Nerve Dentist

Physiology of the nerve for dental student By Dr Yaser Mohamed Ashour Professor of physiology Al Azhar Faculty of Medicine (Assuit)

The human nervous system

Consists of billions of nerve cells (neurons) plus supporting (neuroglial) cells. Neuroglial cells, the major cell type in neural tissue, provide structural integrity to the nervous system and functional support that enables neurons to perform their functions.

Neuroglial cells

• The word 'neuroglia' (nerve glue) • They are not involved in the transfer of the information. • Neuroglia do not typically have synapses at their surface. Classically neuroglial cells are described as existing only in the central nervous system (CNS, i.e. brain and spinal cord) where there are three kinds: • i. astrocyte • ii. oligodendrocyte • iii. microglia

Astrocytes • They are found throughout the CNS. • They are small cells with extensive branching processes. • provide structural support. • Cell bodies of astrocytes are among the largest for the glia, but only overlap the lower end for size of neurons.

Oligodendrocytes • form myelin sheaths around axons in the CNS. • One oligodendrocyte can form myelin sheaths along more than one internode of more than one axon. • They have smaller cell bodies than astrocytes and relatively fewer processes leaving the cell body.

Microglia • Are the main phagocytic cell and antigen presenting cells in the CNS. • They have the smallest cell bodies among the neuroglia. • They are activated by injury and inflammatory process. • They migrate to the site of injury.

The neuron • It is the basic unit of the nervous system. • It is specialized to perform the following functions:-

• 1- Respond to stimuli (such as touch, sound, light, and so on); • 2- Conduct impulses. • 3- Communicate with each other (and with other types of cells like muscle cells).

The neuron-1 • The neuron consists of:– The cell body. – Cell processes: – Axon. – Dendrites.

• The cell body contains: the nucleus, nucleolus, Nissl granules (RNA material), Golgi apparatus, mitochondria and neurofibrils, but no centrosomes. • The cell bodies are located within the grey matter of the CNS.

The neuron- 2 • Cell processes are of two types (dendrites and axon). They are extending out from the cell body. • These processes vary in number & relative length but always serve to conduct impulses (with dendrites conducting impulses toward the cell body and axons conducting impulses away from the cell body).

Axon • The axon is considerably thicker and longer than the dendrites of a neuron. • Larger neurons have a markedly expanded region at the initial end of the axon. • This axon hillock is the site of summation for incoming information. • It carries impulses away from the cell body. • An action potential will be initiated at the axon hillock and propagated along the axon.

Axon • The axon terminates by dividing into a number of branches, the ends of which are enlarged (terminal knobs), making contact with the muscle (neuromuscular junction) or another neuron (synapse).

Axon • The axon may either constitute a part of the long tract of the CNS, forming the white matter, or leave the brain or spinal cord as a peripheral nerve fibre. • On leaving the grey matter, the axon may acquire a thick myelin sheath, to become a myelinated fibre, if not, it is an unmyelinated fibre. • The axon is covered by 2 sheathes:• Neurilemmal sheath and myelin sheath

Axon covers • Neurilemmal sheath - Single layer of Schwann cells. - Cover the all axons. - Covers the whole axon. - Important for regeneration of injured nerve.

• Myelin sheath - It is made of many layers of cell membrane of the Schwann cells - Covers some nerves and leaves the others. - Covers the axon except at its origin, its end and at nodes of Ranvier. - Act as insulator to prevent the spreading of the ions to the surrounding tissue.

The Myelin Sheath • It consists of fat-containing cells that insulate the axon from electrical activity. • This insulation acts to increase the rate of transmission of signals. • A gap exists between each myelin sheath cell along the axon, the signals jump from one gap to the next.

Axon

1.Myelin sheath 2. Neurilemma 3. Node of Ranvier 4. Schwann cell 5. Axon

Electrical prosperities of a neuron 1- excitability. 2- conductivity.

• • • • • • •

The stimulus It is the change in the environment around the nerve. An effective stimulus: it is a stimulus strong enough to excite the nerve. There are different types of stimuli:Electrical Which are made by placing two electrodes over the nerve membrane Chemical By adding a chemical substance to the medium around the nerve e.g. A.Ch. Mechanical By applying mechanical force as pressing or tapping on the nerve. Physical By changing the physical condition of the medium around the nerve e.g. heating

• Why we prefer the electrical stimulus?

• It is similar to the natural stimuli in the body. • They leave the stimulated nerve without injury. • We can control its onset. • We can control its intensity. • We can control its duration.

Factors that determine the effectiveness of Stimuli:

• 1- Intensity of the stimulus: • The Threshold Stimulus: It is an effective stimulus of minimal intensity. • Subthreshold: A stimulus weaker than threshold which does not produce a propagated nerve impulse. • Superathreshold: A stimulus stronger than threshold stimulus.

