Neurons axonal conduction

The distinguishing features of a neuron are the two types of structures, known collectively as neurites, that extend out from its cell body the axon and the dendrites. The function of the axon is to carry messages away from the cell body to recipient cells, including other neurons, an efferent function. The function of the dendrites is to receive messages from other cells, including neurons, and conduct them to the cell body, an afferent function. 

Axon an outgrowth (neurite) of a neuron that conducts action potentials away from the cell body. 

In nonmyelinated neurons, the conduction velocity of an action potential Is roughly proportional to the diameter of the axon, because a thicker axon will have a greater number of ions that can diffuse. The human brain Is packed with relatively small, myelinated neurons. If the neurons In the... 

The core of neuronal axons Is filled with neurofilaments (NFs), each a heteropolymer composed of three polypeptides— NF-L, NF-M, and NF-H—which differ greatly In molecular weight (see Figure 19-32 and Table 19-4). Neurofilaments are responsible for the radial growth of an axon and thus determine axonal diameter, which Is directly related to the speed at which it conducts impulses. The influence of the number of... 

The nerve cell, or neuron, is an unusual looking cell (Fig. 8-1). A neuron may be only 0.1 mm in diameter but, may reach lengths of a meter or more. The threadlike extension from the cell body is the nerve fiber. These fibers interconnect in various special areas of the body, including the brain, the spinal cord, and the peripheral nerves. The function of the neuron is to transmit signals, or impulses. The dendrites specialize in receiving excitations, which may be from environmental stimuli (i.e., from peripheral parts of the body, or from another cell). The axon conducts the excitation away from the dendrite area (e.g., to the brain).1 This excitation, or impulse transmission, may be on a subconscious level. Transmissions of which the subject is not consciously aware are the type that control various body functions such as temperature, respiration, blood pressure, and peristaltic movements of the gastrointestinal tract. In addition to the traditional functions of a cell membrane, the membrane of the nerve fiber (the axon) has the role of transmitting the electrochemical impulses that are received from the dendrites. The additional protective device called the myelin sheath basically serves as an electrical insulator. 

Neurons in the brain appear as two different kinds of tissue. Gray matter is composed of neurons and dendrites (see Section 4.4.3). White matter is made up of neurons and fatty myelin sheaths. The myelin sheaths around neuron axons result in faster and more efficient nerve conduction. 

Oligodendrocytes in the CNS and Schwann cells in the PNS ensheathe neuronal axons with myelin, an insulating structure of multi-layered membranes [14]. Myelin s insulation is required for efficient nerve impulse conduction, but myelin has other profound biological effects. The inability of nerves to regenerate after CNS injury is largely due to the nerve s inability to extend axons when in contact with... 

The glial cells support the neurons physically. Certain glial cells (oligoden-droglial cells) synthesize myelin, a fatty insulation layer wrapped around the axons. Myelin is necessary for the so-called saltatory conduction of electrical... 

Figure 2.12 From voltage-clamp to current-clamp micro-electrode recordings of synaptic current (/, lower trace) and synaptic potential with superimposed action potential (V, upper trace) from a neuron in an isolated rat superior cervical sympathetic ganglion following a single stimulus (S) applied to the preganglionic nerve trunk. The interval between the stimulus and the postsynaptic response includes the conduction time along the unmyelinated axons of the preganglionic nerve trunk. (SJ Marsh and DA Brown, unpublished)...

Myelin A protein and phospholipid sheath that surrounds the axons of certain neurons. Myelinated nerves conduct impulses more rapidly than nonmyelinated nerves. 

The functional significance of myelin is revealed by the neurological deficits observed in patients with multiple sclerosis. This disorder is caused by the demyelination of neurons in the brain, spinal cord, and optic nerve. The loss of myelin disrupts the normal conduction of impulses along the axons of these neurons and results in weakness, numbness, loss of bladder control, and visual disturbances. 

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