Unit 3: Intro to motor systems & spinal cord reflexes pg 85 - 92 Flashcards
What is the function of the lower motor neurons (LMNS)?
Innervate contractile elements in the skeletal muscle
Serve as final common pathway neurons: final link between CNS and skeletal muscle cells
Where are lower motor neurons located?
- spinal cord ventral horn (lamina IX)
- brainstem motor nuclei of cranial nerves
Where do lower motor neurons project their axons?
• via ventral rootlets of all spinal nerves
• via (most) cranial nerves: III, IV, V, VI, VII, IX, X, XI, XII
(all except I, II, VIII)
Where are upper motor neurons located?
Supranuclear motor control areas
The axons that project from upper motor neurons form what pathway and synapse where?
axons form descending motor control
pathways and synapse on lower motor neurons
What kind of control of lower motor neurons do upper motor neurons provide?
Voluntary and involuntary control
Lower motor neurons are also affected by direct sensory input at segmental level in _____ and _____.
- Spinal cord
- Brain regions receiving input from cranial nerves
Direct sensory input from lower motor neurons in the spinal cord mediate what?
Spinal cord reflexes
Direct sensory input form lower motor neurons in the brain regions receiving input from cranial
nerves mediate what?
reflex responses
There are two types of lower motor neurons. Where are they both located?
lamina IX of
spinal cord or cranial nerve nuclei within brainstem
There are two lower motor neuron cell types. What are they?
Alpha motor neurons
Gamma motor neurons
Axons of alpha motor neurons are comprised of what group type of fibers?
A alpha
Axons of gamma motor neurons are comprised of what group type of fibers?
A gamma
Both alpha and gamma motor neurons are multipolar neurons. Which are larger?
Alpha motor neurons
Both alpha and gamma motor neurons exit the CNS with ______ and _______ nerves.
- Spinal
- Cranial
What kind of skeletal muscle fibers do alpha motor neurons (a type of LMNS) innervate?
Extrafusal muscle fibers
What makes up a motor unit?
a single α motor neuron and all extrafusal muscle fibers it innervates
In a small motor unit, 1 α motor neuron may innervate how many
fibers?
Tens
In a large motor unit, 1 α motor neuron may innervate how many
fibers?
Hundreds to thousands
Large muscles that carry out strong but crude actions, such as those in the leg, have what kind of motor units?
Large
Muscles that carry out fine motor movements, such as fingers or eyes, have what kind of motor units?
Small
What kind of skeletal muscle fibers do gamma motor neurons (a type of LMNs) innervate?
intrafusal muscle fibers of skeletal muscle
• part of the neuromuscular spindle
How are α and γ motor neurons arranged within lamina IX of ventral horn of the spinal cord?
somatotopically
• LMNs that innervate extremity muscles are located
more laterally
• LMNs that innervate trunk muscles are located
medially
Lower motor neurons that innervate extremity muscles are located where in lamina IX of the ventral horn of the spinal cord?
More laterally
Lower motor neurons that innervate trunk muscles are located where in lamina IX of the ventral horn of the spinal cord?
More medially
Neuromuscular spindles (NM spindles) are what kind of receptors?
encapsulated mechanoreceptor
Where are NM spindles found?
In skeletal muscle
What is the function of NM spindles?
Monitor amount or velocity of stretch
What is the distribution of NM spindles in skeletal muscles?
uneven; clustered near tendon
‐ more NM spindles in muscles involved in
skilled movement
What is the morphology of NM spindles?
fusiform surrounded by CT capsule attached to CT that encloses extrafusal muscle fibers (endomysium)
Spinal cord reflexes require:
- afferent limb
- efferent limb
- specialized sensory receptors
- contractile elements
What is the afferent limb of spinal cord reflexes?
Sensory input provided by 1˚ sensory
neurons → fibers enter spinal cord via dorsal roots of spinal nerves
What is the efferent limb of spinal cord reflexes?
Motor output provided by lower motor neurons → motor neuron cell bodies located in ventral horn
(lamina IX)
The CT capsule of NM spindles encloses what kind of muscle fibers?
Intrafusal muscle fibers
What are the “main” muscle fibers of skeletal muscle and comprise the mass of the muscle?
Extrafusal muscle fibers
How are extrafusal muscle fibers arranged in relation to neuromuscular spindles?
They are arranged in parallel to the
neuromuscular spindles
What happens when extrafusal muscle fibers are stretched?
Increased tension in NM spindle and increased tension of intrafusal fibers
What happens when extrafusal muscle fibers are contracted?
Decreased tension in NM spindle and decreased tension of intrafusal muscle fibers
NM spindles receive both sensory and motor innervation. Sensory innervation is via?
