1. INTRODUCTION
| External: belonging to or forming the outer surface or structure of something. Internal: of or situated on the inside. Sensory: relating to sensation or the physical senses; transmitted or perceived by the senses. Motor: relating to muscular movement or the nerves activating it. |
The nervous system is a highly complex, integrated system and its function it is to allow the animal to adapt to changes in the external and internal environment. Changes are perceived and actions follow which allow the animal to adapt to the change. In other words, if you put your finger on something hot, you feel pain and quickly take your finger away. The pain has been transmitted to the brain, and the necessary muscles alerted to enable you to pull your finger away from whatever is burning it. The transmission of the pain, and the answering response, is sent through your nervous system.
The nervous system can be divided into two parts; the central nervous system which consists of the brain, and the spinal cord, which is carried down the length of the body inside the vertebrae of the backbone or spine. The peripheral nervous system is made up of the cranial nerves in the head, the spinal nerves which come from the spinal cord, and the autonomic nervous system which deals with reflexes.
The nervous system is composed of specialised cells called neurons which are joined together. Nerve impulses travel along a chain of connected neurons. Neurons can be of two types, sensory (receptor), those that feel the stimulus or pain, and motor (effector), those which take the necessary action, like moving the finger. Sensory neurons conduct impulses towards the central nervous system, and motor neurons conduct impulses away from the C.N.S.
A sensory neuron is shown below. The impulses travel from the nerve ends along the dendron to the cell body and on to the next cell in the chain along the axon.
Figure 1: A sensory nerve
The cell shown below is a motor neuron, and here the impulse from the next neuron in the chain is received by the dentrites which pass through the cell body and along the axon to the motor end plates that are imbedded in the muscle of say, the finger, and cause the muscle to act.
Figure 2: A Motor Nerve
2. THE CENTRAL NERVOUS SYSTEM
The brain of an animal is divided into six main areas, as can be seen from the diagram below.
Figure 3: The Brain of an Animal
THE CEREBRUM
This is the largest part of the brain and it is responsible for intricate, mental activity, such as, voluntary muscle control, interpretation of sensations, reasoning, learning and memory. In other words, this is the thinking part of the brain.
THE OLFACTORY BULB
This part is concerned with the sense of smell, which in an animal is of the greatest importance.
THE CEREBELLUM
This governs the co-ordination, adjustment and the smoothing out of movement.
THE THALAMUS
This acts as a relay centre for the nerve fibres (neurons) that travel from the spinal cord to the cerebrum.
THE HYPOTHALAMUS
This includes the important pituitary gland. This is an endocrine gland and its secretions influence body growth, reproduction, lactation and the water balance in the kidneys through the production of the hormone A.D.H. It also generally influences the activity of cells.
THE MEDULLA OBLONGATA
This is also known as the brain stem. It connects the brain to the spinal cord. A number of the cranial nerves leave the brain at this point. It contains a number of reflex centres which control the heartbeat, circulation, respiration, swallowing and various digestive functions.
THE SPINAL CORD
The spinal cord is the continuation of the medulla oblongata, travelling down the length of the spinal column inside the vertebrae. It is divided into segments, with each segment giving rise to a pair of spinal nerves, which travel from the spinal cord into the body through spaces in the vertebrae. In the centre of the spinal cord there is a canal filled with fluid called the cerebric spinal fluid which circulates up and down the spinal cord and into cavities of the brain.
Impulses are transmitted from the tissues of the body to the brain along the spinal cord which also contains areas which are responsible for many of the reflex actions of the body.
2. THE PERIPHERAL NERVOUS SYSTEM
This system consists of the cranial nerves, the spinal nerves and the autonomic nervous system.
CRANIAL NERVES
There are 12 pairs of cranial nerves, with twelve nerves going to one side of the body, and twelve to the other side. Some are made up of sensory neurons, some are motor neurons and some are mixed. The following table gives the names of these nerves, their type and the areas they serve. This is for general interest only.
