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Human Anatomy, Second EditionMcKinley & O'Loughlin

Chapter 14 Lecture Outline:Nervous Tissue

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The Nervous System The primary communication

and control system of the body.

Fast and homeostatic It can be divided into structural

and functional categories.

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Organization of the Nervous System

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Structural Organization Central nervous system (CNS)

brain and spinal cord Peripheral nervous system

(PNS) cranial nerves (to and from the brain) spinal nerves (to and from the spinal

cord) ganglia (clusters of neuron cell bodies

located outside the CNS)

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Functional Organization: Sensory and Motor Nervous Systems

Three general functions Collecting information Processing and evaluating

information Responding to information

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Sensory Division Somatic sensory components are

the general somatic senses—touch, pain, pressure, vibration, temperature, and proprioception.

Visceral sensory components transmit nerve impulses from blood vessels and viscera to the CNS. The visceral senses primarily include temperature and stretch (of the organ wall).

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Motor Division The somatic motor component (somatic

nervous system; SNS) conducts nerve impulses from the CNS to skeletal muscles. also known as the voluntary nervous system

The autonomic motor component (autonomic nervous system; ANS) innervates internal organs, regulates smooth muscle, cardiac muscle, and glands. also known as the visceral motor system or

involuntary nervous system

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Cytology of Nervous Tissue Two distinct cell types form

nervous tissue. Neurons are excitable cells that

initiate and transmit nerve impulses. Structural units of the nervous system.

Glial cells are nonexcitable cells that support and protect the neurons

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Neurons Neurons have a high

metabolic rate. Neurons have extreme

longevity. Neurons typically are non-

mitotic.

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New Neurons in Adults? A population of immature

progenitor cells in the hippocampus known as neural stem cells can under certain circumstances mature into neurons

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Neuron Structure Neurons come in all shapes and sizes,

but all neurons share certain basic structural features. Some are tiny, less than 1 mm in the

CNS Some are some of the largest cells in

the body A typical neuron has a cell body,

dendrites, and axons.

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Neuron Structure – Dendrites Dendrites tend to be shorter, smaller

processes that branch off the cell body. Some neurons have only one dendrite, while

others have many (100’s often). Dendrites conduct nerve impulses toward

the cell body; they receive input and then transfer it to the cell body for processing.

The more dendrites a neuron has, the more nerve impulses that neuron can receive from other cells.

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Neuron Structure – Cell Body The cell body serves as the neuron’s control

center and is responsible for receiving, integrating, and sending nerve impulses.

Has most of the organelles found in other cells, but no centrioles (no mitosis or regeneration)

Chromatophilic substance, Nissl bodies - rough ER and ribosomes

Lipofuscin inclusions - probably lysosomal wastes. Yellowish-brown. Probably harmless.

Neurofibrils and intermediate filaments form cytoskeleton

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Neuron Structure – Axon The larger, typically longer nerve cell process

emanating from the cell body is the axon, sometimes called a nerve fiber.

Most neurons have one axon, but no more than one. May have collaterals.

The axon transmits a nerve impulse away from the cell body toward another cell.

May be short, absent, or long (3-4 feet) Axon originates at the axon hillock and then tapers.

The nerve impulse, action potential, starts at the first part, the initial segment.

Branches a lot at the end usually, axon terminals, telodendria. May be over 10,000. Ends at synaptic knobs (end bulbs, etc.)

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Neuron Classification Neurons vary widely in morphology

and location. They can be classified according to

either their structure or their function.

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Structural Classification By the number of processes

extending from the cell body. Unipolar

PNS mainly: sensory neurons Bipolar

Rare: sense organs’ receptor cells in the eye, ear, and nose

Multipolar Most common in humans Major type in CNS Most motor neurons and association neurons

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Functional Classification Sensory neurons Interneurons or association

neurons Motor neurons

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Interneurons Interneurons, or association neurons,

lie entirely within the CNS and are multipolar.

They receive nerve impulses from many other neurons and carry out the integrative function of the nervous system.

Thus, interneurons facilitate communication between sensory and motor neurons.

Thousands of types Purkinje cells in the cerebellum Renshaw cells in the spinal cord Pyramidal cells in the cerebral cortex

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Functions of Glial Cells Form a structural network. Replace damaged neurons. Assist neuronal development.