Excitability • It is the ability of the nerve to respond to stimulus. • It respond to a stimulus by generating a propagated nerve impulse. • During the propagation of a nerve impulse along the nerve fibres an excitability changes occurs which passes in a four stages before reaching a zero level • (1) Absolute refractory period (ARP) • (2) Relative refractory period (RRP). • (3) Supernormal period. • (4) Subnormal period.

• (1) Absolute Refractory Period:

- In this period the nerve is inexcitable. - No stimulus can excite it, whatever its strength. - It corresponds to ascending limb of A.P. and the upper l/3 of descending limb of A.P. - Its causes is closing of the voltage gated Na+ channel.

• 2) Relative refractory period: - In this period the excitability is partially recovered. - It correspond the remaining part of descending limb of A.P. - The causes are: - Some voltage gated Na+ channels still inactivated. - voltage gated K+ channels are wide open at this time, thus increasing replorizing force.

• (3) Supernormal period. - The nerve excitability is increased. - The weaker stimuli can excite the nerve. -It corresponds to the area of after depolarization. • (4) Subnormal period: - Nerve excitability is decreased. - The stronger stimuli are required for excitation. - It corresponds to the period of hyperpolarization.

The mechanism of Impulse propagation

• The function of the nerve is to conduct the nerve impulse to or from C.N.S. from or to organs. • There are two ways of conduction along the nerve fibres:• (1) In unmyelinated N.F: sweeping conduction. • (2) In myelinated N.F.: saltatory conduction

1) Sweeping conduction = self propagation.

• On application of a stimulus to unmyelinated nerve fibres an action potential developed at the point of stimulation. • At this point a reversal polarity occur, and this area act as a skink of current). • This sink area draw positive charges from the neighboring areas (This neighboring area becomes depolarized up to the firing level ) the process is repeated and so on. It is a slow type, it consume relatively large amount of energy.

(2) Saltatory conduction

• It is a node to node conduction in which the impulse is conducted from a node of Ranvier to another without depolarizing the membrane of the internodes. • So, the impulse is said to Jump over the internodes from one node to another.

• It is fast type of conduction (50 times more than the previous one) • It consumes less energy during recovery. • However because myelin is an insulator the membrane does not become depolarized until there is a bare area of the axon at the next node of Ranvier. • In a small unmyelinated nerve the conduction velocity is about 0.25 m/sec (0.5 mi/h) while in a large myelinated axon it can reach 100 m/sec (225 mi/h).

Direction of current

• On application of a stimulus to the middle of an axon the propagation of impulse along the nerve fibres may go through one of two ways; - Orthodromic conduction: When the impulse pass toward axon termination. - Antidromic conduction: when the impulse pass toward the soma, the impulse fade at the first node of Ranvier or at the synapse.

Nerve injury • When a nerve exposed to injury and transected there is a degeneration occur to the nerve and there is a degeneration occur to the nerve and there are two types of degenerations:(1) Wallerian degeneration which occur in peripheral part of the nerve. (2) Retrograde degeneration which occur in proximal part of axon and the cell body. Degeneration is followed by regeneration.

Wallerian degeneration -The terminal endings of the nerve fibre swell and any vesicles or granules disappear. - The neurofibrils swell and break into short segments. - The myelin sheath is hydrolyzed and break into oily droplets. - the remnants of the myelin sheath and the nerve fiber are removed by neurilemmal cells and tissue macrophages in the area.

Retrograde degeneration -Similar

changes to Wallerian degeneration occur in the proximal part of the nerve fibre. - Cell body  swell + becomes spherical + disappear of short dendrites. - Nucleus --- eccentric. - Golgi apparatus - fragmented.

Regeneration • When the branch of the proximal stump finds a

distal neurilemmal tube, it proceeds into it to its peripheral termination. Other branches degenerate and disappear. • The nerve grows at a rate of l-4 mm/day. • If the gap between the two ends of the nerve fiber is more than 4 mm - the sprouting nerve Brach fail to find its termination in the neurilemmal tube. • This failed end form a neural lump called (neuroma). • The whole process of regeneration can be helped by surgical suturing of the cut ends of the nerve.

Regeneration of the cell body - Starts 20 days after nerve transaction. - The nucleus returns to the center. - The cell regains its normal size and shape. - The Nissl granules and Golgi apparatus are restorted. - In about 80 days the cell body regeneration is completed.

Regeneration of the nerve fiber - Regeneration of nerve fiber starts

after cell body as follow:- The proximal stumps grows in the proximal neurilemmal tube till its cut end. -The end sprouts into several branches (20 up to 100 ) which proceed in all directions searching for the distal neurilemmal tube