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Neurons and NervesNeurons and Nerves(Hand out)(Hand out)

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The human nervous system has two main The human nervous system has two main divisions (Figure 01a): the central nervous divisions (Figure 01a): the central nervous system (CNS), and the peripheral nervous system (CNS), and the peripheral nervous system (PNS), which includes the system (PNS), which includes the somatic motor nervous system, and the , and the sensory nervous system. .

The CNS consists of the brain and spinal cord. It The CNS consists of the brain and spinal cord. It acts as the central control region of the human acts as the central control region of the human nervous system, processing information and nervous system, processing information and issuing commands. issuing commands.

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The autonomic nervous system (ANS) is the The autonomic nervous system (ANS) is the command network the CNS uses to maintain the command network the CNS uses to maintain the body's homeostasis.body's homeostasis.

It automatically It automatically regulates heartbeat and controls regulates heartbeat and controls muscle contractions in the walls of blood muscle contractions in the walls of blood vessels, digestive, urinary, and reproductive vessels, digestive, urinary, and reproductive tractstracts. It also carries messages that help . It also carries messages that help stimulate glands to secrete tears, mucus, and stimulate glands to secrete tears, mucus, and digestive enzymes. digestive enzymes.

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Figure 01a Figure 01a Nervous SystemNervous System

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The nerves (Figure 01b) that are easily visible to The nerves (Figure 01b) that are easily visible to the unaided eye are not single cells. Rather, the unaided eye are not single cells. Rather, they are bundles of nerve fibers (neurons) each they are bundles of nerve fibers (neurons) each of which is itself a portion of a cell. The fibers of which is itself a portion of a cell. The fibers are all traveling in the same direction and are are all traveling in the same direction and are bound together for the sake of convenience, bound together for the sake of convenience, though the individual fibers of the bundle may though the individual fibers of the bundle may have widely differing functions. have widely differing functions.

There are no cell bodies in nerves; There are no cell bodies in nerves; cell bodies cell bodies are found only in the CNS or in the ganglia.are found only in the CNS or in the ganglia. Ganglia are collections of cell bodies within the Ganglia are collections of cell bodies within the PNS.PNS.

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Figure 01b Figure 01b Neuron and Neuron and

NerveNerve

The main The main portion of portion of the neuron, the neuron, the cell the cell body, is not body, is not too different too different from other from other cells. It cells. It contains a contains a nucleus and nucleus and cytoplasmcytoplasm

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The dendritesThe dendrites

Where it is most distinct from cells of other types Where it is most distinct from cells of other types is that out of the cell body, is that out of the cell body, long threadlike long threadlike projections emergeprojections emerge. Over most of the cell there . Over most of the cell there are numerous projections that branch out into are numerous projections that branch out into still finer extensions. These branching threads still finer extensions. These branching threads are called dendrites ("tree" in Greek). are called dendrites ("tree" in Greek).

At one point of the cell, however, there is At one point of the cell, however, there is a a particularly long extension that usually does not particularly long extension that usually does not branch throughout most of its sometimes branch throughout most of its sometimes enormous lengthenormous length. This is the axon (the axis). . This is the axon (the axis).

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Sensory neuron, motor neuron, and Sensory neuron, motor neuron, and interneuroninterneuron

Figure 01b shows the three parts of the neurons: Figure 01b shows the three parts of the neurons: dendrite(s), cell body, and axon. dendrite(s), cell body, and axon. A dendrites conducts A dendrites conducts nerve impulses toward the cell bodynerve impulses toward the cell body, the part of a neuron , the part of a neuron that contains the nucleus and other organelles. that contains the nucleus and other organelles. An axon An axon conducts nerve impulses away from the cell body.conducts nerve impulses away from the cell body.

There are three types of neurons: sensory neuron, motor There are three types of neurons: sensory neuron, motor neuron, and interneuron. A sensory neuron neuron, and interneuron. A sensory neuron takes a takes a message from the receptors in the sense organ to the message from the receptors in the sense organ to the CNS. A motor neuron sends a message away from the CNS. A motor neuron sends a message away from the CNS to an effector, a muscle fiber or a glandCNS to an effector, a muscle fiber or a gland. An . An interneuron is always found completely within the CNS interneuron is always found completely within the CNS and conveys messages between parts of the systemand conveys messages between parts of the system (Figure 3a). (Figure 3a).

