Lower motor neurons

lower motor neurons definition

spinal ventral horn motor neurons and motor neurons located in cranial nerve somatic motor nuclei whose axons innervate skeletal muscle

activate skeletal muscle - final motor pathway

What are the two types of LMNs and what are the functional differences between them?

alpha motor neurons + gamma motor neurons

alpha motor neurons innervate

extrafusal fibers: tension generating fibers of skeletal muscle

In the AM (alpha motor) sometimes I feel TIGHT (tension/contraction) so i do a little EXTRA (extrafusal) stretching

gamma motor neurons innervate

intrafusal fibers: special type of skeletal muscle found within the muscle spindle (polar ends)

remember what muscle spindles do = detect changed in muscle length

interneurons

axons distribute locally in the spinal gray matter - regulate the activity of alpha and gamma motor neurons;

they can excite or inhibit these motor neurons

LMN pic

How are the local circuit neurons & LMNs topographically organized in the spinal cord?

orderly arrangement between LMN pools and the muscles they supply in the spinal cord

longitudinal arrangement

LMNs for

upper limb: located in the cervical enlargement of spinal cord

trunk: located in thoracic regions

lower limb: lumbar enlargement

Organization of LMNs in the Spinal Gray Matter

flexors are located ____ in the spinal cord

posterior

proximal muscles are located ___ in spinal cord

medial

extensors are located ______ in the spinal cord

anterior

distal muscles are located _____ in spinal cord

lateral

Organization of LMNs in the Spinal Gray Matter pic

medially located local circuit neurons project to ______ spinal segments and cross the midline ____ and communicate with ____ located LMNs for ____

multiple; bilaterally; medially; postural control

laterally located local circuit neurons project to ______ spinal segments and most are _____ and they communicate with ____ located LMNs for the control of _____

fewer; ipsilateral; laterally; skilled limb movement

What is the functional significance of this organization?

This organization allows for efficient control: medial areas manage posture and balance, while lateral areas specialize in skilled, voluntary movements of the limbs

idk CHECKKKKK

where are LMNs located in the brainstem?

in the motor cranial nerve nuclei

axons of LMN will contact skeletal muscle at specialized synapse called

motor end plate or neuromuscular junction

Be able to describe the NMJ and the synaptic transmission at this junction.

1:1 ratio of firing of motor neuron action potential and activation of muscle fiber

alpha motor neuron synapses with motor end plate

1. at active zones there are a large number of docking proteins and acetylcholine located in close proximity to voltage gated calcium channels

2. at subjunctional folds there are clusters of nicotinic acetylcholine receptors which are ionotropic receptors selective for cations

3. activation of the receptor causes influx of Na+ which depolarizes the muscle membrane (quick cross-bridge cycling)

4. acetylcholine is removed from synaptic space by enzymatic breakdown by acetylcholinesterase

NMJ and the synaptic transmission at this junction simple pic

What causes muscular fatigue and weakness in myasthenia gravis?

In myasthenia gravis, the body makes antibodies that block or destroy the acetylcholine receptors at the neuromuscular junction.

Antibodies block ACh receptors → fewer signals reach the muscle → muscle gets weak and tired quickly.

IDK CHECKKK

What is a motor unit and what types of motor units do skeletal muscles contain?

answers after this

motor unit

is the single alpha motor neuron and the muscle fibers it innervates

small motor units

small muscles that generate lower levels of force

large motor units

large muscles that generate higher levels of force

type I motor unit

slow twitch: generate lower levels of tension but for longer periods of time (fatigue resistant) - postural control muscles

type II motor unit

generate higher levels of force but for shorter periods of time

-- IIa: fast twitch oxidative

-- IIb: fast twitch glycolytic

size principle - muscle contractions

grading force and muscle contractions motor unit

smaller motor units are recruited first followed by larger units

rate code - muscle contractions

grading force and muscle contractions motor unit

as the need for greater force and speed increases, synaptic input increases and the firing rate of neurons increase

How do alpha motor neurons encode the force of muscle contraction?

rate coding IDK CHECK

What is the role of peripheral sensory input in motor control?

activity of LMN influenced by what two sources ONE of them being peripheral sensory input

Peripheral sensory input provides feedback to the nervous system about body position, movement, and environment, helping to adjust and guide motor actions for coordination, balance, and accuracy.

