The nervous system

Copyright © 2010 Pearson Education, Inc.

THE NERVOUS SYSTEMCh. 7


Functions of the Nervous System

1. Sensory input

• Sensory receptors gather information about internal and external changes

2. Integration

• Interpretation of sensory input

3. Motor output

• Activation of effector organs produces a response

Copyright © 2010 Pearson Education, Inc. Figure 7.1

Sensory input

Motor output

Integration


Structural Classification of the Nervous System

• Central nervous system (CNS)

• Brain and spinal cord

• Integration and command center

• Peripheral nervous system (PNS)

• Paired spinal and cranial nerves carry messages to and from the CNS


Functional Classification : The Peripheral Nervous System (PNS)

• Two functional divisions

1. Sensory (afferent) division

• Somatic afferent fibers—convey impulses from skin, skeletal muscles, and joints

• Visceral afferent fibers—convey impulses from visceral organs

2. Motor (efferent) division

• Transmits impulses from the CNS to effector organs


Motor Division of PNS

1. Somatic (voluntary) nervous system

• Conscious control of skeletal muscles


Motor Division of PNS

2. Autonomic (involuntary) nervous system (ANS)

• Visceral motor nerve fibers

• Regulates smooth muscle, cardiac muscle, and glands

• Two functional subdivisions

• Sympathetic

• Parasympathetic


Central nervous system (CNS)

Peripheral nervous system (PNS)

ParasympatheticConserves energyPromotes house-keeping functions

during rest

Motor (efferent) division

Sensory (afferent) division

Somatic NSSomatic motor

(voluntary)

skeletal muscles

SympatheticMobilizes bodysystems during

activity

Autonomic NSVisceral motor (involuntary)

cardiac muscles,smooth muscles,

and glands

Skin

Heart

Bladder


Nervous Tissue: Structure and Function

• Two principal cell types

1. Neurons—excitable cells that transmit electrical signals


2. Neuroglia (glial cells)—supporting cells:

• Astrocytes (CNS)

• Microglia (CNS)

• Ependymal cells (CNS)

• Oligodendrocytes (CNS)

• Satellite cells (PNS)

• Schwann cells (PNS)

Nervous Tissue: Structure and Function


(a) Astrocytes are the most abundantCNS neuroglia.

Capillary

Neuron

Astrocyte


(b) Microglial cells are defensive cells inthe CNS.

NeuronMicroglialcell


Brain orspinal cordtissue

Ependymalcells

Fluid-filled cavity

(c) Ependymal cells line cerebrospinalfluid-filled cavities.


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

Nervefibers

Myelin sheath

Process ofoligodendrocyte


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

Schwann cells(forming myelin sheath)

Cell body of neuronSatellitecells

Nerve fiber


(a) Myelination of a nervefiber (axon)

Schwann cellcytoplasm

Axon

Neurilemma

Myelin sheath

Schwann cellnucleus

Schwann cellplasma membrane

1

2

3


Cell Body

• Network of neurofibrils help maintain cell shape

• Axon hillock—cone-shaped area from which axon arises

• Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS


Dendrites(receptive regions)

Cell body(biosynthetic centerand receptive region)

Nucleus

Nissl bodies

Axon(impulse generatingand conducting region)

Axon hillock

NeurilemmaTerminalbranches

Node of Ranvier

Impulsedirection

Schwann cell(one inter-node)

Axonterminals(secretoryregion)

(b)


Processes• Dendrites• Receptive (input) region of a neuron• Convey electrical signals toward cell body

• Axons• Generate & transmits nerve impulses away

from cell body• One axon/cell arising from axon hillock• Long axons=nerve fibers

• Bundles of processes are called: • Tracts in the CNS• Nerves in the PNS


Myelin Sheaths in the CNS

• Formed by processes of oligodendrocytes, not the whole cells

• Nodes of Ranvier are present

• No neurilemma


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

Nervefibers

Myelin sheath

Process ofoligodendrocyte


White Matter and Gray Matter

•White matter

• Dense collections of myelinated fibers

• Gray matter

• Mostly neuron cell bodies and unmyelinated fibers


Structural Classification of Neurons

• Three types:

1. Multipolar—1 axon and several dendrites

• Most abundant

• Motor neurons and interneurons

2. Bipolar—1 axon and 1 dendrite

• Rare, e.g., retinal neurons


Structural Classification of Neurons

3. Unipolar—single, short process that has two branches:

• Peripheral process—more distal branch, often associated with a sensory receptor

• Central process—branch entering the CNS


Functional Classification of Neurons

• Three types:

1. Sensory (afferent)

• Transmit impulses from sensory receptors toward the CNS

2. Motor (efferent)

• Carry impulses from the CNS to effectors


Functional Classification of Neurons

3. Interneurons (association neurons)

• Shuttle signals through CNS pathways; most are entirely within the CNS

The nervous system

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