Chapter 5 Nervous system and special senses

• The functional unit of the nervous system is the neuron. Neurons connect with each other by synapses to create a complex network that conveys electrical impulses all around the body. The function of this system is to take in information from the external and internal environments and to initiate an appropriate response.

• The brain and spinal cord form the control centre of this network and consist of dense accumulations of nervous tissue.

• The brain can be seen to be divided into fore, mid and hindbrains. Each part of the brain performs a different function and is vital for the survival of the animal.

• To ensure that the brain functions normally it is protected externally by a bony cranium and three layers of meninges and internally by the ventricular system consisting of canals filled with cerebrospinal fluid. In addition, every blood vessel entering the brain is wrapped in a layer of protective cells forming the blood–brain barrier, which prevents potentially harmful substances reaching the brain tissue.

• The whole body is supplied by the peripheral nerves. These are given off as cranial nerves from the brain and spinal nerves from the spinal cord.

• There are five special senses, each of which is vital for the survival of the individual.

• External stimuli, e.g. sight and sound, are perceived by specialised receptor cells located within the special sense organs.

• Information from the receptor cells is transmitted by nerve impulses to the brain by cranial nerves. It is interpreted within specific centres in the brain and a response is initiated.

The presence of a nervous system allows an animal to respond, in a coordinated manner, to both the demands of the external environment and the internal changes within its body. The functions of the nervous system are:

• To receive stimuli from the external and internal environment

• To analyse and integrate these stimuli

• To bring about the necessary response.

Although the nervous system works as a well-integrated composite unit within the body, for descriptive purposes, it can be divided into:

• Central nervous system (CNS) – the brain and spinal cord

Nervous tissue

Neuron structure

The main cell of the nervous system is the neuron (see Ch. 2, Fig. 2.16). Neurons are responsible for the transmission of nerve impulses throughout the nervous tissue. Nerve impulses pass from one neuron to another by means of structures known as synapses. All nerve pathways are made up of neurons and synapses. Complex nervous tissue, such as that of the brain and spinal cord, is made of neurons supported by connective tissue and neuroglial cells, whose function is to supply nutrients to and carry waste materials away from the neurons.

Each neuron consists of the following parts:

• A cell body containing a nucleus

• Several short processes called dendrons, which are formed from many finer dendrites. These carry nerve impulses towards the cell body. A single cell body may receive as many as 6000 dendrons from other neurons

• One long process called an axon. These carry nerve impulses away from the cell body. The axon leaves the cell at a point known as the axon hillock.

Nerve impulses from the dendrons are directed across the cell body towards the axon hillock and continue down the axon, reaching their final destination very rapidly. The speed of transmission along the axon is increased by the presence of a myelin sheath. Myelin is a lipoprotein material made by Schwann cells surrounding the axon. Its whitish appearance contributes to the colour of the more visible nerves in the body and to the white matter of the CNS. The myelin sheath is interrupted at intervals of about 1 mm by spaces known as the nodes of Ranvier and it is through these that the axon tissue receives its nutrient and oxygen supply. Non-myelinated fibres are embedded within the Schwann cells (Fig. 5.1) and despite their name are actually covered in a single layer of myelin. Totally uncovered fibres are rare and may be found in areas such as the cornea of the eye.

Fig. 5.1 Cross-section through a Schwann cell showing non-myelinated nerve fibres embedded within the cytoplasm.

Neurons vary in size – the diameter of the axons and dendrons may be a few micrometres and the length depends on the destination of the nerve fibre. This may be anything from a few millimetres to more than a metre long. Neurons also vary in shape (Fig. 5.2).

Fig. 5.2 The variation in shape of neurons. A Bipolar neuron. B Pseudo-unipolar neuron, e.g. a sensory neuron. C Multipolar neuron, e.g. a motor neuron. D Multipolar neuron, e.g. Purkinje cell from the cerebellum of the brain.

Synapses

Each axon terminates in a structure called a synapse (Fig. 5.3). Where an axon terminates on an individual muscle fibre, the synapse is referred to as a neuromuscular junction and the nerve impulse stimulates contraction of the muscle fibre. Within the button-like presynaptic ending are vesicles containing chemical transmitter substances. The most common chemical is acetylcholine but others found in the body include adrenaline (epinephrine), serotonin and dopamine.

Fig. 5.3 The structure of a synapse.

As the nerve impulse travels down the axon, the vesicles drift towards the presynaptic membrane (Fig. 5.3) and release the transmitter into the synaptic cleft. It diffuses rapidly across this gap and combines with the postsynaptic membrane, making the membrane more ‘excitable’ and allowing the transmission of the nerve impulse to continue on down the nerve fibre. The effect is stopped by the release of cholinesterases – enzymes that destroy any acetylcholine remaining in the synaptic cleft – and the synapse returns to its resting state ready for the next nerve impulse. Effective transmission of a nerve impulse across the synapse will only occur in the presence of calcium ions.

