Somatic motor neurons

The cell bodies of somatic motor neurons are found in the ventral horn. The axons of these neurons exit the CNS through the ventral root of the spinal nerve and innervate skeletal muscles. The two types of motor neurons located in the ventral horn are ... 

Each muscle fiber is innervated by a branch of an alpha motor neuron. The synapse between the somatic motor neuron and the muscle fiber is referred to as the neuromuscular junction. Action potentials in the motor neuron cause release of the neurotransmitter acetylcholine. Binding of acetylcholine to its receptors on the muscle fiber causes an increase in the permeability to Na+ and K+ ions. The ensuing depolarization generates an action potential that travels along the surface of the muscle fiber in either direction that is referred to as a propagated action potential. This action potential elicits the intracellular events that lead to muscle contraction. 

Somatic motor and sensory neurons that give rise to PNS axons maintain large fractions of their total protoplasmic bulk within the CNS. Many of the extracellular matrix components and axonal guidance molecules involved in... 

Anatomical differences between the peripheral somatic and autonomic nervous systems have led to their classification as separate divisions of the nervous system. These differences are shown in Figure 9.1. The axon of a somatic motor neuron leaves the CNS and travels without interruption to the innervated effector cell. In contrast, two neurons are required to connect the CNS and a visceral effector cell of the autonomic nervous system. The first neuron in this sequence is called the preganglionic neuron. The second neuron, whose cell body is within the ganglion, travels to the visceral effector cell it is called the postganglionic neuron. 

Lateral efferents of the MPOA innervate septum, amygdala, other hypothalamic regions, ventral tegmental area (VTA) of the midbrain, and midbrain central gray matter (Numan, 1986). Studies have shown that the MPOA-VTA-basal ganglia (BG) circuit is believed to be of importance in maternal behavior because the BG is a major component of the extrapyramidal system (EPS) (Numan, 1986). Perhaps MPOA neurons relevant to maternal behavior via the BG to EPS promote the somatic-motor processes underlying maternal responsiveness (Numan, 1986). 

Nicotinic receptors are part of a transmembrane polypeptide whose subunits form cation-selective ion channels (see Figure 2-9). These receptors are located on plasma membranes of postganglionic cells in all autonomic ganglia, of muscles innervated by somatic motor fibers, and of some central nervous system neurons (see Figure 6-1). 

Figure 11.3 Diagram of autonomic and somatic motor neurons. Presynaptic neurons are depicted with solid cell bodies. Postsynaptic neurons are speckled. The neurotransmitter released by the presynaptic neuron and the type of receptor it activates are listed below each synapse.

The somatic nervous system controls skeletal muscle movement through motor neurons. Alpha motor neurons extend from the spinal cord and terminate on indi-... 

Protein and RNA synthesis inhibitors were subsequentially shown to rescue CNTF-, BDNF-and NGF-dependent neurons from apoptosis in vitro (Scott and Davies, 1990) and to reduce naturally occurring neuronal cell death in chick somatic motor and sensory neurons (Oppenheim et al., 1990). 

Two other CNS neuronal populations that respond to BDNF are dopaminergic neurons and somatic motor neurons. These effects are of particular interest due to the degeneration of dopaminergic and motor neurons in Parkinson s disease and motor neuron disease respectively. Addition of BDNF to cultures of mesencephalic dopaminergic neurons results in enhanced survival and an increase in dopamine uptake (Hyman et al., 1991 Kniisel et al., 1991 Beck et al., 1993). Further-... 

The cell bodies of somatic motor neurons reside in the ventral hom of the spinal cord (see Figure 6-1) the axon divides into many branches, each of which innervates a single muscle fiber, so more than 100 muscle fibers may be supplied by one motor neuron to form a motor unit. At each neuromuscular junction, the axonal terminal loses its myehn sheath and forms a terminal arborization that lies in apposition to a specialized surface of the muscle membrane, the motor end plate (see Figure 9-2). 

