The process by which activities in an organism are made to function as an integrated unit.
All living organisms are able to detect changes in their environment and respond appropriately. Changes in the environment are known as stimuli. A stimulus may either be internal or externals. The ability to respond appropriately to stimuli is known as sensitivity.
In mammals, there are 2 major systems that convey information i.e. the nervous and the endocrine system. These systems co-ordinate all organs and harmonise all life processes.
The nervous system is constituted by a series of conducting cells called neurones responsible for transmitting electrical impulses from one part of the body to another.
The endocrine system comprises of ductless glands which secrete hormones directly into the blood system. The hormones are transported in blood to target organs.
It has 3 main functions;
- Collects or receives information about the internal and external environments.
- Processes and integrates the information often in relation to the previous experiences and takes the massage to various parts of the body.
- Acts upon the information usually by coordinating the organism’s activities.
The nervous system is divided into 2 major parts i.e.
- The central nervous system
- The peripheral nervous system
The CNS is divided into the spinal cord and the Brain while the peripheral NS is divided into the voluntary and somatic nervous system. It is comprised of highly differentiated cells whose function is to detect sensory information and encord it into electrical impulses which are transported to cells which are capable of producing response. All stimuli are detected by sensory receptors e.g. skin, eyes, ears and the information is passed on to the effectors cells.
In between the receptors and effectors are the conducting cells of the nervous system called neurones. A neurone is a functional unit and basic structure of the nervous system
Types of neurons
- i) Sensory (Afferent) Neurones
These transmit impulses from the receptors to the central nervous system
- ii) Motor (efferent) Neurones
Transmit impulses from the CNS to the effectors
(iii) Intermediate Neurones (Relay /associated neurones
Link the receptor and effector neurones within the CNS. All neurones contain similar essential parts
Function of the different parts of a motor neurone
Cell body (centron)
It is the nucleated part of the cell and it is located in the CNS. It has all organelles which control all the activities of the cell.
Slender processes which connect the cell body with neighbouring neurones. They transmit impulses to the cell body.
Axon (nerve fibre)
Contains the axoplasm which transmits impulses from the cell body to the effector muscles.
Protects and insulates the axon and speeds up the transmission of impulses.
It is not part of the neurone but the membrane of another cell (schwann cell)
Increases the rate at which the transmission of impulse takes places.
THE SPEED OF TRANSMISSION
The speed depends mainly upon 2 factors;
- i) Myelin sheath
It is a fatty insulating layer around the axon. At certain intervals, it is absent and this forms the rode of ranvier. Generally, myelinated nerve fibres carry impulses much faster than non-myelinated fibres. This is because ionic charges and depolarization occur only at nodes of ranvier hence the action potential leaps from one node to another. This is referred to as saltatory conduction and it increases the speed of transmission as ionic movements associated with action potential occur less frequently taking less time.
Diameter of axon
The thicker the diameter of the axon, the faster the speed of transmission. This is because of the greater area of the membrane over which ionic exchange can take place.
Invertebrates like crustacean, annelids, molluscs etc lack the myelin sheath so they have developed grant axons in order to increase the speed of transmission. Then vertebrates have a myelin sheath which increases the rate of transmission.
Temperature has an effect on the rate of conduction of nerve impulses and as temperature rises to about 40˚c, the rate of conduction increases.
THE NERVE IMPULSE
The nerve impulse is a small electrical event which is a result of charge differences across the membrane of a nerve fibre. It is based on ion movements through specialized protein pores and an active transport mechanism.
Resting neurone negatively charge internally art outside (polarised)
For a resting neurone, the potential difference that exists a cross the membrane of the axon is responsible for its resting potential. The resting potential is the electrical pd that appears across the membrane that is not transmitting any impulse. The membrane of the axon like any other cell surface is partially permeable the difference in permeability of this membrane to Na+ and K+ sets neurones apart from other cells and gives them their special conducting properties.
The axon membrane is relatively impermeable to Na+ but quite freely permeable to K+. It also contains a Na – K pump which uses ATP to move the Na+ out of the axon and let in K+. The effect of this is to reduce the concentration of Na+ inside the axon because once they are pumped out, they can’t diffuse in. At the same time, K+ are moved in but they diffuse out again along a concentration gradient. As a result, the inside of the cell is left slightly negatively charged relative to the outside i.e. it is polarized and there is a pd across the membrane of -70mV which is known as the resting potential of the cell in its normal state.
When a neurone is simulated by stimulus, the axon shows a sudden and dramatic increase in its permeability to Na+. Na+ diffuse rapidly into axon causing depolarization, specific sodium channels or gates open allowing Na+ to rush in a long both concentration and electrochemical gradients. As a result, the pd across the membrane is briefly reversed and the cell becomes positive inside with respect to the outside. This depolarization lasts for about 1sec and the pd across the membrane is about +40mV and it is known as the action potential.
