Lower Motor Neurons (LMNs)
Function:
Location:
Send axons out of CNS in:
Innervate:
Controlled by
Function: neurons that provide inner action to contractile elements in skeletal muscle tissue
Location: ventral horn of spinal cord gray in laminate IX of rexed & brain stem within motor nuclei of crainial nerves
Send axons out of CNS in: ventral rootless of spinal nerves- all spinal nerves include axons of LMNs & most cranial nerves all but 1,2, 8
Innervate: skeletal muscle cells
Controlled by motor areas of the brainstem and brain, segmental input(sensory info)
Supranuclear motor control areas
Influence activity of LMNs
Comprised of upper motor neurons
Upper motor neurons send axons that synapse on LMNs
Upper motor neurons
Form decending motor control pathways
Provide voluntary and involuntary control of LMNs
The activity of LMNs can be influenced by: direct sensory input at the segmental level (two places)
- In the spinal cord, somatosensory information that enters the cord segment ally can modify the activity of LMNs
- Sensory input to brain from cranial nerves can similarly activate reflex responses involving LMNs of crainial nerves
What are the Two major types of lower motor neurons
- Alpha motor neurons
2. Gamma motor neurons
Alpha motor neurons characteristics
Large multipolar neurons
Receive 1000s of synaptic inputs
Axons=group Aalpha fibre
Exit CNS with spinal and some crainial nerves all but 1,2,8
Provide innervate on to:
- extrafusal muscle fibres of skeletal muscles—> make up the bulk of our muscles
- a single alpha motor neurons and the extrafusal muscle fibres it supplies make up a motor unit
Small motor unit 1alpha motor neurons innervate tens of fibres
Large 1alpha motor neuron innervate hundreds of thousands of fibres
Gamma motor neuron characteristics
Small multipolar neurons
Receive 1000s of synaptic
Axons: Agamma fibres
Exit CNS and some spinal nerves 1,2,8
Provide motor innervate to: intrafusal muscle fibres of skeletal muscles
—>part of neuromuscular spindle alpha and gamma motor neurons are somatopically arranged within laminate IX of ventral horn
Ventral Horn in Lamina IX alpha and gamma motor neurons organization
Somatotopically arranged
- LMNs in a more lateral location within the ventral horn innervate extremity muscles (distal muscles)
-LMNs in a medial location within the ventral horn innervate trunk muscles (proximal muscles)
Spinal cord reflexes
Require specific neural structures: afferent limb (sensory) and efferent limb ( motor)
Afferent/sensory limb is provided by primary sensory neurons that enter the cord through the dorsal roots of spinal nerves.
The efferent/motor limb is supplied by lower motor neurons of lamina IX
Neuromuscular spindles
- Mechanoreceptors (proprioceptors) in skeletal muscle that monitor: amount of stretch in muscle & velocity of stretch (rate of change)
- Spindles are not evenly distributed: most dense in muscle near tendon & higher density in muscles that perform skilled movement
- Morphology: fusiform shape & capsular tissue encloses intrafusal muscle fibres & capsule of CT is attached to CT that encloses extrafusal muscle fibres attached parallel with skeletal muscle
What happens when Extrafusal fibres are stretched
Increased tension in NM spindle
Increased tension of intrafusal fibres
What happens when extrafusal fibres are contracted
Decreased tension in NM spindle
Decreased tension in intrafusal fibres
Innervation of neuromuscular spindles Alpha fibres
Group Aalpha fibres
Type 1a fibres form annulospiral endings of the muscular spindle
Group Abeta fibres
Type II fibres form flower spray ending of the spindle
These fibres activated when muscle stretches
Innervation of neuromuscular spindles by Gamma motor neurons
Gamma motor neurons control the length and tension of NM spindles (of intrafusal fibres) and determines sensitivity of the nm spindle to stretch
An increase in firing of the gamma motor neurons results in contraction of the intrafusal fibres and increases sensitivity of the neuromuscular spindle to a stretch stimulu applied to extrafusal fibres
Stretch reflex=?
= deep tendon reflex (DTR) = myotatic reflex
Stretch reflex how many neurons required and extra info
- 2 neurons required
Afferent limb- sensory neuron, type 1a and II fibres from NM spindle
Efferent limb- alpha motor neuron supplies extrafusal fibres
- Reflex initiated by a high velocity stretch of muscle
- Spindle activation and afferent impulses sent to spinal cord via type 1a and type 2 fibres
- Central processes of primary afferent enter cord and send collaterals in the ascending conscious proprioceptive pathway
- Some collaterals terminates directly on alpha motor neurons within spinal cord
- The pool of alpha motor neurons that are activated supply extrafusal muscle fibres of the homonymous muscle that was stretched
- The muscle contracts/shortens—> results in slight extension of bone