Functions of the Nervous System

The complex activities of the body are controlled jointly by the Endocrine and the Nervous systems. As opposed to the Endocrine system the Nervous system has a more or less instant effect on the body via a complex network of nerves and control centres. The Central Nervous System (CNS) includes the brain and spinal cord, while Peripheral Nervous System (PNS) includes nerves connected to the spinal cord. The nervous system can be further divided into sub-systems, all of which are composed of neurons and connective tissue:

About Neurons

Neurons are specialised cells of the nervous system, they vary greatly in appearance and length, but contain a similar structure:

• Axons are long nerve processes which carry nerve impulses from the Soma to other neurons, they vary in length but can become almost as long as half of the human body.

• The soma (body) of the neuron contains the nucleus which acts as the cell's control centre, these contain many small neurofibrils which project from the nucleus into the dendrites.

• Dendrites are short, thick processes which branch out of the soma in a tree like manor. They conduct nerve impulses to the soma.

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The three categories of neurons:

• Afferent (Sensory) Neurons have the dendrites connected to receptors such as the eyes, ears etc. These receptors change the information they receive into electrical impulses that are transmitted to other neurons. In sensory neurons the axons are connected to other neurons.

• Efferent (Motor) Neurons have the dendrites connected to other neurons, the axons are connected to effectors. Effectors are either glands or a muscle cell that is the receiving end of the nerve impulse. The nerve, when excited will cause the effector to react (move, contract, or secrete etc).

• Internuncial Neurons have both the dendrites and the axons are connected to other neurons. They are sometimes referred to as connector neurons. Internuncial neurons are found throughout the body, but especially in the spinal cord and brain.

Properties and characteristics of Neurons:

• Normally the electrical impulses (messages) travel through a neuron in only one direction.

• The axon may be surrounded by a 'coat' of lipids (fats) and proteins known as the myelin sheath which acts as an insulator.

• Neurons are specialist cells that have lost the ability to reproduce themselves. Once the soma of a neuron has died the entire neuron dies, and can never be replaced.

• Repair of damaged neurons only occurs in myelinated neurons.

• white matter are coloured by myelin, consisting of many neurons supported by neuroglia.

• grey matter is soma and dendrites or bundles of unmyelinated axons and neuralgia.

Nerves, Neuroglia, and Ganglia

A nerve is a bundle of fibres (axons and/or dendrites) outside the CNS.

Neuroglia are cells of the nervous system that help protect and support it.

Ganglia are groups of nerve cell bodies lying outside the CNS.

The Spinal Chord

A spinal tract is a bundle of fibres in the CNS that travel long distances up or down the spinal chord. Ascending tracts carry impulses up the chord to the brain, while descending tracts carry impulses down the chord from the brain. Tracts run along the spinal canal inside the protective spinal column, conveying sensory and motor (movement ) information to and from the brain. Spinal meninges are tough tubes of tissue which protect the chord.

The Brian

The brain is highly complex, it contains about 1000 billion neurons, and weighs about 3 lbs in adults.

There are four main areas of the brain:

1. The brain stem is at the base of the brain where it joins the spinal chord (contains the medulla, pons, and mid brain)

2. The diencephalon is above the brain stem (contains the thalamus and hypothalamus)

3. The cerebrum is above the diencephalon and forms the majority of the brain

4. The cerebellum is the lower back of the brain

The brain has two hemispheres , there are functional differences, for example the left had side of the brain controls the right hand side of the body and visa versa (lateralisation).

Neurotransmitters are substances which excite or inhibit the neurons of the brain, facilitating communication between brain cells. These include endorphins, neuropeptides.

Cerebrospinal fluid circulates around the brain and spinal tracts to provide protection in addition to that provided by the meninges and protective bones of the spine and skull. A lumbar puncture (spinal tap) is where a needle is placed between the vertebra in the lower back. A sample of cerebrospinal fluid might be taken to see if cancerous cells have entered the CNS, or chemotherapy might be administrated to prevent or combat CNS involvement.

The Sensory Systems

A receptor or sense organ picks up stimulus and converts it into a nerve impulse. This impulse is then conducted along a neural pathway to the brain, where the signal is converted into a sensation. There are various receptors:

• Cutaneous (skin) senses. The skin contains specialist receptors for touch, pressure, vibration, hot, cold, and pain.

• Proprioceptive (muscle) sensations inform us of the activities and current posture of the muscles.

• Integrative sensations are not well defined but include memory, sleep, emotions etc.

• Olfactory sensations (smell). Olfactory neurons have dendrites that are connected to fine hairs in the nose that react to odours.

• Gustatory sensations (taste) there are about 2000 taste buds, mostly on the tongue and a few in the throat. Taste buds contain gustatory cells which contain sensitive hair like processes.

• Ophthalmic sensations (sight). The retina of the eye converts light into nerve impulses which are transmitted to the optic nerve. Retinoblastoma is a rare tumour in the cells of the retina.

• Auditory senses (hearing). Sound waves cause fine hairs in the inner ear to vibrate generating nerve impulses.

• Equilibrium (sense of balance) the ear also contains receptors that give a sense of static equilibrium (position of the head) and dynamic equilibrium (sudden movements).

The Autonomic Nervous system (ANS)

The nerves of the ANS activate the involuntary smooth muscles, cardiac muscles, and some glands.

Roots, suffixes, and prefixes

component	meaning	example
ASTRO-	star
astrocyte = star shaped brain cell.
CRANI-	skull
cranial radiation = radiation to the head.
CEPHAL-	head
encephal = the brain, en(in) cephal (the head).
MENING-	membrane
meningitis = inflammation of the membranes of the spinal chord.
NEUR-	nerve
neuroblast = an immature nerve cell.
VENTRICULO-	cavity
ventricles are small cavities in the brain & spinal cord. Ventriculscopy = examination of ventricles.
-MALACIA	softening
neuromalacia = morbid softening of the nerves.
-GRAM	record
electroencephalogram (EEG) = brain scan.

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