Test 123  (Physiology)

1. The following are true about an action potential in a nerve fibre: 

a. it occurs when its membrane potential is hyperpolarized

b. it is associated with a transient increase in membrane 
    permeability to sodium

c. there is a decreased in membrane permeability to 
    potassium

d. it has an amplitude which varies directly to the strength 
    of stimulus

e. it causes propagation of electrical current along the 
   nerve fibre

Answera.F  b.T  c.T  d.F  e.T
In an action potential, the membrane potential is reduced to its threshold for firing.
Increased membrane permeability to sodium ions causes rapid depolarization whereas potassium efflux from increased potassium permeability contributes to membrane repolarization. 
The impulse configuration of a given action potential obeys the all or nothing law, therefore, the action potential is independent of stimulus strength.
 

2. Increased intracranial pressure causes:

a. sixth nerve palsy 

b. cupping of the optic disc

c. absent venous pulsation

d. increased cerebral blood flow

e. increased blood pressure

Answera.T   b.F  c.T  d.F  e.T
The optic disc appears swollen and elevated. The cerebral blood flow is decreased.

 
3. Muscle tone is reduced by:

a. lower motor neurone lesion

b. curare

c. cerebellar lesion

d. lesion of the motor cortex

e. intravenous edrophonium

Answera.T   b.T  c.T  d.F  e.F
Cerebellum is involved in maintaining the muscle and lesion causes reduced muscle tone.
Upper motor lesion such as lesion of the cerebral cortex causes spasticity. 
Intravenous edrophonium inhibits the anticholinesterase in the synapses and thus increase impulses travelling through the neuromuscular junction.
.

4. Optic disc swelling occurs in:

a. optic disc drusen

b. hypotony

c. central retinal vein occlusion

d. retrobulbar neuritis

e. posterior uveitis

Answera.F  b.T  c.T  d.F  e.T
Optic disc drusen causes pseudo-papilloedema and not true swelling. Retrobulbar neuritis by definition refers to optic neuritis behind the optic nerve head and does not cause optic disc swelling.

5. Compared with myelinated nerve fibres, non-myelinated nerve 
    fibres :

a. do not contain Schwann cell sheaths

b. have a higher threshold for stimulation

c. have a longer refractory period 

d. transmit impulses at a lower frequency

e. are found only in sensory nerve fibres

Answera.F   b.T  c.T  d.T  e.F
Although unmyelinated, Schwann cell sheaths are found in non-myelinated nerve fibres. They are found in sensory fibres serving pain and temperature and also in the autonomic nerve fibres.

 
 

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