Refractory Period:
After an axon has transmitted an impulse it is impossible for it to transmit another one for a short period - known as the refractory period. Axon has to recover, ionic movements have to occur and membrane has to be repolarised - lasts approx. 3 millisecs.
- Absolute refractory period - axon completely incapable of transmitting an impulse.
- Relative refractory period - possible to generate an impulse provided stimulus stronger than usual.
This determines frequency at which an axon can transmit impulses - range from 500 - 1000/sec/
A strong stimulus produces a great number of impulses (no difference in speed or size of action potential). Brain interprets intensity of stimulus from number of impulses arriving along a neuron per unit time.
Transmission Speed
Depend upon type of neuron and animal. Mammals can be over 100m/sec., while for many inverts, 0.5m/sec or less.
Myelin sheath - causes action potential to leap from node to node of Ranvier thereby speeding up transmission.
Axon diameter - in general, the greater the cross sectional area of the axon, the faster it will transmit impulses.
Synapse
Where one nerve cell connects with another.
End feet contain numerous mitochondria and sec-like vehicles. When on impulse arrives at the synaptic knob (end-feet) it causes a synaptic vesicle to move towards the pre-synaptic membrane and discharge its contents (Ach = acetylcholine). This diffuses across the synaptic cleft to the postsynaptic membrane. If sufficient Ach is secreted, an action potential is generated in the neuron. If Ach is to be effective, it must not be allowed to linger. The moment Ach has done its job, it is inactivated by the enzyme cholinesterase.
Nerve-muscle junction
Essentially similar to a nerve - nerve synapse.
Synapses result in an appreciable delay, up to one millisec. Therefore slows down transmission in nervous system. Synapses are highly susceptible to drugs and fatigue e.g.:-
Function of Synapses
Nor-adrenaline
This is another transmitter substance which may be in some synapses instead of Ach, e.g. some human brain synapses & sympathetic nervous system synapses. Mescaline and LSD produce their hallucinatory effect by interfering with nor-adrenaline.
Reflexes
A quick automatic response to a particular stimulus which do not require conscious control, e.g. knee jerk, blinking.
Minimum number of neurons is two, e.g. knee jerk, however usually three. Not as simple as they appear. Connector neurons also transmit impulses to brain which can override the reflex action, e.g. pick up a hot valuable object.
Function
Complete automation of all protective and avoiding reactions, also internal regulation mechanisms. Leaves higher centres of nervous system free to deal with more complex problems involved in coping successfully with the environment. These reflexes are not learned, i.e. unconditional reflexes.
Conditioned reflexes (learned reflexes). e.g. Pavlov experiment with dogs.
Conditioned reflex theory (stimulus response theory)
Attempt to explain learning. In reality learning process not just a matter of conditioned reflexes, much more complicated.
Effectors
Structures that respond directly or indirectly to a stimulus, e.g. muscle, glands.
Properties
MUSCLE
Muscle is composed of many elongated cells, called muscle fibres, which are all able to contract and relax. Each has its own nerve supply.
Histologically (histology - study of tissues and cells at microscopic level)
3 distinct types:
Skeletal
Attached to bone in at least two places, by touch, relatively inextensible (non-elastic) tendons (connective tissue comprised almost entirely of collagen)
Muscles can only produce contraction. Therefore at least two muscles of sets of muscles must be used to move a bone into one position and back again (called antagonistic muscles) e.g., biceps and triceps.
In order for the CNS to co-ordinate movement it must be able to continually monitor the state of contraction of all the body’s muscles. This is achieved by several types of sense organs located within the muscle itself. The most sophisticated are the muscle spindles. These monitor the extent of contraction of a particular muscle and provide information about how rapidly it is changing length. The simpler Golgi tendon organs merely detect the tension the muscle is under.
Since skeletal muscle is a neurogenic muscle (only contracts when externally stimulated by a nerve) other neurons (motor) must carry the necessary information from the CNS to the muscle.
Most muscles also possess a well developed blood supply.
The muscle cells are relatively uniform in appearance. They consist of long, thin, cylindrical cells arranged parallel to the long axis of the muscle and, therefore, the direction of contraction. The cells are 0.01 to 0.1mm in diameter, several cm long and multi nucleated (nuclei located near the surface of each fibre).
The main components of the muscle cell:
Myofibrils
These are made up of two sets of filaments, thick and thin, which slide past one another during a contraction causing the myofibril to shorten (the filaments do not shorten during a contraction). When the myofibril is relaxed dark bands are produced in the regions where the thick and thin filaments overlap. The full contracted myofibrils are composed of two proteins - actin (thin filaments) and myosin (thick filaments).
Each myofibril is divided by cross-partitions called Z lines into numerous compartments called sarcomeres.
Isolated actin-myosin filaments contract when ATP applied to them. As ATP is always present, an inhibitor prevents continuous contraction. The inhibitor is neutralised by calcium ions (Ca2+). In relaxed muscle, Ca2+ is pumped out of the muscle cells into the tissue fluid. The membranes of the cells are thus polarised. Depolarisation occurs when the muscle is stimulated (action potential arrives along a motor neurone) and Ca2+ enters the cells. Here the Ca2+ neutralises the inhibitor. The ATP then provides the energy for actin and myosin to interact, resulting in muscle contraction.
Impulses spread rapidly all over the muscle in a similar way as nerve impulses are transmitted. Causes contraction of the muscle.
Using energy from ATP the bonds between actin and myosin break and reform near each Z line. The Z lines are thus pulled closer together as actin and myosin do not stretch.
Bridges, seen connecting the thick and thin filaments. Bonds form between the bridges and the actin filament. On contraction the bridge swings through an arc, pulling the actin filament past the myosin filament. After is has completed its movement, each bridge detaches itself from the actin filament and re-attaches itself at another site further along. The cycle is repeated. Shortening of muscle thus brought about by the bridges going through a kind of ratchet mechanism.
Just as the transmission of an action potential by a neuron is ‘all or nothing’ event, so are the contractions of the muscle cells they innervate. This means that an individual cell is either relaxed or fully contracted. However, muscles are capable of differing strengths of contraction. This is achieved by varying the number of muscle cells involved in the contraction, i.e. whereas as the muscle cells will be used in a strong contraction, only a few will be used in a weak one.
