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NEURAL TISSUE

Introduction

1. The nervous system includes all of the neural tissue in the body. Neural tissue carries information or instructions from one region of the body to another.

2. The primary functions of the nervous system include the sensation of internal and external environments, integration of sensory inputs, coordination of motor outputs, and regulation or control of peripheral systems.

3. Integration and coordination occurs within the brain and spinal cord, components of thecentral nervous system. The peripheral nervous system includes all of the neural tissue outside of the CNS.

HISTOLOGY OF NEURAL TISSUE

1. Neurons are cells responsible for information transfer in the nervous system. A typical neuron has dendrites, an axonthat ends in synaptic terminals, and a cell body, or soma.

2. Neural tissue includes neurons and supporting cells, or neuroglia. Neuroglia isolate the neurons, provide a supporting framework for the tissue, and act as phagocytes.

Neuroglia

1. Neuroglia far outnumber neurons; the greatest variety of glial cells is found within the CNS, where they account for roughly half of the total volume of neural tissue.

Neuroglia of the Central Nervous System

1. Astrocytes provide a structural framework and their expand feet surround capillaries and nerve cells.

2. Oligodendrocytescontact nerve cells and form the neurilemma around axons in the CNS. They are also responsible for the myelination of axons in the brain and spinal cord.

3. Microglia are phagocytic white blood cells that have migrated into the neural tissues.

4. Ependymal cellsline the central canal of the spinal cord and the ventricles of the brain. They may assist in the circulation of the enclosed fluids and also play a sensory role.

Neuroglia of the Peripheral Nervous System

1. Amphicytes surround the nerve cells of peripheral ganglia.

2. Schwann cells produce the neurilemma and myelin of peripheral axons.

Neuronal structure

1. Neurons have a variety of shapes. General categories include multipolar, bipolar, unipolar, and anaxonic neurons.

Multipolar Neurons

1. The dendrites receive information from other neurons or specialized receptors. They are often highly branched and may account for most of the total surface area of the neuron.

2. The nerve cell body, or soma, contains most of the metabolic machinery as well as the nucleus. Important organelles in the perikaryon include mitochondria, Golgi apparatus, Nissl bodies (ribosomes), neurofilaments, neurotubules, and aggregations of pigments such as melanin or lipofuschin.

3. The soma does not contain a centrosome, and neurons cannot undergo mitosis.

4. The axon begins at the axon hillock, a specialized portion of the soma. Theinitial segment of the axon is adjacent to the axon hillock.

5. The axoplasm contains an abundance of neurofilaments and neurotubules, as well as other organelles.

6. The axon may or may not have collateralbranches. At its distal tip the axon gives rise to a number of finer terminal branches, the telodendria.

The Synapse

1. Each telodendrial branch ends at a synapse, a region of intercellular communication. At a synapse the activity of one neuron affects the membrane characteristics of another cell

2. A synapse where a neuron communicates with another cell-type represents a neuroeffector junction.

3. Communication normally occurs from the presynaptic neuron to the postsynaptic neuron or effector cell. Most communication occurs across chemical synapses.

4. The presynaptic membrane and the postsynaptic membrane are separated by a narrow synaptic cleft . The diffusion of the neurotransmitter across this cleft accounts for the observed synaptic delay.

5. The soma exports transmitter molecules, nutrients, lysosomes, and mitochondria along the axon via axoplasmic transport.Retrograde flow brings materials from the synaptic terminals backalong the axon and into the soma.

THE PHYSIOLOGY OF NEURAL TISSUE

The Transmembrane Potential

1. The transmembrane potential of a typical neuron averages around -70 mV.Nerve cells may hyperpolarize or depolarize in response to environmental stimuli. Graded changes are localized alterations in the transmembrane potential that are proportional to the magnitude of the stimulus.

2. Depolarization of the axon hillock to threshold produces a conducted change, or action potential, in the membrane.

Action Potentials and Nerve Cell Membranes

1. An action potential begins when an excitable membrane depolarizes to threshold. The sequence of events then proceeds: (1) sodium channels open, and the membrane depolarizes rapidly to +30mV; (2) sodium channels close; (3) potassium channels open, and (4) the membrane repolarizes.

2. Until repolarization has been completed the membrane is in arefractory period, and will not respond normally to additional stimuli.

3. Action potentials in a nerve cell membrane last only around 0.5 msec instead of the 5 msec characteristic of skeletal muscle membranes.

4. Only the axon conducts action potentials comparable to those observed in skeletal muscle cells. This insulates the dendrites and soma from the action potentials carried by the axon.

5. The neuron may be inhibited orfacilitated by setting the transmembrane potential of the axon hillock to some value other than 70 mV.

6. The highest rates of action potential generation are seen when the transmembrane potential at the axon hillock remains above threshold.The maximum recorded frequencies range from 500-1,000 impulses per second.

Conduction Velocity

1. Action potentials may travel along the axon via continuous or saltatory conduction . In saltatory conduction the action potentials jump from node to node at high speed. This can only occur along myelinated axons.

