & initial segment where action potential arises so is called the trigger zone

Parts of an Axon

axoplasm: cytoplasm of an axonaxolemma: plasma membrane of axonaxon collaterals: side branches along

length of axon (most @ 90°)axon terminals: axon divides into many

fine processes

Synapse

site of communication between 2 neurons or between a neuron & effector cell

synaptic end bulbs: tips of some axon terminals swell into bulb-shaped structures

synaptic vesicles: store neurotransmitter◦many neurons have >1 neurotransmitter, each with different effects on postsynaptic cell

Axonal Transport

2 types:◦for moving materials from cell body axon

terminals

1. slow◦ 1-5 mm/d◦ replenishes new axoplasm to developing or

regenerating axons

2. fast◦ 200 – 400 mm/d◦ moves materials to/from cell body

organelles or membranes needed in axon terminal

Types of Neurons

Functional Classification

Structural Classification

SensoryInterneuronsMotor

use # processes extending from cell body

1. Multipolar neurons2. Bipolar neurons3. Unipolar neurons

Multipolar Neurons

several dendrites with 1 axonincludes most neurons in brain & spinal

cord

Bipolar Neuron

1 main dendrite & 1 axonretina, inner ear, olfactory area of brain

Unipolar Neuron

are sensory neurons that begin in embryo as bipolar

during development axon & dendrite fuse then divide into 2 branches (both have characteristic structure & function of an axon)

1 branch ends with dendrites (out of CNS)2nd branch ends in axon terminal (in CNS)cell bodies of most found in ganglia

Unipolar Neuron

Purkinje Cells

found in cerebellum

Pyramidal Cells

in cerebral cortex of brain

Neuroglia (Glia)

~50% vol of CNS“glue”do not generate or propagate action

potentialsmultiply & divide in mature nervous

systemsglioma:

◦ brain tumors derived from glial cells◦very malignant, grow rapidly

Glial Cells of the CNS

1. ASTROCYTES2. OLIGODENDROCYTES3. MICROGLIA4. EPENDYMAL CELLS

Astrocytes

star-shapedlargest & most numerous of glial cellsfunctions:1. physically support neurons2. assist in blood-brain-barrier (bbb)3. in embryo: regulate growth, migration,

&interconnections between neurons4. help maintain appropriate chemical

environment for propagation of action potentials

Oligodendrocytes

“few trees”smaller & fewer branches than astrocytesFunctions:1. form & maintain myelin sheath on axons

in CNS2. 1 oligo. myelinates many axons

Microglia

small cells with slender processes giving off many spine-like projections

function:1. phagocytes

◦ remove cellular debris made during normal development

◦ remove microbes & damaged nervous tissue

Ependymal Cells

single layer of cuboidal to columnar cellsciliated & have microvillifunction:1. line ventricles of brain & central canal of

spinal cord2. produce, monitor, & assist in circulation

of cerebrospinal fluid (CSF)3. form bbb

Neuroglial Cells of the PNS

Schwann cellsSatellite cells

Schwann Cells

functions:1. myelinate axons in PNS

◦ 1 Schwann cell myelinates 1 axon

2. participate in axon regeneration

Satellite Cells

flat cells that surround cell bodies of neurons in PNS ganglia

functions:1. structural support2. regulate exchange of materials between

neuronal cell bodies & interstitial fluid

Myelination

myelin sheath: made up of multilayered lipid & protein (plasma membrane) covering

function:1. electrically insulates axon2. increases speed of nerve impulses

Myelinated & Unmyelinated Axons

Nodes of Ranvier

gaps in myelin sheath1 Schwann cell wraps axon between

nodes of Ranvier

Sheath of Schwann = Neurolemma

Myelin

amount increases from birth to maturity infant‘s responses slower & less

coordinated as older child or adult in part because myelination is a work in progress thru infancy

Demyelination

loss of myelin sheathsee in disorders:

◦multiple sclerosis◦Tay-Sachs◦side effect of radiation therapy &

chemotherapy

Gray Matter of the Nervous System

contains:◦ neuronal cell bodies◦dendrites◦unmyelinated axons◦axon terminals◦neuroglia

White Matter of the Nervous System

composed of:◦myelinated axons