LMNs

Question 1

Types of motor coordination (from MOST to LEAST automatic)

Answer 1

1. Reflex
2. Rhythmic/Repetitive (walking)
3. Skilled Voluntary (writing, typing)

Question 2

Cell bodies of LMNs are located where?

Answer 2

Ventral horn
OR
Cranial Nerve motor nuclei

Question 3

LMNs receive input from

Answer 3

UMNs
Sensory N
Interneurons

Question 4

Motor N Pools are located where

Answer 4

ventral horn

Question 5

Medial pools - which muscles?

Answer 5

Axial

Question 6

Lateral pools - which muscles?

Answer 6

Distal

Question 7

___ interneurons are long-distance circuits

Answer 7

Medial

Question 8

___ interneurons are short-distance circuits

Answer 8

Lateral

Question 9

This type of interneuron crosses the midline (symmetrical movements)

Answer 9

Medial

Question 10

This type of interneuron terminates unilaterally (fine control, manipulative tasks)

Answer 10

Lateral

Question 11

2 types of LMNs found in pools

Answer 11

Alpha

Gamma

Question 12

Alpha motor neurons invx ___ fibers

Answer 12

extrafusal

Question 13

Gamma motor neurons invx ___ fibers

Answer 13

intrafusal

Question 14

This type of Alpha Motor N is easily excitable, but low velocity

Answer 14

Small

Question 15

This type of Alpha Motor N is hard to excite, but fast velocity

Answer 15

Large

Question 16

Small Alpha N invx ____ muscle fibers, forming a ____ MU

Answer 16

Red (Type 1) muscle fibers

Slow (S) MU

Question 17

Large Alpha N invx ____ muscle fibers, forming a ____ MU

Answer 17

Pale (Type IIb) muscle fibers
Fast Fatigueable (FF) MU

Question 18

Intermediate Alpha N invx ____ muscle fibers, forming a ____ MU

Answer 18

Type IIa muscle fibers

Fast Fatigue Resistant (FR) MU

Question 19

Muscle force is regulated by these 2 mechanisms

Answer 19

1. Size Principle: S recruited first, then FR, then FF.

2. Frequency of APs: greater firing rate = increased force of contraction.

Question 20

Types of contractions with low to high AP frequencies

Answer 20

Single twitch (lowest)
Temporal summation
Unfused tetanus
Fused tetanus (highest)

Question 21

2 types of afferent axons in muscle spindles

Answer 21

1. Group 1a (rate of stretch)

2. Group 2 (degree of stretch)

Question 22

Gamma motor neuron function

Answer 22

When activated, intrafusal fiber contracts

Question 23

Why is co-activation of Alpha & Gamma important?

Answer 23

Allows spindle to function at all lengths.
If only alpha was stimulated, 1a afferent response decreases as muscle contracts.
When both are stimulated, 1a afferent activity continues sending info to CNS about muscle length.

Question 24

Gamma Bias/Gain

Answer 24

Gamma N activity is constantly adjusted to meet functional requirements.
Higher activity during difficult, precise, rapid movements.
Lower activity during rest.

Question 25

GTO invx by ____ afferent

Answer 25

Group 1b

Question 26

Purpose of 1b Afferents

Answer 26

Project to Inhibitory Interneurons, which decrease Alpha Motor N activity to the same muscle. Prevents muscle from contracting too much (prevents tearing).

Question 27

Flexion Reflex

Answer 27

Withdraw limb from painful stimulus. | A-Delta nociceptive N stimulated --> excites ipsi flexor & inhibits ipsi extensor

Question 28

Crossed Extension Pathway

Answer 28

Opposite reaction of Flexion Reflex to contralateral limb to maintain balance. Excites contra ext & inhibits contra flex.

Question 29

LMN Syndrome can result from damage to...

Answer 29

LMN cell body OR Their peripheral axons

Question 30

Symptoms of LMN Syndrome

Answer 30

Paralysis or Paresis Areflexia or Hyporeflexia Atrophy or loss of muscle tone Fibrillations/Fasciculations

Question 31

Examples of LMN diseases

Answer 31

Polio - viral destruction of motor Ns in ventral horn. Trauma/laceration of N (e.g. Bells Palsy). ALS (Lou Gehrigs) - neurodegenerative disease of ventral horn.

Question 32

Characteristics of ALS

Answer 32

Initially affects LMNs, eventually affects UMNs. CN motor nuclei affected too. Sensory & cognition remain intact.