Spinal Cord

Question 1

the final common path

Answer 1

motoneuron in the spinal cord

Question 2

Where do nociceptors synapse in the SC?
where do cutaneous mechanoreceptors synapse?
where do Ia afferent proprioceptors (muscle spindle) synapse?
where do Ib and II afferent (Golgi Tendon Organ) synapse?

Answer 2

LI - LII
LIII -LIV
LV, LVI, LVII, LIX
LV, LVI, LVII

Question 3

spinal cord histology

Answer 3

10 laminae (gray matter)
dorsal horn: I - V
ventral horn: VII - IX
dorsal column
ventral funiculus
lateral funiculus

dorsal: sensory in
ventral: motor out

Question 4

define motor unit and its innervation ratio

Answer 4

motoneuron + all muscle fibers innervated

fine movement = low innervation ratio
strength movement = high innervation ratio

Question 5

what is the typical organization of motor pools in SC?

Answer 5

motor pools are organized in columns

proximal muscle > ventral in SC
distal muscle > dorsal in SC
flexors > lateral
extensors > medial

Question 6

what is the difference between alpha and gamma motoneuron?

Answer 6

alpha motoneuron innervates extrafusal muscle fibers (GTO), heavily myelinated, initiate contraction

gamma motoneurons innervate intrafusal muscle fibers (muscle spindles), maintain spindle tone during contraction, small, thinly myelinated

alpha/gamma co-activation during voluntary movement; maintenance of spindle sensitivity during contraction

Question 7

types of motor unit

Answer 7

slow (S): contraction slow, strength low, no fatigue
fast (FF): contraction fast, strength high, fast fatigue
fast (FR): contraction intermediate, strength intermediate, resistant fatigue

check the slides for the shape of recordings

Question 8

describe motor unit recruitment

Answer 8

1. non-fatiguing slow muscle fibers (small motor units) get recruited first, produce little force
2. fast fatiguing muscles get recruited last. Produce a lot of force but get fatigued.
3. firing rate is modulated (up to 22 HZ b/c that is the freq of muscle contraction)

Question 9

why do smaller muscles get recruited first?

Answer 9

big and small fiber innervated by the same neuron. Small fiber has large input resistance, so it reaches a voltage threshold easily according to Ohm’s law. Large fiber has small input resistance so it takes a lot of synaptic current to reach a voltage threshold.

Question 10

describe recruitment gain

Answer 10

1. sharpening of recruitment curve
2. bias introduced by sensory system
3. sensory system can excite the larger motor neuron and inhibit the smaller one

check the slide for the graph (slope = gain)

Question 11

define motor pool

Answer 11

all motoneurons innervating a single muscle

Question 12

types of muscle fibers

Answer 12

I: motor unit S, myosin ATPase low, myoglobin high, mitochondria many, ATPase slow
IIb: motor unit FF, myosin ATPase high, myoglobin low, mitochondria low, ATPase many
IIa: motor unit FR, myosin ATPase high, myoglobin high, mitochondria many, ATPase fast

Question 13

“the force”

Answer 13

innervation ratio: FF > FR > S
force exerted by the MHC: FF > FR > S
cross section of individual muscle fibers: FF > FR > S

Question 14

two ways of regulating muscle force

Answer 14

1. motor unit recruitment
   meaning of size principles:
   simplifies modulation of force by higher centers;
   saves energy, as only S-type units are recruited during postural tasks and low energy motor tasks like walking
2. modulation of discharge frequency