Nervous system

Topic: Anatomy Organ Systems Chapter: Nervous system

Nervous system

In this chapter you will learn about: •

Structure and function of the nervous system

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Some common diseases and disorders of the nervous system

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Influence of anaesthetic and sedative medication on the nervous system

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Nervous system Dagmar Sens-Kirchenbauer & Hans Geyer

Introduction The nervous system enables animals and humans to orientate themselves in their environment, pick up stimuli, process them and then respond to them. Furthermore, the nervous system monitors and regulates the other organ systems. As a rule, the horse reacts extremely fast to stimuli from the environment; from an evolutionary point of view this can be explained by the flight instinct. If a threat is perceived by a flight animal such as the horse, the instantaneous flight response is more conducive to survival then spending valuable seconds considering how to react! Horses are also conscious emotional beings capable of learning, and they possess a well-developed space-time awareness with an excellent sense of direction.

Building blocks of the nervous system Nerve cells The basic unit of the nervous system is the neuron. This is a single nerve cell together with its many finger-like protrusions known as nerve processes. The nerve cell body is relatively large and is the metabolic apparatus of the cell. A neuron has two different types of processes; a multitude of highly branching dendrites and a single axon. Axons are also known as nerve fibres; they can be over 1m in length and often branch at the end. A neuron is capable of generating and transmitting electrochemical signals. Signals can be passed on from one neuron to another or from a neuron to an effector organ, e.g. to a muscle fibre. Signals are usually transmitted along the axon and received by a dendrite; in this way, neurons can be highly interconnected. The site of signal transmission between individual neurons or between neurons and effector cells is called a synapse. Synapses can convert electrical signals into chemical signals using transmitter substances, mostly acetylcholine. The speed at which a signal is conducted along an axon can vary according to the type of nerve cell; e.g. the signal to contract a muscle is passed on at a speed of up to 100 m/s (360 km/h), however a pain stimulus is transmitted through a different nerve at

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“only” 20 m/s (72 km/h). Most axons are surrounded by an insulating layer of fatty tissue known as the myelin sheath, which improves the rate of transmission.

Nerves A nerve consists of a bundle of axons or nerve fibres held together by connective tissue. Large nerves are up to several millimetres thick. From about 1 millimetre in diameter they can be picked out with the naked eye. Their fibrous structure and yellow colour as well as their soft consistency differentiate nerves from tendon fibres. The longest nerve in the body of the horse is the laryngeal nerve (N. recurrens), which runs from the brain stem to the heart and back to the larynx and thus attains a length of more than 2 m. The largest and strongest nerve is the sciatic nerve (N. ischiadicus), the diameter of which in the horse is approx. 2cm in the pelvic area. It too is of considerable length as it runs from the spinal cord in the lumbo-sacral area right down to the digit of the hind leg.

Types of nerves Nerves which transmit signals from the central nervous system (brain and spinal cord) towards the effector organs are known as efferent nerves. The individual nerve fibres which these nerves contain belong to motor neurons (nerve cells occupied with movement). These nerves conduct signals for example from the brain to the muscles. Nerves which pick up stimuli from the environment (e.g. from the surface of the body or from organs) and conduct them to the central nervous system for processing, are called afferent nerves. They contain nerve fibres belonging to sensory neurons. They register, for example, pain or heat stimuli and conduct them to the spinal cord or the brain so that the body can organise a response.

Organisation of the nervous system In vertebrates there is only one nervous system; however, this is divided into several subsystems. Each subdivision has unique structural and functional features, but they are all highly interlinked and the nervous system can only function if all subdivisions work together. The main subdivisions are as follows:

Central and peripheral nervous systems The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord. The cerebral cortex

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(outer part of the brain) is the seat of consciousness. The PNS includes all other nerve pathways, which on the one hand transmit incoming sensory stimuli from the environment to the CNS, and on the other transmit outgoing motor stimuli from the CNS to effector organs such as the muscles.

Somatic and autonomic nervous systems The nerves of the PNS belong either to the somatic nervous system or to the autonomic (or visceral) nervous system. Both systems are regulated by control centres in the brain. The somatic nervous system is under voluntary control, whereas the autonomic nervous system regulates non-voluntary processes such as heart rate or movement of the gastrointestinal tract; this is therefore a predominantly functional division.