Table 1: Types of Nerves
| Cranial Nerves | ||||
| No. | Name | Type | Area Served | |
| I | Olfactory | Sensory | Mucous membrane inside the nose | |
| II | Optic | Sensory | Retina of the eye | |
| III | Oculomotor | Motor | Muscles of the eye | |
| IV | Trochlear | Motor | Muscles of the eye | |
| V | Trigeminal | Mixed | Sensory | Muscles of the eye and face |
| Motor | Muscles of the jaw | |||
| VI | Abducens | Motor | Muscles of the eye | |
| VII | Facial | Mixed | Sensory | The ear and tongue |
| Motor | Muscles of the face | |||
| VIII | Acoustic | Sensory | The ear | |
| IX | Glossopharyngeal | Mixed | Sensory | The throat and tongue |
| Motor | Muscles of the throat | |||
| X | Vagus | Mixed | Sensory | The throat and voice |
| Motor | Muscles of the voice | |||
| XI | Spinal Accessory | Motor | Muscles of the shoulder and neck | |
| XII | Hypoglossal | Motor | Muscles of the tongue |
SPINAL NERVES
These emerge in pairs from the spinal cord, one nerve going to the one side of the body and the other to the opposite side. Each nerve contains both sensory and motor neurons which divide when they reach the spinal cord, the sensory neuron entering via the dorsal root and the motor neuron through the ventral root. They connect up inside the spinal cord to form a reflex arc. This is shown in the diagram below. The spinal nerves serve various parts of the body and are connected to the brain through the spinal cord.
Figure 4: Spinal Nerves
REFLEX ACTIONS
A reflex action is the automatic and unconscious response of a motor neuron to a stimulus from a sensor neuron. The common example of a reflex action occurs when you cross your legs and give yourself a tap just below the kneecap. The leg which you have tapped gives an automatic jerk. Simple reflexes are the spinal reflexes which take place through the link between the sensory and motor neurons which connect in the spinal cord. In more complex reflexes, impulses from the sensory neurons enter the spinal cord and travel to the reflex centres of the brain, where the message for action is relayed back down the spinal cord and out to the appropriate part of the body.
The medulla oblongata has reflex centres which control the actions of the heart, expansion and contraction of blood vessels and such actions as swallowing, vomiting, coughing, and sneezing. The centres in the cerebellum are concerned with movement and posture, and those in the hypothalmus, with temperature regulation and water balance.
Figure 5: The Central Nervous System of the Horse
Source: wikieducator
AUTONOMIC NERVOUS SYSTEM
This system deals with responses in the body which are automatic, not consciously controlled by the animal. Messages are received by the system and the necessary action taken without the animal having to think about it at all. Examples of this are the action of the muscles of the heart and intestines and the functions of the glands.
The autonomic nervous system is, in fact, made up of two separate systems, the sympathetic system and the parasympathetic system, and most of the organs not controlled by the ‘will’ receive nerves from each system. The reason for this is that the two sets of nerves have opposite effects, one stimulating the organ, the other, calming it down. Between them they maintain the body in a stable condition. Some examples of the two systems in operation are given in the table. For example, the sympathetic system causes the blood vessels to contract and the parasympathetic system causes them to expand, so that the two working together help to pump the blood around the body.
Table 2: Actions of Autonomic Stimulation
| Organ | Sympathetic Portion Causes | Parasympathetic Portion Causes |
| Sweat Glands | Secretion | No effect |
| Salivary Glands | Mucus secretion | Serous secretion |
| Digestive Glands | Inhibition of secretion | Secretion |
| Muscles of Hair Follicles | Contraction (hair erection) | No effect |
| Muscles of Digestive Tract | Inhibition of peristalsis | Peristalsis |
| Contraction of sphincters | Relaxation of sphincters | |
| Muscles of Bronchi | Relaxation | Contraction |
| Muscles of Bladder | Contraction of sphincter | Relaxation of sphincter |
| Muscles of Uterus | Contraction | Inhibition of contraction |
| Muscles of Blood Vessels | Vasoconstriction | Vasodilation |
| Muscles of eye: | ||
| a) Iris | Contraction of radial muscles | Contraction of circular muscles |
| b) Ciliary Muscle | Relaxation (distant vision) | Contraction (near vision) |
