Lecture 10

Question 1

Neural structures involved in motor systems (4 subsystems, 6 total structures)

Answer 1

1) Basal Ganglia: starting & stopping
2) Cerebellum: coordinating & ongoing movement
3a) Descending → motor cortext: planning, initialting, guiding
3b) → brainstem centers: posture & automatic movement
4a) Spinal cord & brainstem: → local circuit neurons: integration & patterns
4b) → MN pools: lower motor neurons

Question 2

Motor systems path (basic)

Answer 2

(1) (2)
↓ ↓
[(3a) (3b)]
↓ ↓
[4a] [4b] →[skeletal muscles]
↑
[sensory inputs]

(see 10.2 if needed)

Question 3

Motor neuron pool, lumbar vs cervical enlargement

Answer 3

MNp: All of the MNs that innervate a single muscle

Lumbar enlargement innervates leg
Cervical enlargement innervates arm

Question 4

Lower MN pools (medial, lateral, axial)

Answer 4

Medial: proximal muscles
Lateral: distal muscles
Axial: postural trunk

(see 10.4 for diagram)

Question 5

Long distance local circuit neurons

Answer 5

Medial ventral horn
Control axial muscles
Long distance signal
Bilateral
Control position & locomotion

Question 6

Short distance local circuit neurons

Answer 6

Lateral ventral horn
Control distal muscles
Short distance signal
Unilateral
Fine Control distally

Question 7

γ (gamma) MNs

Answer 7

Small diameter
Innervate intrafusal fibers
Send info about length & △ length
Sensory receptors in capsules (intrafusal)

Question 8

α (alpha) MNs

Answer 8

Large diameter (size ∝ motor unit size & force)
Innervate extrafusal fibers
Generate force for movement & posture. striated muscles
Each neurons branches to 10-1000 muscle fibers
α MNs + its muscle fibers = MOTOR UNIT
> fibers distributed throughout muscle
> same muscle cluster & motor nucleus

Question 9

Motor units (3 types)

Answer 9

Slow (s):
fatigue resistant
sustained muscle activity (ex: posture)
high myoglobin/mitochondria

Fast fatigue-resistant (FR):
intermediate alpha MN, in between

Fast fatiguable (FF):
easily fatigued
low mitochondria
good for force (ex: running)

Question 10

Motor units and:
- fore
- size
- recruitment
- stimulation

Answer 10

SLOW FR FF
SIZE —(smaller)—————(larger)—→
FORCE ————————————–→
Recruitment —-(first)————(last)-→
___________________________________
Stimulation:
—→ (tension stimulation rate)

Force ↑z (see 10.10)

Question 11

Intrafusal fibers

Answer 11

Spindles = 8 - 10 intrafusal fibers
Small diameter
γ MNs
Stretch on extension, shorten on flexion
Chain or bag fibers –(ch.)–{(bag)}— 2x chains

Question 11

Extrafusal fibers

Answer 11

Outside spindle
Large diameter
α MNs
Produce movement

Question 12

Intrafusal stretch receptors (2 types)

Answer 12

Group 1a sensory afferents
- primary, bag & chain
- phasic (fast adapting)
- small stretches
- dynamic
- poly synaptic α MN connection + monosynaptic α MN connection (excitatory)

Group 2 sensory afferents
- secondary, mainly chain
- tonic (slow adapting)
- proportional to stretch
- static
- poly-synaptic α MN connections (excitatory)

Question 13

Intrafusal innervation (γ axons, 2 types)

Answer 13

Dynamic: dynamic bag fibers termination (1 or 2 axons)

Static: static bag & chain fiber termination (1 axon)

Question 14

Stretch reflex

Answer 14

• see knee jerk reflex
  Monosynaptic
  Inc. α MN & group 1 afferent activity

EXCITE synergistic neurons directly → contraction
INHIBITS antagonists muscles indirectly → relaxation
EXCITE & INHIBITS → reciprocal innervation

Load → length change in fiber → spindle receptors inc. afferent discharge to α MN

Question 15

Gain & tone

Answer 15

Gain: force produced in response to stretch
Tone: steady level of tension in muscles
> mediated by group- 2 afferents

Constantly modulated !
Gain regulares by γ axons & α MNs

Question 16

Golgi tendon organs (location, important for, innervated by)

Answer 16

Important for: reflex regulation of MU activity
Location: junction of muscles & tendon
Innervated by: group 1b sensory afferents

Also important: counteracts fatigue, protects muscles when large force

Question 17

GTOs:
- contraction
- connections

Answer 17

Contraction: → collagen fibrils tension inc. → compressed sensory nerve endings
> ACTIVE contraction, insensitive to passive

Connections:
1b afferents → inhibitory INs → ↓α MN activity
1b afferents → excitarory INs → ↑ α MN in opposing muscle

Negative feedback on muscle tension

Question 18

GTOs vs spindles

Answer 18

Spindles:
Rapid (tonix) during passive
Slow (phasic) during active
Monitor & maintain
LENGTH

GTOs:
Slow (phasic) during passive
Rapid (tonic) during active
Monitor & maintain
FORCE

Question 19

Flexion reflex

Answer 19

Nociceptor stimulation in foot:

IPSILATERAL
-> flexor excitation
-> extensor inhibitions

CONTRALATERAL
-> flexor inhibition
-> extensor excitation

Question 20

Rhythmic movements & locomotion

Answer 20

Controlled by local spinal cord circuits
> central pattern generators

Locomotion: stance (extend/contact) &swing (flex & lift)

Coordinated movements don’t require input from higher centers