Lower motor neurons pt. 1

Lower motor neurons

final common path for initating movement, initiates movements, directly innervate skeletal muscles

spinal cord

where are lower motor neurons located

ventral horn of spinal cord

where are cell bodies of LMN’s located?

muscle fibers in a single muscle

what do lower motor neurons innervate?

motror neuron pool

LMN groups

column

howe are LMN’s organized?

ipsilateral

is LMN ipsilateral or contralateral to the ventral horn?

medial topographic organization

innervate proximal muscles 

Lateral topographic map

innervate distal muscles

posture and locomotion

what does the medial part of the spinal cord control?

• cover segments vertically

• bilateral

what are the characteristics of medial interneurons

fine movement and lower extremities

hat does the lateral part of the spinal cord control?

• strictly local

• ipsilateral

what are characteristics of lateral interneurons

alpha motor neurons

large, innervate force generated extrafuscal fibers, posture and movement

gamma motor neurons

small, innervate intrafusal fibers(muscle spindals), regulate spindle length 

motor unit

1 motorneuron + muscle fiber targets

contraction

what does an action potential of motor neuron cause

slow (s) mototr neurons

small motor units comprising small muscle fibers that contract slowly and generate relatively small forces, fatigue resistant, many mitochondria

sustained muscular contraction (standing, maintainng upright posture

what are S motor units used for?

Fast Fatiguable motor Units

large comprising large pale muscle fibers, large amt force, easily fatigued 

brief, large force exertions (running, jumping)

what are FF used for

Fast Fatigue-resistant (FR) motor units

intermediate size, not as fast as FF, but twice force of S, resistant to fatigue

One motor neuron

each muscle fiber is innervated by…

multiple muscle fibers in same muscle 

each motor neuron can innervate 

motor units are recruited in size order

what is the size principal?

Ia & II afferents

largest and fast axons in peripheral nerves

middle region of intrafusal fibers

where are group Ia afferents located

group Ia afferents 

respond phasically to small stretches 

velocity of fiber stretch

what are Ia sensitive to

group II afferents

innervate static nuclear bag fibers and the nuclear chain

level of sustained fiber stretch

what do group II afferents signal

tonically, porportinal to stretch, dynamic senitivity 

how do group II afferents fire 

• sensor

• intgrator

• effector

what does a reflex need?

sensory neurons

what is the sensor of a spinal reflex

spinal cord interneurons

what is the integrator of a spinal cord reflex?

motor neuron/muscle

what is the effector of a spinal reflec

• myotatic/stretch

• withdrawl

• cross extensor

what are the 3 types of spinal reflexes

stretch, activate muscle spindles, fire group Ia afferent, direct synape with a-motor neuron, activate same muscle, reciprocal inhibition

what are the steps of a stretch reflex?

reciprocal innervation 

mediates the simultaneous relaxation of antagonists during contraction of agonists 

tap tendon, muscle spindle stretch, Ia afferents fire

what is the “stretch of a myotatic reflex?

myotatic reflex 

what is the reflex name for knee-jerk response 

• sensory neuron goes to spinal cord

• excites motor and interneuron

what is the signal step of a myotatic reflex?

extensor contracts (excites), reflector relaxes (inhibits interneurons)

what is the repsonse step of a myotatic reflex

kick

what is the action step of a myotatic reflex

• patellar tapped and stretch quad

• muscle spindles activate Ia afferents

• sensory neuron synaose with Ia affernt which excites motor neuron in spinal cord

• quad contracts from efferent impulse

• inhibit interneuron to relax hamstring

• kick-leg extends

what are the steps of a monosynaptic stretch?

delta motor neurons

modulate the excitability of muscle spindles

encapsulated afferent nerve endings at juntion of muscle + tendon

what is the location of GTO?

1 GTO: 1 Ib sensory axon

How are GTO’s innervated?

• muscle contraction increases tendon tension

• collagen compresses sensory endings 

what is the trigger step of GTO sensory feedback affects

Ib axons activate inhibitory interneurons (GABA)

what is the signal step of sensory feedback affects

inhibit a-motor neurons of same muscle

what is the effect step of GTO sensory feedback affects

Negative feedback (decrease activation of muscle)

what kind of circiut is GTO?

passive stretch

Both MS and GTO are activated, but MS to a larger extent

active muscle contraction

MS are unloaded and activity decreases and GTO are more and more activated

muscle length

what do muscle spindles monitor

muscle force

what do golgi tendon systems monitor?

cross-extensor reflex

connections to the motor neurons for the antagonistic muscles on the contolateral half of the body for bilateral coordination

polysynaptic

what type of reflex is a withdrawl/ flextion reflex?

CPG’s

control timing and coordination of complex patterns of movement and adjusting them in response to altered circumstances

rythmic (walking, swimming)

what activities are generated by local circuts in the spinal cord and brain stem?

LMN syndrome

damage to lower motor neuron cell bodies or peripheral axons can lead to what?

• Paralysis

• Pariesis (weakness),

• loss of reflexes (areflexia) due to interruption of the efferent motor limb of the sensorimotor reflex arcs

• loss of muscle tone

• muscle twitching (Fasiculations)

symptoms of LMN syndrome

long-term denervation and missuse

what causes atrophy?

ALS

slow degeneration of lower and upper motor neurons, progressive muscle weakness and wasting of skeletal muscles 

• glutamate-mediated excitotoxicity 

• increased oxidative stress 

• mitochondrial dysfunction 

• accumulation of toxic protein and RNA aggregates

• impaired axonal transport 

• activation of pro-inflammatory microglia

mechanisms implicated by als