The nervous symptoms of eclampsia in the bitch and milk fever in the cow are caused by low levels of calcium in the blood, affecting the transmission of nerve impulses.

Generation of a nerve impulse

Nerve impulses transmitted along axons and dendrites can be considered to be electrical phenomena. An impulse changes the electrical charge of the neuron by altering the relationship between the negative charge of the cell contents and the positive charge of the cell membrane. This results from a change in the permeability of the cell membrane to sodium and potassium ions, causing an exchange of ions between the inside and the outside of the neuron (Fig. 5.4).

Fig. 5.4 The alterations in electrical charge that occur in an axon as a nerve impulse travels along it from right to left. The charge differences are brought about by movement of sodium and potassium ions across the axon membrane.

For a short time after the impulse has passed, the nerve fibre becomes refractive, i.e. it cannot be reactivated by another impulse until the permeability of the cell membrane returns to normal. This ensures that the flow of impulses travels only in one direction.

A nerve impulse is an all-or-nothing phenomenon – the nerve is either stimulated or it is not; there is no gradation of impulse. A particular neuron will always transmit with the same power or the same speed. The different effects of the nervous system depend on:

• The number of nerve impulses transmitted per second – a weak stimulus produces few impulses at long intervals, while a strong stimulus produces rapid impulses at shorter intervals

• The number of neurons activated by a single stimulus – a strong stimulus produces activity in a large number of neurons and a weak stimulus produces activity in a small number

• The type of neuron stimulated – some have an excitatory effect while others have an inhibitory effect.

Classification of nerves

Peripheral nerves can be classified according to their function and to the structures they supply.

• Sensory nerves – carry impulses towards the CNS

• Motor nerves – carry impulses away from the CNS

• Mixed nerves – carry both sensory and motor fibres.

An intercalated neuron is one that lies between a sensory neuron and a motor neuron. Such neurons may not always be present within a nerve pathway.

• Afferent nerves – carry impulses towards a structure, usually in the CNS

• Efferent nerves – carry impulses away from a structure, usually in the CNS

Afferent and efferent are also terms used within other systems, e.g. the blood vascular system, to describe blood and lymphatic capillaries going to and leaving organs.

Nerve fibres may be further classified according to the organ with which they are associated.

• Visceral sensory and motor nerves – nerves that are associated with the visceral body systems, i.e. the heart, digestive, respiratory, urinary and reproductive systems. Stimuli are carried by visceral sensory nerves from blood vessels, mucous membranes and the visceral organs to the CNS. Impulses are transmitted from the CNS by visceral motor nerves to smooth muscle and glandular tissue, where they initiate a response.

• Somatic sensory and motor nerves – nerves that are associated with the somatic structures in the body, i.e. receptors in the skin, muscles, joints and tendons and in specialised somatic organs such as the ear and eye. Stimuli are carried by somatic sensory nerves towards the CNS and nerve impulses are carried by somatic motor nerves to skeletal muscles, where they initiate a response.

The central nervous system

During embryonic development of the CNS, a hollow neural tube forms from the ectodermal layer of the inner cell mass of the embryo. This neural tube runs along the dorsal surface of the embryo and, as it develops, nerve fibres grow out laterally extending to all parts of the body and eventually forming the peripheral nervous system. The anterior end of the tube becomes the brain and the remaining tube becomes the spinal cord. The brain and spinal cord are hollow and are filled with cerebrospinal fluid (CSF).

The brain

The function of the brain is to control and coordinate all the activities of the normal body. The brain is a hollow, swollen structure lying within the cranial cavity of the skull, which protects it from mechanical damage. In many species, three distinct areas can be identified – these are the forebrain, midbrain and hindbrain. In mammals, this arrangement may be difficult to recognise because the right and left cerebral hemispheres of the forebrain are enlarged and overlie the midbrain (Fig. 5.5).

Fig. 5.5 Left lateral view of the canine brain.

Forebrain

This consists of the cerebrum, thalamus and hypothalamus.

The cerebrum – or right and left cerebral hemispheres – takes up the greater part of the forebrain and contains up to 90% of all the neurons in the entire nervous system (Fig. 5.5). The right and left sides are linked by the corpus callosum. This consists of a tract of white matter whose nerve fibres run across the brain between the right and left hemispheres. It is found in the roof of the third ventricle, which is part of the hollow ventricular system inside the brain and contains CSF.

The surface of the hemispheres is deeply folded, which enables a large surface area to be enclosed within the small cranial cavity and allows nutrients in the CSF to reach the cell bodies of neurons lying deep inside the brain tissue. The folds are known as gyri (upfolds), sulci (shallow depressions) and fissures (deep crevices). The two hemispheres are divided by the longitudinal fissure. Each cerebral hemisphere consists of four lobes each of which contains specific centres or nuclei with specific functions related to conscious thought and action.

