Nervous System PDF

Nervous system Dr.sidra shafique RIU Lahore  Nervous system INTRODUCTION The nervous system detects and responds to changes inside and outside the body. Together with the endocrine system, it coordinates and controls vital aspects of body function and maintains homeosta...

Nervous system Dr.sidra shafique RIU Lahore  Nervous system INTRODUCTION The nervous system detects and responds to changes inside and outside the body. Together with the endocrine system, it coordinates and controls vital aspects of body function and maintains homeostasis.  Nervous system The nervous system consists of the brain, the spinal cord and peripheral nerves. central nervous system (CNS), consisting of the brain and the spinal cord peripheral nervous system (PNS), consisting of all the nerves outside the brain and spinal cord.  PNS It comprises paired cranial and sacral nerves or spinal nerves some of these are sensory (afferent) transmitting impulses to the CNS, some are motor (efferent) transmitting impulses from the CNS. mixed. (spinal nerves)  PNS It is useful to consider two functional parts within the PNS: sensory division motor division The motor division has two parts: Somatic Nervous system, which controls voluntary movement of skeletal muscles Autonomic Nervous system, controlling involuntary processes such as heartbeat, peristalsis and glandular activity. The Autonomic Nervous System has two divisions: Sympathetic Parasympathetic.  Cells and tissues of the nervous system There are two types of nervous tissue, neurons neuroglia Neurones (nerve cells) are the working units of the nervous system that generate and transmit nerve impulses. Neurones are supported by connective tissue, collectively known as neuroglia, which is formed from different types of glial cells. There are vast numbers of both cell types, trillion glial cells and 10 times fewer neurones.  Neurones(nerve cells) Each neurone consists of a cell body and its processes, one axon and many dendrites 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.  structure of neurons Cell bodies Nerve cells vary considerably in size and shape but they are all too small to be seen by the naked eye. 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. Groups of cell bodies are called nuclei in the central nervous system and ganglia in the peripheral nervous system.  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. dendrites  Axons Each nerve cell has only one axon, which begins at a tapered area of the cell body, the axon hillock. They carry impulses away from the cell body and are usually longer than the dendrites, sometimes as long as 100 cm  Structure of an axon. The membrane of the axon is called the axolemma and it encloses the cytoplasmic extension of the cell body. Myelinated neurones 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. Between the layers of plasma membrane is a small amount of fatty substance called myelin.  The outermost layer of the Schwann cell plasma membrane is the neurilemma. There are tiny areas of exposed axolemma between adjacent Schwann cells, called nodes of Ranvier which assist the rapid transmission of nerve impulses in myelinated neurones.  Unmyelinated neurones In this type a number of axons are embedded in one Schwann cell The adjacent Schwann cells are in close association and there is no exposed axolemma. The speed of transmission of nerve impulses is significantly slower in unmyelinated fibres. dendrite  Dendrites These are the many short processes that receive and carry incoming impulses towards cell bodies. They have the same structure as axons but are usually shorter and branching. In motor neurones dendrites form part of synapses and in sensory neurones they form the sensory receptors that respond to specific stimuli.  The synapse and neurotransmitters There is always more than one neurone involved in the transmission of a nerve impulse from its origin to its destination, whether it is sensory or motor. There is no physical contact between two neurones. The point at which the nerve impulse passes from the presynaptic neurone to the postsynaptic neurone is the synapse  At its free end, the axon of the presynaptic neurone breaks up into minute branches that terminate in small swellings called synaptic knobs, or terminal boutons. These are in close proximity to the dendrites and the cell body of the postsynaptic neurone. The space between them is the synaptic cleft.  Synaptic knobs contain spherical membrane bound synaptic vesicles, which store a chemical, the neurotransmitter that is released into the synaptic cleft Synaptic junction  Neurotransmitters are synthesised by nerve cell bodies, actively transported along the axons and stored in the synaptic vesicles. They are released by exocytosis in response to the action potential and diffuse across the synaptic cleft. They act on specific receptor sites on the postsynaptic membrane.  Somatic nerves carry impulses directly to the synapses at skeletal muscles, the neuromuscular junctions stimulating contraction. In the autonomic nervous system efferent impulses travel along two neurones(preganglionic and postganglionic) and across two synapses to the effector tissue, i.e. cardiac muscle, smooth muscle and glands, in both the sympathetic and the parasympathetic divisions.  Thank you