Nerve and muscle

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Higher animals have four basic tissue types: epithelial tissue, connective tissue, nervous tissue, and muscle. Of these, nerve and muscle are grouped together as ‘excitable cells’ because the cell membrane has the ability to vary membrane ion conductance and membrane voltage so as to transmit meaningful signals within and between cells. Within excitable cells information is transmitted using either an amplitude-modulated (AM) code using slow, electrotonic potentials, or a frequency-modulated (FM) code when signalling is by action potentials. Much of the signalling between excitable cells occurs at chemical synapses where a chemical neurotransmitter is released from presynaptic cells and then interacts with postsynaptic membrane receptors. Clinical symptoms can arise when the release of chemical neurotransmitters is disturbed, or when availability of postsynaptic receptors is altered. Thus, a reduction in dopamine release from basal ganglia substantia nigra cells is found in Parkinson’s disease, while myasthenia gravis results from loss of nicotinic acetylcholine receptors at the neuromuscular junction of skeletal muscle. Sometimes transmission from cell to cell is not by chemical neurotransmitter but by electrical synapses, where gap-junctions provide direct electrical connectivity. Transmission between cardiac muscle cells occurs in this way. Some cardiac arrhythmias, such as Wolff –Parkinson–White syndrome, are a consequence of an abnormal path of electrical conduction between cardiac muscle fibres. Sensory cells on and within the body pass information via afferent pathways from the peripheral nervous system into the central nervous system (CNS). CNS processes and sensory information are integrated to produce outputs from the CNS. These outputs pass by various efferent routes to the effector organs: skeletal muscle, cardiac muscle, smooth muscle, and glands. It is through these effectors that the CNS is able to exert control over the body and to interact with the environment. Alterations of function anywhere in the afferent, integrative, or efferent aspects of the system, as well as defects in the effectors themselves, are likely to lead to significant clinical symptoms and signs. The efferent outflow from the CNS has two major components. One, the somatic nervous system, innervates only skeletal muscle. The other is the autonomic nervous system (ANS), which innervates cardiac muscle, smooth muscle, and the glands of the viscera and skin.