Somatic Motor System

Question 1

Muscle Spindle Components

Answer 1

-on skeletal muscles to sense stretch

Intrafusal Muscle in non-contractile regions
-Dynamic nuclear bag (1a, dynamic y)
-Static Nuclear Bag (1a, II, static y)
-Nuclear Chain (1a, II, static y)

Large Diameter myelinated sensory receptors
-central regions of fibers

Small Diameter myelinated motor endings
-innervate polar contractile regions

Question 2

Extrafusal Muscle Fibers

Answer 2

Outer skeletal muscle that generate movement by attaching to tendon
-innervated by alpha motor neuron

Question 3

Intrafusal Muscle Fibers

Answer 3

Inner skeletal muscles that form spindle
-non-contractile
-proprioceptors (length and velocity)

Question 4

Nuclear Bag Fibers

Answer 4

Type of Intrafusal Fibers
-centrally located, only 1
-large
-innervated by Group 1a fibers: detect change in length
-innervated by gamma motor at ends

Static

Dynamic

Question 5

Nuclear Chain Fibers

Answer 5

Type of intrafusal fiber
-smaller, more abundant
-length dependent
-innervated by Group 1a fibers: detect change in length
-innervated by Group 2 fibers: at ends, detect length
-innervated by gamma motor at ends

-activate alpha motor neurons

Question 6

Ia Fibers

Answer 6

-primary sensory on muscle spindle
-phasic and tonic discharges
-changes to length

Question 7

IIa

Answer 7

-secondary sensory endings
-tonic discharges
-responds to steady state length at ends of muscles
- in nuclear chain only

Question 8

Gamma Motor Neuron

Answer 8

-keeps muscle spindle fibers sensitive at different lengths by contracting intrafusal fibers sto avoid going limp

-innervate polar regions and stretch
-motor supply to intrafusal fibers

Dynamic: innervate dynamic NB, increase sensitivity of Ia

Static: innervate NB and NC, increase tonic activity both, decrease dynamic sensitivity Ia

Question 9

Alpha-Gamma Co-activation

Answer 9

Gamma: innervate polar regions and initiate stretch causing; mechanical gated channels to depolarize on intrafusal fibers

Alpha: …. contract the muscle to maintain length-tension relationship

Stretch sensed by 1a fibers < travels to SC < gamma neurons contract intrafusal to maintain sensitivity to muscle spundle < alpha motor neuron contracts muscle

Question 10

Reciprocal Inhibition

Answer 10

-1 muscle contracts and opposing muscle is inhibited

1. Intrafusal fiber senses stretch
2. Info travels to dorsal horn
3. Interneuron
   4.Travels to ventral horn
4. Opposite muscle contracts

Question 11

Golgi Tendon Organ

Answer 11

-proprioceptive structures at junction between muscle fibers and tendon
-sensitive to stretch in tendon/contraction of muscle
-tension in tendon < innervated by Ib fibers that send AP to SC < inhibits muscle

Question 12

Motor Neuron Pools

Answer 12

-clusters MNs in SC
-innervate a single muscle

Medal MN: innervate axial and proximal limb muscles, laterally, distal

Ventrally MN: innervate extensors and dorsal flexors

Question 13

Small MNs

Answer 13

-innervate slow twitch fibers
-less force
-long periods of time

Question 14

Large MNs

Answer 14

-fast twitch fibers
-large forces
-fatigue quickly

Question 15

Rate Coding

Answer 15

-MN signals amount of force exerted by muscle
-increased rate increases force (force summation)

Question 16

Size Principle of MNs

Answer 16

-smaller MNs are recruited and fire before larger
Ohm’s Law: V=IR (voltage= current x resistance)
- smaller motor units have larger excitatory potential, small current can cause an action potential faster (but the conduction is slower and smaller motor units)

Question 17

Transverse Tube

Answer 17

(T-tube)
-surrounds sarcoplasmic reticulum and propagates AP

Question 18

Sarcoplasmic Reticulum

Answer 18

-released acetylcholine
-stores Ca+

Question 19

Z Line

Answer 19

-fibrous disc at end of each sarcomere

Question 20

M line

Answer 20

-holds together fibers at sarcomere center

Question 21

Titin

Answer 21

-maintains actin/myosin positions to prevent them from being pulled apart

Question 22

Troponin

Answer 22

-calcium binding causes tropomyosin to move and uncover sites on actin

Question 23

Muscle Tone

Answer 23

Resistance to stretch of a muscle

Question 24

Muscles immobilized in shortened position

Answer 24

-lose sarcomeres

Question 25

Muscles immobilized in lengthened position

Answer 25

Gain sarcomeres

Question 26

Phasic Stretch Reflex

Answer 26

-DTR
-muscle contraction in response to quick stretch
-monosynaptic

Question 27

Cutaneous Withdrawl Reflex

Answer 27

-monosynaptic
-response to pain, withdrawal/flexion before consciously aware of pain

Question 28

Muscle Cramps

Answer 28

-painful contractions due to overstimulation of nerve tracts

Question 29

Fasciculations

Answer 29

-fast twitches of all motor fibers in a MU
-eyelash twitching

Question 30

Myoclonus

Answer 30

-brief contractions of a group of muscles

Question 31

Tremors

Answer 31

-involuntary rhythmic movements
-with movement or at rest

Question 32

Polio and post-polio syndrome (types)

Answer 32

1. normal MNs
2. death of selected MNs
3. Neuroplasticity cause giant motor units
4. Muscle fatigue and pain