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Chapter 12Nervous Tissue

The Nervous System Objectives

Describe the two anatomical divisions of the nervous system.

List the general functions of both divisions.

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Copyright 2009 John Wiley & Sons, Inc.

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Major Structures of the Nervous System

The Nervous System The nervous system functions to:

monitor the internal and external environment

integrate sensory information

coordinate voluntary and involuntary responses of other organ systems

Functions are performed by neurons, the functional unit of the nervous system.

Neuroglia support and protect the neurons.

Two major subdivisions of the nervous system are:

central nervous system (CNS)

peripheral nervous system (PNS)

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Central Nervous System

consists of the brain and the spinal cord.

integrates and coordinates sensory data and motor commands.

is the seat of higher functions (intelligence, memory, and emotion)

Peripheral Nervous System

provides communication between the CNS and the rest of the body.

includes all the neural tissue outside the CNS.

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afferent division - brings sensory information to CNS

Ex. From touch receptor to brain

efferent division - brings motor commands to muscles and glands

Ex. From brain to triceps brachii

Peripheral Nervous System

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Subdivisions of the PNS Somatic (voluntary) nervous system (SNS)

motor neurons to skeletal muscle tissue – controls skeletal muscle contractions

Autonomic (involuntary) nervous systems (ANS) motor neurons to smooth & cardiac muscle and glands –

involuntary control

sympathetic division (speeds up heart rate)

parasympathetic division (slow down heart rate)

Enteric nervous system (ENS) involuntary sensory & motor neurons control GI tract neurons function independently of ANS & CNS

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Copyright 2009 John Wiley & Sons, Inc.

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Organization of the Nervous System

Cellular Organization

Objective: Distinguish between neurons and

neuroglia on the basis of their structure and

function.

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Cellular Organization All neural tissue consists of two kinds of cells. Neurons

functional unit of the nervous system all neural functions involve the communication of

neurons with each other and with other cells Cannot divide

Neuroglia regulate the environment around the neurons provide a support framework for neural tissue act as phagocytes retain the ability to divide much smaller and much more numerous than

neurons

Dendrites

Cell body

Axon

Nuclei ofneuroglia

cell body

several branching sensitive dendrites receive incoming signals

elongated axon carry outgoing signals

one or more synaptic terminals communicates with other

cells

Neurons - Structure

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Neurons - Structure

Cell body large nucleus with large nucleolus

mitochondria

free and fixed ribosomes

nissl bodies - gray clusters of RER and free ribosomes

usually no centrioles

Neurons

Membrane of the cell body and dendrites are sensitive to chemical, mechanical, and electrical stimulation.

Stimulation results in an action potential from the axon hillock.

Axon may branch into collaterals each with a synaptic terminal.

Synapse is the site where communication occurs.

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Structure of a Multipolar Neuron

Structural & Functional Classification of Neurons Structural

multipolar neuron

unipolar neuron

bipolar neuron

Functional

sensory neurons

motor neurons

interneurons

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Multipolar Neuron

Multiple processes extending away from the cell body.

Very common in the CNS

Unipolar Neuron

Dendrites and axon are continuous and the cell body lies to one side.

Action potential begins at the base of the dendrites and the rest of the process is considered an axon.

Most sensory neurons of the PNS are unipolar.

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Bipolar Neurons

Cell body lies between the one dendrite and one axon.

Rare, occur in special sense organs such as the eye and ear.

Sensory Neurons (afferent)

There are about 10 million neurons in the afferent division. These are all sensory neurons

Receptors are categorized based on the information that they carry somatic sensory receptors

external receptors proprioceptors

visceral receptors

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Motor Neurons (efferent)

There are about .5 million motor neurons of the efferent division.

There are two efferent divisions of the PNS:

Somatic motor neurons of the somatic nervous system innervate the skeletal muscles.

Visceral motor neurons of the autonomic nervous system innervate smooth muscle, cardiac muscle, and glands

Interneurons

There are about 20 billion interneurons.

Located within the brain and spinal cord.

Interconnect other neurons.

Responsible for the distribution of sensory information and the coordination of motor activity.

A stimulus that requires a more complex response involves a greater number of interneurons.

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Neuroglia Objectives

Describe the locations of neuroglia.

Explain the functions of each type of neuroglia.

Neuroglia

found in the CNS and PNS.

The CNS has the greatest variety of glial cells.

There are four types of glial cells in the CNS.

astrocytes

oligodendrocytes

microglia

ependymal

Schwann cells are the major glial cells in the PNS.

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Copyright 2009 John Wiley & Sons, Inc.

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Neuroglia of the CNS

Ependymal Cells

Epithelial cells that line the central canal of the spinal cord and the ventricles of the brain.

Produce cerebrospinal fluid (CSF) in certain regions of the brain.

Cilia help to circulate the CSF in brain ventricles.

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Astrocytes

Largest and most numerous glial cells.

Secrete chemicals that maintain blood brain barrier.

Create a structural framework for the CNS.

Perform repairs in damaged neural tissue.

Direct growth and interconnection for developing neurons in the embryonic brain.

Adjust composition of ECF.

Oligodendrocytes

Cytoplasmic extensions create a myelin sheath around axons.

Myelin increases the speed of an action potential.

nodes of Ranvier (nodes): gaps between adjacent oligodendrocytes

internodes: areas covered in myelin.

Lipid rich myelin causes axons to appear glossy white.

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Microglia

Smallest and rarest of the neuroglia in the CNS.

Phagocytic cells.

Schwann Cells

Most important glial cells in the PNS.

Cover every axon outside the CNS whether myelinated or not.

The Schwann cell covering of the axon is called the neurilemma.

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Neuroglia of the PNS

Demyelination

The progressive destruction of myelin sheaths.

inflammation

axon damage

scarring of neural tissue

Results in gradual loss of sensation and motor control.

numbness / weakness

paralysis

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Anatomical Organization

Neurons and their axons form bundles with distinct anatomical boundaries.

PNS neuron cell bodies are located in ganglia axons are bundled in nerves

spinal nerves connected to spinal cord cranial nerves connected to brain

Anatomical Organization CNS

group of neuron cell bodies with a common function is a center (or nucleus if it has a distinct boundary).

brain surface is covered by a thick layer of gray matter called neural cortex

the most complex integration centers, nuclei, and cortical areas in the brain are called higher centers.

white matter containing bundles of axons sharing common origins, destinations, and functions are tracts

tracts in the spinal cord form larger groups called columns. Pathways link the centers of the brain with the rest of the

body sensory (ascending) pathways take info to the brain motor (descending) pathways result in motor control at

the skeletal muscles

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?????

1. What would damage to the afferent division of the nervous system affect?

2. Examination of a tissue sample shows unipolar neurons. Are these more likely to be sensory neurons or motor neurons?

3. Which type of glial cell would you expect to be present in large numbers in brain tissue from a person suffering from an infection of the central nervous system?

?????

1. Afferent division of the nervous system is composed of nerves that carry sensory information to the brain and spinal cord. Damage would interfere with a persons ability to experience a variety of sensory stimuli.

2. Sensory neurons of the peripheral nervous system are usually unipolar. This tissue is most likely associated with a sensory organ.

3. Microglial cells are small phagocytic cells found in increased number in damaged and diseased areas of the CNS.