Copyright © 2009 Pearson Education, Inc. Neurons and Neurological Cells: The Cells of the Nervous...

Copyright © 2009 Pearson Education, Inc.

Neurons and Neurological Cells: The Cells of the Nervous System

The nervous system Integrates and coordinates many of the

body’s activities

The nervous system is divided into The central nervous system (CNS)

The brain and spinal cord The peripheral nervous system (PNS)

All of the nervous tissue in the body besides the brain and spinal cord


Cells of the Nervous System

The nervous system is composed of two types of specialized cells

Neurons Excitable cells that generate and transmit

messages

Neuroglial cells (also called glial cells) More numerous and provide structural

support, growth factors, and insulating sheaths around the nerves


Cells of the Nervous System

Sensory neurons Carry information toward the CNS from

sensory receptors

Motor neurons Carry information away from the CNS to an

effector

Interneurons Located between sensory and motor

neurons within the CNS, where they integrate and interpret sensory signals


Neurons Have Dendrites, a Cell Body, and an Axon

The shape of a typical neuron is specialized for communicating with other cells. It has: Many short, branching projections called

dendrites (one or more) A single long extension of the neuron called

an axon (one) A cell body (one) which contains the nucleus

and other organelles, and functions to maintain the neuron


Neurons Have Dendrites, a Cell Body, and an Axon

Dendrites Carry information toward the cell body of a

neuron

A single long axon Carries information away from the cell body


Dendrites, Cell Body, and Axon

Nerves Consist of parallel axons, dendrites, or both

from many neurons Covered with tough connective tissue Classified as sensory, motor or mixed

(sensory and motor together) depending on the type of neurons they contain



Most axons not found in the CNS, and some of those within, are electrically insulated by a myelin sheath which increases the rate of conduction of a nerve impulse



In the PNS, Schwann cells form the myelin sheath, insulating it and allowing messages to travel faster as they jump from one node of Ranvier to the next in a type of transmission called saltatory conduction



The myelin sheath also facilitates nerve repair outside the CNS

When an axon in the PNS is cut, the Schwann cells take part in regeneration as they wrap around an axon



Multiple sclerosis (MS) Results from the destruction of the myelin

sheath that surrounds axons found in the CNS

The resulting scars (scleroses) interfere with the transmission of nerve impulses

Can result in paralysis and loss of sensation, including loss of vision


The Nerve Impulse Is an Electrochemical Signal

A nerve impulse, or action potential, is a bioelectrical signal involving sodium ions (Na+) and potassium ions (K+) that cross the cell membrane through ion channels

Each ion channel is designed to allow only certain ions to pass through


The Nerve Impulse

Ions are also transported across the membrane actively by the sodium-potassium pump (3Na+ out, 2K+ in)

When a neuron is not conducting a nerve impulse, it is in a resting state There is a slight difference in charge across

the membrane which is called the resting potential (mainly due to action of sodium-potassium pump)


The Nerve Impulse

When the neuron is stimulated, there is a sudden reversal of charge across the membrane because the sodium gates open and sodium ions enter the cell The minimum charge that causes the sodium

gates to open is called the threshold


The Nerve Impulse

Next, the potassium gates open and potassium ions rush out of the cell, causing the cell to return to the original state, or repolarize

Lastly, resting potential must be restored Note the charge across the membrane

actually dips below the resting potential as the potassium gates are slow to close (hyperpolarization)


The Nerve Impulse

An action potential is the sudden reversal of the charge across the membrane followed immediately by its restoration These changes occur in a wave along the

axon

For a very brief period following an action potential, the nerve cannot be stimulated again This is called the refractory period


Synaptic Transmission Is Communication between Neurons

Communication between neurons is by neurotransmitters, which are chemicals that cross the gap between two neurons

Synapse The junction between a neuron and another

cell Between two neurons Presynaptic neuron sends a message to the

postsynaptic neuron


Communication between Neurons

A signal travels down the axon Calcium ions cause the membrane of the

synaptic vesicles to fuse with the plasma membrane, and to release the neurotransmitter substances into the synaptic cleft



Neurotransmitters diffuse across the synaptic cleft to the other side, where they bind with specific receptors on the postsynaptic cell, which causes ion channels to open E.g., if another neuron is to be stimulated

sodium channels will open and this will cause the axon membrane to depolarize



If neurotransmission occurs at an excitatory synapse, and enough receptor sites bind with neurotransmitter substances to cause depolarization to the threshold value, an action potential is generated in the postsynaptic cell



However, in an inhibitory synapse, the postsynaptic cell becomes more negatively charged, and there is no action potential generated


Summation

A neuron may have as many as 10,000 synapses with other neurons at the same time

Some have excitatory effects and some inhibitory effects

Summation is the combined effects of excitatory and inhibitory effects at any given moment to determine whether an action potential is generated



After the neurotransmitter crosses the membrane, it is quickly broken down or pumped back into the synaptic knob of the presynaptic axon The enzyme acetylcholinesterase removes

acetylcholine from synapses



Acetylcholine Acts in both the PNS and the CNS as a

neurotransmitter Causes voluntary muscles to contract

Myasthenia gravis is an autoimmune disease that attacks the acetylcholine receptors, resulting in little muscle strength



The neurotransmitters dopamine, norepinephrine, and serotonin affect our emotional state

Alzheimer’s disease, depression, and Parkinson’s disease are caused by deficiencies of these chemicals in the brain

Copyright © 2009 Pearson Education, Inc. Neurons and Neurological Cells: The Cells of the Nervous...

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