Lecture#5-A The Nervous System (1) (2) PDF

 COURSE CODE: SHS.132 RESOURCE PERSON : DR. MAHAM KHALID (S-2024) REFERENCE TEXT: ROSS AND WILSON ANATOMY AND PHYSIOLOGY IN HEALTH AND ILLNESS The nervous system INTRODUCTION the central nervous system (CNS), consisting of the brain and the spinal cord the periph...

 COURSE CODE: SHS.132 RESOURCE PERSON : DR. MAHAM KHALID (S-2024) REFERENCE TEXT: ROSS AND WILSON ANATOMY AND PHYSIOLOGY IN HEALTH AND ILLNESS The nervous system INTRODUCTION the central nervous system (CNS), consisting of the brain and the spinal cord the peripheral nervous system (PNS) consisting of all the nerves outside the brain and spinal cord. INTRODUCTION The PNS comprises paired cranial and sacral nerves — some of these are sensory (afferent), some are motor (efferent) and some mixed. It is useful to consider two functional parts within the PNS: 1. the sensory division 2. the motor division INTRODUCTION In turn the motor division is involved in activities that are: 1. voluntary —the somatic nervous system (movement of voluntary muscles) 2. involuntary — the autonomic nervous system (functioning of smooth and cardiac muscle and glands). The autonomic nervous system has two parts: 1. sympathetic 2. parasympathetic NEURONS The nervous system consists of a vast number of cells called neurons, supported by a special type of connective tissue, neuroglia. Each neuron consists of a cell body and its processes, one axon and many dendrites. Neurones are commonly referred to simply as nerve cells. NEURONES Bundles of axons bound together are called nerves. Neurones cannot divide and for survival they need a continuous supply of oxygen and glucose. Unlike many other cells, neurones can synthesise chemical energy (ATP) only from glucose. NEURONES The physiological 'units' of the nervous system are nerve impulses, or action potentials, which are akin to tiny electrical charges. Properties of neurones Neurones have the characteristics of irritability and conductivity. Irritability is the ability to initiate nerve impulses in response to stimuli from: 1. outside the body 2. inside the body Conductivity means the ability to transmit an impulse. Cell bodies Nerve cells vary considerably in size and shape but they are all too small to be seen by the naked eye. Groups of cell bodies are called nuclei in the central nervous system and ganglia in the peripheral nervous system. Cell bodies form the grey matter of the nervous system and are found at the periphery of the brain and in the centre of the spinal cord. Axons and dendrites Axons and dendrites are extensions of cell bodies and form the white matter of the nervous system. Axons are found deep in the brain and in groups, called tracts, at the periphery of the spinal cord. They are referred to as nerves or nerve fibres outside the brain and spinal cord. Axons Each nerve cell has only one axon, carrying nerve impulses away from the cell body. They are usually longer than the dendrites, sometimes as long as 100 cm. Structure of an axon The membrane of the axon is called axolemma and it encloses the cytoplasmic extension of the cell body. Large axons and those of peripheral nerves are surrounded by a myelin sheath. This consists of a series of Schwann cells arranged along the length of the axon. Each one is wrapped around the axon so that it is covered by a number of concentric layers of Schwann cell plasma membrane. Structure of an axon Between the layers of plasma membrane there is a small amount of fatty substance called myelin. The outermost layer of Schwann cell plasma membrane is sometimes called neurilemma. There are tiny areas of exposed axolemma between adjacent Schwann cells, called nodes of Ranvier, which assist the rapid transmission of nerve impulses. Dendrites The dendrites are the many short processes that receive and carry incoming impulses towards cell bodies. They have the same structure as axons but they are usually shorter and branching. In motor neurones they form part of synapses and in sensory neurones they form the sensory receptors that respond to stimuli. The nerve impulse (action potential) An impulse is initiated by stimulation of sensory nerve endings or by the passage of an impulse from another nerve. Transmission of the impulse, or action potential, is due to movement of ions across the nerve cell membrane. The nerve impulse (action potential) At rest the charge on the outside is positive and inside it is