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No. 21 1. Introduction of Nervous System 2. Spinal Cord (1) PART Ⅵ THE NERVOUS SYSTEM Chapter 1 Introduction The nervous system is a master system in the living body; it regulates and integrates the activities of all the bodily systems for the benefit of the organism as a whole. Ⅰ. Divisions of the Nervous System The nervous system consists of central part (central nervous system) and peripheral part (peripheral nervous system). The central nervous system is composed of brain and spinal cord. The peripheral nervous system includes cranial nerves (12 pairs), spinal nerves (31 pairs), and visceral nerves. The somatic and visceral nerves: According to the functions of organs innervated by the nerves, the peripheral nervous system is also divided the somatic nerves and visceral nerves. The somatic nerves supply the body surface, bones, joints, and skeletal muscle. The visceral nerves are distributed in the viscera, heart, vessels, and smooth muscles. In the peripheral nerves (the somatic and visceral nerves), there are two nerve fibers called afferent nerves (sensory nerves) and efferent nerves (motor nerves). The efferent (motor) part of visceral nerves is called the vegetative nervous system or autonomic nervous system. The autonomic nervous system innervates the smooth muscle and the glands of viscera, and the smooth muscle of blood vessels, together with the cardiac muscle, and further divided into sympathetic nerve and parasympathetic nerve. Ⅱ. Organization of nervous system The nervous system is composed of nervous tissue that consists of billions of nerve cells (neurons) and supported by a special variety of connective tissue known as neuroglia. Ⅰ) Neurons The neurons are independent structural unit of the nervous system and are functional specialized for reception, integration, and transmission of coded information. 1. Morphology Each neuron possesses a nucleated cell body and two types of processes; an axon which conducts impulse away from the cell body and one or more dendrite which conduct impulses towards the cell body. 2. Structure ① Cell body Like other cells, the cell body of the neuron serves as metabolic center of the entire unit and consists of a large, pale nucleus and cytoplasm, organelles, cell membrane, and Nissl body. Nissl body and neurofibril: The organelles contained within the cytoplasm are common to other cell in the body, but there are abundant granular endoplasmic reticulum which constitutes the Nissl body, a protein synthesis apparatus. Neurofibrils have role of supporting neuron and involved in transmission of substance in the cell. ② Processes The axon: It is a slender process. It may transfer the nerve impulses from the beginning part (axon hillock) to the end (axon terminal). Because the axoplasm does not contain RNA and ribosome, proteins synthesis cannot take place in the axon. All axonal proteins, therefore, must come from the cell body, and the products are transported by a perpetual axoplasmic motion. Some organelles, structural protein and neurotransmitters contained within cytoplasm are carried by axoplasmic flow which moves in both directions and with varying velocity. This phenomenon is called axoplasmic transport. The dendrites: The main or primary dendrites arise from the cell body and then branch repeatly in a tree –like manner to form a complex dendrite tree. Dendrite spines are on the dendrites, which are structures specialized for synaptic contact, receiving nerve impulses. 3. Classification of neurons ①According to the number of their processes, they are described as: Unipolar neuron, Bipolar neuron, Multipolar neuron. ②According to their functions and the direction of transportation, they are described as: Sensory (afferent) neuron Motor (efferent) neuron Intermediate (association) neuron. 4. Nerve fiber The nerve fibers are the longer processes of neurons which are enveloped by myelin sheath and nerve membrane. 5. The synapses Within the nervous system impulses are conducted from one part to another along a chain of neurons. The terminal arborizations of the axon of one neuron ramify in close contact with the cell body or dendrites, less frequently with axonic terminals of many others. These structural and functional areas of contact are termed synapses. Chemical synapse transports the impulses through the chemical substance neurotransmitter. The chemical synapse is the most common type in the mammalian nervous system. The chemical synapse includes three parts: Presynaptic element, Postsynaptic element. Synaptic cleft. The presynaptic element contains numerous synaptic vesicles in which the neurotransmitter is present and presynaptic membrane. When an impulse arrives at the presynaptic element, the neurotransmitter diffuse cross the synaptic cleft and bind to the receptor molecules in the postsynaptic membrane. As a result, the postsynaptic neuron is activated and impulse is conducted from one neuron to the others. Ⅱ). Neuroglia It includes the central and peripheral nervous systemic neuroglia. 