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Neurons

Chapter 11

The Nervous System

The Nervous System

The nervous system is responsible for all of our behaviors, memories, & movements

The nervous system…

• Cooperates with endocrine system

• Senses environment

• Responds to changes in environment

• Maintains homeostasis

Functions of the Nervous System

1. Sensory input

– Information gathered by sensory receptors about internal and external changes

2. Integration

– Interpretation of sensory input

3. Motor output

– Activation of effector organs (muscles and glands) produces a response

Works through rapid and specific

electrical and chemical signals

to produce immediate responses

Sensory input

Motor output

Integration

Organization

• Central Nervous System

– Brain

– Spinal Cord

Organization

• Peripheral Nervous System

– Afferent (sensory)

– Efferent (motor)

• Somatic

• Autonomic

– Sympathetic

– Parasympathetic

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Copyright 2009 John Wiley & Sons, Inc. 7

Major Structures of the Nervous System

Central nervous system (CNS)

Brain and spinal cord

Integrative and control centers

Peripheral nervous system (PNS)

Cranial nerves and spinal nerves

Communication lines between the

CNS and the rest of the body

Parasympathetic

division

Conserves energy

Promotes house-

keeping functions

during rest

Motor (efferent) division

Motor nerve fibers

Conducts impulses from the CNS

to effectors (muscles and glands)

Sensory (afferent) division

Somatic and visceral sensory

nerve fibersConducts impulses from

receptors to the CNS

Somatic nervous

system

Somatic motor

(voluntary)Conducts impulses

from the CNS to

skeletal muscles

Sympathetic division

Mobilizes body

systems during activity

Autonomic nervous

system (ANS)

Visceral motor

(involuntary)Conducts impulses

from the CNS to

cardiac muscles,

smooth muscles,

and glands

Structure

Function

Sensory (afferent)

division of PNS

Motor (efferent)

division of PNS

Somatic sensory

fiber

Visceral sensory fiber

Motor fiber of somatic nervous system

Skin

StomachSkeletal

muscle

Heart

BladderParasympathetic motor fiber of ANS

Sympathetic motor fiber of ANS

Histology of Nervous Tissue

• Two principal cell types1. Neurons

• Excitable cells that transmit electrical signals

2. Accessory cells (neuroglia)• Non-excitable supporting cells

� Only about 20% empty space!

Neurons and Nerves

• Neurons

– Individual nerve cells

• Nerves

– Parallel bundles of neurons carrying

impulses in the PNS (called tracts in the CNS)

• Types– Sensory (afferent)

– Motor (efferent)

– Association (interneurons)

Nerves

� Endo = within

� Peri = next to; around

� Epi = above

Neurons

• Special characteristics of neurons

– Long lived

– Amitotic

– High metabolic rate

– Excitable

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The Structure of a Neuron

• Body

• Dendrites

• Axon (only one)

Copyright © 2010 Pearson Education, Inc.

Figure 11.4 Structure of a motor neuron.

Dendrites

(receptiveregions)

Cell body

(biosynthetic centerand receptive region)

Nucleolus

Nucleus

Nissl bodies

Axon

(impulsegeneratingand conductingregion)

Axon hillock

NeurilemmaTerminalbranches

Node of Ranvier

Impulsedirection

Schwann cell(one inter-node)

Axon terminals(secretoryregion)

Dendriticspine

Neuron cell body

(a)

(b)

The Structure of a Neuron

• Unipolar neurons– Found mostly in ANS, although some in CNS

The Structure of a Neuron

• Pseudounipolar neurons– Found chiefly in PNS as sensory neurons

The Structure of a Neuron

• Bipolar neurons– Rare, but found in the special sense organs (retina, olfactory mucosa)

The Structure of a Neuron

• Multipolar neurons– Most common type in humans (>99%)

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Accessory Cells

• Neuroglia

• About 50% of cellular mass in nervous tissue

• Do not conduct impulses

• Most retain capacity to divide

• Different types found in PNS and CNS

Accessory Cells

• Neuroglia of the CNS

1. Astrocytes - support

2. Oligodendrocytes - myelin

3. Microglial cells - defense

4. Ependymal cells - CSF

Accessory Cells

• Astrocytes

– Most abundant of CNS neuroglia

– Contact blood vessels

– Blood Brain Barrier (BBB) regulates passage of

molecules

Figure 11.3a

Astrocytes are the most abundant of

CNS neuroglia. Provide most structural support

Capillary

Neuron

Astrocyte

Accessory Cells

• Oligodendrocytes

– Branched cells

– Processes wrap around CNS neurons

• Form insulating myelin sheaths

Figure 11.3d

Oligodendrocytes have processes that form

myelin sheaths around CNS nerve fibers.

Nerve

fibers

Myelin sheath

Process of

oligodendrocyte

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Myelin insulates neuronal axons, much the way

insulation protects wires.

Accessory Cells

– Ependymal cells

• Vary in shape from squamous to columnar

• Many are ciliated

• Line the central cavities of the brain and spinal column

• Form cerebrospinal fluid

• Separate the CNS interstitial fluid from the cerebrospinal fluid

in the cavities

Figure 11.3c

Brain or

spinal cord

tissue

Ependymal

cells

Fluid-filled cavity

Ependymal cells line cerebrospinal

fluid-filled cavities.

Accessory Cells

• Microglia– Small, ovoid cells with thorny processes

– Migrate toward injured neurons

– Phagocytize microorganisms and neuronal debris• Why do you think this would be so important?

Add these to your list!

Figure 11.3b

(b) Microglial cells are defensive cells in

the CNS.

Neuron

Microglial

cell

Accessory Cells

• Neuroglia of PNS

1. Satellite cells

• Surround neuron cell bodies in the PNS

2. Schwann cells

• Surround peripheral nerve fibers and form myelin sheaths

– Axon + myelin = nerve fiber

• Vital to regeneration of damaged peripheral nerve fibers

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Figure 11.3e

Satellite cells and Schwann cells (which

form myelin) surround neurons in the PNS.

Schwann cells

(forming myelin sheath)

Cell body of neuronSatellite

cells

Nerve fiber

Accessory Cells

• Schwann cells cont.

– Myelin sheath

– Neurilemma

– Nodes of Ranvier

Figure 11.5a

(a) Myelination of a nerve

fiber (axon)

Schwann cell

cytoplasm

Axon

Neurilemma

Myelin sheath

Schwann cellnucleus

Schwann cell

plasma membrane

1

2

3

A Schwann cell

envelopes an axon.

The Schwann cell then

rotates around the axon,

wrapping its plasma

membrane loosely around

it in successive layers.

The Schwann cell

cytoplasm is forced from

between the membranes.

The tight membrane

wrappings surrounding

the axon form the myelin

sheath.

Multiple Sclerosis

• Autoimmune disease of young adults

• Variety of clinical signs

• No known cure

Antibodies produced in multiple sclerosis attack

myelin made by oligodendrocytes, and lead to

demyleination in the CNS.