Topics(Nervous System|Part 2)
Excitability is defined as the physiochemical change that occurs in a tissue when stimulus is applied. Stimulus is defined as an external agent, which produces excitability in the tissues. Chronaxie is an important parameter to determine the condition of nerve fiber. Nerve fibers have a low threshold for excitation than the other cells.
1.Action potential or nerve impulse
Adequate strength of stimulus, necessary for producing theaction potential in a nerve fiber is known as threshold or minimal stimulus.
Fig.1 Action potential in nerve fiber
2. Electrotonic potential or local potential
When the stimulus with subliminal strength is applied, only electrotonic potential develops and the action potential does not develop.
ACTION POTENTIAL OR NERVE IMPULSE
Action potential in a nerve fiber is similar to that in a muscle, except for some minor differences
ELECTROTONIC POTENTIAL OR LOCAL POTENTIAL
Electrotonic potential or local potential is a non-propagated local response that develops in the nerve fiber when a subliminal stimulus is applied.
Properties of Electrotonic Potential
1. Electrotonic potential is non-propagated
2. It does not obey all-or-none law.
Normally in the body, the action potential is transmitted through the nerve fiber in only one direction. Conductivity is the ability of nerve fibers to transmit the impulse from the area of stimulation to the other areas.
MECHANISM OF CONDUCTION OF ACTION POTENTIAL
Depolarization occurs first at the site of stimulation in the nerve fiber. It causes depolarization of the neighboringareas. Like this, depolarization travels throughout the nerve fiber. Depolarization is followed by repolarization.
CONDUCTION THROUGH MYELINATED
NERVE FIBER – SALTATORY CONDUCTION
Saltatory conduction is the form of conduction of nerve impulse in which, the impulse jumps from one node to another.
Mechanism of Saltatory Conduction
The entry of sodium from extracellular fluid into nerve fiber occurs only in the node of Ranvier, where the myelin sheath is absent. It causes depolarization in the node and not in the internode. Thus, depolarization occurs at successivenodes. So, the action potential jumps from one node to another. Hence, it is called saltatory conduction
Refractory period is the period at which the nerve does not give any response to a stimulus.
TYPES OF REFRACTORY PERIOD
1.Absolute Refractory Period
Absolute refractory period is the period during which the nerve does not show any response at all, whatever may be the strength of stimulus.
2.Relative Refractory Period
It is the period, during which the nerve fiber shows response, if the strength of stimulus is increased to maximum.
All-or-none law states that when a nerve is stimulated by a stimulus it gives maximum response or does not give response at all.
When a nerve fiber is injured, various changes occur in the nerve fiber and nerve cell body. All these changes are together called the degenerative changes.
Causes for Injury
Injury to nerve fiber occurs due to following causes:
1. Obstruction of blood flow
2. Local injection of toxic substances
3. Crushing of nerve fiber
4. Transection of nerve fiber.
DEGREES OF INJURY
It is caused by applying pressure overa nerve for a short period leading to occlusion of blood flow and hypoxia. ow and hypoxia. By first degree of injury, axon is not destroyed but mild demyelination occurs. The function returns within few hours to few weeks. First degree of injury is called Seddon neuropraxia.
Second degree is due to the prolonged severe pressure, which causes Wallerian degenerationHowever, the endoneurium is intact. Repair and restoration of function take about 18 months. Second degree of injury is called axonotmesis. THIRD DEGREEIn this case, the endoneurium is interrupted. After degeneration, the recovery is slow and poor or incomplete. Third, fourth and fifth degrees of injury are called neurotmesis.
This type of injury is more severe. Epineurium and perineurium are also interrupted.
Fifth degree of injury involves complete transaction of the nerve trunk with loss of continuity.
DEGENERATIVE CHANGES IN THE NEURON
Accordingly, degenerative changes are classified into three types:
1. Wallerian degeneration
2. Retrograde degeneration
3. Transneuronal degeneration.
WALLERIAN DEGENERATION OR ORTHOGRADE DEGENERATION
Changes in Nerve
1.Axis cylinder swells and breaks up into small pieces.
