Chapter 12 - Nervous System

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connects to the brain through the foramen magnum of the skull and is encircled by the bones of the vertebral column.

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spinal cord

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are located outside the brain and spinal cord, and are classified by their origin.

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Peripheral nerves

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emerge from the base of the brain; spinal nerves emerge from the spinal cord. Each nerve follows a defined path and serves a specific region of the body

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Cranial nerves

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A group of neuronal cell bodies lying outside the central nervous system.

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Ganglia

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Functions

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Sensory Function.
Integrative Function.
Motor Function

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PNS may be subdivided further into:

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somatic nervous system (SNS) (somat- = body), an autonomic nervous system (ANS) (auto- = self; -nomic = law), and an enteric nervous system (ENS)

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convey information to the CNS from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste, and smell. Includes motor neurons that conduct impulses from the CNS to skeletal muscles only

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SNS

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visceral organs such as the stomach and lungs. The ANS also consists of motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands

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ANS

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100 million neurons in enteric plexuses that extend the length of the gastrointestinal (GI) tract. Enteric sensory neurons monitor chemical changes within the GI tract and the stretching of its walls The operation of the ENS, the “brain of the gut,” is involuntary. Many neurons of the enteric plexuses function independently of the CNS to some extent, although they may communicate with the CNS via sympathetic and parasympathetic neurons.

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ENS

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nerve cell, consisting of a cell body, dendrites, and an axon.

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Neurons

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Cells of the nervous system that perform various supportive functions

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Neuroglia

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Most neurons have three parts

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(1) a cell body, (2) dendrites, and (3) an axon

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clusters of rough endoplasmic reticulum,
protein synthesis

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Nissl bodies

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bundles of intermediate filaments that provide the cell shape and support, and microtubules, which assist in moving materials between the cell body and axon

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neurofibrils

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The portion of the axon closest to the axon hillock is the initial segment. In most neurons, impulses arise at the junction of the axon hillock and the initial segment, called

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trigger zone

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cytoplasm of an axon

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axoplasm

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several dendrites and one axon (Figure 12-4a). Most neurons in the brain and spinal cord are of this type.

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Multipolar

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one main dendrite and one axon (Figure 12-4b). They are found in the retina of the eye, in the inner ear, and in the olfactory (olfact- = to smell) area of the brain.

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Bipolar neurons

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dendrites and one axon that are fused together to form a continuous process that emerges from the cell body (

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Unipolar neurons

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action potential in its axon and the action potential is conveyed into the CNS through cranial or spinal nerves.

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Afferent Neurons

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onvey action potentials away from the CNS to effectors (muscles and glands) in the periphery (PNS) through cranial or spinal nerves

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Motor or Efferent Neurons.

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largest and most numerous of the neuroglia. The processes of astrocytes make contact with blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord). Astrocytes cling to and support neurons. Astrocytes help to maintain the appropriate chemical environment for the generation of nerve impulses by providing nutrients to neurons, removing excess neurotransmitters (described shortly), and regulating the concentration of important ions

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Astrocytes

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esponsible for forming and maintaining the myelin sheath (described shortly) around CNS axons.

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Oligodendrocytes

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phagocytize microbes and damaged nervous tissue.

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Microglia

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produce and assist in the circulation of cerebrospinal fluid. They also form the blood–cerebrospinal fluid barrier

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Ependymal cells-

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form the myelin sheath around axons in the PNS.

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Schwann cells-

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structural support, satellite cells regulate the exchanges of materials between neuronal cell bodies and interstitial fluid.

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Satellite cells-

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multilayered lipid and protein covering called

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multilayered lipid and protein covering called

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produce myelin sheaths around axons in the CNS.

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oligodendrocytes

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Schwann cells begin to form myelin sheaths around axons during

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Schwann cells

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The outer, nucleated cytoplasmic layer of the Schwann cell enclosing the myelin sheath is

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neurolemma

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aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon

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neurolemma

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The production of graded potentials and action potentials depends on two basic features of the plasma membrane of excitable cells:

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the existence of a resting membrane potential and the presence of specific types of ion channels

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flow of charged particles is called

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current

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always open to ions

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Leakage channels

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opens and closes in response to a specific ligand, a molecule that binds to a receptor (

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A ligand-gated channel

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opens cation channels that allow sodium ions (Na+) and calcium ions (Ca2+) to diffuse inward and potassium ions (K+) to diffuse outward.