IDK CHECKKK

reflex definition

is an automatic, involuntary response to a stimulus, that happens without conscious control. IDK CHECK

reflex pathways can produce

complex, coordinated activity at multiple joints

reflexive movements are ____ but descending input can

involuntary; alter the threshold for their activation or their gain (strength of response)

example: when think a plate is hot you pull your hand, or when a plate is hot and you carry it to the table instead of following instinct to drop it

muscle tone definition

steady level of tension in muscles - felt as a resistance of the muscle to passive stretch

function of the muscle spindle

proprioceptors that monitor muscle length and mediate monosynaptic stretch reflexes

structure of a muscle spindle

group 1a fibers = afferent endings that respond to small stretches and depend on velocity

group 2 fibers = afferent endings that respond to sustained stretch

intrafusal fibers of muscle spindle

nuclear bag and nuclear chain fiber innervated by gamma motor neurons (efferent)

sensory endings of muscle spindle

group Ia afferents: responsive to small, phasic stretches; velocity dependent

group II afferents: responsive to sustained stretch

What is alpha-gamma co-activation, and why is it important?

during voluntary movement both alpha and gamma motor neurons will be activated simultaneously

activation of the alpha motor neuron results in muscle fiber contraction

activation of the gamma motor neuron maintains the sensitivity of the muscle spindle during ongoing movement by keeping it tense enough to sense change (allows the system to provide feedback regarding muscle length during movement)

What is gamma bias?

(gain) amount of force generated in response to a muscle fiber stretch

*bias is modulated by UMN's to meet functional demands

high gain

small amount of stretch produces large increase in # of motor units recruited and increased firing rate resulting in large increases in tension

low gain

greater stretch needed to produce the same level of tension (experience during yoga)

What is the gamma loop and what happens to this loop when descending control is lost?

activation of gamma motor neurons can indirectly cause activation of an alpha motor neuron there by mediating muscle contraction

if descending control is lost = HYPERTONUS (muscle stiffness) + AREFLEXIA (diminished/absent reflexes)

Review the major brainstem reflex circuits covered in lecture.

Jaw jerk circuit

Corneal circuit

Pharyngeal circuit

jaw jerk reflex circuit

afferent limb: proprioceptive fibers of the mandibular division of the trigeminal nerve (mesencephalic nucleus)

efferent limb: motor fibers from the trigeminal motor nucleus traveling in the mandibular division of the trigeminal nerve

what CN does the jaw jerk reflex involve

CN V

jaw jerk reflex pic

corneal reflex circuit

afferent limb: nociceptive fibers of the opthalmic division of the trigeminal nerve project to the spinal trigeminal nucelus (pain)

2nd order neurons of spinal trigeminal nucleus project to facial motor nuclei

efferent limb: motor fibers from the facial motor nuclei travelling in the facial nerves project to orbicularis oculi muscle

what CN are involved in the corneal reflex

CN V and CN VII

corneal reflex pic

pharyngeal reflex circuit

afferent limb: nociceptive fibers of glossopharyngeal and vagus nerves project to the spinal trigeminal nucleus

2nd order neurons of spinal trigeminal project to the nucleus ambiguus bilaterally

efferent limb: motor fibers from nucleus ambiguus traveling in the glossopharyngeal and vagus nerve project to posterior oral and pharyngeal muscle

what CN are involved in the pharyngeal reflex

CN IX and X

what is the purpose of the pharyngeal reflex

gag reflex, prevent choking

the corneal reflex and pharyngeal reflex are _____ and ______

direct and consensual

pharyngeal reflex pic

are lower motor signs seen on the ipsilateral or contralateral side?

ipsilateral

lower motor neuron signs

damage to the final motor pathway will cause some combination of the following

-weakness and eventual muscle atrophy

-fibrillations or fasciculations: spontaneous twitches due to -involuntary contractions of one motor unit (fibrillation) or group of motor units (fasciculation)

-hypotonia: decreased tone in skeletal muscles

-areflexia or hyporeflexia: lack of reflex activity or weak reflex activity

lower motor signs pic

f you have a lesion of a CN nuclei or a peripheral nerve (cranial or spinal), would the deficitsbe ipsilateral or contralateral to the lesion?

ipsilateral (same side as the lesion)

Because LMNs go directly from the brainstem or spinal cord to the muscles on the same side of the body or face. If the LMN is damaged, the muscles it serves on that same side are affected