Central nervous system (CNS) The central nervous system includes the brain and the spinal cord. On the one hand, the brain (specifically the cerebral cortex or outer part of the brain) is the seat of consciousness; on the other hand, it is the seat of the integrative and controlling centres which govern all bodily functions. Therefore the CNS exerts influence over both the somatic and the autonomic nervous systems.

Brain The brain of the horse is smaller and lighter than that of the human. Nevertheless, it enables the animal to have a very broad range of functions. In a horse weighing 500-600kg the brain weighs roughly 600-700g. The brain lies protected inside the cranial vault of the skull. Its rear boundary to the spinal cord lies at the junction of the base of the skull with the 1st cervical vertebra; at the front it ends at the ethmoid bone plate which forms the boundary to the rear of the nasal cavity and the base of the nose. Seen in cross-section the grey substance towards the outer surface constitutes cerebral cortex and the white substance on the inside the cerebral medulla.

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The brain is divided into 5 main components which are closely interconnected and interact with each other: In ascending sequence these are: 1. The medulla oblongata 2. The cerebellum and the pons (together known as the metencephalon) The above components make up the hindbrain 3. The midbrain or mesencephalon as the connection between the hind and forebrain parts of the brain The above components (without the cerebellum) make up the brainstem 4. The diencephalon with the thalamus, the hypothalamus and the pituitary gland (hypophysis) 5. The telencephalon or forebrain with the cerebral hemispheres. For basic understanding, some selected structures and functions of the brain are discussed (in ascending sequence):

Medulla oblongata The medulla oblongata is the segment connecting the spinal cord to the brain. It contains many nerve pathways as well as respiratory, cardiac and vasomotoric centres responsible for the regulation of breathing, heart rate and blood pressure.

Metencephalon The metencephalon is made up of a protruding structure known as the cerebellum (little brain in Latin) and the pons, situated beneath the cerebellum in the continuation of the medulla oblongata. The cerebellum contains centres responsible for coordination and balance and processes sensory information from the ear and proprioceptors in muscles and joints. The pons plays a role in the regulation of breathing and sleep (amongst other things).

Mesencephalon or Midbrain The midbrain or mesencephalon contains centres controlling unconscious movement, sleep, vision, hearing (amongst other things).

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Diencephalon The diencephalon (occasionally known as the interbrain) contains several notable structures including the thalamus, the hypothalamus and the pituitary gland. In this area, many sensory inputs are processed and “fed” to other parts of the brain which then coordinate a suitable reaction. The hypothalamus regulates many processes which are controlled by the autonomic nervous system for example, the body temperature. The hypothalamus is also an important link between the nervous system and the endocrine system (the body’s chemical controlling system) as it connects directly to the pituitary gland or hypophysis. This is a superordinate endocrinal gland which regulates many endocrine functions via numerous signalling hormones which in turn affect other endocrine systems within the body. For example, the length of day light is registered in the hypothalamus, which transmits signals to the hypophysis, which in turn secretes hormones which affect sexual activity; this is highly relevant for animals, like horses, that are seasonal breeders.

Telencephalon, cerebral hemispheres The telencephalon or cerebrum consists of two halves, the so-called hemispheres, which are connected to enable an exchange of information. The cerebrum contains the outer cerebral cortex where conscious thought is processed; the cerebral medulla contains various structures including the hippocampus. The hippocampus and the region in the direct vicinity are regarded as the seat of emotions such as rage, aggression and above all anxiety. In the centre of each brain hemisphere there is a fluid-filled cavity known as the cerebral ventricle.

