Nervous system summary
- 1. THE NERVOUS SYSTEM The nervous system, along with the endocrine system, responds to changes in the external or internal environment. The nervous system is the co-ordination system for rapid actions. INITIAL EVOLUTION OF THE NERVOUS SYSTEM To coordinate the initial rod shaped organisms some form of communication system would have been required. Chemical messengers that spread by diffusion might suffice initially but, for rapid responses, a nervous system would be better and, probably at much the same time as the rod organisms were being evolved in a segmental fashion, a midline coordinating structure (the spinal cord) developed. Vertebrates eventually evolved from such segmented ancestors (possibly flatworms). Segmentation was useful to allow sequential contraction of the muscle groups (myotomes), for defensive reflexes and for propulsion in water or on land. The development of similar segments also minimized the need for genetic information - all that was required was a genetic instruction “repeat this basic pattern n times.” Presumably vertebrates living on the seafloor were governed by gravitational considerations, and because muscles which conferred mobility work best if attached to a rigid structure like the backbone, and because muscle groups became ventrally suspended from the backbone, it made sense for the motor output to skeletal muscles to be ventrally situated in the spinal cord. The sensory nerves would mostly serve the outside of the rod-like animal and would have been dorsal to avoid pushing through the muscle groups (Fig. 1). Initially there was a neural tube, the front of which then divided into three functional areas (Fig.2). Three important early features in mammalian evolution were: • bilateral symmetry (humans are mostly symmetrical)
- 2. • segmentation (to allow sequential contraction for swimming purposes) • cephalization (a major focalisation of organisation in a head)
- 4. Nearly all nervous pathways, except cerebellar and vestibular pathways,cross the midline either in the spinal cord, the brainstem or in the brain. There seem to be three possible explanations. Firstly the primitive coiling reflex to noxious stimuli (Fig. 3) required that nerve fibres crossed the midline. Impulses from the sensory nerve fibres had to be conducted to the motor neurones on the immediate opposite side of the spinal cord to allow the primitive rod-like animal to bend away from the noxious stimulus.
- 5. Secondly stereoscopic sight using two eyes (which allows accurate distance appreciation) would have offered great evolutionary advantages. Integration of the information from the two eyes demands that information crosses the midline (Figs. 4a, 4b and 4c). A threat to the right of the head is appreciated by the left hand side of each retina and information from one eye has to cross the midline (in the optic chiasma) and the visual input from both retinas is coordinated in the left occipital cortex (Fig. 4c). Thus if there is to be a coordinated response, then a quicker and more efficient response to the threat demands that the right side (specifically the right arm) be moved. To move the right arm the left motor cortex has to initiate action and to do this information from the left occipital cortex has to cross the midline.
- 7. Thirdly swimming or walking requires coordination of the two sides and this requires that nervous information crosses the midline. The basic structure of the human nervous system is illustrated in Fig. 2. In anatomical terms the central nervous system comprises the brain and spinal cord (Fig. 5 ) whilst the peripheral nervous system comprises 12 pairs of cranial nerves and 31 pairs of nerves arising from the spinal cord.
- 9. The brain and spinal cord are surrounded by three membranes, called the meninges. The outer layer is the dura mater, the middle the arachnoid mater (interior to which is the cerebrospinal fluid) and the inner pia mater. Blood or infection in the cerebrospinal fluid causes meningeal irritation which results in neck stiffness. The capillary blood vessels supplying the central nervous system are anatomically and functionally different from capillaries elsewhere in the body. They constitute the blood-brain barrier which prevents certain large molecules and certain drugs from crossing into the central nervous system. On electron microscopy the CNS endothelial cells are joined by tight junctions which are impermeable to large molecules. The are fenestrations (holes) between endothelial cells outside the CNS. NERVES IN GENERAL
- 11. The 1011 nerve cells and their processes are the building blocks of the nervous system. Nerve cells, once dead, cannot be replaced. Each nerve cell body has one or more dendrites which collect impulses from other nerve cell bodies or from sensory receptors. Axons are long outgrowths from nerve cells which transmit impulses away from the nerve cell. Axons may branch at their terminations and impact upon other nerve cells or their dendrites. In the central nervous system such impulses can be excitatory or inhibitory. In some nerve cells a single processemerges which splits into two in a T-shaped manner - (pseudo) unipolar nerve cells. For example the cells of the sensory spinal ganglia lateral to the dorsal surface of the spinal cord have neurones which have sensory gathering nervous fibres extending peripherally and a nerve fibre extending centrally (Fig. 7). Nerve cells do not divide except in the embryo. There are four types of nerve fibres: • Myelinated with neurolemma sheaths (mostly in peripheral nerves) • Myelinated with no neurolemma sheaths (in brain and spinal cord) Axon (=hillock): an outgrowth, usually a single outgrowth, from a nerve cell which conducts impulses along the nerve cell to the other nerve cells or their dendrites. Synonymous with nerve fibre Dendrite (=tree): the branching process of a nerve cell which conducts impulses to synapses on other nerve cells Decussation: nerve fibres which cross the midline in bundles of fibres (commisures pass to the same side) Ganglia (=knotlike) collections of nerve cell bodies outwith the central nervous system (except the basal ganglia which are within the central nervous system). They are the seat of visceral motor reflexes which affect some internal organs Neurone: nerve cell, axon, and dendrites Nerve fibre: axon and its myelin sheath.