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Glial Cells (Neuroglia) Occur within both the CNS and the PNS.

4 of the 6 types are within the CNS. Smaller and capable of mitosis. Do not transmit nerve impulses. Physically protect and help nourish neurons,

and provide an organized, supporting framework for all the nervous tissue.

Far outnumber neurons (about 10x). Account for roughly half the volume of the

nervous system. Almost 1/2 of brain tumors are gliomas.

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Astrocytes exhibit a starlike shape due to projections from their surface.

Astrocytes are the most abundant and largest glial cells in the CNS, and they constitute over 90% of the tissue in some areas of the brain.

Help from the blood-brain barrier (BBB) that strictly controls substances entering the nervous tissue in the brain from the bloodstream.

Regulate tissue fluid composition (ionic balance). Forming a structural network. Replacing damaged neurons. Assisting neuronal development.

Glial Cells of the CNS

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Glial Cells of the CNS Ependymal cells line the

ventricles and spinal cord and produce and move CSF

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Glial Cells of the CNS Microglial cells are brain

macrophages Rarest and smallest of the glial

cells

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Glial Cells of the CNS Oligodendrocytes myelinate fibers

in the CNS One oligodendrocyte may help

myelinate 60 axons Cell body not involved in wrapping (like

an octopus) No neurilemma Large nodes of Ranvier

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Glial Cells of the PNS Satellite cells are found in the

ganglia around the cell bodies. Physically separate cell bodies

from the surrounding interstitial fluid and regulate exchanges

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Glial Cells of the PNS Neurolemmocytes or Schwann

cells myelinate in the PNS Have neurilemma around myelin

sheath Many Schwann cells/axon Smaller nodes of Ranvier

(neurofibril nodes)

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Myelination of Axons Main activity of axons is nerve

impulse conduction. Nerve impulse (action potential) is

the rapid movement of an electrical charge along a neuron’s plasma membrane. A voltage (potential) change that moves

along the plasma membrane The ability to travel along an axon is

affected by myelination

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Myelination Myelination is the process by which part of

an axon is wrapped with a myelin sheath, a protective fatty coating that gives it glossy-white appearance (white matter).

The myelin sheath supports, protects, and insulates an axon.

No change in voltage can occur across the membrane in the insulated portion of an axon.

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Nerve Impulse Conduction In a myelinated axon, the nerve impulse

“jumps” from neurofibril node to neurofibril node and is called saltatory conduction.

In an unmyelinated axon, the nerve impulse must travel the entire length of the axon and is called continuous conduction and is slower. Pain stimuli carried this way.

A myelinated axon produces a faster nerve impulse and it requires less energy.

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Development of the Myelin Sheath Starts during late fetal development

and first year Increases to maturity Increase in conduction rate

with increase myelination Explains why infants are slower and

less coordinated in their responses

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Regeneration of PNS Axons PNS axons are vulnerable to cuts, crushing

injuries, and other trauma. A damaged axon can regenerate, however,

if at least some neurilemma remains. PNS axon regeneration depends upon three

factors. the amount of damage neurolemmocyte secretion of nerve growth

factors to stimulate outgrowth of severed axons

the distance between the site of the damaged axon and the effector organ

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Regeneration of CNS Axons Very limited Oligodendrocoytes do not release a nerve

growth factor Large number of axons crowded together Astrocytes and connective tissue

coverings may form some scar tissue that obstructs axon regrowth

No neurilemma to act as a regeneration tube

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Demyelination Progressive destruction of myelin

sheath is accompanied by inflammation, axon damage, and scarring of neural tissue

Usually results in a gradual loss of sensation and motor control

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Causes of Demyelination Heavy metal poisoning Diptheria Multiple sclerosis Guillain-Barre syndrome

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Nerves A nerve is a bundle of parallel axons. Like a muscle, a nerve has three successive

connective tissue wrappings. endoneurium - a delicate layer of loose

connective tissue perineurium - a cellular and fibrous connective

tissue layer that wraps groups of axons into bundles called fascicles

epineurium - a superficial connective tissue covering

A thick layer of dense irregular fibrous connective tissue encloses the entire nerve, providing both support and protection

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Nerves Nerves are a component of the peripheral

nervous system. Sensory (afferent) nerves convey sensory

information to the CNS. Motor (efferent) nerves convey motor

impulses from the CNS to the muscles and glands.