Spinal cord The spinal cord is also a part of the CNS. It consists of large bundles of nerves (nerve fibres and nerve cells) together with their support cells. It runs through the vertebral canal in the vertebral column and extends from the brain (the medulla oblongata is at its cranial end) down to the level of the 1st sacral vertebra. The main function of the spinal cord is the transmission of information between the brain and the rest of the body (the peripheral nervous system). Some reactions which are processed only in the spinal cord occur unconsciously (reflexes). In reactions which involve conscious thought, the signal must first be conducted to the brain, where it is processed in the cerebral cortex before a signal is returned to the effector organ via the spinal cord. In cross-section, the spinal cord is oval in shape and consists of a central butterfly-shaped region of grey matter in which the nerve cell bodies are found together with nutrient cells,

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and the surrounding white matter which consists of nerve fibres together with their companion cells. In the centre is a small central canal which is an extension of the brain ventricles. The spinal cord contains both sensory and motor neurons. The cell bodies of the motor neurons are found in the ventral horn (lower-most section) of the grey matter. The cells in this region coordinate impulses to effector muscles. The smaller dorsal horn (upper-most section of the grey matter) contains the cell bodies of neurons involved in the transmission of sensory stimuli; however, the cell bodies of the primary sensory neurons (the first to register a stimulus) are located in the spinal ganglia outside the actual spinal cord. Ganglia are accumulations of nerve cell bodies. The white matter contains many bundles of nerves which transmit signals up and down the spinal cord. It has a larger volume than the grey substance and appears white because of the fatty layers and supporting cells surrounding the nerve fibres (myelin sheaths). These pathways can be compared to major “trunk roads” or “motorways” which ensure a rapid connection between distant sites (segments of the spinal cord and the brain).

Segments of the spinal cord The spinal cord is arranged in segments, each corresponding to a vertebra. In the space between two vertebrae, a pair of spinal nerves branch out of the spinal cord in each segment through two openings, known as the intervertebral foramina, one to the left and one to the right. Each spinal nerve has a dorsal root for incoming sensory stimuli and a ventral root for exiting motor stimuli. The openings of the intervertebral foramina are just large enough to accommodate the spinal nerves and the spinal ganglia (see above).

Spinal reflex A reflex is a type of reaction mediated by a reflex arc, which enables the body to react rapidly and unconsciously to a potentially dangerous situation without having to first process a signal in the brain. A good example is the withdrawal reflex, which is triggered upon contact with a hot surface. The heat stimulus is conducted to the spinal cord by the afferent sensory nerves. In the spinal cord the sensory stimulus is passed straight to a motor neuron, which then causes a rapid contraction of the muscles in the area concerned via the efferent executing nerves. This results in the pulling back of the hand from the hot surface split seconds before the brain has even registered the heat.

Disorders of the vertebral column which affect the spinal cord As the spinal cord and the spinal nerves branching off it are snugly enclosed in the bony structures of the vertebral column, it is not hard to imagine that any structural damage,

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swelling or arthritic bony proliferation will rapidly impinge on the nervous tissue encased within. Fractures of the vertebrae or a haematoma within the spinal canal that may occur after a serious fall can cause a compression of the spinal cord, leading to a loss of continuity of the nerves within. The section of the body lying ‘behind’ the compressed section may then be partially or completely paralysed (known as paraplegia). Bony proliferation that accompanies osteoarthritis of the small inter-vertebral joints can lead to a reduction of the size of the intervertebral foramina. This leads to a compression and inflammatory irritation of the spinal nerves which pass through them, which can be very painful.

Meninges and cerebrospinal fluid The brain and the spinal cord are surrounded by 3 protective layers of connective tissue: the meninges. The outermost layer, the dura mater, is an extremely tough layer of connective tissue. Around the brain it is attached firmly to the periostium of the skull, around the spinal cord however, there is a space between the bone of the vertebral canal and the dura mater; this is the epidural space and is the space into which anaesthetics are injected to anaesthetise spinal nerves. The middle layer is the thin arachnoid layer and the innermost layer is the pia mater, which is firmly attached to the brain and the spinal cord. Between the middle and inner layers (the arachnoid and the pia mater) there is a space (the subarachnoid space) which contains a small amount of fluid. This is the cerebrospinal fluid which forms a mechanical and a thermal protective layer around the brain and spinal cord. It is the same fluid that fills the ventricles of the brain and is very watery with a low cell content. Collection of this fluid is known as a spinal tap, which is done to check for infection or bleeding.