negative. The principal ions involved are: 1. sodium (Na+ ) the main extracellular cation 2. potassium (K+ ) the main intracellular cation. The nerve impulse (action potential) Resting state Depolarisation Nerve impulse or action potential Repolarisation Sodium pump Saltatory conduction Simple propagation The nerve impulse (action potential) In the resting state there is a continual tendency for these ions to diffuse along their concentration gradients, i.e. K+ outwards and Na+ into cells. When stimulated, the permeability of the nerve cell membrane to these ions changes. Initially Na+ floods into the neurone from the ECF causing depolarisation, creating a nerve impulse or action potential. The nerve impulse (action potential) During repolarization process K+ floods out of the neurone and the movement of these ions returns the membrane potential to its resting state. As the neurone returns to its original resting state, the action of the sodium pump expels Na+ from the cell in exchange for K+ The nerve impulse (action potential) In myelinated neurones, the insulating properties of the myelin sheath prevent the movement of ions. Therefore electrical changes across the membrane can only occur at the gaps in the myelin sheath, i.e. at the nodes of Ranvier. When an impulse occurs at one node, depolarization passes along the myelin sheath to the next node so that the flow of current appears to 'leap' from one node to the next. This is called saltatory conduction. The nerve impulse (action potential) The speed of conduction depends on the diameter of the neurone: the larger the diameter, the faster the conduction. Myelinated fibres conduct impulses faster than unmyelinated fibres because saltatory conduction is faster than the complete conduction, or simple propagation. Types of nerves 1. Sensory or afferent nerves 2. Motor or efferent nerves 3. Mixed nerves Sensory or afferent nerves When action potentials are generated by sensory receptors on the dendrites of these neurones, they are transmitted to the spinal cord by the sensory nerve fibres. The impulses may then pass to the brain or to connector neurones of reflex arcs in the spinal cord. Sensory receptors Specialised endings of sensory neurones respond to different stimuli (changes) inside and outside the body. 1. Somatic, cutaneous or common senses 2. Proprioceptor senses 3. Special senses 4. Autonomic afferent nerves Motor or efferent nerves Motor nerves originate in the brain, spinal cord and autonomic ganglia. They transmit impulses to the effector organs: muscles and glands. There are two types: 1. somatic nerves — involved in voluntary and reflex skeletal muscle contraction 2. autonomic nerves (sympathetic and parasympathetic) — involved in cardiac and smooth muscle contraction and glandular secretion Mixed nerves In the spinal cord, sensory and motor nerves are arranged in separate groups, or tracts. Outside the spinal cord, when sensory and motor nerves are enclosed within the same sheath of connective tissue they are called mixed nerves. The synapse and neurotransmitters The point at which the nerve impulse passes from one to another is the synapse. Presynoptic neurone Synaptic knobs or terminal boutons Postsynaptic neurone Synaptic cleft Synaptic vesicles Neurotransmitter The neuromuscular junction The axons of motor neurones, conveying impulses to skeletal muscle to produce contraction, divide into fine filaments terminating in minute pads called motor end-plates. Each muscle fibre is stimulated through a single motor end-plate, and one motor nerve has many motor end-plates. The motor end-plate and the sensitive area of muscle fibre through which it is stimulated is analogous to the synapse between neurones and is known as the neuromuscular junction. The nerve impulse is passed across the gap between the motor end-plate and the muscle fibre by the neurotransmitter, acetylcholine. The neuromuscular junction The group of muscle fibres and the motor end-plates of the nerve fibre that supplies them constitute a motor unit. Nerve impulses cause serial contraction of motor units in a muscle and each unit contracts to its full capacity. The strength of the contraction depends on the number of motor units in action at a particular time. The endings of autonomic nerves supplying smooth muscle and glands branch near their effector structure and release a neurotransmitter which stimulates or depresses the activity of the structure.