The tissue of the cerebral hemispheres consists of:

• An outer layer of grey matter known as the cerebral cortex, containing millions of cell bodies of neurons. Deeper in the brain are groups of cell bodies forming centres or nuclei, which act as relay stations collecting information and sending out impulses to a variety of areas

• An inner layer of white matter made of tracts of myelinated nerve fibres linking one area to another. Histologically the tissue appears white because of the high proportion of the white lipoprotein myelin.

The thalamus is found deep in the tissue of the posterior part of the forebrain. Its function is to process information from the sense organs and relay it to the cerebral cortex.

The hypothalamus lies ventral to the thalamus and has several functions:

1 It acts as a link between the endocrine and nervous systems by secreting a series of releasing hormones that are then stored in the pituitary gland

2 It helps to control the autonomic nervous system by influencing a range of involuntary actions such as sweating, shivering, vasodilation and vasoconstriction

3 It exerts the major influence over homeostasis in the body by influencing osmotic balance of body fluids, the regulation of body temperature, and control of thirst and hunger centres in the brain.

On the ventral surface of the forebrain (Fig. 5.6) the following structures can be seen:

• Optic chiasma – nerve impulses, carried by the optic nerve (cranial nerve II) from the right eye pass to the right side of the brain and to the left side via the optic chiasma (and similarly for the left eye). This ensures that information from each eye reaches both sides of the brain

• Pituitary gland – an endocrine gland attached below the hypothalamus by a short stalk (see Ch. 6)

• Olfactory bulbs – these are a pair of bulbs which form the most rostral part of the brain. They are responsible for the sense of smell or olfaction. Their size is determined by the sense of smell in a particular species. Fish have very large olfactory bulbs and hence an excellent sense of smell, while humans have very small olfactory bulbs. The cat has larger olfactory bulbs than the dog as, in fact, cats have a much better sense of smell.

Fig. 5.6 View of the ventral surface of the canine brain.

Midbrain

This is a short length of brain lying between the forebrain and the hindbrain. It is overhung by the cerebral hemispheres and is not easy to see in the gross specimen. It acts as a pathway for fibres running from the hindbrain to the forebrain carrying the senses of hearing and sight.

Hindbrain

This consists of the cerebellum, pons and medulla oblongata.

The cerebellum lies on the dorsal surface of the hindbrain (Fig. 5.5). Its name means ‘little brain’ as it was originally thought to be a smaller version of the cerebrum. It has a globular appearance and is covered in deep fissures. In cross-section, the tissue is very obviously divided into an outer cortex of grey matter and an inner layer of white matter. The grey matter consists of numerous Purkinje cells (Fig. 5.2), each of which forms thousands of synapses with other neurons.

The cerebellum controls balance and coordination. It receives information from the semicircular canals of the inner ear and muscle spindles within skeletal muscles. Voluntary movements are initiated by the cerebrum and fine adjustments are made and coordinated by the cerebellum.

The pons lies ventral to the cerebellum and forms a bridge of nerve fibres between the cerebellar hemispheres. It contains centres that control respiration.

The medulla oblongata extends from the pons and merges into the spinal cord (Fig. 5.5). It contains centres responsible for the control of respiration and blood pressure.

Injury or malformation of the cerebellum results in incoordination and spasticity. Kittens whose dam was infected with the feline enteritis virus during pregnancy may be born with cerebellar hypoplasia (underdevelopment) and will never be able to walk in a normal coordinated fashion.

The medulla oblongata and the pons form part of the brain stem that is responsible for the vital control of respiration and blood pressure. If the brain stem is damaged, e.g. by brain stem herniation through the foramen magnum, which may occur if the brain swells as a result of inflammation or traumatic damage, then respiration and blood pressure will be affected and the animal will die – often described as being ‘brain stem dead’.

Protection of the brain

Normal brain function is essential for the maintenance of an animal’s life. If it is damaged mechanically or chemically, function will be impaired and the animal may die. Several structures have evolved within the brain to protect it from harm.

Cranium

This bony structure forms a tough outer shell to protect the soft brain tissue from physical damage.

Ventricular system

The ventricular system (Fig. 5.7) is derived from the hollow neural tube of the embryo. The lumen of this tube develops into a series of interconnecting canals and cavities or ventricles found inside the brain and spinal cord. The ventricles and the central canal are filled with cerebrospinal fluid (CSF). This also surrounds the outside of the brain, lying in the subarachnoid space (Fig. 5.8). CSF is secreted by networks of blood capillaries known as choroid plexuses lying in the roofs of the ventricles. It is a clear fluid, resembling plasma, but has no protein in it – it is an example of a transcellular fluid. The function of CSF is to protect the CNS from damage by sudden movement or knocks and to provide nutrients to the nervous tissue of the brain and spinal cord.