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Copyright © 2007 Pearson Prentice Hall, Inc.

Neurons: Histology of

the Nervous System

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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 peripheral nervous system (PNS)

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Cells of the Nervous System

Neurons are excitable cells that generate and transmit messages

The Neuron: the most basic unit in the brain, and more generally, the nervous system

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Cells of the Nervous System

When skeletal muscles are viewed under a microscope Neuroglial cells (also called glial cells) are more numerous and provide structural support, growth factors, and insulating sheaths around the nerves They are able to reproduce, unlike neurons Many types of glial cells: Astrocytes,

microglial, ependymal, and oligodendrocytes.

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Glial cells provide support and protection for neurons, the other main type of cell in the central nervous system. They are thus known as the "glue" of the nervous system.

The four main functions of glial cells are to surround neurons and hold them in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons.

Astrocytes can be visualized in culture because, unlike other mature glia, they express glial fibrillary acidic protein.

Glial cells, commonly called neuroglia or simply glia (greek for "glue"), are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, glia are estimated to outnumber neurons by about 10 to 1

Neuroglial cells

Types of Glial Cells

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Cells of the Nervous System

Sensory (or afferent) neurons carry information toward the CNS from sensory receptors

Association neurons (or interneurons) are located between sensory and motor neurons within the CNS where they integrate and interpret sensory signals

Motor (or efferent) neurons carry information away from the CNS to an effector

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Peripheral Nervous System (PNS)

The nerves and cells of PNS carry messages in two directions: away from the CNS or to the CNS.

Afferent Division Nerves that carry messages to the

brain or spinal cord are part of the afferent division of the PNS.

Efferent Division Nerves that carry messages from

the brain or spinal cord are part of the efferent division of the PNS.

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Types of Neurons

A message comes from the cell body of a motor neuron, travels down the extension of it, and then gets transferred to a muscle causing the muscle to contract.

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Neurons Have Dendrites, a Cell Body, and an Axon Dendrites carry information toward the cell

body of a neuron and a single long axon carries information away from the cell body

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Dendrites, Cell Body, and Axon

A nerve consists of parallel axons, dendrites, or both from many neurons

Nerves are covered with tough connective tissue

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Dendrites, Cell Body, and Axon

Most axons not found in the CNS are electrically insulated by a myelin sheath

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Dendrites, Cell Body, and Axon

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

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Dendrites, Cell Body, and Axon

The myelin sheath also facilitates nerve repair outside the CNS

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The myelin sheath forms from multiple wrappings of Schwann cells

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Dendrites, Cell Body, and Axon

Multiple sclerosis (MS) results from the destruction of the myelin sheath that surrounds axons found in the CNS

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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 the ion channels

http://youtube.com/watch?v=uK_cLTJLIXo

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The Nerve Impulse

They are also transported across the membrane by the sodium-potassium pump

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The Nerve Impulse

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 called the resting potential

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The Nerve Impulse

The minimum charge that causes the sodium gates to open is called the threshold

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

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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

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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

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The Nerve Impulse

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

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Communication between

neurons is by

neurotransmitters,

chemicals that cross the

gap between two

neurons

Synaptic Transmission Is Communication between Neurons

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Communication between Neurons

A synapse is the junction between the presynaptic neuron, which sends a message to the postsynaptic neuron

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Communication between Neurons

Calcium ions then cause the membrane of the synaptic vesicles to fuse with the plasma membrane and to release the neurotransmitter substances

They diffuse across the synaptic cleft to the other side where they bind with receptors on the postsynaptic cell causing the ion channels to open

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Communication between Neurons

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

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Nerve Impulse Across Neurons

The three units involved in the transmission of the nerve impulse across neurons:

1. The presynaptic neuron (sending neuron)

2. The synapse (gap between the sending and receiving neurons)

3. The postsynaptic neuron (receiving neuron).

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Structure and Function of a Synapse

The signal passes across the synaptic cleft as a chemical called neurotransmitter which is released from vesicles by exocytosis.

Neurotransmitter is a chemical that is secreted into a synaptic cleft by a neuron that affects another neuron or an effector by binding with receptors on it.

The sending cell mal also be capable of taking theneurotransmitter back into itself by a process known as “reuptake.”

Neurotransmitters include:Acetycholine, serotonin, dopamine, and others.

http://youtube.com/watch?v=HXx9qlJetSU

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Communication between Neurons

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

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Communication between Neurons

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

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Communication between Neurons

Acetylcholine causes voluntary muscle to contract

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

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Communication between Neurons

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

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