It is reasonably proven analogy that the cell bodies present in the somatic motor nerves invariably lie within the spinal cord and, therefore, veiy much within the CNS. It has been observed that the prevailing activity of motor neurons is mostly affected by a host of such cardinal factors, such as facilitatory and inhibitory modulation through feedback Ifom contralateral and ipsilateral stretch, besides other receptors various centres of the brain. 

Loewi s discovery led to a chain reaction which resulted in the rapid spread of the principle of chemical transmission to all the peripheral synapses in the autonomic nervous system, and to the discovery of chemical transmission from the somatic motor nerve to the skeletal muscles and from one neuron to another, first in the autonomic and then in the central nervous system (studies of Samoilov, the classical experiments of the Dale and Cannon groups, the investigations of Ecdes, and many others). Finally, the chemical theory was confirmed by modem experiments using the latest methods electron microscopy, histochemical methods, the microelectrode technique, and others ... 

The afferent division carries sensory information toward the CNS and the efferent division carries motor information away from the CNS toward the effector tissues (muscles and glands). The efferent division is further divided into two components (1) the somatic nervous system, which consists of motor neurons that innervate skeletal muscle and (2) the autonomic nervous system that innervates cardiac muscle, smooth muscle, and glands. 

Transmission of impulses to somatic efferent motor neurons in spinal cord to initiate voluntary contraction of skeletal muscle... 

Skeletal muscle is neurogenic and requires stimulation from the somatic nervous system to initiate contraction. Because no electrical communication takes place between these cells, each muscle fiber is innervated by a branch of an alpha motor neuron. Cardiac muscle, however, is myogenic, or self-excitatory this muscle spontaneously depolarizes to threshold and generates action potentials without external stimulation. The region of the heart with the fastest rate of inherent depolarization initiates the heart beat and determines the heart rhythm. In normal hearts, this "pacemaker region is the sinoatrial node. 

The nervous system is divided into two parts the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists of all afferent (sensory) neurons, which carry nerve impulses into the CNS from sensory end organs in peripheral tissues, and all efferent (motor) neurons, which carry nerve impulses from the CNS to effector cells in peripheral tissues. The peripheral efferent system is further divided into the somatic nervous system and the autonomic nervous system. The effector cells innervated by the somatic nervous system are skeletal muscle cells. The autonomic nervous system innervates three types of effector cells (1) smooth muscle, (2) cardiac muscle, and (3) exocrine glands. While the somatic nervous system can function on a reflex basis, voluntary control of skeletal muscle is of primary importance. In contrast, in the autonomic nervous system voluntary control can be exerted, but reflex control is paramount. 

The nervous system is conventionally divided into the central nervous system (CNS the brain and spinal cord) and the peripheral nervous system (PNS neuronal tissues outside the CNS). The motor (efferent) portion of the nervous system can be divided into two major subdivisions autonomic and somatic. The autonomic nervous system (ANS) is largely independent (autonomous) in that its activities are not under direct conscious control. It is concerned primarily with visceral functions such as cardiac output, blood flow to various organs, and digestion, which are necessary for life. The somatic subdivision is largely concerned with consciously controlled functions such as movement, respiration, and posture. Both systems have important afferent (sensory) inputs that provide information regarding the internal and external environments and modify motor output through reflex arcs of varying size and complexity. 

The efferent portion of the peripheral nervous system can be further divided into two major functional subdivisions, the somatic and autonomic systems (see Figure 3.1). The somatic efferents are involved in voluntarily controlled functions such as contraction of the skeletal muscles in locomotion. The autonomic system functions involuntarily to regulate the everyday needs and requirements of the body without the conscious participation of the mind. It is composed primarily of visceral motor (efferent) neurons that innervate smooth muscle of the viscera, cardiac muscle, vasculature and the exocrine glands. 

The efferent somatic nervous system differs from the autonomic system in that a single myelinated motor neuron, originating in the CNS, travels directly to skeletal muscle without the mediation of ganglia. As noted earlier, the somatic nervous system is under voluntary control, whereas the autonomic is an involuntary system. 

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