brain stem

brain stem The brain within the skull and the spinal cord within the vertebral column constitute the central nervous system. The cerebral hemispheres (cerebrum) of the brain occupy the larger, front part of the cavity of the skull. And at the base of the cerebrum, emerging like the stalk from a mushroom cap, is an elongated structure, the brain stem. With the cerebellum behind and above it, it occupies the smaller, back part of the skull cavity.

The brain stem forms a bridge between cerebrum and spinal cord and also carries the major pathways (peduncles) for signals to pass to and from the cerebellum. The brain stem itself consists of three distinct parts. From above down these are the midbrain; a bridge-like structure, the pons; and the medulla oblongata, which merges below with the spinal cord.

From the brain stem emerge pairs of cranial nerves, analogous to the spinal nerves that innervate the limbs and trunk. These contain motor nerves to skeletal muscle fibres that move the eyes, to the facial muscles responsible for the familiar expression or ‘set’ of a face, and to those controlling the movements of the jaw, the tongue, and the larynx. Damage to the seventh cranial (facial) nerve on one side for example, creates the characteristic asymmetry of the ‘set’, smile or grimace in ‘Bell's Palsy’, while motorneuron disease affecting other cranial nerves interferes with speech.

Most of these nerves also carry incoming information: the massive fifth cranial nerve (trigeminal) has a rich abundance of sensory fibres contributing to the exquisite tactile sensitivity of the facial skin at the mouth, eyes, cornea and, less agreeably, ones from the dental pulp and gums to cause dental pain. The eighth (auditory) cranial nerve carries mainly sensory fibres from two highly specialized structures, the cochlear and vestibular apparatus, concerned respectively with hearing and balance. The ninth (glossopharyngeal) and tenth (vagus) nerves also contain the special system of ‘parasympathetic’ motor fibres of the autonomic nervous system that innervate not only structures around the head and neck (such as the salivary glands) but also, in the case of the vagus nerves, the thoracic and abdominal organs (heart, bronchi, gut). These two nerves also carry important information from receptors in the lungs, heart, and blood vessels essential to the reflex regulation of these structures.

By virtue of its nerve connections the brain stem mediates important reflexes, including protection of the eyes by closure of the lids, protection of the throat by gagging, and elimination of irritant bodies by sneezing and coughing.

The brain stem is very much more than simply a viaduct for the long nerve fibre tracts directly linking brain and spinal cord. It is also a relay for certain categories of movement commanded by the motor cortex, as instanced by signals related to hand clenching conveyed in the ‘rubrospinal tract’, with its neuronal cell bodies at the upper end of the brain stem. Similarly, signals relating to the sense of touch or limb movement (somatic sensation and proprioception) are relayed in cells within the medulla.

More importantly still, the brain stem is the origin of a multitude of fibre systems, ascending to higher levels as well as those descending into the spinal cord, passing into the cerebellum, or, just as richly, terminating elsewhere within the brain stem.

Networks within the brain stem are at the heart of rhythm generators, for movements such as mastication and the rhythmic eye movements (nystagmus) that occur during rotation or when the eye attempts to fixate on a moving object. Most vital of all is the network that generates the rhythm and the command for breathing movements and that mediates the central and reflex adjustment of these to match metabolic demands. Closely related networks are responsible for maintaining vascular tone and serve as centres for the reflex regulation of heart rate and blood pressure.

Other networks in the brain stem have been linked to pain and its control. Neurons in the ‘periaqueductal grey matter’ (PAG), which surrounds the channel (aqueduct) in the midbrain for cerebrospinal fluid, are sites of extensive convergence of sensory information from all over the body. In turn, these signal back to the spinal cord and reduce nerve transmission in ascending pathways carrying signals with the potential of causing pain. This and other systems are engaged when the opiate morphine is used to abolish acute pain and, it is thought, by the body's endogenous production of opioids following severe acute trauma. The PAG also has substantial connections to the cerebrum and activity in these pathways may ultimately form the basis of ‘affect’; the emotional adjunct to human behaviour.

Whilst the cerebrum is absolutely essential for sensory perception and conscious, willed behaviour, the brain stem is absolutely essential for life in the absence of artificial life support. Even should the entire brain be destroyed above the midbrain, the brain stem itself, providing the motor pathways to the respiratory motor neurons in the spinal cord are intact, will sustain a living body (though not a ‘life’ as we normally know it) until death ensues due to starvation, infection, or cardiac arrest.

Tom Sears

See nervous system.See also brain; brain death; breathing; life support; vegetative state.