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neurotransmitter acetylcholine

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response to mechanical stimulation such as sound waves, pressure, or tissue stretching

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mechanically gated channel

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response to a change in membrane potential (voltage)

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voltage-gated channe

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small buildup of negative ions in the cytosol along the inside of the membrane, and a buildup of positive ions in the extracellular fluid along the outside surface of the membrane

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Resting Membrane Potential

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The resting membrane potential arises from three major factors:

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Unequal Distribution of Ions Across the Plasma Membrane

Inability of Most Anions to Leave the Cell

Electrogenic Nature of the Sodium–Potassium Pump.

maintain the resting membrane potential by pumping out Na+ as fast as it leaks in

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small deviation from the membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative).

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graded potential-

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When the response makes the membrane more polarized (increases the difference in the charge between inside and outside),

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hyperpolarizing graded potential -

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When the response makes the membrane less polarized

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depolarizing graded potential -

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occurs when a stimulus causes ligand-gated or mechanically gated channels to open or close in an excitable cell's plasma membrane

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graded potential

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process by which graded potentials add together





graded potentials resulting from a neurotransmitter binding to the dendrites or cell body of a neuron are called postsynaptic potentials, and graded potentials occurring in sensory receptors are termed receptor potentials

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summation

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nerve impulse is a sequence of rapidly occurring events that reverse the membrane potential and then eventually restore it to the resting state

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Action Potential-

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the negative membrane potential becomes less negative, reaches zero, and then becomes positive

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depolarizing phase-

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restores the membrane potential to the resting state of -70 mV.

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repolarizing phase -

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a stimulus that is strong enough to depolarize the membrane above threshold.

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suprathreshold stimulus,

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period of time after an action potential begins, during which an excitable cell cannot generate another action potentia

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Refractory Period

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occurs from the time the Na+ channel activation gates open to when the Na+ channel inactivation gates close (steps 2–3 in Figure 12-19). During this period, even a very strong stimulus cannot initiate a second action potential because inactivated Na+ channels must return to the resting state before they can reopen

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absolute refractory period

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period of time during which a second action potential can be initiated, but only by a larger-than-normal stimulus.

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relative refractory period

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To communicate information from one part of the body to another, action potentials must travel from where they arise at a trigger zone to the axon terminals





propagates in only one direction, toward the axon terminals

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propagation

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which involves step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane as ions flow through each voltage-gated channel along the membrane. Unmyelinated axons propagate action potentials by

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continuous conduction-

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propagation that occurs along myelinated axons, occurs because of the uneven distribution of voltage-gated channels. Voltage-gated channels are present primarily at the nodes of Ranvier (where there is no myelin sheath) rather than in regions where a myelin sheath covers the plasma membrane.

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Saltatory conduction -

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The action potential appears to leap from node to node as each nodal area depolarizes to threshold, thus the name

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“saltatory

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moves action potentials much faster than continuous conduction in an unmyelinated axon of the same diameter.

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saltatory conduction

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Factors that Affect the Speed of Propagation

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Amount of Myelination

Axon Diameter.

Temperature

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how often they are generated at the trigger zone.

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frequency of action potentials—

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neuron sending the signal

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presynaptic neuron

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neuron receiving the message

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postsynaptic neuron-

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(from axon to axon),

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axoaxonic

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from axon to dendrite)

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axodendritic

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from axon to cell body

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axosomatic

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presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter). The postsynaptic neuron receives the chemical signal and, in turn, generates an electrical signal

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postsynaptic potential-

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reaches threshold, it triggers an action potential in the postsynaptic neuron.

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depolarizing postsynaptic potential

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action potentials conduct directly between adjacent cells through gap junctions, which act like tunnels to connect the cytosol of the two cells





Gap junctions are common in visceral smooth muscle, cardiac muscle, and the CNS.

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Electrical Synapses

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Removal of the neurotransmitter from the synaptic cleft

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Diffusion

Enzymatic Degradation. Certain neurotransmitters are inactivated through enzymatic degradation. For example, the enzyme acetylcholinesterase breaks down acetylcholine in the synaptic cleft.

Uptake by Cells.

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capability to change based on experience.

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plasticity

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capability to replace destroyed cells

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regeneration

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damage to dendrites and myelinated axons may be repaired if the cell body remains intact and if the Schwann cells that produce myelination remain active can occur in

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PNS

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Nissl bodies break up into fine granular masses

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chromatolysis

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Within days the part of the axon distal to the damaged region breaks up into fragments and the myelin sheath deteriorates (Figure 12-24b). Degeneration of the distal portion of the axon and myelin sheath

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Wallerian degeneration

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phagocytize the debris.

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Macrophages

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orm a regeneration tube across the injured area (Figure 12-24c). The tube guides growth of a new axon from the proximal area, across the injured area, into the distal area occupied by the original axon, and eventually toward receptors and effectors previously contacted by the neuron

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Schwann cells - mitosis