1. In the central nervous system The neuroglial cells is the interstitial cells or supporting cells. According to their shape, they are divided into four types, i.e.: Macroglia, including Astrocytes, Oligodendrocyte,Microglia, Ependymal cell. 2. In the peripheral nervous system Schwann cell,Satellite cell. Ⅲ. Activated Way of Nervous System The way of activity of nervous system is reflex. Reflex: A reflex is an automatic, stereotyped reaction, such as movement, that is performed without conscious volition in response to an appropriate stimulus. The basic structure is reflex arc. Reflex Arc: The reflex arc, a linkage of afferent and efferent neurons, is defined as the entire neural pathway that is involved in a reflex. The effector, e. g. a muscle, is supplied by an efferent nerve, and between the afferent and efferent components there may be one or more connector or interneuron. These elements-afferent neurons, interneurons and efferent neurons-are the basis of reflex nervous activities. Receptor→afferent (sensory) nerve →center→efferent (motor) nerve→effector. 3. Gray matter and white matter Gray matter: In the CNS the part that contains aggregations of nerve cell bodies embedded in a network of delicate nerve processes is known as gray matter, it has a gray color during the fresh condition. White matter: In the CNS the part that contains mainly bundles of nerve fibers are white matter and the white color is due to a rich content of fatty myelin sheath. 4. Cortex and medullary substance Cortex: The cortex is the outermost layer of gray matter in the cerebral hemispheres or in the cerebellum. The cell bodies in the cortex are arranged in more or less welldefined laminae or layers. Medullary substance: It a central core of white matter beneath the cortex of the cerebrum and cerebellum. Ⅳ. Some Usual Terminology of nervous system The neuroanatomical terms in common usage are as follows: 1. Nucleus and ganglion Nucleus: Nerve cells with the same shape, function and connections within the central nervous system (CNS) are grouped together into nucleus. The nucleus may originate, relay, modify, or amplify neural signal within the nervous system. Ganglion: Nerve cells with the same shape, function and connections outside the CNS often are grouped together into ganglion. 2. Nerve and nerve fiber Nerve fiber: Nerve fibers are mainly axons, some of which are enveloped by myelin sheath. Fasciculus: In CNS, a distinct collection of nerve fibers with common origins, destinations and functions are referred to fasciculus, or tract. Nerve: In the peripheral nervous system (PNS), the nerve fibers are grouped into bundles to form the nerve trunk. Most of nerves have a whitish appearance because of their myelin content. Chapter 2 The Central Nervous System Section 1 The Spinal Cord Ⅰ. Location and Length The spinal cord, a long cylindrical structure, is located in the vertebral canal and invested by meninges. It extends from the foramen magnum, where it continues with the medulla oblongata, to the lower border of the first lumbar vertebra, about 40~45 cm in length. Diameters of the spinal cord are not equal at various levels. Ⅱ. External Features Ⅰ) Two enlargements and conus medullaris The spinal cord displays two prominent enlargements. The cervical enlargements, The lumbosacral enlargements. Each enlargement associates with the nerve roots that make up the brachial plexus and lumbosacral plexus, which innervate the upper and lower extremities, respectively. Caudal to the lumbosacral enlargement, the spinal cord tapers gradually and becomes the conical termination known as conus medullaris. Ⅱ) Film terminale and cauda equina A condensation of pia mater forms the film terminale which descends the conus medullaris to the level of the second sacral vertebra, from here it is enveloped by the dura mater and continues to the posterior surface of the coccyx. Since the spinal cord is markedly shorter than the vertebral column, the lumbosacral roots descend for varying distances within the terminal cisterna before reaching their corresponding intervertebral foramina. They form a divergent sheaf of spinal roots surrounding the film terminale, and is called the cauda equina. Ⅲ) Fissure and sulci Six longitudinal sulci are shown on the surface of the naked spinal cord. The anterior median fissure is on the median line of the anterior surface, where the anterior spinal artery and the companion vein are lodged. On the posterior surface there is the shallow posterior median sulcus. On each side of the posterior median sulcus there is a pair of posterior lateral sulci into which filaments of the posterior roots enter the spinal cord and a pair of posterior spinal arteries and veins run along the sulci. On each side of anterior median fissure there is a pair of anterolateral sulci which mark the exit of the anterior root fibers. Ⅲ. External segments Thirty-one pairs of spinal nerves arise from the spinal cord and pass through the intervertebral froamina between adjacent vertebrae of the vertebral column. Each portion of the cord that gives rise to a pair of spinal nerves is called a spinal segment. Each spinal segment is functionally correlated with the related cutaneous area, skeletal musculature and viscera. Because the vertebral column grows at a faster rate than the spinal cord, so the spinal segments are not equal to the vertebral segments except for the CS1-4. Table 1. The correlation between spinal segments (PS) and vertebrae Spinal segments (SS) Vertebrae CS1-4 CV1-4 CS5-8, ST1-4 -1 TS5-8 -2 TS9-12 LS1-5 SS1-5, CoS -3 TV10-12 TV12, LV1