- 12. • No myelination but a neurolemma sheath (autonomic nerves and fine efferents of corticospinal nerves) • No myelination and no neurolemma sheath (the gray matter of brain and spinal cord) There are three types of glial cells within the CNS, astrocytes, oligodendrocytes and ependymal lining cells. These support neurones structurally and metabolically. In the peripheral nervous system neurones are supported by Schwann cells which ensheath axons. In the CNS the oligodendrocytes produce myelin which ensheaths CNS axons. Whilst all axons have Schwann cells surrounding them the speed of propagation along axons is enhanced if the axons are wrapped up with a myelin sheath by Schwann cells (Fig. 6). This “plastic covering of the electric wire” is interrupted at intervals by Nodes of Ranvier. Myelinated axons appear white (constituting the white matter of the brain and spinal cord) whereas unmyelinated axons appear gray (constituting the gray matter (neuronal cell bodies) of the brain and spinalcord). White matter conducts impulses but gray matter processes and interprets. The cortex of the cerebral hemispheres and cerebellum are mostly infolded sheets of nerve cells (gray matter). Areas of the brain called nuclei are (more or less) spherically shaped collections of functionally related gray matter in the central nervous system. Multiple sclerosis produces sensory and motor symptoms and signs caused by patchy demyelination “plaques” in medullated axons “white matter” of the central but not the peripheral nervous system. There are 31 pairs of spinal nerves - eight cervical, 12 thoracic, five lumbar, five sacral and one rudimentary coccygeal. Peripheral nerves are formed by the unification of the dorsal (sensory) and ventral (motor) roots. In the cervical and lumbar regions these roots join to form the cords of the brachial and lumbrosacral plexuses and from these arise mixed sensory and motor peripheral nerves. The motor fibres supply voluntary (striated muscle). Autonomic nerves arise from sympathetic plexuses to supply cardiac muscle, smooth muscle of the gut and blood vessels, and the salivary and some other glands. Depending on which roots are predominantly affected there may be: • Segmental muscle wasting • Reflex loss • Dermatomal pattern sensory loss (Fig. 8a, 8b for peripheral nerve losses and Fig. 8c for the likely explanation of the actual dermatome distributions) • Radiating root pain (localization often poor) • Tingling • Oversensitive skin (hyperaethesiae)
- 15. If nerve root problems are caused by pathologies that tether the root then anything which moves the root (movements, coughing, straining) will exacerbate symptoms, especially pain. Depending on the
- 16. pathology there may also be signs of spinal cord compression. If the nerve roots leaving the end of the spinal cord (conus and cauda equina) are involved then bladder sensation may be reduced and there may be difficulty voiding with a poor stream (motor) and rectal function may be similarly affected leading to incontinence and faecal soiling.
