Afferent Health Dictionary

The term applied to vessels which convey away blood or a secretion from a body part, or to nerves which carry nerve impulses outwards from the nerve-centres. (Opposite: AFFERENT.)... efferent

Also known as a nerve cell, this is the basic cellular building-block of the NERVOUS SYSTEM, which contains billions of neurones linked in a complex network and acting in di?erent combinations to keep the body informed about the outside world, and then to organise and activate appropriate responses. There are three main types of neurone:

Sensory These carry signals to the central nervous system (CNS) – the BRAIN and SPINAL CORD – from sensory receptors. These receptors respond to di?erent stimuli such as touch, pain, temperature, smells, sounds and light.

Motor These carry signals from the CNS to activate muscles or glands.

Interneurons These provide the interconnecting ‘electrical network’ within the CNS.

Structure Each neurone comprises a cell body, several branches called dendrites, and a single ?lamentous ?bre called an AXON. Axons may be anything from a few millimetres to a metre long; at their end are several branches acting as terminals through which electrochemical signals are sent to target cells, such as those of muscles, glands or the dendrites of another axon.

Axons of several neurones are grouped

together to form nerve tracts within the brain or spinal cord or nerve-?bres outside the CNS. Each nerve is surrounded by a sheath and contains bundles of ?bres. Some ?bres are medullated, having a sheath of MYELIN which acts as insulation, preventing nerve impulses from spreading beyond the ?bre conveying them.

The cellular part of the neurones makes up the grey matter of the brain and spinal cord – the former containing 600 million neurones. The dendrites meet with similar outgrowths from other neurones to form synapses. White matter is the term used for that part of the system composed of nerve ?bres.

Functions of nerves The greater part of the bodily activity originates in the nerve cells (see NERVE). Impulses are sent down the nerves which act simply as transmitters. The impulse causes sudden chemical changes in the muscles as the latter contract (see MUSCLE). The impulses from a sensory ending in the skin pass along a nerve-?bre to affect nerve cells in the spinal cord and brain, where they are perceived as a sensation. An impulse travels at a rate of about 30 metres (100 feet) per second. (See NERVOUS IMPULSE.)

The anterior roots of spinal nerves consist of motor ?bres leading to muscles, the posterior roots of sensory ?bres coming from the skin. The terms, EFFERENT and AFFERENT, are applied to these roots, because, in addition to motor ?bres, ?bres controlling blood vessels and secretory glands leave the cord in the anterior roots. The posterior roots contain, in addition to sensory ?bres, the nerve-?bres that transmit impulses from muscles, joints and other organs, which among other neurological functions provide the individual with his or her

proprioceptive faculties – the ability to know how various parts of the body are positioned.

The connection between the sensory and motor systems of nerves is important. The simplest form of nerve action is that known as automatic action. In this, a part of the nervous system, controlling, for example, the lungs, makes rhythmic discharges to maintain the regular action of the respiratory muscles. This controlling mechanism may be modi?ed by occasional sensory impressions and chemical changes from various sources.

Re?ex action This is an automatic or involuntary activity, prompted by fairly simple neurological circuits, without the subject’s consciousness necessarily being involved. Thus a painful pinprick will result in a re?ex withdrawal of the affected ?nger before the brain has time to send a ‘voluntary’ instruction to the muscles involved.

Voluntary Actions are more complicated than re?ex ones. The same mechanism is involved, but the brain initially exerts an inhibitory or blocking e?ect which prevents immediate re?ex action. Then the impulse, passing up to the cerebral hemispheres, stimulates cellular activity, the complexity of these processes depending upon the intellectual processes involved. Finally, the inhibition is removed and an impulse passes down to motor cells in the spinal cord, and a muscle or set of muscles is activated by the motor nerves. (Recent advances in magnetic resonance imaging (MRI) techniques have provided very clear images of nerve tracts in the brain which should lead to greater understanding of how the brain functions.) (See BRAIN; NERVOUS SYSTEM; SPINAL CORD.)... neuron(e)

(ARF) a primary disorder of gaseous exchange (as distinct from failure of the mechanical process of breathing). The prototype of ARF is *adult respiratory distress syndrome, but the term sometimes also refers to disruption of any other part of the respiratory system, including the respiratory control centre in the brain with its *efferent and *afferent pathways.... acute respiratory failure

adj. describing impulses travelling ‘the wrong way’ in a nerve fibre. This is rare but may happen in shingles, when the irritation caused by the virus in the spinal canal initiates impulses that travel outwards in normally afferent nerves. The area of skin that the sensory nerves supply (usually a strip on the trunk) becomes painfully blistered. Antidromic impulses cannot pass *synapses, which work in one direction only.... antidromic

(JGA) a microscopic structure within the kidney that is important in regulating blood pressure, body fluid, and electrolytes. It is situated in each nephron, between the afferent arteriole of the glomerulus and the returning distal convoluted tubule of the same nephron. The JGA consists of specialized cells within the distal tubule (the macula densa), which detect the amount of sodium chloride passing through the tubule and can secrete locally acting vasoconstrictor substances that act on the associated afferent arteriole to induce a reduction in filtration pressure (tubuloglomerular feedback). Modified cells within the afferent arterioles secrete *renin in response to a fall in perfusion pressure or feedback from the macula densa and form a central role in the renin-*angiotensin-aldosterone axis. Mesangial cells support and connect the macula densa and the specialized cells in the afferent arteriole and have sympathetic innervation, facilitating the renin response to sympathetic nervous stimulation.... juxtaglomerular apparatus

n. hardening of the arteries and arterioles of the kidneys. Benign nephrosclerosis is associated with essential hypertension. There is preferential involvement of the preglomerular arterial vessels, primarily the afferent arteriole and the interlobular artery. The classic arterial lesion, which is termed arteriolosclerosis, involves replacement of smooth muscle cells in the media of the vessel by connective tissue. There is often evidence of ischaemia in the glomerulus and *tubulointerstitium. Functionally there may be some degree of renal impairment. End-stage renal failure is uncommon, but more likely to occur in Afro-Caribbeans. Malignant nephrosclerosis is the hallmark of *malignant hypertension, with arterioles showing mucoid change, endothelial cell swelling, and fibrinoid necrosis. The lumen of the vessel is reduced and red cells fragmented in their passage through the narrowing. The kidney shows petechial haemorrhage on the subcapsular surface, with mottling and areas of infarction. Malignant nephrosclerosis can lead to a very rapid destruction of renal function and is recognized as a potential cause of acute renal failure.... nephrosclerosis