The following is a basic diagram of a neurone
(Adds et al, 2000)
The axons conduct impulses away from the cell body to other neurones or effectors, whereas dendrites carry impulses from specialised receptors or from adjacent neurones with which they can form synapses. (Purves et al, 2001)
Depending on the arrangement of the axons and the dendrites there are 3 main groups of neurones, which carry out different roles in the nervous system. The first group is multipolar neurones, which occurs n large amounts in the nervous system and have numerous amounts of dendrites branching of the cell body. More than likely multipolar neurones are typical effector (motor) neurones. The second group is bipolar neurones, which only consist of a single neurone coming directly from the cell body opposite the axon. These act as receptors for the senses such as sight and smell. Finally the third type of neurones is the pseudo-unipolar neurones, which consist of the cell body and from the stem of the cell body a single dendrite and axon branch. These are typically sensory neurones. (Adds et al, 2000)
The structure and the function of the human nervous system are efficient to its information flow. Information is passed through the nervous system in the form of a nerve impulse. In order to pass information in the form of a nerve impulse, Schwann cells surround most axons, forming a covering, which is referred to as the myelin sheath and the axons covered by them are said to be myelinated. Between each adjacent Schwann cell there are short gaps, which are not covered by the sheath and these are the nodes of Ranvier, which allow nerve impulses to be conducted. (Purves et al, 2001)
As the diameter of the axon increases the conduction velocity of the nerve impulses also increases, but when an axon becomes myelinated the conduction velocity of an impulse is faster than a non-myelinated axon of the same diameter. So therefore a reaction to the information happens a lot faster. (Adds et al, 2000)
The following is a section of an axon and the myelin sheath formed by Schwann cells
(Adds et all, 2000)
For information to pass to the brain or to the spinal cord it has to be passed in the form of nerve impulses through the neurone. The function of the axon aids the information flow. A nerve impulse involves the movement of ions through the axon membrane. When a nerve impulse is taking place sodium ions are moving into the axon and potassium ions are moving out. When a neurone does not conduct an impulse it is at resting state and the inside of the axon has a negative electrical charge. (Adds et al, 2000)
When positively charged sodium ions enter the inside becomes depolarised and an action potential is generated, then the potassium ions move out making the inside of the axon negatively charged again and so therefore restoring the resting potential. (Toole et al, 1991)
The ion pumps and channels in the axons cause the resting and action potentials when information is passed through. When a nerve impulse is generated, the membrane of the axon increases permeability to sodium ions by opening more sodium channels, causing sodium ions to enter faster than they can leave the axon. The positive charge in the axon increases causing an action potential. Then the permeability of the membrane to sodium ions decreases and increases in permeability to potassium ions by opening more potassium channels. This causes potassium ions to flow out and the potential value inside the axon returns to its negative charge. The potassium channels remain open until the resting potential is restored, but at times there may be a potassium overshoot causing hyperpolaristaion, where the membrane potential is lower than the normal resting value, but this is only minor so as soon as the potassium and sodium ions go back to their resting concentrations the resting potential is restored. (Purves et al, 2001)
On the membrane of the axon these changes occur in small parts and the nerve impulse is an action potential moving at a constant velocity along the membrane. As mentioned before in a myelinated axon, the action potential leaps from one node of Ranvier to the next and this is known as salutatory conductions. This increases the velocity of the conduction of the impulse so therefore increasing the speed that information is passed through the nervous pathway to stimulate a response. (Toole et al, 1991)
The nervous system has specialised junctions to aid information flow. Due to the fact that no neurone is in direct contact with other neurones specialised junctions called synapses have to aid their pathways. There are 2 types of synapses; they can either be electrical or chemical. In an electrical synapse, the membranes of adjacent neurones are very close and so the wave of excitation can pass from one neurone to the next. Whereas in a chemical synapse the gap is relatively large so a direct excitation cannot be passed and so when an impulse is generated in the neurone a presynaptic neurone triggers the release of a chemical transmitter substance to help transmit the impulse from one neurone to the next. (Adds et al, 2000)
The following is a diagram showing the structure of a chemical synapse
(Adds et al, 2000)
The brain and the spinal cord decide the appropriate response to a particular stimulus. The brain is developed from a structure in the neural plate forming the neural tube, expanding into the forebrain, midbrain and hindbrain, which are the 3 main parts of the brain. (Adds et al, 2000)
The forebrain is made of the left and right cerebral hemispheres and hypothalamus. The hemispheres make up the largest part of the brain, the cerebrum and are connected by nerve fibres (corpus callosum). The cerebrums job is to receive impulses from sensory receptors then initiate and control contractions of skeletal muscles for movement and to help with mental cavities linked with consciousness such as speech, emotions and memory. (Toole et al, 1991)
The hypothalamus’s job is to syntheses the hormones secreted by the posterior pituitary gland, help with osmoregulation and maintaining the body temperature. It also contains neurones that function as the endocrine glands. (Adds et al, 2000)
The midbrain’s job is to conduct impulses between hindbrain and midbrain. It contains centres associated with visual and auditory reflex actions. (Toole et al, 1991)
The cerebellum works with the cerebrum and helps coordinate movement of skeletal muscles and maintenance of balance and body posture. (Purves et al, 2001)
The medulla oblongata has nerve fibres between the brain and spinal cord, it controls the rate and force of the heartbeat. It also controls the rate and depth of breathing and the diameter of blood vessels. It also controls reflexes such as sneezing and coughing. (Purves et al, 2001)
The function of the spinal cord is linked with the medulla oblongata and is made of a central area of grey matter and external layer of white matter. In the centre of the spinal cord there is the central canal with the cerebrospinal fluid. (Adds et al, 2000)
The grey matter has cell bodies of nerve cells and transmits impulses to the skeletal muscle and to the unmyelinated connector neurones. Whereas the white matter has nerve fibres which conduct impulses to and from the brain. (Toole et al, 1991)
In conclusion there is a relationship between the structure of the human nervous system, its function and information flow. The human nervous system is a very complex system and a lot of things are occurring in the different departments to make it work and the things described in this essay are just some of the things going on.
REFERENCES
Purves, Sadava, Orians & Heller, 2001
“Life: The Science Of Biology – 6th Edition”
Sinauer Associates Publishing Inc. USA
Adds. J, Larkcom. E & Miller. R, 2000
“Exchange and Transport, Energy and Ecosystems”
Nelson Publishing Inc. UK
Toole. G & Toole. S
“Understanding Biology”
Hutchinson Education Publishing Inc. UK
