More information about the muscles and nerves

Smooth muscle, also called involuntary muscle, is that contained in organs and tissues that are not under our direct control, such as the muscle in the walls of blood vessels or the gut. It's called smooth because under the microscope it does not look striped. Each fibre contains myofibrils and a single nucleus.

Cardiac muscle is found in the heart. It has fibres which are striated, like those of voluntary muscle, but each fibre contains only one single nucleus. They are joined end to end and in between them are structures called intercalated discs which allow transmission of the electrical impulse, or action potential, throughout the heart in order that it contracts in a coordinated way.

Nerves outside the brain and spinal cord, which together are called the central nervous system, make up two distinctly different nerve systems. One of these is under our control, called the voluntary nervous system: it controls the movement of muscles when we want to move them; the other, called the autonomic nervous system, oversees the control of organs whose function is automatic, such as the beating of the heart or the movement of the gut.

The voluntary nervous system consists of a number of nerves which arise either from the brain (the 12 cranial nerves) or from the spinal cord (the 31 spinal nerves, each associated with a particular vertebra). These both carry signals to the brain or spinal cord, (and because they sense things are called sensory), and convey signals from the brain to the muscles (and make them move, and so are called motor nerves). Each nerve trunk serves a particular area of skin - called a dermatome - and an individual set of muscle fibres. Transmission along the nerves is by electrical conduction using currents generated within the nerve itself, mainly by changes in concentrations of sodium and potassium ions. The speed of conduction and the maintenance of the signal is enhanced by the mode of progression: rather than going down the nerve as it would an electric wire it jumps along using a series of what are called nodes, situated every millimetre or so down the nerve. Using this means of conduction nerves can transmit signals at speeds of up to 125 miles per hour.

We sense things because of the activation of specialised receptors situated in the skin, the muscles, the joints and the tendons. Skin receptors respond to pain, touch, temperature, pressure, itch and tickle; muscle and tendon receptors respond to stretch, and receptors in joints to the position of the joint. There are other receptors which mediate taste and smell for example, described in the section on the nose and mouth.

We move muscles by causing them to contract in response to a signal down the motor nerve which end in the muscle at a region called the end plate. Here, when the electric signal arrives, the end of the nerve secretes a chemical called acetylcholine, which acts as a transmitter (because it transmits the signal): this causes a contraction by binding to a place on the muscle called a receptor. The way the muscle contracts is described in the muscle section.

The autonomic nervous system consists of two parts which usually have opposing effects on tissues, called the sympathetic and parasympathetic nervous system. The sympathetic nervous system arises from a chain of structures called ganglia alongside the vertebrae in the chest and the lumbar region. The chemical it uses as a transmitter is mostly noradrenalin, and in general it increases the activity of the organ that it acts on - the heart rate increases and blood vessels contract (apart from those in muscles), for example, as we prepare for fight or flight. The parasympathetic nervous system has fibres that arise from the brain or the end of the spinal cord, at the sacrum. The chemical used as a transmitter is acetylcholine and in general it decreases the activity of the organ it acts on: for example it causes the heart rate to slow. This is the system that prepares us to rest and digest. The relative levels of activity of the two parts of the autonomic nervous system are governed, as is the activity of all the nervous system, by the brain.