Nervous Tissue:Neurons and Support Cells

(glial cells)Pages 227-234

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Astrocytes◦ Brace/anchor neurons, provide chemical barrier◦ Most abundant glial cells

Microglia◦ Destroy threatening particles/cells; phagocytes

Ependymal cells◦ Line ventricles and central canal; cilia circulate

CSF Oligodendrocytes

◦ Produce myelin sheath around axons (in the CNS)

Support Cells: Neuroglia

Figure 7.3a Supporting (glial) cells of nervous tissue.

Capillary

Neuron

Astrocyte

(a) Astrocytes are the most abundantand versatile neuroglia.

ASTROCYTES

Figure 7.3b Supporting (glial) cells of nervous tissue.

NeuronMicroglialcell

(b) Microglial cells are phagocytes thatdefend CNS cells.

MICROGLIA

Figure 7.3c Supporting (glial) cells of nervous tissue.

Ependymalcells

Brain orspinal cordtissue

Fluid-filled cavity

(c) Ependymal cells line cerebrospinalfluid-filled cavities.

EPENDYMAL CELLS

Figure 7.3d Supporting (glial) cells of nervous tissue.

Myelin sheath

Process ofoligodendrocyte

Nervefibers

(d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers.

OLIGODENDROCYTES

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Satellite cells◦ Protect neuron cell bodies

Schwann cells◦ Form myelin sheath around axons (in the PNS)

Support Cells: PNS glial cells

Figure 7.3e Supporting (glial) cells of nervous tissue.

Satellitecells Cell body of neuron

Schwann cells(forming myelin sheath)

Nerve fiber

(e) Satellite cells and Schwann cells (whichform myelin) surround neurons in the PNS.

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specialized to transmit messages Major regions of neurons

◦ Cell body— contains the nucleus metabolic center of the cell

◦ Processes—fibers that extend from the cell body Dendrites : pick up sensory stimuli Axons : receive impulse from cell body

Impulses are UNIDIRECTIONAL along the axon

Neurons (nerve cells)

Figure 7.4a Structure of a typical motor neuron.

Mitochondrion

Nucleus

DendriteCell body

Axon

One Schwann cell

Node of Ranvier

Schwann cells,forming the myelinsheath on axon

Axonterminal

(a)

Figure 7.4b Structure of a typical motor neuron.

Neuroncell body

Dendrite

(b)

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End in axon terminals◦ Terminals contain vesicles filled with

neurotransmitters

◦ Neurons are separated from other cells by a gap called the synaptic cleft:

Synapse—junction between nerves Neurons NEVER touch each other

Axons and the Synaptic Cleft

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The Myelin Sheath• whitish, fatty material covering

axons• Nodes of Ranvier

– gaps in myelin sheath along the axon

– impulse jumps from one node to the next• The current cannot flow across

the axon where there is myelin so it jumps between the myelin bundles

• The presence of the myelin sheath speeds the nerve impulse transmission

Myelin sheath becomes destroyed Replaced with hardened scleroses

◦ Sclerosis: hardening of body tissue Incomplete/staggered transmission of

impulse Autoimmune disease- potientially caused

by:◦ Genes◦ Smoking◦ Viral infections◦ Vitamin D deficiency

Multiple Sclerosis

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Sensory (afferent) neurons◦ Carry impulses from sensory receptors to the CNS

Cutaneous (skin) sense organs Pain receptors Temperature receptors Touch/pressure receptors

Proprioceptors—detect stretch or tension Muscle spindle in muscles Golgi tendon organ in tendonw

Classification of Neurons

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Motor (efferent) neurons◦ Carry impulses from the central nervous system

to viscera, muscles, or glands

Interneurons (association neurons)◦ Located in the spinal cord (CNS)◦ Connect sensory and motor neurons◦ Play role in reflex arc – allow quicker response

without involving the brain

Functional Classification of Neurons

Figure 7.6 Neurons classified by function.

Dendrites Peripheralprocess (axon)

Ganglion

Cellbody

Sensoryneuron

Central process (axon)

Spinal cord(central nervous system)

Interneuron(associationneuron)

Afferenttransmission

Peripheralnervous system

ReceptorsEfferent transmission

Motor neuron

To effectors(muscles and glands)