2. Axons may be categorized as Type A, B, or C on the basis of diameter, myelin content, and conduction velocity.

The Presynaptic Membrane and Neurotransmitter Release

1. Many synapses release acetylcholine (ACh) at the presynaptic membrane. Such synapses are called cholinergic.

2. Each synaptic knob contains mitochondria, vesicles, and small components of the endoplasmic reticulum. Theneurotransmitter is found within the synaptic vesicles and dissolved in the cytoplasm.

3. The stimulus for neurotransmitter release is the depolarization of the presynaptic membrane. In multipolar or unipolar neurons this occurs following the arrival of an action potential.

4. Depolarization leads to the entry of calcium ions into the cytoplasm, triggering the release of neurotransmitter into the synaptic cleft by exocytosis and diffusion through special membrane channels.

5. The acetylcholine entering the synaptic cleft is attacked by the enzyme acetylcholinesterase. Recycling of the breakdown products and neurotransmitter molecules provided by axoplasmic transport prevent premature synaptic fatigue.

The Effects of Acetylcholine on the Postsynaptic Membrane

1. A depolarization of the postsynaptic membrane creates an excitatory postsynaptic potential, or EPSP.

2. EPSPs may combine to bring the axon hillock to threshold via temporal summation orspatial summation.

The Effects of Other Important Neurotransmitters

1. Neurotransmitters may hyperpolarize or depolarize the postsynaptic membrane.A graded hyperpolarization represents an inhibitory postsynaptic potential, or IPSP. IPSPs can also summatetemporally or spatially.

2. A typical neuron synthesizes and releases a single neurotransmitter. ACh usually produces an EPSP in the postsynaptic membrane, but in some locations it produces an IPSP. The effect depends on the nature of the receptors on the postsynaptic surface.

3. Norepinephrine (noradrenaline) is important in he brain and in portions of the autonomic nervous system. Synapses releasing NE are called adrenergic .

4.Dopamine (DOPA), gamma aminobutyric acid (GABA), and serotonin are neurotransmitters that are usually inhibitory, producing IPSPs in the postsynaptic membrane.

5. Several dozen neurotransmitters and other active compounds are released at synapses in the nervous system. Neuromodulators are compounds that modify the response of the postsynaptic membrane to specific neurotransmitters.

Neurons and Metabolic Processes

1. Neurons have a high demand for ATP to support synthetic and active transport operations. They obtain energy through the aerobic breakdown of glucose, and do not maintain glycogen reserves

2. As a result, these cells are totally dependent on the oxygen and glucose delivered by the circulation, and any interruption in the circulatory supply may damage or destroy neurons.

INFORMATION PROCESSING

Neurotransmitters and Processing at the Cellular Level

1. A neuron in a processing center of the brain may be influenced by as many as 80,000 synapses. Some of these synapses produce EPSPs, others IPSPs.

2. For any portion of the membrane, an IPSP can counteract the effects of an EPSP. The activity of the neuron is determined by the transmembrane potential at the axon hillock, and at any given moment this reflects the interactions between EPSPs and IPSPs. 3.Electrical synapses, or gap junctions, involve direct links between the presynaptic and postsynaptic neurons. These synapses are relatively uncommon.

4. Because the cells at an electrical synapse are physically connected, there is no synaptic delay.There are also fewer opportunities for varying the response of the postsynaptic neuron to stimulation.

Functional Patterns of Neural Organization

1. Sensory neurons bring information concerning the external or internal environments to the central nervous system. Motor neurons control or modify the activity of peripheral effectors. Interneurons, or association neurons, are interposed between sensory and motor neurons inside the CNS.

An Introduction to Reflexes

1. Reflexes are rapid and automatic motor responses to stimuli. A reflex arc begins with the activation of a sensory neuron and ends with the activation of a motor neuron.Interneurons may or may not be involved.

2.Somatic reflexes produce skeletal muscle contractions; visceral (autonomic) reflexes alter the activities of visceral systems.

3. There are monosynapticand polysynaptic reflexes; polysynaptic reflexes involve one or more interneurons.

An Introduction to Neural Processing

1. The roughly 20 billion interneurons can be sorted into at most a few thousandneuronal pools . Each pool performs processing functions comparable to those underway on the soma of an individual neuron.

2. The output of a neuron within a pool may influence other nerve cells within the same or different pools. The nature of the influence depends upon whether the nerve cell lies within the discharge zone or the facilitated zone of the active neuron.

3. Stimulating a single neuron may secondarily activate many other neurons.This process, divergence , is characteristic of sensory pools.

4. Convergence implies that a single neuron is receivinginputs from several other neurons in the same or different pools. Convergence is often encountered when tracing the output of neuronalpools concerned with motor control.

5. In a reverberating circuit, collaterals provide additional stimulation following the initial activation of the neuron or neuronal pool.This process can increase the duration of the neural response to a particular stimulus.

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