Lecture 13: Motor neurons, units, cortex

Elasticity

Returns to its original resting state after being stretched. [Muscle fiber characteristic]

Extensibility

Capable of being stretched. [Muscle fiber characteristic]

Responsiveness (excitability)

Responsiveness to chemical signals, stretch, and electrical charges across plasma membrane. [Muscle fiber characteristic]

Conductivity

Local electrical change triggers wave of excitation that travels along the muscle fiber. [Muscle fiber characteristic]

Pontine reticulospinal

________: Ventromedial pathway; enhances antigravity reflexes.

Rubrospinal

________: Lateral pathway; [what it controls]

Characteristics of muscle fibers

Responsiveness (excitability), conductivity, contractibility, extensibility, elasticity

Size principle

Neurons with a smaller motor unit usually have smaller axon diameters and cell bodies (i.e

Motor unit

A motor neuron and the muscle it innervates

Lateral pathways (how many and what are they?)

(2) Corticospinal, rubrospinal

Ventromedial pathways (how many and what are they?)

(4) Vestibulospinal, Tectospinal, Pontine reticulospinal, Medullary reticulospinal

What controls the lateral pathways? What do they control??

○ Corticospinal & Rubrospinal
○ Under direct cortical control
○ Control voluntary movement

Lesion of lateral pathways

Lesion of ____ pathways:
Could not move shoulders, elbows, wrists, and fingers independently.
Could sit or stand with normal posture.

What controls the ventromedial pathways? What do they control?

○ Vestibulospinal, Tectospinal, Pontine reticulospinal, Medullary reticulospinal
○ Under brain stem control
○ Control posture & locomotion balance

Pontine reticulospinal

○ Ventromedial pathway
○ enhances antigravity reflexes

Medullary reticulospinal

○ Ventromedial pathway
○ liberates antigravity muscle from reflex control

Area 4

Primary motor cortex, M1

Area 6

"higher" motor area
Actions converted into signals specifying how actions will be performed

Lateral region of area 6

premotor area (PMA)

Medial region of area 6

supplementary motor area (SMA)

Area 5

inputs from areas 3, 1, and 2

Area 7

inputs from higher order visual cortical areas such as MT

Anterior frontal lobes

abstract thought, decision making, and anticipating consequences of action

Alpha motor neurons

○ innervate extrafusal muscle fibers
○ control muscle contraction
○ involved in voluntary movement

Gamma motor neurons

○ innervate intrafusal muscle fibers
○ control external forces acting on the muscle
○ proprioception