Axons terminate as they contact other neurons, muscle cells, or gland cells.

An axon transmits a nerve impulse at a specialized junction called a synapse.

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Nerve Regeneration and Spinal Cord Injuries Spinal injuries at the neck used to be fatal

because of respiratory failure Much better survival now from early steroid use

and antibiotics Reconnection and partial restoration of function

of severed cords in rats Olfactory nerve bridge spans injured area and acts as a

guide (not in humans yet) Neural stem cells may be able to regenerate CNS axons

Christopher Reeve, paralyzed below the second cervical vertebra, made tremendous progress and raised funds and awareness

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Synapses Presynaptic neurons transmit nerve

impulses along their axonal membranes toward a synapse.

Postsynaptic neurons conduct nerve impulses through their dendritic and cell body membranes away from the synapse.

Axons may establish synaptic contacts with any portion of the surface of another neuron, except those regions that are myelinated.

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Synaptic Communication

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Electrical Synapses Electrical synapses are not very common in

mammals. In humans, these synapses occur primarily

between smooth muscle cells where quick, uniform innervation is essential.

Electrical synapses are also located in cardiac muscle and the developing embryo.

Use gap junctions. Advantages

Faster communication Can synchronize the activity of a group of

neurons or muscle fibers

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Chemical Synapses The most numerous type of synapse is the

chemical synapse. It facilitates most of the interactions between

neurons and all communications between neurons and effectors.

At these junctions, the presynaptic membrane releases a signaling molecule called a neurotransmitter, such as acetylcholine (ACh), into the synaptic cleft.

Other types of neurons use other neurotransmitters.

Synaptic delay of about 0.5 msec

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Neurotransmitters Are released only from the plasma membrane of

the presynaptic cell. It then binds to receptor proteins found only on

the plasma membrane of the postsynaptic cell. A unidirectional flow of information and

communication takes place. Two factors influence the rate of

conduction of the impulse: the axon’s diameter and the presence (or absence) of a myelin sheath.

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Graded Potentials Graded potentials occur on the

dendrites and cell body (no polarity reversal, a local depolarization)

Spreads from the receptive zone to the axon hillock (the trigger zone). It decreases in strength as it travels.

If sufficient graded potentials at the initial segment, then it will initiate an action potential (all or none) if the voltage threshold is exceeded.

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Synapses The typical CNS neuron receives input

from 1,000 to 10,000 synapses! Synapses may be excitatory and/or

inhibitory. Integration of inputs on post-synaptic

neuron can result in: 1 or more nerve impulses if the threshold is

reached or surpassed No impulse if inhibitory effects are greater than

the excitatory effects Facilitated: if subthreshold stimulation. More

easily can generate an impulse.

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Synapses The neurotransmitter affects the post-

synaptic neuron as long as it remains in the synaptic cleft.

Three ways to remove Diffusion Destroyed by enzymes Reuptake into presynaptic neuron or

transport to neighboring neuron Example, Prozac is a selective serotonin

reuptake inhibitor (SSRI)

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Synapses Synapses are essential for

homeostasis by transmitting certain impulses and inhibiting others.

Often brain and psychiatric disorders result from disruption of synaptic communication.

Also site for most drugs that affect brain, therapeutic and addictive

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Neural Integration and Neuronal Pools Billions of interneurons within the CNS are

grouped in complex patterns called neuronal pools (or neuronal circuits or pathways).

Neuronal pools are defined based upon function, not anatomy, into four types of circuits: converging diverging reverberating parallel-after-discharge

A pool may be localized, or its neurons may be distributed in several different regions of the CNS.

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Nervous System Disorders Amyotrophic lateral sclerosis Multiple sclerosis Parkinson disease Guillain-Barre syndrome

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Neural Tube Defects Serious developmental deformities

of the brain, spinal cord, and meninges

Anencephaly Spina bifida

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Development of the Nervous System Begins in the third week in the

embryo from the ectoderm.