Christopher L.-H. Huang

Nerve and Muscle: The activation of skeletal muscle

Skeletal muscle contraction is ultimately initiated by activity in the nervous system. Muscle receives both sensory and motor nerve fibres. The sensory nerves convey information about the state of the muscle to the nervous system. This includes information about muscle length detected by the muscle spindles and tension detected by the Golgi tendon organs. There are also a variety of free nerve endings in the muscle tissue, some of which convey sensations of pain. Of the motor fibres in mammals, the γ-motoneurons provide a separate motor nerve supply for the muscle fibres of the muscle spindles. However, the bulk of the muscle fibres are supplied by the α-motoneurons. Each α-motoneuron innervates a number of muscle fibres, from less than ten in the extraocular muscles, which move the eyeball in its socket, to over a thousand in a large limb muscle. The complex of one motoneuron plus the muscle fibres which it innervates is called a motor unit . Since they are all activated by the same nerve cell, all the muscle fibres in a single motor unit contract at the same time. However, muscle fibres belonging to different motor units may well contract at different times. Thus, most mammalian muscle fibres are contacted by a single nerve terminal, although sometimes there may be two terminals originating from the same nerve axon. Muscle fibres of this type are known as twitch fibres , since they respond to nervous stimulation with a rapid twitch.

Structural organization of the nervous system

One of the characteristics of higher animals is their possession of a more or less elaborate system for the rapid transfer of information through the body in the form of electrical signals, or nervous impulses. At the bottom of the evolutionary scale, the nervous system of some primitive invertebrates consists simply of an interconnected network of undifferentiated nerve cells. The next step in complexity is the division of the system into sensory nerves responsible for gathering incoming information, and motor nerves responsible for bringing about an appropriate response. The nerve cell bodies are grouped together to form ganglia. Specialized receptor organs are developed to detect every kind of change in the external and internal environment; and likewise there are various types of effector organ formed by muscles and glands, to which the outgoing instructions are channelled. In invertebrates, the ganglia which serve to link the inputs and outputs remain to some extent anatomically separate, but in vertebrates the bulk of the nerve cell bodies are collected together in the central nervous system. The peripheral nervous system thus consists of afferent sensory nerves conveying information to the central nervous system, and efferent motor nerves conveying instructions from it. Within the central nervous system, the different pathways are connected up by large numbers of interneurons which have an integrative function. Certain ganglia involved in internal homeostasis remain outside the central nervous system. Together with the preganglionic nerve trunks leading to them, and the postganglionic fibres arising from them which innervate smooth muscle and gland cells in the animal’s viscera and elsewhere, they constitute the

Nerve and Muscle: Synaptic transmission in the nervous system

The functioning of the nervous system depends largely on the interactions between its constituent nerve cells, and these interactions take place at synapses. In most cases synaptic transmission is chemical in nature, so that, as in neuromuscular transmission, the pre-synaptic cell releases a chemical transmitter substance which produces a response in the post-synaptic cell. There are a few examples of electrically transmitting synapses, which we shall consider briefly at the end of this chapter. Acetylcholine is only one of a range of different neurotransmitters. Figure 8.1 shows some of the variety found in the central nervous system. For a long time it was thought that any one cell would only release one neurotransmitter, but several cases where two of them are released at the same time are now known. Different chemically transmitting synapses differ in the details of their anatomy, but some features are common to all of them. In the pre-synaptic terminal the transmitter substance is packaged in synaptic vesicles. The pre- and post-synaptic cells are separated by a synaptic cleft into which the contents of the vesicles are discharged. There are specific receptors for the neurotransmitter on the post-synaptic membrane. Just as with the neuromuscular junction, our knowledge of how synapses work was greatly affected by the invention of the intracellular microelectrode. Much of the fundamental work with this technique was performed by J. C. Eccles and his colleagues on the spinal motoneurons of the cat, so it is with these that we shall begin our account of synapses between neurons.

Nerve and Muscle: The ionic permeability of the nerve membrane

Structure of the cell membrane All living cells are surrounded by a plasma membrane composed of lipids and proteins, whose main function is to control the passage of substances into and out of the cell. In general, the role of the lipids is to furnish a continuous matrix that is impermeable even to the smallest ions, in which proteins are embedded to provide selective pathways for the transport of ions and organic molecules both down and against the prevailing gradients of chemical activity. The ease with which a molecule can cross a cell membrane depends to some extent on its size, but more importantly on its charge and lipid solubility. Hence the lipid matrix can exclude completely all large water-soluble molecules and also small charged molecules and ions, but is permeable to water and small uncharged molecules like urea. The nature of the transport pathways is dependent on the specific function of the cell under consideration. In the case of nerve and muscle, the pathways that are functionally important in connection with the conduction mechanism are: (1) the voltage-sensitive sodium and potassium channels peculiar to electrically excitable membranes, (2) the ligand-gated channels at synapses that transfer excitation onwards from the nerve terminal, and (3) the ubiquitous sodium pump, which is responsible in all types of cell for the extrusion of sodium ions from the interior. The essential feature of membrane lipids that enables them to provide a structure with electrically insulating properties, i.e. to act as a barrier to the free passage of ions, is their possession of hydrophilic (polar) head groups and hydrophobic (non-polar) tails.

Nerve and muscle: Structural organization of the nervous system

Nervous systems One of the characteristics of higher animals is their possession of a more or less elaborate system for the rapid transfer of information through the body in the form of electrical signals, or nervous impulses. At the bottom of the evolutionary scale, the nervous system of some primitive invertebrates consists simply of an interconnected network of undifferentiated nerve cells. The next step in complexity is the division of the system into sensory nerves responsible for gathering incoming information, and motor nerves responsible for bringing about an appropriate response. The nerve cell bodies are grouped together to form ganglia . Specialized receptor organs are developed to detect every kind of change in the external and internal environment; and likewise there are various types of effector organ formed by muscles and glands, to which the outgoing instructions are channelled. In invertebrates, the ganglia which serve to link the inputs and outputs remain to some extent anatomically separate, but in vertebrates the bulk of the nerve cell bodies are collected together in the central nervous system . The peripheral nervous system thus consists of afferent sensory nerves conveying information to the central nervous system, and efferent motor nerves conveying instructions from it. Within the central nervous system, the different pathways are connected up by large numbers of interneurons which have an integrative function. Certain ganglia involved in internal homeostasis remain outside the central nervous system.