- 17. Synapses (=junction) The junction between a “sending” axon and a “receiving dendrite or nerve cell is a synapse (Fig.6). Synapses are only found in the central nervous system or in autonomic ganglia and only conducts information in one direction. Some axons terminate with a single synapse whilst others may have thousand of terminations. A typical motor neurone in the spinal cord has about 10,000 synaptic contacts. Whether the receiving nerve cell responds usually depends on the number of inputs from other axons that it is receives. The more synapses that impinge on a nerve cell or its dendrites the more modifying influences there are on the final output of the receiving nerve cell. This is the basis of the integrated neural networking that forms the brain. Frequent use of particular synapses in certain situations may result in the establishment of a regular response pattern. In the central nervous system this constitutes a basis for learning (consciously or unconsciously). Synapse damage leads to forgetting. Nerves pass on information by putting out chemicals - neurotransmitters - at their synaptic endings. There are four main groups of neurotransmitters (Fig. 6): • Acetylcholine (in cholinergic neurones) • Amines Noradrenaline (in noradrenergic neurones) Serotonin (an important transmitter in sensory channels relevant to emotions) Dopamine (in the motor systems, limbic system and in the hypothalamus) • Amino acids Glutamic acid (always excitatory) Gamma aminobutyric acid (always inhibitory) • Peptides Enkephalins Endorphins These transmitters can be reabsorbed, broken down, or competitively inhibited or totally blocked before they can act. Nitric oxide (a gas) has recently been shown to be a neurotransmitter. Neuromuscular junctions When a motor nerve arrives at its muscle it loses its myelinated sheath and the axon forms a pre-synaptic terminal (motor end plate). Acetylcholine is released to bridge the gap (Fig. 9). When sufficient acetylcholine accumulates the muscle fibre depolarizes and an action potential spreads across the muscle to cause contraction. Acetycholine release from the pre-synaptic membrane is enhanced by calcium and its action is limited by destruction by an enzyme, cholinesterase, within the neuromuscular junction. Problems may arise if acetycholine release is modified, its action blocked, or if acetyl choline is broken down by cholinesterase. The final result is muscle weakness.
- 18. Antibodies may damage the acetylcholine receptor and prevent acetylcholine binding causing poor depolarisation of nerve impulses to muscle which leads to muscle fatigue and weakness on repeated use (myasthenia gravis). This may be focal (often causing drooping of the eyelids), or may affect other brainstem-derived cranial nerves or may be more widespread. In the Eaton-Lambert syndrome there are antibodies to calcium channels in the pre-synaptic terminals leading to failure of acetylcholine release (and perhaps also in the autonomic nerves to cause parasympathetic failure). Often the weakness improves on repeated use. With botulism the toxin binds to the terminals of nerves such that acetylcholine cannot be released and there is widespread muscle paralysis and autonomic dysfunction. Nerve impulses are electric waves of depolarisationwhich propagate along axons. Normally sodium is pumped out of the axon whereas potassium is kept at a high concentration inside the axon. This causes a 60 mVolts positive charge outside the axon. If the axon is excited sodium briefly enters the axon, the inside briefly becoming 40 mvolts positive, and a current flows ahead and initiates further sodium penetration “down the line” and thus a wave of depolarisation travels along the axon. An axon either transmits an impulse or not, “all or nothing,” and information conveyed depends on alterations in the frequency of impulses plus the nature and situation of the sending nerve cell body or receptor. On arrival centrally the sensory inputs are allowed to progress depending on their density and timing from the periphery. This wave of depolarisation is followed by a refractory period in which the nerve is absolutely or relatively resistant to the passage of other impulses. After conducting an impulse the larger nerve fibres recover in over one millisecond (up to 1,000 vibrations per second can be appreciated). In practice most sensory fibres can conduct 300-400 impulses per second and motor fibres can conduct at 100-150 impulses a second. If the axon is unmyelinated then the wave flows continuously to its destination but if there are nodes of Ranvier the wave jumps from node to node, increasing the velocity of conduction. Unmyelinated axons can conduct as slowly as 50 cms per second whereas myelinated fibres can conduct at about 100 metres per second (Fig. 10).
