Test 159  (Ocular anatomy )

1. The following are true about the extraocular muscle fibres:

a. they are poorly fasciculated

b. they are all supplied by single motor end plates

c. they contain a higher ratio of nerve axons to muscle 
    fibres compared with the skeletal muscles

d. the outer (orbital) fibres have a larger diameter than
    the inner (global) fibres.

e. in the orbital zone, the majority of fibres are of the
    fibrillenstruktur type
 

Answera.T   b.T   c.T    d.F  e.T
Compared with skeletal muscle, extraocular muscle fibres have a higher ratio of nerve axons to muscle fibres (1:5 in extraocular muscles vs 1:100 in skeletal muscle). There are two types of fibres in the extraocular muscles: the fast twitch type (fibrillenstruktur) which are supplied by single motor end plates and tonic type (felderstruktur) which are innervated by multiple endings. The inner (global) fibres have a larger diameter than the outer (orbital) fibres.
 
 

2. Felderstruktur fibre:

a. produces tonic contraction in response to acetylcholine

b. contains poorly defined myofibrils

c. contains straight Z-line

d. contains grapelike nerve endings

e. has no M-line
 

Answera.T   b.T  c.F  d.T  e.T
Felderstruktur fibre contains zig-zag Z-line and has multiple grape-like nerve ending.
 
 

3. Fibrillenstruktur fibre:

a. is rich in mitochondria 

b. has well developed sarcomere

c. contains granular synaptic vesicles

d. contains centrally located nuclei

e. is absent in the global zone of the extraocular muscle
 

Answer
a.F   b.T   c.F   d.F  e.F
Fibrillenstruktur fibre is fast twitch and rich in mitochondria. It is found in higher concentration in the orbital zone than the globe zone. The synaptic vesicles are agranular and the nuclei are located peripherally.
 

4. The following are true about the eyelid:

a. its skin is the thinnest in the body

b. the palpebral aperture can be widened by an 
    additional 2 mm through the use of frontalis.

c. the upper tarsus is about three time higher than the 
    lower tarsus

d. the muscle of Riolan is part of the orbicularis muscle

e. facial nerve palsy causes upper lid retraction

Answer
a.T   b.T  c.T  d.T  e.T
The tarsal plate is 11mm in height in the upper lid compared with 4 mm in the lower lid.
The unopposed action of levator muscle due to paralysis of orbicularis oculi in facial nerve palsy causes upper lid retraction.
 
 

5. Muller cells:

a. are glial cells

b. extend vertically from the inner limiting membrane to 
    the outer plexiform layer

c. have nuclei in the inner nuclear layer

d. contain proteins that bind retinaldehyde

e. produce insulin and growth hormone
 

Answer
a.T   b.F  c.T   d.T  e.T
Muller cells extend vertically from the inner to the external limiting membrane.

 

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