Chapter 5 Nervous system and special senses
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:
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.
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.
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.
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.
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.
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:
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 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).
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.
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.
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.
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’.
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.
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.
Samples of CSF can be collected from the cisterna magna, lying between the cerebellum and the medulla oblongata. The patient is anaesthetised and restrained in sternal or lateral recumbency, with its chin touching its chest. A spinal needle is slowly advanced into the space until CSF begins to drip from the hub.
This is a modification of the neuroglial tissue that connects and supports all the neurons within the nervous tissue. Different types of neuroglial cells surround the blood capillaries, creating an almost impermeable layer, to protect the brain from substances that are harmful to or not needed by the brain. These include urea, certain proteins and antibiotics. Other materials such as oxygen, sodium and potassium ions and glucose can pass rapidly through the barrier to be used for brain metabolism.
The action of general anaesthetic agents relies on their ability to pass through the blood–brain barrier and affect the neurons in the brain. Drugs that affect the brain must be in a lipophilic form, i.e. soluble in lipids, so that they are able to pass through the phospholipid cell membranes of the neuroglial cells.