Nervous Tissue Help
The Nervous System
On the basis of structure, the nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is composed of the brain and the spinal cord. The PNS is composed of cranial nerves from the brain and spinal nerves from the spinal cord. In addition, ganglia, clusters of cell bodies of neurons, and plexuses, networks of nerves, are found within the PNS.
The autonomic nervous system (ANS) is a functional division of the nervous system. Structures within the brain are ANS control centers, and specific nerves are the pathways for conduction of autonomic nerve impulses. The ANS functions automatically to speed up or slow down body activities.
Neurons and Neuroglia
A neuron is a nerve cell found in both the CNS and the PNS. Although neurons vary considerably in size and shape, they are generally composed of a cell body, dendrites, and an axon (Figure 9-1). At the ends of the branched axon are slight enlargements, axon terminals, that contain synaptic vesicles that produce and secrete neurotransmitter chemicals in the synapses.
Myelin is an insulating sheath of a fatlike lipid that wraps around the axon of many neurons. This sheath is produced by specific neuroglia cells. In the PNS, there are small gaps between segments of the sheath. The myelin sheath insulates nerve fibers and speeds up transmission of an impulse along the axon.
Neuroglia are specialized cells of the nervous support neurons. There are six different types of neuroglia, all mitotically divide, and are about five times more abundant than neurons.
Physiology of Nerve Conduction
In a non-conducting ("resting") neuron, a voltage, or resting potential, exists across the cell membrane. This resting potential is due to an imbalance of charged particles (ions) between the extracellular and the intracellular fluids. The mechanisms responsible for the membrane having a net positive charge on its outer surface and a net negative charge on its inner surface (Figure 9-2) are as follows:
- Asodium-potassium pump actively transports sodium ions (Na+) to the outside and potassium ions (K+) to the inside, with three Na+ moved out for every two K+ moved in.
- The cell membrane is more permeable to K+ than to Na+, so that the K+ , which is more concentrated inside the cell, diffuses outward faster than the Na+, which is more concentrated outside the cell, diffuses inward. Na+ and K+ move through the membrane using different channels.
- The cell membrane is essentially impermeable to the large (negatively charged) anions that are present inside the neuron, therefore fewer negatively charged particles move out than positively charged particles.
Nerve impulses carry information from one point of the body to another by progression along the neuron membrane of an abrupt change in the resting potential. This "traveling disturbance," called an action potential, is described below.
- A stimulus (chemical-electrical-mechanical) is sufficient to alter the resting membrane potential of a region of the membrane.
- The membrane's permeability to sodium ions (Na+) increases at the point of stimulation.
- Na+ rapidly moves into the cell through the membrane; the membrane becomes locally depolarized (membrane potential = 0).
- Na+ continues to move inward; the inside of the membrane becomes positively charged relative to the outside (reverse polarization).
- Reverse polarization at the original site of stimulation results in a local current that acts as a stimulus to the adjacent region of the membrane.
- At the point originally stimulated, the membrane's permeability to sodium decreases, and its permeability to K+ increases.
- K+ rapidly moves outward, again making the outside of the membrane positive in relation to the inside (repolarization).
- N+ and K+ pumps transport Na+ back out of, and K+ back into the cell. The cycle repeats itself, traveling in this manner along the neuron membrane.
An action potential will be produced in response to a threshold stimulus. The resting membrane potential is about –70mV. If a stimulus raises the membrane potential –55 mV, a threshold potential has been reached, complete depolarization and repolarization occur, and an action potential is generated. (See Figure 9-3.)
Synapse and Synaptic Transmission
A synapse is the specialized junction through which impulses pass from one neuron to another (synaptic transmission), via chemical messengers (neurotransmitters). Refer to Figure 9-4 and steps below.
- An action potential reaches the axon terminal.
- An influx of Ca2+ causes synapatic vesicles containing neurotransmitter to fuse with the presynaptic membrane.
- Neurotransmitter is released by exocytosis into the synaptic cleft.
- The neurotransmitter diffuses across the cleft to the postsynaptic membrane and bind to specific receptors located there.
- The permeability of postsynaptic membrane is altered, initiating an impulse on the second neuron.
- The neurotransmitter is removed from the synapse.
Neurotransmitters may be excitatory, causing the postsynaptic neuron to become active by producing an excitatory postsynaptic potential (EPSP), or may be inhibitory, preventing the postsynaptic neuron from becoming active by producing an inhibitory postsynaptic potential (ISPS).
Practice problems for these concepts can be found at:
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