central nervous system

central nervous system The central nervous system (CNS) consists of the brain (inside the skull) and spinal cord (inside the vertebral column), which derive from a single, continuous tube of neural tissue that forms at an early stage in the embryo. The head end of the tube develops into the cerebral hemispheres, the cerebellum, and the brain stem. The lowest part of the brain stem, the medulla oblongata, merges with the spinal cord at the large hole (foramen magnum) in the base of the skull. The spinal cord in an adult human is about 45 cm long, tapering to a cone at its lower end. It has two swellings, the cervical and lumbar enlargements, which are due to accumulations of nerve cells (neurons) responsible for innervating the upper and lower limbs.

Both the brain and the spinal cord are ensheathed by three protective meninges (from the Greek for ‘membranes’). The outer one, the dura mater (‘dura’ because it is relatively hard and strong; ‘mater’ because it protects like a mother) lines the skull and the tunnel that runs through the centre of the vertebrae. The delicate, innermost pia mater (‘pia’ is Latin for soft) envelops the brain and the spinal cord closely down to the level of the upper lumbar vertebrae. Between the pia and dura is a space, particularly voluminous below where the cord itself ends. The dura is lined by the arachnoid mater, which is more fragile than the dura, being likened to a spider's web, the Greek origin of its name. So the space between the pia and dura, which contains cerebrospinal fluid (CSF), is called the subarachnoid space.

Twelve pairs of cranial nerves are attached to the brain, at various levels. Some, such as the olfactory and optic nerves, are purely sensory. Others, such as those supplying the muscles that move the eyeball, are motor. The tenth cranial nerve, called the ‘vagus’ (Latin for wanderer), carries sensory information from some of the viscera and also contains the outflow of parasympathetic fibres (part of the autonomic nervous system) that innervate the heart, the bronchial tree, the smooth muscle of much of the gut, and various glands.

Evolutionarily, the central nervous system is derived from the repetitive, segmented chain of nerve cells found in invertebrates, and this segmental pattern is still clear in the human spinal cord, and even in the lower parts of the brain. It is most evident in the spinal nerves, 31 pairs in all, that sprout from the sides of the spinal cord. Strictly, these spinal nerves are part of the peripheral nervous system, but their organization is best understood in relation to the cord itself. For each vertebra, on each side of the cord is a dorsal root (‘dorsal’ means on the top), containing sensory nerve fibres from the periphery of the body, which are destined to end in the cord or the base of the brain. The neuron cell bodies for these fibres are in swellings on the dorsal roots (dorsal root ganglia). There is a corresponding pair of ventral roots (‘ventral’ literally meaning on the side nearer the stomach), containing axons from motor neurons in the cord, on their way to skeletal muscles. Additional fibres leave the ventral roots in the middle levels of the cord to innervate smooth muscle (e.g. that of blood vessel walls and the gut), glands, and the heart. These are the sympathetic fibres and are also part of the autonomic nervous system. Just outside each vertebra, the dorsal and ventral roots unite to form a spinal nerve on each side.

If you cut a spinal cord transversely, you can see, even with the naked eye, a central, butterfly-shaped core of darker material. If such a section is examined with a microscope it becomes clear that this core consists of grey matter (grey because of a concentration of cell bodies), surrounded by columns of white matter (white because it consists largely of nerve fibres — axons), running up and down the cord. The dorsal part of the grey matter receives fibres of the dorsal root, relaying information about touch, temperature, pain, and also position sense. The ventral part of the grey matter contains motor neurons that send out their axons in the ventral root to reach the skeletal muscles.

Imagine what happens as nervous impulses arrive at the cord through fibres of the dorsal root. This sensory information is of several types. Firstly, it is either somatic (from skin, muscles, and joints) or visceral (from the internal organs) in origin. Secondly it may give rise to conscious sensation, which presupposes that the information is transmitted from the spinal cord to higher brain centres, and ultimately the cerebral cortex. Alternatively it remains unconscious, in which case it may be handled by brain centres such as the cerebellum, or it may simply feed a pathway within the spinal cord, ultimately resulting in signals passing out to cause muscle reactions (a spinal reflex).

Fibres concerned with touch, temperature, and pain end on nerve cell bodies in the dorsal grey matter, which in turn send axons across to the other side of the cord and then up to the brain. Many of them reach the thalamus, projecting thence to, amongst other regions, the somatic sensory cortex — a strip at the front edge of the parietal lobe of the cerebral hemispheres. Fibres that convey conscious position sense and fine discriminative touch also enter the cord by dorsal roots, but they behave differently in that they immediately turn upward on the same side of the cord and run all the way up through a tract of white matter called the dorsal columns, merely sending branches into the dorsal grey matter of the spinal cord along the way. These fibres end on groups of cells in the medulla of the brain stem, whose axons cross to the other side and run up to the thalamus, from where axons run up to the somatic sensory cortex.

The basic function of the central nervous system is to generate appropriate reactions to sensory signals, from inside or outside the body. The simplest form of such a reaction is a ‘reflex’ — an involuntary response to a sensory stimulus. The circuit of nerve cells and axons responsible is called a reflex arc. The simplest form of reflex arc involves an incoming fibre, which traverses the dorsal grey matter of the spinal cord to terminate at a synapse on a motor neuron in the ventral grey matter, whose axon runs out to a muscle. Since this circuit contains only one synaptic connection, it is called a monosynaptic reflex. The best known example is the ‘tendon jerk reflex’: when a muscle is suddenly stretched it reflexly contracts, to oppose the stretching. For instance, when the tendon just below the knee is tapped, stretching the thigh muscles to which this tendon is attached, the same muscles contract, causing the leg to kick. Such tendon jerks are tested as part of a routine neurological examination, to assess the state of synaptic connections. This very simple type of reflex arc is relatively rare, most reflexes being complex or multisynaptic. This implies that the circuit between incoming sensory fibre and motoneuron includes other nerve cells. As these may innervate several levels of the cord, or even cross to the other side, these reflexes can be much more sophisticated than simple ones. For example, burning the tip of a finger may result in reflex withdrawal of the whole upper limb.

Some reflexes, although involuntary, almost certainly involve connections running through the cerebral cortex, or through the cerebellum, which is particularly involved in the learning and execution of motor skills, especially highly automated ones whose operation does not intrude into consciousness.

As well as major ‘ascending’ pathways carrying sensory information up to the corresponding regions of the cerebral cortex, the white matter of the brain stem and spinal cord also contains many tracts of fibres running downwards. The largest of these is the corticospinal, or pyramidal, tract, which originates in large neurons in motor areas of the cerebral cortex, and descends to the lower brain stem, where most of its axons cross over and enter the spinal cord to end, without interruption, on motoneurons in the grey matter.

This brief account leaves the impression that the central nervous system is little more than a set of cables running up and down, with something akin to a telephone switchboard in between. In reality, the human central nervous system is a monstrous biological computing instrument (although many would contest the analogy with a conventional computer), which is somehow capable of capturing the meaning of events in the outside world, representing them in memories and as conscious experiences, and making decisions that go far beyond automatic reactions to immediate events.

Laurence Garey

See also autonomic nervous system; brain; nervous system; reflexes; spinal cord.