Nervous System PDF

Fahd Bin Sultan University Faculty Of Medicine Med 115 Anatomy Department Nervous system By Dr. Mohamed Sonbol, PhD Intended Learning Outcomes (ILOs): By the end of this Lecture, the s...

Fahd Bin Sultan University Faculty Of Medicine Med 115 Anatomy Department Nervous system By Dr. Mohamed Sonbol, PhD Intended Learning Outcomes (ILOs): By the end of this Lecture, the student should be able to: 1. Identify the main components of the nervous system. 2. Recognize the functional unit of the nervous system. 3. Differentiate between central, peripheral and autonomic nervous system. The nervous system is formed of: 1. Central nervous system (CNS): includes the brain and spinal cord. 2. Peripheral nervous system (PNS): includes -12 pairs of cranial nerves and the sensory ganglia related. - 31 pairs of spinal nerves and the dorsal root ganglia. Ganglia: are group of nerve cells outside the central nervous system 3. Autonomic nervous system (ANS): is formed of the sympathetic and parasympathetic systems. ANS supplies; smooth muscles, heart, and glands. The nerve cell (neuron): Neuron is the anatomical, functional and embryological unit of the NS. It is formed of a) Cell body: it contains the nucleus. b) Processes: are two types The axon (nerve fibre): single, long process which carries nerve impulses away from the cell body. Outside the central nervous system, the axons run in groups forming the nerves. Dendrites: short, multiple processes which carry impulses towards the cell body. Synapse: site where impulses pass from one neuron to another by contact of the axon of one neuron with the dendrites of the other Central nervous system 1. The CNS (brain & spinal cord) Brain and spinal cord are protected by bones, fluid (cerebro-spinal fluid, C.S.F.), and meninges. The brain is protected by skull while the spinal cord is protected by vertebral column The brain and spinal cord are surrounded by three covering membranes called the meninges. From inside - out, are pia, arachnoid and dura mater. The pia is adherent to the brain & spinal cord. The arachnoid is separated from pia by the subarachnoid space that contains the CSF. The dura is separated from arachnoid matter by the subdural space Subdivisions of the brain: Cerebrum: it consists of two cerebral hemispheres and the diencephalon (thalamus and hypothalamus). Brain stem: it consists of midbrain, pons and medulla oblongata. Cerebellum: it consists of two cerebellar hemispheres. In cross section (transverse section), the brain shows: a. Outer grey matter: dark areas mainly on the surface of the cerebrum (cerebral cortex). It contains cell bodies of the nerve cells. b. Inner white matter: light areas in the centre of cerebrum. It contains axons (nerve fibers) of nerve cells. The fibers run inside the C.N.S. as; bundles or tracts. 2.Spinal cord: It is the downward continuation of the medulla oblongata at the foramen magnum. It has a central narrow cavity called the central canal. It is divided into cervical, thoracic, lumbar, sacral and coccygeal segments In cross section (transverse section), the spinal cord shows: a. Outer layer of white matter: It contains the ascending fibers (sensory tracts) that carry sensations to the brain; & the descending fibers (motor tracts) that carry the higher motor orders from the brain. b. Inner layer of grey matter: Contains neurons disposed in the form of H-shaped area. Two anterior limbs are termed the anterior horns (motor) & two posterior limbs are the posterior horns (sensory) of grey matter. The horizontal connecting region is known as the grey commissure, through which runs the central canal. Lateral horn is present in thoracic and upper two lumber segments; it is the only source of sympathetic fibers. Peripheral nervous system a) Cranial nerves: These are 12 pairs which are named as follows b) Spinal nerves: These are 31 pairs; 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and one coccygeal spinal nerves.  Structure of the spinal nerve: - Each spinal nerve is connected to the spinal cord by two roots; anterior& posterior roots. - The anterior (ventral) root is motor i.e. carrying nerve impulses away from CNS. - The posterior (dorsal) root is sensory i.e. carrying sensations towards CNS. - The anterior & posterior roots unite to form the spinal nerve trunk nerve (mixed). - Once spinal nerve emerges from the intervertebral foramen, it divides into a large anterior primary ramus & a small posterior primary ramus. - The posterior rami are small, pass posteriorly to supply muscles & skin of the back and have no tendency to form plexuses. - The anterior rami are larger, pass anteriorly to supply muscles & skin of the anterolateral body wall and all muscles & skin of the limbs and have tendency to form plexuses. - The anterior primary rami of all thoracic & upper two lumbar spinal nerves sends white rami communicants to sympathetic ganglia and receive grey rami communicants from them  Autonomic nervous system Definition: It is the component of the nervous system concerned with the control of the involuntary functions of smooth muscles, heart and glands. The ANS is composed of the: 1- Sympathetic system 2- Parasympathetic system. The ANS needs two sets of neurons: a) Preganglionic neurons: they carry impulses from the C.N.S. to ganglia outside the C.N.S. b) Postganglionic neurons: they carry impulses from the ganglia to the smooth muscles and glands. In the ganglia, the pre and postganglionic neurons meet by making synapses The sympathetic system: It is formed of: 1. Preganglionic fibers: arise from neurons of the lateral horns of grey matter of the spinal cord in the following segments: a) All thoracic segments b) Upper 2 lumbar segments (Hence called thoracolumbar outflow) The fibers run in the ventral root, trunk, ventral ramus of corresponding spinal nerve. Finally, they leave the ventral ramus as white ramus communicant to reach the ganglia of the sympathetic chain which extends alongside of the vertebral column. 2. Postganglionic fibers: are the axons of cell bodies of the sympathetic ganglia. The fibers leave the ganglia as grey rami communicants to be distributed to their target organs in close relations to the arteries. The parasympathetic system: It is formed of: 1. Preganglionic fibers: they arise from C.N.S. in two sites: a) Cranial nerves number 3, 7, 9, 10 (1973). b) Neurons of 2nd, 3rd, 4th sacral segments of spinal cord. (Hence it is called the craniosacral outflow). 2. Parasympathetic ganglia: they are found close to the parts to be innervated. 3. Postganglionic fibers: axons of the cell bodies leave the ganglia to reach the target organs.  Terms to remember. Nucleus & ganglion: Nucleus is a collection of cell bodies that have the same function within the CNS. Outside the CNS, a collection of cell bodies is called ganglion. Tract: a bundle of nerve fibers within the CNS having the same origin, same termination and same function. Nerve: Bundles of nerve fibers outside the CNS. Pathway: a chain of successive tracts having the same function (e.g. carrying pain sensation). Lemniscus: collection of ascending fibers in the brainstem. Commissure: a band of white or grey matter connecting the right & left sides of the CNS across the midline. Decussation: a point at which an ascending or descending tract crosses the midline. Afferent: input i.e., going towards a certain structure. Efferent: output i.e., going away from a certain structure. Contralateral: Opposite side & ipsilateral: same side Synapse: site of contact between two neurons; usually between the axon of one neuron and the dendrites of the other neuron. References: 1- Gray's Anatomy: The Anatomical Basis of Clinical Practice, 41e 41st Edition (2016) by Susan Standring, PhD, DSc (Author). 2- Gray's Anatomy for Students. Textbook by Richard Drake (2014). 3- Clinical Anatomy by Regions. Textbook by Richard S. Snell, 12th edition (2011). 4- Clinically oriented anatomy. Book by Keith L. Moore (1985). thank you