Blood supply to brain and spinal cord The blood supply to the brain and spinal cord has to be very good as the CNS has the highest oxygen requirements in the body. A drop in blood pressure or sudden loss of blood from blood vessels in the brain causes unconsciousness. An insufficient supply of oxygen for more than 5 minutes can lead to irreversible damage to the brain and spinal cord and in case of severe damage to the brain, to death (in humans this is known as a stroke). The backflow of blood from the brain is also important. The veins around the brain play a part in temperature regulation by removing heat away. If too little heat is conducted away, e.g. in extremely hot conditions, heatstroke can occur.

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Peripheral nervous system The peripheral nervous system or PNS includes all the nerves that extend from the CNS (brain and spinal cord) into the periphery (all other parts of the body). Nerves which originate from the brain are known as cranial nerves (12 in the horse), nerves which originate from the spinal cord are known as spinal nerves (42 in the horse). As previously mentioned, there are afferent or sensory nerves which transmit signals from the periphery to the CNS and efferent or motor nerves which transmit signals from the CNS to the periphery. The efferent nerves can be further subdivided into somatic motor nerves (which control voluntary activity) and autonomic motor nerves (which control involuntary activity). In connection with the hoof of the horse, the peripheral nerves to the limbs and the hoof are of particular importance and are individually described in other chapters.

The autonomic nervous system Whereas the somatic nervous system receives the incoming stimuli from the environment and activates the muscles that are used to respond (e.g. to run away from a perceived threat), the autonomic nervous system (ANS) is responsible for the involuntary reactions to a stimulus (e.g. the increase in heart beat when a threat is recognised). It regulates all vital functions such as heartbeat, breathing, blood pressure, digestion and metabolism and endocrine function. Other organs or organ systems regulated by the autonomic nervous include the sexual organs, sweat glands, and the internal eye muscles (pupil reaction). The autonomic nervous system can be structurally and functionally subdivided into 2 systems: the sympathetic nervous system and its counterpart the parasympathetic nervous system. For many functions the sympathetic nervous system is the instigator of activity (e.g. increasing heart rate and breathing), whilst the parasympathetic nervous system curbs activity and regulates resting functions (e.g. decreasing heart rate and regulation of digestion). The ANS’s control of the organ systems takes place for the most part unconsciously and can only be influenced to a limited extent by external stimuli. The two systems are as different structurally as they are functionally. To expand on this would however be beyond the scope of this chapter.

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Sympathetic nervous system The sympathetic nervous system prepares the body functionally for situations such as flight or fight. Sympathetic stimuli cause the heart to beat faster; they expand the bronchi in the lungs and inhibit peristalsis (gut movement) and gland activity in the digestive system. Stimuli from the sympathetic nervous system cause the constriction of blood vessels so that a sufficiently high blood pressure is maintained and not too much heat is lost. Impulses from the sympathetic nervous system bring about the dilation of the coronary vessels around the heart so that heart muscles working at a faster rate receive enough oxygen. Sympathetic stimuli cause the pupil in the eye to expand in situations such as pleasure or fear.

Parasympathetic nervous system The parasympathetic nervous system regulates functions associated with resting and digesting. Parasympathetic stimuli lower the heart rate and constrict the bronchi; they promote gland (e.g. salivary or intestinal gland) secretion and stimulate peristalsis in the oesophagus, stomach and intestines or cause the constriction of the pupils in the eyes. The most important parasympathetic nerve in the body is the vagus nerve, a cranial nerve which runs from the brain to the heart and to the abdominal organs.

Medical influence on the nervous system Many medical (or veterinary) interventions are aimed at influencing the nervous system, be this by desensitising a region to sensory stimuli or by sedating a nervous individual. Some of the most important procedures are listed below.

Nerve block or regional anaesthesia In nerve block or regional anaesthesia an anaesthetic is injected close to one or several sensory nerves. The anaesthetic interrupts the conductivity of the nerves in question so that the area which they innervate becomes numb. This is used to locate a source of pain which causes lameness: if a horse is lame before the nerve block is applied and is sound after application then the cause of the pain must be located in the area innervated by the nerve which was blocked. The direct application of anaesthetic into joints or around a wound area is known as infiltration anaesthesia. This numbs the sensitive nerve endings of the nerve fibres in the joint capsule or in the area surrounding the wound rather than numbing an entire nerve.