- 19. In some situations there can be pain appreciation without touch (from the bladder for example) but in other situations touch becomes pain if afferent nerves convey an excessive number of similar impulses. Within limits, the larger the diameter of the axon the quicker the impulse conduction. Large nerve fibres mostly transmit touch, pressure, two point discrimination, and joint position sensation but usually not pain and temperature. Smaller fibres (which are more likely to be affected by diabetes) mostly transmit pain and temperature but not other modalities (thus neuropathic foot ulcers caused by loss of pain sensation may result). Axons normally only transmit away from the nerve cell. Nerves that carry important messages tend to have myelinated axons and a minimum of synapses to allow rapid conduction (for example in the corticospinal tract). Nerve cell bodies in the central nervous system, usually in the cortex, can give rise to unusual, paroxysmal, electrical activity which may spread to cause epilepsy, the manifestations of which depend upon the initial area of gray matter involved and the extent of spread. Localized seizure discharges in the frontal lobe motor cortex produce a localized jerking of skeletal muscle on the contralateral side but generalized epilepsy produces tonic (continuous contraction) and then clonic (jerking) contraction of all skeletal muscle along with loss of consciousness. Epileptic foci in areas of the brain that do not normally cause skeletal muscle action may present in other ways (temporal lobe epilepsy for example may present with autonomic sensations of taste, smell or déjà vu with loss of awareness without loss of consciousness. Peripheral neuropathy Peripheral nerves are surrounded by neurilemma (Fig. 11), groups of axons by perineurium, and groups of groups (the nerves) are surrounded by epineurium which also contains the nourishing blood vessels. The nerve fibres of brain and spinal cord have no neurolemma and are incapable of regeneration whereas peripheral nerve axons can regrow along the “empty” neurolemmal tubes after nerve damage.
- 20. The nutrient-providing blood vessels may be damaged by metabolic disorders (such as diabetes mellitus) and cause axonal degeneration or demyelination. The longer the nerve then the more cumulative low grade damage, explaining why symptoms and signs of neuropathy (nerve damage without inflammation) often begin in the feet. Nerve compression often causes both axonal degeneration and demyelination. Neuropathy may affect individual nerves or nerves (mononeuropathy) or all nerves (generalized neuropathy). In either motor symptoms may include: • Weakness • Wasting • Fasciculation (twitching of small areas of muscle caused by spontaneous electrical activity in the motor unit so that individual muscle fasciculi contract) In either sensory symptoms may include: • Loss of sensation (but only in the area exclusively supplied by an individual nerves in a mononeuropathy) • Pain • Tingling With a generalized sensory neuropathy there will be glove and stocking sensory loss (Fig. 12). Reflexes may be reduced by damage to either sensory or motor nerve fibres or both.
- 21. Peripheral nerves usually contain a mixture of sensory (afferent= to carry to) and motor (efferent= to carry away) fibres. If a perip heral nerve is cut then the result usually include: • Flaccid paralysis of the muscles receiving motor innervation, followed by muscle wasting and contractures • Loss of cutaneous sensation in the areas exclusively supplied by the nerve (most cutaneous areas exhibit overlap of cutaneous sensation) • Loss of reflexes (if there are reflexes associated with the nerve concerned) • Loss of sympathetic and parasympathetic responses (causing initial vasodilatation, lack of sweating and piloerection) The cell bodies of the motor nerves (motor horn cells) may be affected in isolation within the spinal cord (polio specifically damages the anterior horn cells of the lower motor neurone) to produce lower motor neurone signs. If a peripheral nerve is damaged the neurone fibres degenerate proximally, usually up to the proximal node of Ranvier, and distally. The Schwann cells (Fig. 6) then remain, surrounding an empty tube. The intact proximal neurone then grows distally, hopefully along its previous tube, at a rate of about one mm a day. If a nerve is cut but the two ends remain approximately opposed an axon may regrow along the wrong tube and this can cause partial or altered sensation or false motor innervation leading to persisting dysfunction.
- 22. Malignant tumours of nerves are rare but benign tumours are not uncommon, either in isolation (as neuromas, notably acoustic neuromas) or as part of a more generalized problem (e.g. neurofibromatosis). Generalized peripheral neuropathy can be post-infective, metabolic, or related to malignancy. • diabetes mellitus • vitamin deficiency (notably vitamin B12 or B1) • infections (including leprosy and HIV) • toxic (including alcohol) • drug induced • hereditary • vasculitis • compression at certain points: wrist (median nerve), elbow (ulnar nerve), supper arm (radial nerve), head of fibula(lateral popliteal nerve), groin (lateral cutaneous nerve of thigh) and buttock (sciatic nerve). Groups of functionally similar axons are called tracts, fascicles, peduncles, or lemnisci. The first part specifies the site of origin, the second the destination e.g. corticospinal = from the cortex to the spinal cord