Introduction to the Nervous System

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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 responder• action potentials

• slow, prolonged response

• releases hormones

Structures of the Nervous System

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

• Skull• Spinal Cord• Spinal Nerves• Cranial Nerves• Ganglia• Enteric Plexus• Special Senses & other Sensory Receptors

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: process• nervous 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) neurons• carry info from brain/spinal cord

effectors (muscle or gland) thru cranial or spinal nerves

• results in muscles contraction or gland secreting

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

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 neuron• usually, short, tapering, highly

branched• their 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 axon• axolemma: plasma membrane of

axon• axon 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

• Sensory• Interneurons• Motor

• use # processes extending from cell body

1. Multipolar neurons2. Bipolar neurons3. Unipolar neurons

Multipolar Neurons

• several dendrites with 1 axon• includes most neurons in brain &

spinal cord

Bipolar Neuron

• 1 main dendrite & 1 axon• retina, 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

Pyramidal Cells

• in cerebral cortex of brain

Neuroglia (Glia)• ~50% vol of CNS• “glue”• do not generate or propagate action

potentials• multiply & divide in mature nervous

systems• glioma:– brain tumors derived from glial cells– very malignant, grow rapidly

Glial Cells of the CNS

1. ASTROCYTES2. OLIGODENDROCYTES3. MICROGLIA4. EPENDYMAL CELLS

Astrocytes • star-shaped• largest & most numerous of glial cells• functions: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

astrocytes• Functions: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

cells• ciliated & have microvilli• function: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 cells• Satellite 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 sheath• 1 Schwann cell wraps axon between

nodes of Ranvier

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 sheath• see 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