Behavioral Neuroscience, lecture on Turtle Efferent Eyeblink Pathway
Eyeblink Conditioning
VI. Neuromuscular Function
A. ACh secreted from motor neuron projecting from the accessory
principle abduscens to the pyramidalis, rectus,
and retractor bulbi muscles
B. ACh binds to Nicotinic receptors at a neuromuscular synapse
1. stumulates action potential (= end-plate potential or EPP) by opening
transmitter-gated Na+ and K+ channel (even Ca++ passes; a large
diameter channel)
a. along sarcolemma and into transverse tubules
i. opens voltage gated Ca++ channels in
sarcoplasmic reticulum
(1) binds to troponin
(a) conformational change in
tropomyosin uncovers binding
site for myosin on actin
(i) myosin binds
to actin
(ii) conformational change
(muscle contracts)
(iii) ATP allows unbinding
of actin
ii. Ca++ATPase returns Ca++ to cisternae
of sarcoplasmic reticulum
C. Useful Movement: retraction of the eyes
by the retractor bulbi muscles
1. Contractile force summates for each muscle cell
a. requires repeated EPP's (before fiber relaxes)
i. for eyeblink this is R1 + R2
ii. \requires Ca++ restoration to cisternae
iii. \contractile force depends on initial length
2. the Nervous System Grades the force of muscle contraction
a. single motor axon innervates a single muscle fiber (cell)
b. one motor neuron (many axons) innervates a number
of muscle fibers = Motor Unit
i. smallest functional unit
(1) magnitude of contractile function depends
on innervation ratio (fibers/neuron)
c. graded force is dependent upon motor unit recruitment
i. R1 + R2
d. motor units are recruited in a fixed order
i. weakest to strongest
(1) allows fine motor actions
(2) like eyeblinks
e. ñ firing rate (like R1 + R2) ñ force
i. more effective summation (see C 1)