Peripheral motor function

Purves et al., chapter 16, 1, 9
The Biology Department at SLU has a faculty member, Dr. Fisher, who does research on muscle
Note that the Washington University Medical School's Neuroscience Tutorial has good coverage on this topic:
Spinal motor structures

Review of some muscle physiology mostly not in book

Sliding filament - well covered in Bio 106 & cell - only reviewed here
Ca2+ binding to troponin gets tropomyosin off actin sites
myosin can bind actin, ATP unbinds - explaining rigor mortis in ATP depletion
Duchenne (and Becker) muscular dystrophy X linked
additional protein - dystrophin - also in brain axon terminals

Excitation - contraction coupling

Fig (section opener)
Axon and collaterals go to the huge NMJs of one motor unit

Here is a picture from our histology course of the neuromuscular junction.

Here is a transmission electron micrograph of a portion of a neuromuscular junction. Note the folds, increasing the area on the muscle cell. Note the space with electron density in the cleft. Note the numerous vesicles.

t-tubules get excitation to near sarcoplasmic reticulum
dyhydropyridine (blocking drug) receptor in t-tubule
homology to sodium channel - voltage sensitive
ryanodine receptor in sarcoplasmic reticulum same family as IP3 receptor
coupled with t-tubule

Nervous control of muscle

Fig. 16.6

In BL A347 (General Physiology Lab) one lab goup stimulated the forearm of subject Joel with increasing frequency and obtained this record of finger twitches using a sensitive force transducer; this was our non-invasive equivalent of a tetanus experiment.

Recall that "tetanus" was the term for the disorder caused by the clostridial toxin that cleaved synaptobrevin (vSNARE).

twitches summate (to tetanus)
Types of muscle (review) - best seen in turkey
slow, actually tonic, oxidative (and hence dark meat because of hemoglobin, myoglobin and cytochrome)
fast fatigable, phasic, glycolytic
and intermediate
It is possible to stain, in this case for ATPase, to show mixed muscle cells in a muscle (dark is slow, aerobic).
autonomic nervous system (controls smooth muscle and influences cardiac muscle)

Fig. 16.5
Motor units
In 1932, Sir Charles Sherrington won the Nobel Prize. He originated our understanding of the motor unit..

(see also Fig (section opener))
One spinal motor neuron connects to several muscle cells scattered through muscle
How many cells innervated depends on how fine vs gross the muscle's control:
13 muscle cells per nerve in extraocular muscle
1730 in calf

Fig. 16.2BC
Motor unit pool - motor units to one muscle.
Spinal motor neuron cell bodies are labeled by injection of marker into the muscle (soleus vs gastrocnemius)

Also (this is a different point) each motor neuron innervates only one type (white meat, dark meat) of muscle.

Reflex

Fig. 16.10 B
Stretch reflex - simplest behavior
Ia sensory -> alpha motor neuron -> muscle
alpha motor neuron to striated muscle
gamma motor neuron to intrafusal muscle (fusimotor system) to preset stretch on stretch receptor

Fig. 1.7 A,B

Fig 9.7A
This pathway was also described for proprioception and in introduction
With inhibitory interneuron, there is an inhibition of the antagonistic muscle

Complex behaviors
Up to and beyond fixed action pattersn (FAPs)
built up from complex of reflexes - with many other influences
Sir Charles Sherrington Integrative action of the nervous system (1906)
1932 Nobel Prize (with Adrian) "discoveries regarding the function of neurons"

Fig. 16.14
how this integrates in spinal cord
crossed extensor reflex

Fig. 16.12
Golgi tendon organ

Fig. 16.13
via Ib (slower than Ia) acts through inhibitory interneuron
to mediate the clasp-knife reflex -give up if stretch is too strong

More on the anatomy of the spinal cord

Fig. 16.3
"mototopic" organization
axial (proximal) vs distal muscles - medial vs lateral
flexors vs extensors - dorsal vs ventral

Fig3. A2 & A5A (appendix)
cervical vs. lumbar enlargements
- for all the extra motor neurons for the arms vs legs respectively

ALS

Amyoropic Lateral Sclerosis (ALS) Box 16.D
Lou Gehrig's disease - he died in 1941 after playing baseball for the New York Yankees (retired in 1939) and (until recently) holding the record for consecutive games played
a familial variety is on chromosome 21 and codes for copper/zinc superoxide dismutase (SOD)

Recent paper

P. Aebischer & A.C. Kato, Playing defense against Lou Gehrig's Disease, Scientific American, November 2007, pp. 86-93
Cells die from cell terminal back in to cell body
Interesting that bladder and eye movement spared
Most die in a few years, Physicist Stephen Hawkins lived 4 decades

Exam questions from 2005 - 2007 relating to this outline

What cells degenerate in ALS (amyotrophic lateral sclerosis, Lou Gehrig's disease)?

spinal motor neurons

Relative to adjacent areas, what would be more plentiful in the cervical enlargement?

spinal motor neurons

In the ventral horn of the spinal cord, where are motor neurons controlling the hand relative to those controlling the shoulders?

lateral

Where is the cell body for the stretch receptor involved in the monosynaptic reflex?

dorsal root ganglion

What are gamma motor neurons used for?

to preset stretch in intrafusal muscle fibers

The sensitivity for the reflex can be preset by the fusimotor system. What type of nerve and what type of muscle are used in this efferent system?

gamma motor neuron to intrafusal system

What muscles are excited and inhibited in the crossed-extensor reflex?

ipsi + flexor to withdraw, - extensor, contra + extensor to support and - flexor

Consecutive muscle twitches that come close enough together in time fuse to a steady and stronger contraction. What is this called?

tetanus

One motor neuron connects to quite a few muscle cells. What is this called?

motor unit

There's a muscle protein called "dystrophin." Why did they give it that name?

it is the product of a gene that, if mutant, leads to Duchenne (or Becker) muscular dystrophy

Name a Ca2+ channel relevant to excitation-contraction coupling.

dihydropyridine receptor and ryanodine receptor (also synaptic calcium channel)

I compared 13 for extraocular muscle and 1730 for gastrocnemius. 13 and 1730 what?

muscle cells per neuron in a motor unit

In the knee-jerk reflex, the flexor is inhibited. How?

there is an inhibitory interneuron


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