Neurons efferent

The third component of the nervous system is the motor division. Appropriate signals are transmitted from the CNS to various body parts or effector tissues by way of efferent neuronal pathways. These effector tissues, which include organs, muscles, and glands, carry out the appropriate physiological responses to bring the variable back to within its normal limits. 

Distinguish among the three types of neurons afferent neurons, efferent neurons, and interneurons... 

Afferent neurons lie predominantly in the PNS (see Figure 6.1). Each has a sensory receptor activated by a particular type of stimulus, a cell body located adjacent to the spinal cord, and an axon. The peripheral axon extends from the receptor to the cell body and the central axon continues from the cell body into the spinal cord. Efferent neurons also lie predominantly in the PNS. In this case, the cell bodies are found in the CNS in the spinal cord or brainstem and the axons extend out into the periphery of the body where they innervate the effector tissues. By way of convergence, the centrally located cell bodies may receive inputs from several different regions of the brain that will influence their activity. 

Figure 6.1 Types of neurons. Afferent neurons, which transmit impulses toward the CNS and efferent neurons, which transmit impulses away from the CNS, lie predominantly in the peripheral nervous system. Intemeurons, which process sensory input and coordinate motor responses, lie entirely within the central nervous system.

Figure 7.1 Cross-sectional view of the spinal cord. In contrast to the brain, the gray matter of the spinal cord is located internally, surrounded by the white matter. The gray matter consists of nerve cell bodies and unmyelinated intemeuron fibers. This component of the spinal cord is divided into three regions the dorsal, lateral, and ventral horns. The white matter consists of bundles of myelinated axons of neurons, or tracts. Each segment of the spinal cord gives rise to a pair of spinal nerves containing afferent and efferent neurons. Afferent neurons enter the spinal cord through the dorsal root and efferent neurons exit it through the ventral root.

Figure 7.3 Components of a reflex arc. As illustrated by the components of the reflex arc, reflexes may be processed entirely at the level of the spinal cord with no need for input from the brain. A monosynaptic reflex has a single synapse between afferent and efferent neurons a polysynaptic reflex has two or more synapses between these neurons. In this case, intemeurons lie between the sensory and motor neurons. The more intemeurons involved, the more complex the response is.

At the end of this processing sequence, some of the interneurons connect with motor efferent neurons. These motor efferents leave the CNS and stimulate the peripheral effectors. Most of the effectors are muscles of various types smooth,... 

Sensory afferent neuron Synapse Synapse Motor efferent neuron... 

Figure 1.13 Communication to and from the cardiovascular centre in the brain. The cardiovascular centre controls changes in the output from the heart (cardiac output) and the flow of blood through peripheral b ssues and organs. It is the efferent neurones that transfer informab on from the brain to the heart and peripheral vessels. The afferent neurones transfer informab on from the heart and other b ssues, e.g. muscle, to the centre. Informab on transfers from the major arteries, the coronary arteries and peripheral muscles to the brain. There is also informab on transfer within the brain and within the muscle.

Cell bodies of somatic efferent neurons are found at all levels of the spinal cord... 

Target tissues receive two (sympathetic and parasympathetic) efferent neurons The neurons between the CNS and the organ are myelinated preganglionic fibers and unmyelinated postganglionic fibers... 

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. 

A. The autonomic nervous system is composed entirely of efferent neurons. 

Correct choice = E. The parasympathetic nervous system is essential for life. Visceral motor (efferent) neurons innervate smooth muscle of the viscera, cardiac muscle, and the exocrine glands. 

Caspar P, Bloch B, Le Moine C (1995) D1 and D2 receptor gene expression in the rat frontal cortex cellular localization in different classes of efferent neurons. Eur J Neurosci 7 1050-1063. 

Multipolar neurons have many processes that extend from the cell body (Figure 14.4). However, each neuron has only one axon. Most motor (or efferent) neurons are this type and account for 9% of all neurons. Multipolar neurons have a diversity of shapes. 

Efferent neuron— A motor neuron that carries an impulse from the central nervous system to muscles or glands. 

Sensory neurons transduce physical stimuli, such as smell, light, or sound, into action potentials, which are then transmitted to the spinal cord or brain. Sensory neurons, which bring information into the central nervous system, are also referred to as afferent neurons. Motor neurons transmit nerve impulses away from the brain and spinal cord to muscles or glands and are also called efferent neurons. Intemeurons transmit nerve impulses between sensory neurons and the motor neurons. Interneurons are responsible for receiving, relaying, integrating, and sending nerve impulses. Intemeurons are found exclusively in the central nervous system and account for almost 99% of all the nerve cells in the body. 

Another way in which hormonal secretion is regulated involves a neuroendocrine reflex, which differs from a neural reflex in that the efferent neuron is replaced by a... 

Denis-Donini S, Estenoz M. 1988. Intemeurons versus efferent neurons Heterogeneity in their neurite outgrowth response to glia from several brain regions. Dev Biol 130 237-249. 

Bernard V, Dumartin B, Lamy E, Bloch B (1993) Fos immunoreactivity after stimulation or inhibition of muscarinic receptors indicates anatomical specificity for cholinergic control of striatal efferent neurons and cortical neurons in the rat. Eur. J. Neurosci, 5, 1218-1225. 

As can be seen in Figure 16.2, the sensory fibers (afferent neurons) course together in bundles with the motor fibers (efferent neurons) from the periphery to the spinal cord (2). The cell bodies of the sensory fibers are found at the point at which the nerve enters the vertebra, and they are seen as enlargements on the nerve bundles. The cell bodies of the motor fibers are found within the spinal cord. The bundles of sensory and motor fibers outside the spinal cord are wrapped in a connective tissue sheath, the epineurium. Groups of fibers are found within this nerve" in small bundles, each of which is surrounded by connective tissue known as perineurium and, in even smaller tubes of connective tissue, called endoneurium. 

The basic unit of integrated activity is the reflex arc. This arc consists of a sense organ, an afferent neuron, one or more synapses in a central integrating station (or sympathetic ganghon), an efferent neuron, as well as, an effecdor. The simplest reflex arc is the monosynaptic one, which only has one synapse between the afferent and efferent neuron. With more than one synapses the reflex arc is called polysynaptic. In each of these cases, activity is modified by both spatial and temporal facihtation, occlusion, and other effects (2,3]. 

Primary afferents and related ejferents A model comprising a fast sodium current, a delayed rectifier potassium current, a calcium current, and a calcium-dependent potassium current was used to model primary afferents as well as intemeurons and efferent neurons connected to the afferents by Saxena et al. [1997]. A modification of the HH model was used in Amir et al. [2002] to model A-type dorsal root ganglion neuron ceU bodies. 

Swadlow, Fl.A. Monitoring the excitability of neocortical efferent neurons to direct activation by extracellular current pulses. /. Neurophysiol. 1992 68 605-619. 

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