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In addition to neurons, nervous tissue contains glial cells In addition to neurons, nervous tissue contains glial cells such as the Schwann cells covering the neurons with such as the Schwann cells covering the neurons with sheath. These cells maintain the tissue by supporting sheath. These cells maintain the tissue by supporting and protecing the neurons. and protecing the neurons.

They also provide nutrients to neurons and help to keep They also provide nutrients to neurons and help to keep the tissue free of debris. the tissue free of debris. The neurons require a great The neurons require a great deal of energy for the maintenance of the ionic deal of energy for the maintenance of the ionic imbalance between themselves and their surrounding imbalance between themselves and their surrounding fluids, fluids, which is constantly in flux as a result of the which is constantly in flux as a result of the opening and closing of channels through the neuronal opening and closing of channels through the neuronal membranes. membranes.

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Figure 01c Neurons Figure 01c Neurons

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the energy consumptionthe energy consumption

Thus while the brain is only 2% of our Thus while the brain is only 2% of our body weight, it consumes 20% of our body weight, it consumes 20% of our energy and moreover 80% of this energy energy and moreover 80% of this energy consumption is devoted to maintain the consumption is devoted to maintain the imbalance. imbalance.

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Figure 01d Action Potential

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Neurons are Neurons are dynamically dynamically polarizedpolarized, so that information , so that information flows from the fine dendrites into flows from the fine dendrites into the main dendrites and then to the main dendrites and then to the cell body, where it the cell body, where it is is converted into all-or-none signalsconverted into all-or-none signals, , the action potentials, which are the action potentials, which are relayed to other neurons by the relayed to other neurons by the axon, a long wire like structure. axon, a long wire like structure.

The neuron is actually a very poor The neuron is actually a very poor conductor; the signal drops to 37% of conductor; the signal drops to 37% of its original strength in only about 0.15 its original strength in only about 0.15 mm. Thus it needs amplification all mm. Thus it needs amplification all along its length in the form of along its length in the form of sodium-sodium-potassium pumps and gatespotassium pumps and gates (see (see Figure 01d). Figure 01d).

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prof. azaprof. aza

The amplificationThe amplification

The amplification is initiated by detection of The amplification is initiated by detection of small changes in voltage across the membrane small changes in voltage across the membrane with the opening of voltage-sensitive sodium with the opening of voltage-sensitive sodium channels in the membrane of the neuron. channels in the membrane of the neuron.

Sodium ions rush into the neurons from the extra Sodium ions rush into the neurons from the extra cellular fluid, cellular fluid, resulting in a transient change in resulting in a transient change in the voltage difference between the neuron and the voltage difference between the neuron and the surrounding environmentthe surrounding environment. The action . The action potential travels like a wave from the cell body potential travels like a wave from the cell body down the neuron via the repeating down the neuron via the repeating amplifications. amplifications.

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the action potentialthe action potential Thus, the action potential enables the neuron to Thus, the action potential enables the neuron to

communicate rapidly with other neurons over communicate rapidly with other neurons over sizable distances, sometime more than a meter sizable distances, sometime more than a meter away with a speed from away with a speed from 20 -200 m/sec20 -200 m/sec. .

When the action potential When the action potential reaches an axon reaches an axon terminalterminal (the synapse), it causes the terminals (the synapse), it causes the terminals to to secrete a chemical messenger secrete a chemical messenger (neurotransmitter),(neurotransmitter), generally an amino acid or its generally an amino acid or its derivative, which binds to receptors in the post-derivative, which binds to receptors in the post-synaptic neurons on the far side of the synaptic synaptic neurons on the far side of the synaptic cleft. When the postsynaptic potential cleft. When the postsynaptic potential has has reached a specific value an action potential is reached a specific value an action potential is triggered and the signal is passedtriggered and the signal is passed to the next to the next neuron. neuron.