NERVOUS SYSTEM
Nerve tissue of vertebrates consists of:(a) Three types of cell:(i) Neurons, many million, function transmit messages (impulses).
(ii) Schwann cells, associate with neurons in PNS
(iii) Neurological cells, found within CNS
(b) Connective tissue and blood vessels.
Neurons
Described as unipolar, bipolar and multipolar, according to how many processes
project from the cell body.
Three types:-
(i)
(ii)
(iii)
Motor
Sensory
Connector (intermediate, relay or inter)
1 Motor (efferent) neurons
Transmits impulses from CNS to effectors e.g. muscles. Cell body located CNS.
Axon enters a peripheral nerve and terminates in a muscle. May be over a metre
long. A peripheral nerve may contain several thousand axons. Axon enclosed
within a fatty myelin sheath. At about 1mm intervals are constrictions called nodes
of Ranvier. Function of sheath is protection, insulates axon and speeds up
transmissions of impulses. Nodes allow exchange of materials between axoplasm
and surrounding tissue.
Some vertebrates have axons which are non-myelated, however majority are
myelated.
Both myelated and non myelated neurons associated with Schwann cells, which
produces the myelin sheath in the case of myelinated neurons. Both types
surrounded by a thin neurilemma which is part of the Schwann cell.
Nissi’s granules are groups of ribosomes (rich in RNA) and are concerned with
protein synthesis.
2 Sensory (afferent) neurons
Transmit impulses from receptors to CNS. Usually myelinated and associated with
Schwann cells. Cell body situated dorsal root ganglion of a spinal nerve.
3 Connector neurons
Confined to CNS. Connect sensory and motor neurons with each other and other
neurons in CNS. Their structure shows considerable variation e.g. bipolar and
multipolar. Non-myelinated and not associated with Schwann cells.
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Investigating the nerve impulse
When the nerve is stimulated an impulse passes along the nerve from the site of the
stimulus. This consists of a wave of electrical current called an ‘Action Potential’.
Effect can be displayed on an oscilloscope.
Impulse travels at a definite velocity (e.g. frog 25m sec-1 ).
decrease in temperature.
Velocity falls with a
Majority of axons extremely fine, rarely exceed 20 m in diameter. (Squid contains a
series of giant axons 1mm diameter - so these often used in experiments).
There is a difference in potential of 60 millivolts between the inside and outside of the
neuron (inside - 60mv). This polarity is known as the resting potential. Stimulation
of a receptor causes a rapid depolarisation of the resting potential and is called the
action potential (inside + 60mv positive compared to outside).
Properties of nerves and nerve impulses
Stimulation:
In normal circumstances, impulses are set up in nerve cells as a result of excitation of
receptors. (However, an axon can also be executed by direct application of any
appropriate stimulus that causes local depolarisation of the membrane, e.g. electrical,
mechanical).
All or Nothing Law:
If stimulus above threshold, a full sized potential is produced; further increase in
intensity does not give a larger potential, i.e. all or nothing.
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.
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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.:1 Curare (poison used by S.American Indians) and atropine stops Ach from
depolarising the post-synaptic membrane , i.e. become paralysed.
2 Strychnine and some nerve gases inhibit or destroy cholinesterase formation.
Prolongs and enhances any stimulus, i.e. leads to convulsions, contraction of
muscles upon the slightest stimulus.
3 Cocaine, morphine, alcohol, ether and chloroform anaesthetise nerve fibres.
Function of Synapses
1 Prevents impulses travelling in the wrong direction. An impulse can pass along
an axon in either direction, but can only cross a synapse in one direction because
the synaptic vesicles are only found in the synaptic knobs and end plates.
2 A vast number of synaptic connections allow for great flexibility. They are
equivalent to the switchboard in an elaborate telephone exchange enabling
messages to be diverted from one line to another and so on.
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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.
Reflex arc
-
the pathway of a reflex impulse
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.
1 Innate reflexes (born with) e.g., sucking reflex - baby will suck almost any object
placed in its mouth. Vital reflex which activates expulsion of milk from mother’s
mammary glands during suckling.
2 Acquired reflexes - young infants acquire additional reflexes and later override
them at certain stages of their growth and development, e.g. grasping reflex.
Conditioned reflexes (learned reflexes). e.g. Pavlov experiment with dogs.
(a)
Primary stimulus: FOOD
Response salivation
(b)
Primary stimulus: FOOD
secondary stimulus
ringing bell
(c)
Secondary stimulus only
ringing of bell
Response
salivation
-
Innate reflex
Response
salivation
-
conditioned reflex
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.
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Properties
1 Effect of stimulus - all or nothing response.
2 Summation - when two or more stimuli, close together above the threshold
reach the muscle the effects they produce add together or summate, e.g.
3rd stimulus
2nd stimulus
1st stimulus
3
Repeated stimulation produces tetanus, i.e. continual contraction e.g.
produced by strychnine poisoning, lockjaw.
4
Fatigue - after long period contraction muscle will not contract again
immediately when neuron stimulated due to synapse transmitter substance,
Ach, being temporarily used up.
More difficult to cause fatigue if muscle directly stimulated, eventually all ATP
used up.
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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:
1 Skeletal (striated, striped, voluntary)
Attached to bone. Concerned with locomotion.
quickly. Innervated by voluntary nervous system.
Contract quickly and fatigue
2 Smooth (unstriated, unstriped, plain, involuntary)
Found walls of tubular organs, e.g. intestines, blood vessels, and concerned with
movement of materials through them. Contract slowly and fatigue slowly.
Innervated by autonomic nervous system.
3 Cardiac (myogenic)
Contracts spontaneously, without fatigue. Innervated by autonomic NS
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).
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The main components of the muscle cell:
MYOFIBRILS
These are the actual contractile elements within the muscle cell.
These are also arranged parallel to the axis of pull of the muscle.
SARCOPLASMIC
RETICULUM
A double membrane ‘jacket’ wrapped around the myofibrils. It
controls their contraction by regulating the levels of free calcium
within the cell.
TRANSVERSE
TUBULES
Long tubular invaginations of the outer membrane of the muscle
cell running deep into its interior where they come into close
contact with sarcoplasmic reticulum. They initiate contraction
by conducting action potentials throughout the cell.
MITOCHONDRIA
Provide the large amounts of ATP required to power the muscle.
SARCOPLASM
Myofibril cytoplasm.
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.
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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.
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