2. Myelin sheath is slowly disintegrated into fat droplets.
3. Neurilemmal sheath is unaffected, but the Schwann cells multiply rapidly.
Changes in Nerve Cell Body
i. First, the Nissl granules disintegrate into fragments by chromatolysis
ii. Golgi apparatus is disintegrated
iii. Nerve cell body swells due to accumulation of fluid and becomes round
iv. Neurofibrils disappear followed by displacement of the nucleus towards the periphery
v. Sometimes, the nucleus is extruded out of the cell.
If an afferent nerve fiber is cut, the degenerative changes occur in the neuron with which the afferent nerve fiber synapses. It is called transneuronal degeneration.
REGENERATION OF NERVE FIBER
It starts as early as 4th day after injury, but becomes more effective only after 30 days and is completed in about 80 days.
CRITERIA FOR REGENERATION
1. Gap between the cut ends of the nerve should not exceed 3 mm
2. Neurilemma should be present; as neurilemma is absent in CNS, the regeneration of nerve does not occur in CNS
3. Nucleus must be intact
4. Two cut ends should remain in the same line.
STAGES OF REGENERATION
1. First, some pseudopodia like extensions grow from the proximal cut end of the nerve.
2. Fibrils move towards the distal cut end of the nerve Fiber
3. Some of the fibrils enter the neurilemmal tube of distal end and form axis cylinder
4. Schwann cells line up in the neurilemmal tube and actually guide the fibrils into the tube.
5. Axis cylinder is fully established inside the neurilemmal tube.
6. Myelin sheath is formed by Schwann cells slowly. Myelination is completed in 1 year.
7. Diameter of the nerve fiber gradually increases.
8. In the nerve cell body, first the Nissl granules appear followed by Golgi apparatus
9. Cell looses the excess fluid; nucleus occupies the central portion
10. Though anatomical regeneration occurs in the nerve, functional recovery occurs after a long period.
Neuroglia or glia (glia = glue) is the supporting cell of the nervous system. Neuroglial cells are non-excitable and do not transmit nerve impulse
CLASSIFICATION OF NEUROGLIAL CELLS
Accordingly the neuroglial cells are classified into two types:
A. Central neuroglial cells
B. Peripheral neuroglial cells
CENTRAL NEUROGLIAL CELLS
Neuroglial cells in CNS are of three types:
Fig.2 Neuroglial cells in CNS
Astrocytes are star-shaped neuroglial cells present in all the parts of the brainTwo types of astrocytes are found in human brain:
This type of astrocytes play an important role in the formation of blood-brain barrier by sending processes to the blood vessels of brain.Tight junction in turn forms the blood-brain barrier.
Functions of Astrocytes
Twist around the nerve cells and form the supporting network in brain and spinal cordiii. Maintain the chemical environment of ECF around CNS neuronsv. Regulate recycling of neurotransmitter during synaptic transmission.
These cells are derived from monocytes and enter the tissues of nervous system from blood. These phagocytic cells migrate to the site of infection or injury and are often called the macrophages of CNS.
Functions of Microglia
Engulf and destroy the microorganisms and cellular debris by means of phagocytosisii. Migrate to the injured or infected area of CNS and act as miniature macrophages.
Oligodendrocytes are the neuroglial cells, which produce myelin sheath around the nerve fibers in CNS.
Functions of Oligodendrocytes
1.Provide myelination around the nerve fibers in CNS where Schwann cells are absent
2. Provide support to the CNS neurons by forming a semi-stiff connective tissue between the neurons.
PERIPHERAL NEUROGLIAL CELLS
Neuroglial cells in PNS are of two types:
1. Schwann cells
2. Satellite cells.
Functions of Schwann Cells
Satellite cells are the glial cells present on the exterior surface of PNS neurons.
Functions of Satellite Cells
1.Provide physical support to the PNS neurons
2.Help in regulation of chemical environment of ECF around the PNS neurons.