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Analgesics, sedatives and general anaesthetics In general, analgesic drugs or pain-killers have an inhibitory effect on the initiation or transmission of pain stimuli. This results in a reduction in the amount of pain stimuli that reach the brain (the cerebral cortex), and therefore in a reduction in the perception of pain (although the actual source of the pain is still there). Sedatives have a dampening effect on the reception and/or transmission of all sensory stimuli so that only a reduced number of stimuli reach the cerebral cortex at all. This does not cause any numbness of specific regions but reduces general perception of threatening stimuli. Painful stimuli are generally still received and can still lead to a rapid and violent reaction even though the horse appears depressed. Major interventions are usually carried out on a horse under general anaesthetic. In this the anaesthetic is administered via the blood or the respiratory system, which leads to strong reduction in activity of the cerebral cortex; consciousness and pain perception are “switched off”. The musculature is also relaxed. The respiratory centre remains capable of functioning so that the patients breathe spontaneously. If too high a dose of anaesthetic is given, the respiratory centre is also disabled and death will occur unless the horse is put on an artificial respirator.

Euthanasia of a horse When euthanizing a horse with the help of a captive bolt gun, the bolt hitting the cerebrum causes instant loss of consciousness and leads to the animal collapsing. To bring about death rather than just loss of consciousness, blood must then be drained by opening large blood vessels (at the entrance to the chest). When the blood is drained the blood supply to the brain is cut off, thus removing the brain’s supply of oxygen; respiratory arrest and death ensues. If blood is not drained, an animal may start to breathe again after a few moments as the respiratory centre is still being supplied with blood, even though it is unlikely to regain consciousness. When a horse is euthanized with an anaesthetic (put to sleep), a lethal dose of an anaesthetic drug is injected so that consciousness is lost and the respiratory and cardiac centres are inactivated. This leads to rapid death.

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Examples of disorders of the nervous system Paralysis of the radial nerve This is the paralysis of a peripheral nerve, the nervus radialis. The cause of the paralysis is mostly mechanical in nature. The nerve runs along the upper arm directly on the upper and lateral surface of the bone (humerus). This exposes the nerve to damage through the application of pressure over a long period (long-term lateral recumbency during an operation) or through the effect of a violent blow (e.g. a kick). In an affected horse the paralysis causes the loss of use of the extensor muscles in the forelimb so that the horse stands with a hanging limb, resting the toe of the hoof on the ground. Depending on the extent of the damage, the nerve may repair itself or not.

Ataxia This is a disorder of the central nervous system, which can either be inherited or acquired. Depending on the location of the problem one differentiates between spinal ataxia (spinal cord), cerebellar ataxia (cerebellum) or cerebral ataxia (brain). In each case nerve tissue damage has occurred. This damage can appear in the course of growth or also arise as the result of accidents with bleeds or fractures. The symptoms affect movement and are various, appearing in different degrees of severity; the disorder is characterised by a lack of coordination of varying extent. In mild cases the horse may have difficulties walking backwards or down steep slopes. Tight turns and uneven terrain may cause it to stumble or fall. In more severe cases, the horse will stagger and sway when led up on even ground and in advanced cases, horses may be recumbent and unable to rise. There are many causes of this disorder – it is a symptom rather than a disease in its own right. Certain infections (e.g. EHV) can cause ataxia as can developmental disorders of the vertebral column (e.g. wobbling).

Rabies This is a viral disease which is transmitted via the saliva of an infected animal through a bite. The disease causes encephalitis, an inflammation of the brain. A rabies infection often has a fatal outcome. All warm-blooded animals, including man, are susceptible. After the bite the virus multiplies and reaches the brain via the peripheral nerves; once there it continues to multiply and then in turn reaches the mucous membranes and glands via the neural pathways so that it can be excreted via the saliva again. In people it is customary to carry out a vaccination after a suspect bite; this measure is usually successful if the virus has not yet reached the brain.

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Pets and people, who live or work in areas at risk, should definitely have a prophylactic vaccination. For dogs and cats travelling abroad a valid rabies vaccination is compulsory more or less worldwide.

Questions to check learning Which two large nervous systems do you know? Which two main anatomical components form the central nervous system? The autonomous nervous system consists of which two major functional components? How does the sympathetic nervous system affect the heart? And the parasympathetic nervous system? Describe a reflex arc.

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