Introduction to Nervous System PDF

 Lecture 1 Introduction to the Nervous System  This course Aims: By the end of the 1- course the students Electrophysiological should be familiar Examination. w...

 Lecture 1 Introduction to the Nervous System  This course Aims: By the end of the 1- course the students Electrophysiological should be familiar Examination. with: ELECTRODIAGNOSIS 2- Basics of 3- Normal and Electrodiagnostic abnormal findings in tests. NCS and EMG.  The Nervous System ▪The Nervous System is compromised of two major parts or, Subdivisions: ▪The Central Nervous System (CNS), ▪The Peripheral Nervous System(PNS). -The CNS includes the brain and the Spinal cord. -The brain is the body's “Control Center” The Nervous System has Three main functions: ▪Sensory input, 2. data integration, 3.Motor output ▪ The nerve conduct impulses from sensory receptors(touch, pressure, temperature, pain, vibration) to the brain and spinal cord. ▪ The data is then processed by way of integration of data, which occur only in the brain. ▪ After the brain has proceeded the information, impulses are then conducted from the brain and spinal cord to muscle and glands which are called motor output. Integration of sensory information from different parts of the body and processing them then, Response generation.  The Peripheral Nervous System Consist of 12 pairs of Cranial Nerves and 31 pairs of Spinal Nerves, With their associated ganglion make up the peripheral nervous System. A) Motor (or efferent) fibers of peripheral nervous system which are of two types: Somatic motor fibers that terminate in skeletal muscles. Autonomic fibers that innervate cardiac, smooth muscle and glands. The termination of the somatic fibers in the skeletal muscle occur at the motor end plate which resembles (synapse). B) The sensory (or afferent) fibers of peripheral nerves. Transmit signals from receptors of various types. Each afferent fibers conduct impulses toward the brain and spinal cord from particular receptor with which it is connected.  The Peripheral Nervous System Elements of peripheral nervous system The peripheral nervous system includes the nerve roots, peripheral nerves, primary sensory neurons, neuromuscular junctions (NMJs), and muscles. The primary motor neurons ( anterior horn cells), which are located in the spinal cord, are often included as part of the peripheral nervous system as well.  The motor root originating from anterior horn cell(AHC), leaves the cord ventrally, Whereas the sensory root enter the cord on the dorsal side. Motor and sensory roots at each spinal level unite distal to the DRG to become a mixed spinal nerve. There are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal. Each spinal nerve divides into a dorsal and ventral ramus. The dorsal and ventral rami both contain motor and sensory fibers.  The dorsal ramus runs posteriorly to supply sensory innervation to the skin over the spine and muscular innervation to the paraspinal muscles at that segment. The ventral ramus differs, depending on the segment within the body. In the thoracic region, each ventral ramus continues as an intercostal nerve. In the lower cervical to upper thoracic (C5–T1) region, the ventral rami unite to form the brachial plexus. In the mid-lumbar to sacral regions, the ventral rami intermix to form the lumbosacral plexus  All muscles supplied by one spinal segment (one nerve root) are known as a myotome, All cutaneous areas supplied by a single spinal segment are known as a dermatome. For both myotomes and dermatomes, there is considerable overlap between adjacent segments. Because of the high degree of overlap between spinal segments, a single root lesion seldom results in significant sensory loss and never in anesthesia. Likewise, on the motor side, even a severe single nerve root lesion usually results in only mild or moderate weakness and never in paralysis.  Remember : Brachial plexus: The brachial plexus is divided into five parts; roots, trunks, divisions, cords and branches. Roots → C5, C6, C7, C8, and T1. At the base of the neck, the roots of the brachial plexus converge to form three trunks. Superior trunk – a combination of C5 and C6 roots. Middle trunk – continuation of C7. Inferior trunk – combination of C8 and T1 roots. Each trunk divides into two divisions; anterior and posterior. Divisions combine together to form three cords, named by their position relative to the axillary artery. The lateral cord is formed by: The anterior division of the superior trunk and the anterior division of the middle trunk The posterior cord is formed by: The posterior division of the superior trunk, the posterior division of the middle trunk and the posterior division of the inferior trunk The medial cord is formed by: The anterior division of the inferior trunk. Branches from roots: - C5 → Dorsal scapular to rhomboids and levator scapulae - C5,6,7 → Long thoracic to serratus anterior. From trunks: Upper trunk (C5, 6)→Suprascapular nerve to supra-& infraspinatus muscles and Subcalvian nerve to subclavius. From cords: Lateral cord (C5-7) Lateral pectoral nerve to pectoralis , Musculocutaneous nerve to biceps brachii (continued to lateral antebrachial cutaneous nerve) Lateral head of median nerve. Posterior cord (C5-8, T1) upper Subscapular nerve to subscapularis (upper part) Thoracodorsal nerve to latissimus dorsi, Axillary nerve to deltoid, teres minor, Radial nerve lower Subscapular nerve to subscapularis (lower part) and teres major, Medial cord (C8, T1) Medial pectoral nerve to