Medical brain notes 24

  Basic Neuronal Arrangements CLASSIFICATION OF NEURONS AND OF NERVE FIBRES IN PERIPHERAL NERVES ACCORDING TO FUNCTION        The nerve fibres that make up any peripheral nerve can be divided into two major types as follows. a.                Fibres that carry impulses from the CNS to an  effector organ  (e.g., muscle or gland) are called  efferent  or  motor  fibres. b.                Fibres that carry impulses from peripheral structures (e.g., skin) to the CNS are called  afferentfibres . Some afferent fibres carry impulses that make us conscious of sensations like touch or pain:such fibres may, therefore, be called  sensory fibres . Other afferent fibres convey information which is not consciously perceived, but is necessary for reflex control of various activities of the ...

  Somatic Efferent Neurons         These neurons supply striated muscle. In the spinal cord the cell bodies of these neurons lie in the ventral grey column. They are often referred to as anterior horn cells. The neurons are large and multipolar and their Nissl substance is prominent. They are designated as  alpha neurons  to distinguish them from smaller anterior horn cells called  gamma neurons  (Fig.3.1).         The axon of a somatic efferent neuron leaves the spinal cord through a ventral nerve root to enter the spinal nerve concerned. During its course through the spinal nerve (and its branches) the axon divides into a variable number of branches each one of which ultimately ends by supplying one muscle fibre. The region of junction between a terminal branch of the axon and the muscle fibre has a special structure and is called the  motor end plate  or  neuromuscular juncti...

  Special Visceral Efferent Neurons (Branchiomotor Neurons)        These are seen only in relation to cranial nerves. The cell bodies of these neurons are located in the branchial efferent nuclei of the brainstem. Their axons pass through the fifth, seventh, ninth, tenth, and eleventh cranial nerves to supply striated muscle derived from the branchial arches. The relationship of these neurons to striated muscle is the same as that of somatic efferent neurons.

  General Visceral Efferent Neurons        These are the neurons that constitute the autonomic nervous system (sympathetic and parasympathetic). They supply smooth muscle or glands. The nerves to glands are called  secretomotor  nerves. The pathway for the supply of smooth muscle or gland always consists of two neurons that synapse in a ganglion (Fig. 3.2). The first neuron carries the impulse from the CNS to the ganglion and is, therefore, called the  preganglionic  neuron. The second neuron carries the impulse from the ganglion to smooth muscle or gland and is called the  postganglionic  neuron.         The cell bodies of preganglionic neurons of the sympathetic nervous system are located in the lateral grey column of the spinal cord in the thoracic and upper two lumbar segments (Fig. 19.3). Their cell bodies are multipolar, but are smaller than those of somatic efferent neurons. The Ni...

  Afferent Neurons        We have seen that afferent nerve fibres can be divided into four categories  viz .,  general somatic afferent, special somatic afferent, general visceral afferent and special visceral afferent. The basic arrangement of the neurons that give origin to all four categories of afferent fibres is similar, and the description that follows applies to all of them.         The cell bodies of neurons that give rise to efferent fibres of peripheral nerves are located within the brain and spinal cord. In contrast, the cell bodies of neurons that give rise to afferent fibres are located outside the CNS. In the case of spinal nerves the cell bodies lie in the spinal ganglia; and in the case of the cranial nerves they lie in sensory ganglia (e.g., the trigeminal ganglion) associated with these nerves. We may illustrate the arrangement with reference to a spinal nerve (Fig. 3.3). The cells of the d...

  Arrangement of Neurons within the Central Nervous System So far we have considered the arrangement of neurons: a.                having cell bodies that lie within the brain and spinal cord, and sending out efferent processes that leave the CNS to form the motor fibres of peripheral nerves; and b.                having cell bodies located in ganglia outside the CNS, but sending processes into it.         The bulk of the CNS is, however, made up of neurons that lie entirely within it. As explained earlier, the cell bodies of these neurons are invariably located in masses of grey matter. The axons may be short, ending in close relation to the cell body ( short axon neurons  or  Golgi type II neurons) , or may be long ( long axon neurons  or  Golgi type I neurons)  and may travel to oth...

  Projection, Association and Commissural Fibres         When a considerable number of fibres pass from a mass of grey matter in one part of the brain to another mass of grey matter in another part of the brain or spinal cord these are referred to as  projection fibres.  Fibres interconnecting different areas of the cerebral cortex or of the cerebellarcortex are called  association fibres , while fibres connecting identical areas of the two halves of the brain are called  commissural fibres.          All fibres crossing from one side of the brain or spinal cord to the opposite side are not commissural. When fibres originating in a mass of grey matter in one half of the CNS end in some other mass of grey matter in the opposite half they are referred to as  decussating fibres,  and the sites where such crossings take place are referred to as  decussations.