PATHOPHYSIOLOGY OF NERVE

Dr. Ayisha QureshiProfessor

MBBS, MPhil

EFFECTS OF CUTTING A MIXED NERVE:

What happens when you cut a mixed nerve?

1. LOSS OF MOTOR ACTIVITY: • Affected muscles become paralyzed.• Muscles become flaccid (NO TONE).• Rapidly undergo loss of mass (WASTING).• Muscles start showing the ERB’S REACTION OF DEGENERATION:

altered response of the muscles to electrical stimulation seen after loss of motor supply.

2. LOSS OF SENSATIONS:• NO sensation in the area of the cut sensory nerves.

3. LOSS OF AUTONOMIC NERVE ACTIVITY: • Area becomes blue & cold.• Activity of the sweat glands comes to an end.

4. LOSS OF TROPHIC ACTION: (growth-promoting action)• Atrophy (loss of mass & wasting) of the muscles that were

supplied by the motor nerves.

DEGENERATION & REGENERATION OF NERVE FIBRES

How can the nerve fibers be damaged?Nerve fibers may be damaged by being severed or crushed (as during a traumatic event, such as a vehicle wreck, a gunshot wound, or a diving accident) or by being deprived of their blood supply (as during a stroke).What happens when the nerve fiber is damaged?When damaged, the affected axon is unable to conduct action potentials. If the cell body dies when a neuron is injured, the entire neuron dies. If the cell body is intact and only the axon is severed, most of the neuron survives.

DEGENERATION:

When a nerve fibre is cut or severely crushed, degenerative changes take place at 3 levels:1. Changes in the NERVE CELL BODY2. RETROGRADE DEGENERATION: Changes in the

central/proximal stump. 3. WALLERIAN DEGENERATION: Changes in the distal stump.

(also called the Secondary Degeneration)

1. Changes in the nerve cell body:

• Cell body swells.• Nissl granules undergo dissolution.

(CHROMATOLYSIS)

• Nucleus is pushed to one side.• Mitochondria, Golgi apparatus, ribosomes &

lysosomes show structural changes.• If the axon is cut quiet close to the cell body,

the neuron may die....

2. RETROGRADE DEGENERATION

Retrograde Degeneration is the degeneration that occurs in the central or proximal segment of an injured neuron. • Degenerated area may extend upwards for one or

more nodes.• The degenerating area swells as substances carried into

the axon by axonal transport accumulate.• Chemical factors produced by Schwann cells near the

injury site move by retrograde transport to the cell body, carrying the information that injury has occurred.

• Degeneration my be followed by regeneration (if this part is still attached to the cell body).

REGENERATION:Regeneration of injured nerve fibres will take place in 2 cases:1. If the injury was slight and/ or away from the cell body.2. If the injured nerve fibre was part of the PNS. Cut axons in the peripheral

nervous system (PNS) can regenerate, whereas those in the central nervous system (CNS) cannot.

WHAT HAPPENS:• The nissl granules reappear & the nucleus resumes its central position.• Full recovery may take up to 3-6 months (myelination takes upto 1 year to

complete).• The endo-neural tube is formed by the Schwann cells themselves. This

tube guides the regenerating nerve fibre to its proper destination. • At the beginning of the process, axon in the central end of the cut nerve

fibres elongate & give rise to large no. of fibrils that enter into the endo-neural tube.

NO REGENERATION IN CNS:• Regeneration DOES NOT take place in optic nerve and in the

CNS due to the following reasons:1. The endoneural tubes are absent in the CNS as there are no

Schwann cells present; so the regenerating axons cannot be guided.

2. The oligodendrocytes cannot aid in regeneration as they secrete growth inhibiting factors instead of the growth promoting factors (unlike the Schwann cells).

3. The activity of the astrocytes results in the formation of scar tissue.

MULTIPLE SCLEROSISGOING, GOING, GONE…..

Multiple Sclerosis (MS) is an auto-immune disease in which nerve fibers in various locations throughout the nervous system lose their myelin. This leads to disruption of nerve impulse conduction.

Multiple Sclerosis: Cause:Exact cause is still unknown. It is suspected that a foreign agent such as a virus alters the immune system so that the antibodies of the immune system mistakenly attack and destroy Schwann cells, thus also destroying the myelin sheath. The destroyed part is replaced with scar tissue. When myelin sheath is destroyed, the nerve impulses travelling along the nerve fibers slows down. When more and more nerves are affected, the person experiences interference with functions that are controlled by the nervous system such as speech, memory, vision, walking etc.

Signs & symptoms:Patchy destruction of myelin in the CNS→ slow & abnormal conduc on of the nerve

impulses in the neurons↓

1. Visual disturbances2. Tingling & numbness

3. Muscle weakness & Fatigue4. Gradual paralysis

5. Bladder & bowel problems

Myelin destruction in MS

DIAGNOSIS & TREATMENT DIAGNOSIS

Physicians take detailed histories and perform complete physical and neurological examinations.• MRI• Nerve Conduction Studies• Cerebrospinal fluid exam (spinal tap,

lumbar puncture)

TREATMENT• Debilitating disease but not fatal. The

quality of life is affected…. Death may occur when the paralysis reaches the respiratory muscles and the person cannot breathe.

• No treatment as yet.• Only symptomatic treatment.1. Immunosuppressive therapy: as

autoimmune disease so you try to suppress the immune system to prevent further damage.

2. Corticosteroids3. Alternative therapy: as homeopathy,

ayurvedic treatment etc.