- Group A α (type Ia) fibers -> form annulospiral endings
* Group A β (type II) fibers -> form flower spray endings
How are fibers and endings of NM spindles activated?
when muscle is stretched -> tension in NM spindle increases -> NM spindles are activated -> endings
stimulated -> proprioceptive data produced
extrafusal muscle fibers stretched= ________ firing rate of sensory fibers
Increased
extrafusal muscle fibers contracted= _______ firing rate of sensory fibers
Decreased
NM spindles receive both sensory and motor innervation. Motor innervation is via?
γ motor neurons
γ motor neurons control length/tension of ________ within NM spindles and determine sensitivity to stretch of ________.
- intrafusal fibers
- extrafusal fibers
I.e. increased firing rate of gamma motor neurons = contraction of intrafusal fibers; increased sensitivity to stretch of extrafusal fibers
Stretch Reflex (aka deep tendon reflex; DTR) requires 2 neurons:
- ‘afferent limb’: sensory neuron (type Ia, II fibers) - - ‘efferent limb’: alpha motor neuron -> supplies extrafusal fibers
What is a monosynaptic reflex?
two neurons, one synapse as seen in stretch reflex
High velocity muscle stretch initiates _____.
Stretch Reflex (aka deep tendon reflex; DTR)
Tapping the quadriceps/infrapatellar tendon will cause what chain of events?
- high velocity muscle stretch
- spindle (sensory endings) activation
- sensory info relayed to spinal cord via Type Ia and II fibers
- some collateral processes join ascending pathways
- some collateral processes synapse on alpha motor neurons
- alpha motor neurons cause contraction of extrafusal fibers of homonymous muscle
During muscle contraction caused by deep tendon reflex, why does the contraction not last indefinitely?
Because, during the contraction, the spindle tension decreases which decreases firing of sensory fibers, loss of activation of alpha motor neurons and muscle relaxation.
What kind of receptors are Golgi tendon organs (GTOs; neurotendinous spindles)?
encapsulated proprioceptors that contain thin capsule of CT that surrounds
some collagenous fibers of the tendon
Where are Golgi tendon organs (GTOs; neurotendinous spindles) most numerous?
near attachments of tendons to muscle
Golgi tendon organs (GTOs; neurotendinous spindles) are supplied by what group of fibers?
type Ib fiber type sensory
afferents
What do Golgi tendon organs (GTOs; neurotendinous spindles) monitor in muscle-tendon units?
Tension
What happens to GTO tension during muscle contraction? What does this do to the firing of the type Ib fibers?
GTO tension increases = Increased type Ib fiber firing
UMNs are able to influence motor activity (muscle tone) by two mechanisms:
- directly synapsing on γ (gamma) motor neurons
- indirectly synapsing on α (alpha) motor neurons
What does a poly synaptic reflex involve?
More than 2 neurons and more than 1 synapse
What is a flexor reflex?
protective reflex involving withdrawal of a body part from a noxious stimulus
Interneurons of the flexor reflex will synapse on two things:
- α motor neurons of flexor muscles to stimulate them
- α motor neurons of extensor muscles to inhibit them
What is reciprocal innervation?
activation of flexor muscles, inhibition of antagonist muscles (extensors)
In the flexor reflex, how is significant muscle activation achieved?
Motor neurons at multiple cord levels must be activated via collaterals traveling to superior and inferior segments
What two pathways can the flexor reflex travel to different spinal cord levels in order to involve motor neurons at multiple levels for sufficient muscle activation in the flexor reflex?
- collaterals of Lissauer’s tract
- interneurons within fasciculus proprius (propriospinal tract)
Where is fasciculus proprius located?
immediately adjacent to spinal cord gray
The neurons that travel in fasciculus proprius are termed________.
Spinospinalis neurons
What is crossed extension/extensor reflex?
some dorsal horn neurons (interneurons) have axons that decussate and synapse on neurons in the contralateral ventral horn for activation of contralateral ventral horn neurons to induce muscle contraction of contralateral extensor muscles
When GTOs ‘fire’ due to increased tension and send sensory impulses through type Ib fibers along ascending
conscious pathways and onto interneurons in the spinal cord gray to do what?
inhibit alpha motor neurons that
supply the homonymous muscle
What is the gamma reflex loop?
“functional complex’ of structures that play an
important role in controlling muscle tone
What are the components of the gamma reflex loop?
a. γ motor neurons
b. intrafusal muscle fibers of the NM spindle
c. type Ia, II fibers
d. α motor neurons
e. extrafusal muscle fibers
Interruption of the gamma reflex loop results in:
- no paralysis
- decreased muscle tone
- loss of associated DTR
- loss of stretch reflex
- lack of strength and precision of muscle contraction
descending motor control fibers from higher centers that influence LMNs terminate on gamma motor neurons resulting in:
increased activity of γ motor neurons → increased type Ia, II fiber firing → info travels to spinal cord → increased facilitation of α motor neurons → increased tone of extrafusal fibers → increased muscle tone