pectoralis Medial brachial nerve to medial arm (sensory), Medial antebrachial n to medial forearm (sensory), Ulnar nerve, Medial head of median nerve. The Nervous System Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons They are responsible for receiving, processing and transmitting information. They are electrically active and release chemical signals for target cells. Glial cells or , glia, They are known to play a supporting role for nervous tissues. Neurons are important, but without glial cells they would not be able to perform their function. Neurons Neurons or, nerve cell is the structural and functional unit of nervous system. The nervous system of a human is made up of innumerable neurons. A neuron primarily consists of the cell body and processes, which are of two kind ( the axon and the dendrites). Structures of a Neuron Structures of a Neuron The main part of a neuron is: Cell body Which is known as the soma (Soma= “body”). The cell body contains the nucleus and most of major organelles. The axon, The axon is the single longer process of the nerve cell. It varies in length from a few micron to one meter. It arises from the conical extension of the cell body called axon hillock. The axon terminates by dividing into a number of branches, each ending in a number of synaptic knobs known as a terminal buttons. Structures of a Neuron Cont. Myelin Sheath Is present a round the axon in the so- called myelinated nerve fibers. Myelin Sheath which consist of protein lipid complex is produced by glial cells called Schwann cells, which encircle the axon forming around it a thin sleeve. Each Schwann cells provides the myelin sheath for a short segment of the axon. Myelination of axons increase the speed of conduction, but greatly increase their diameter. At the junction of any two segment, there is a short gap. These gap are called the Node of Ranvier. Axons perform the specialized function of conducting impulses away from the cell body Structures of a Neuron Cont. Dendrites, The dendrites are multiple small branched process. Dendrites are the receptive process of the neuron receiving signals from other neuron via their synapse with the axon terminals. The dendrites are highly branched process, providing location for other neurons to communicate with the cell body. Information flow through a neuron from a dendrites, across the cell body, and down to the axon. This give a neuron a polarity- meaning that information flow in one direction. Excitability Excitability is that property of the nerve that respond by generating a nerve signals or so called- action potential(when it is stimulated by suitable stimulus (threshold stimulus) which may be electrical, thermal, chemical or mechanical. Theses signals between neurons are transmitted via specialized connections called synapses. Action Potential Is a very rapid change in membrane potential that occurs when a nerve cell membrane is stimulated. Specifically the membrane potential goes from the resting potential (typically -70) to some positive value (typically +30) in a very short period of time (just a few millisecond). The minimum stimulus needed to achieve action Potential is called the threshold stimulus. The threshold stimulus causes the membrane potential to be less negative.  Action Potential Cont. Because a stimulus, no matter how small causes a few sodium channels to open and allow some positively - charged sodium ions to diffuse inside a cell to start Depolarization phase. Because there is more sodium ions on the outside than inside the cell membrane, sodium then diffuse rapidly in to the nerve cell. All these positively -charged sodium rushing inside the cell causing the membrane potential to become more positive (the inside of a membrane is now positive relative to the outside). The sodium channels open only briefly, and then close again. Action Potential Cont. The Potassium channels then open, and because there is more potassium inside the membrane than outside, positively charged potassium ions diffuse out. As these positive ions goes out, the inside of the membrane once again becomes negative with respect to the outside (Repolarization phase). Separation of charges across the membrane = resting potential (polarized state) Phases of Action Potential Propagation of Action Potential An Action potential occurring in a local area of a membrane is to depolarize that area. Local current spreading from that area depolarize the surrounding resting membrane to an extent greater than necessary to cause neighboring voltage-gated sodium and potassium channels to open, resulting in the changes in the sodium and potassium conductance that then produce the action potential in these neighboring areas. This wave of depolarization occurs in a continuous fashion in un myelinated fibers.  Propagation of Action Potential Cont. In un myelinated Nerve: The action potential travels in a continuous manner along these axons. Because of a relatively uniform distribution of voltage -sensitive Na+ and K+. In myelinated Nerve: The voltage -sensitive Na+ and K+ channels are not distributed uniformly. Na+ channels are clustered in high density in the axon membrane at the Node of Ranvier. K+ channels , on the other hand , tend to be localized in the “Internodal” area. Propagation of Action Potential Cont. Because the current flow through the insulating myelin is very slow and physiologically negligible, the action potential in myelinated axons jumps from one node to the next in a mode of conduction termed saltatory conduction.