Neurological conditions are commonplace in day-to-day veterinary practice. But to provide an effective and efficient treatment plan, it is important to understand the basics of the nervous system.
The role of the nervous system
The nervous system permeates all areas and parts of the body and can be divided up in a number of ways, including by function or position. Nerves are made up of bundles of neurons, and each nerve can carry more than one type of fibre, meaning impulses can travel in both directions.
The role of the nervous system is to:
- Receive stimuli from the animal’s internal and external environment
- Process the stimuli it receives
- Initiate a response to the stimuli. This can be a conscious response, such as movement away from painful stimuli, or unconscious, such as secretion of saliva
The nervous system can be broken down into the central and peripheral nervous systems. The central nervous system (CNS) includes the brain and spinal cord. The peripheral nervous system (PNS) consists of the autonomic nervous system (the involuntary system for which you have no conscious control) and the somatic nervous system (the voluntary system, which is under conscious control). The autonomic system can be further broken down into sympathetic and parasympathetic systems.
Nerves are made up of neurons (Figure 1), which conduct electrical impulses. Connective tissue, known as neuroglia, runs between the neurons. Neurons may be unipolar, bipolar or multipolar, depending on how many are joined together.
Each neuron is made up of the following elements:
- A cell body with a central nucleus
- Dendrites – several short processes protruding from one end of the cell body. This is where nervous impulses enter the cell
- Axon – the long central core that the nervous impulses travel along
- Neurilemma – the sheath of connective tissue which surrounds the whole axon
- Schwann cells – the fatty cells that make up the myelin sheath, which insulates the axon. They form little plates which the impulses can jump on and, therefore, move more quickly
- Node of Ranvier – the gaps in the myelin sheath. It is thought that this is where the nerve cell takes in oxygen and nutrients from surrounding tissues
- Nerve endings – projections at the opposite end of the cell body to the dendrites. These are branching and transmit the impulse to the dendrites of the next neuron
Nerves tend to be either:
- Afferent nerves that receive stimuli from the external receptor and deliver them to the central nervous system
- Efferent nerves that transmit the response from the central nervous system to the desired end location
The transmission of stimuli occurs via certain receptors, which can be further categorised by type of stimulus.
Special senses receptors include photoreceptors, which respond to light (sight); mechanoreceptors, which respond to noise (hearing); chemoreceptors which respond to odours (smell); and chemoreceptors which respond to taste.
Skin receptors include mechanoreceptors, which respond to touch; thermoreceptors, which respond to heat; and nociceptors, which respond to pain.
There are also internal receptors involved in homeostasis. For example, baroreceptors respond to changes in blood pressure, while thermoreceptors respond to changes in core body temperature.
The central nervous system
There are two types of tissue found within the central nervous system:
- Grey matter is found centrally in the medulla of the cerebrum and of the spinal cord; it is also located in the cortex of the cerebellum
- White matter is found in the medulla of the cerebellum and the cortex of the spinal cord and cerebrum
The brain is divided into three regions: the forebrain, midbrain and hindbrain.
The forebrain is made up of the cerebrum – also known as the telencephalon – and the diencephalon.
The cerebrum is the largest part of the brain and is divided into two cerebral hemispheres connected by a mass of white matter known as the corpus callosum. Grey matter, known as the cerebral cortex, makes up the surface of each hemisphere and is folded to increase the surface area. The cerebral cortex helps with perception and memory. The basal nuclei is a bundle of nuclei located within the grey matter of the cortex, which receives information from the cortex to regulate skeletal movement and higher motor functions.
Important structures relating to hormone production and regulation are found in the diencephalon region of the forebrain, namely the hypothalamus, thalamus and epithalamus.
The midbrain – also known as the mesencephalon – comprises the tectum, tegmentum and cerebral peduncle. It is a relatively small portion of the brain, and it is involved in sleep, thermoregulation, vision and hearing.
The hindbrain is made up of the medulla oblongata, the pons and the cerebellum and is located in the lower part of the brainstem. This is the most crucial part of the brain, controlling many life-maintaining factors. It is also the location where 10 of the 12 cranial nerves originate (Figure 2).
The medulla oblongata is responsible for cardiovascular functions, including regulating blood pressure and controlling heart rate. The pons affects respiratory motions and digestive processes. The cerebellum’s general role is to interpret any movements in process or any movements being considered. It does so by receiving messages from muscles, the vestibular system and motor centres. While it does not initiate movements, the cerebellum feeds messages back to minimise any difference in the intended and actual movements.
Cerebrospinal fluid is found in the central nervous system, and its role is to cushion, protect and provide nutrients to the nerve tissues. The fluid is produced by the vascular plexuses housed in the ventricles – there are four located throughout the brain.
Three membrane layers protect the CNS and keep the cerebrospinal fluid in place. These layers are known as meninges and are called:
- the dura mater (outermost layer)
- the arachnoid mater (middle layer)
- the pia mater (the innermost and thinnest layer)
The spinal cord reaches from the medulla oblongata all the way through the centre of the spinal column, which is made up of the vertebrae, and finishes just before the end of the spinal column. The end of the spinal cord is an area called the cauda equina, and is a sequence of thin nerves.
At every intervertebral junction, a pair of spinal nerves exit, one on the left and one on the right. One side is the dorsal root, where sensory nerves enter the spinal cord, and the other is the ventral root, where motor nerves exit on the way to somatic muscles and visceral organs. This structure means spinal nerves are mixed nerves as they contain both a sensory and a motor root. Due to this combination, they can form a reflex arc, which allows a sensory message to travel via the spinal cord so a motor function is performed without involving the brain.
Some common reflex arcs include:
- Pedal reflex – the pain receptors in the skin are stimulated, then the muscles of the limb flex to move the limb away
- Panniculus reflex – the twitching reflex seen along the back when pinching/stimulating the skin
- Patellar reflex – tapping on the patellar ligament results in extension of the lower limb
- Anal reflex – another twitch reflex, this time in response to touch/stimulation of the perianal skin
Automatic nervous system
It is also important to consider the autonomic nervous system, which is the unconscious visceral system that provides motor innervation to most of the vital organs, such as the heart, intestines and bladder. It also acts on endocrine and exocrine glands.
The autonomic nervous system comprises two parts: the sympathetic nervous system, responsible for “fight, flight, fright” responses, and the parasympathetic nervous system, which generally opposes the sympathetic system.
This may seem a little confusing, so here is an example – the sympathetic effect on the bladder is to relax the bladder muscle and increase bladder sphincter tone, whereas the parasympathetic effect is to contract the bladder muscle and decrease sphincter tone. The same goes for the eyes, as the sympathetic effect dilates the pupil, and parasympathetic effect constricts the pupil.