At the end of this brief depolarisation, the Na channels close again and the excess Na+ are rapidly pumped out by the Na pump. In addition, the permeability of the membrane to K+ is temporarily increased so that they move out along a concentration gradient.
It takes a few ms before the resting potential is restored so that the nerve is able to carry another impulse. This period is known as the refractory period which ensures that the nerve impulse travels only in one direction.
This is the recovery period required by an axon membrane before it can be able to transmit another impulse after a pervious one has passed.
This period involves repolarisation of the membrane and re-distribution of the ions. It is in 2 stages;
- Absolute refractory period
- Relative refractory period
Absolute refractory period
During this period, it isn’t possible for an axon to generate an impulse even if the stimulus intensity is increased. The sodium channels are completely blocked and the resting potential has not been restored.
Relative refractory period
During this stage, it is possible to generate an impulse before the membrane completely recovers if a strong intensity stimulus is applied.
Importance of the refractory period
- Limits the frequency with which impulses will flow within a nerve
- Ensures that impulses flow in only one direction along the nerves making it possible to have motor and sensory systems without internal confusion. This is because the previously active area undergoes a recovery period during which it will not respond to any external stimulus.
For an action potential to be produced, the stimulus must reach a certain threshold intensity. Increase in the strength of the stimulus does not increase the amplitude of the action potential.
For an impulse to be generated, the intensity of the stimulus must be able to cause depolarization and bad to generation of an action potential. Increasing the intensity of the stimulus will have no affect all on the action potential. This is called the all – or- nothing law which states that: the response of an excitable axon is independent of the intensity of the stimulus. I.e. the size of the action potential is independent of the size of the stimulus. A stimulus is either able to produce a full size action potential or no action potential at all.
This is a junction between to neurones or it is a precise point where one nerve cell connects to another.
Synapses exist between fine terminal branches of the axon of one neuron and the dendrites or cell body of another neurone.
Some synapses can be categorized as electrical if transmission across the synapse is electrical or chemical if the transmission across the synapse is chemical.
The most common transmitter substances are acetylcholine and noradrenaline.
Structure of a synapse
The axon terminals of neurones end in small bulb-like structures known as synaptic knobs. The nerve ending in the synaptic knob along which the impulse travels to reach the synapse is known as the presynaptic neurone. The neurone after the synapse into which the impulse travels is known as the postsynaptic neurone. The surface membranes of the 2 neurones are known as the presynaptic and postsynaptic membrane respectively.
The membrane of the past synaptic neurone is ticked and contains numerous protein molecules to act as receptor sites for the transmitter substance. It also contains a number of channels and pores through which the ions move into the postsynaptic neurone.
The presynaptic membrane is separated from the postsynaptic membrane by a small gap called the synaptic cleft. The synaptic knob contains many mitochondria, synaptic vesicles, smooth endoplasmic reticulum and micro filaments. The synaptic vesicles contain the transmitter substance.
There are two categories of synapses i.e.
- Nerve to nerve synapse
- Nerve to muscle (neuro muscular) synapse.
The synapses have a few differences in structure but they function in the same way.
When an impulse arrives at a synaptic knob, it causes depolarisation of the presynaptic membrane. The depolarisation increases the permeability of the presynaptic membrane to ca2t. The ca2t enter the synaptic knob and cause the synaptic vesicles to migrate to the presynaptic membrane.
The vesicles fuse with the membrane and release the transmitter substance into the synaptic cleft. The transmitter substance diffuses across the different and gets attached to the protein receptor sites in the postsynaptic membrane.
The receptor sites undergo changes resulting into the opening of the channels.
Nat enter while k t leave leading to the depolarisation of the postsynaptic membrane. An action potential called excitatory postsynaptic potential (EPSP) is generated in the post synaptic neurone and is propagated as an impulse along the neurone.
After the transmitter substance has caused changes in the postsynaptic membrane, it immediately diffuses out of the cleft and is reabsorbed by the presynaptic membrane or it is hydrolysed by the enzyme acetylcholinesterase (AChE) also called cholinesterase to choline and ethanoic acid. These products are absorbed into the synaptic knob and are used to synthesize more acetylcholine which is stored in new synaptic vesicles.
The synthetic path way requires ATP which is provided by the mitochondria.
The hydrolysis of the transmitter substance makes its effects temporary.
This prevents generation of many impulses in the post-synaptic membrane from one impulse that arrives at the knob
Nerve –nerve synapse