Muscles--Twitch and Control by Proprioception; Movement

I.                    Categories

A.    Smooth-internal organs, blood vessels, and glands-ANS

B.     Striated-skeletal-SNS (upper motor neuron and lower)

C.    Cardiac-ANS

II.                 Neuromuscular Junction

            A.      Innervation Ratio=Number of fibers per motor neuron as few as 3:1 to as many as 1,000’s to 9:1 lips; 2000:1 gastrocnemius (calf).

-1.   gives rise to control therefore precision. The smaller the ratio, the greater (finer) the control.

-2. A single motor neuron plus the fibers it innervates=Motor Unit

-   Excitable Membrane of fiber=Sarcolemma

3.      NMJ contains axon terminal and Motor End Plate

4.      NT=Ach, receptors=Nic (nicotinic), always excitatory

5.      Recruitment is important=how many motor units are called into action (how many a movement requires).

III.               Control By Proprioceptors

A.    Golgi Tendon Organ (GTO)-Sensory neuron

1.      embedded in tendon

2.      it’s a mechanoreceptor (stretch)

3.      activated when a muscle actively contracts

4.      sends a message to an interneuron that inhibits the motor neuron.

5.      causes a contraction to cease.

B.     Muscle Spindles – Maintains muscle tone

1.      intrafusal fibers (muscle itself is made of extrafusal fibers) + gamma M.N.

2.      Contains a sensory neuron (spiral mechanoreceptor)

3.      muscle relaxes-lengthens

4.      spindle also lengthens

5.      stretching of sensory fiber, will depolarize sensory fiber

6.      sends a burst of excitatory messages into the cord

7.      excitatory message activates motor neuron

8.      causes contraction of muscle and spindle

9.      spindle stops firing, stops the muscle contraction

10.  muscle relaxes (in other words goes back to step 3)

    IV.        Summary of muscle twitch

       1.      action potential reaches the axon terminal

       2.      Ca2+ channels open – trigger NT release

       3.      NT binds at the motor end plate, results in depolarization (Na+ channels open)

       4.      depolarization spreads along membrane and throughout the fiber via t-tubules.

       5.      depolarization reaches sarcoplasmic reticulum

       6.      Sarcoplasmic reticulum releases lg. quantities of Ca2+

       7.      Ca2+ binds to troponin, activates the tropomysin, baring binding site.

       8.      myosin head binds to actin, then tugs on it, then disengages (bcs. Of ATP, it’s dependent).

    V.        Filaments

                A.     Thick-myosin

                B.     Thin-actin

    VI        Motor Pathways

          A.         Corticospinal Tract –sometimes called pyramidal system (book refers to these as the lateral pathways)

       1.      originates in the primary motor cortex

       2.      ventral mid brain

       3.      passes into the cerebral peduncles of medulla

       4.      then passes to the medullary pyramids → info. Decussates (crosses over)

       5.      travels down cord (descends) along the dorsal lateral tract.

           B.           Extra Pyramidal Tracts (book refers to these as Ventromedial Pathways)

       1.      Vestibulospinal Tract

a.   originates in the vestibular nuclei –helps with balance, head movements, and stability of head

        2.      Tectospinal Tract

a.   tectum (superior colliculi)

b.  responsible for eye movements with or without head movements

c.   visual orienting responses

        3.      Reticulospinal

a.   reticular formation

b.  anti-gravity and postural reflexes

Your book has a very simple but excellent diagram on page 459.