Lecture 18 Lower Motor Neurons and Spinal Reflexes Flashcards
Clinical connection: BLANK is an autoimmune disorder in which
serum antibodies target acetylcholine receptors on the BLANK of the BLANK resulting in BLANK and
BLANK muscle weakness that improves with rest
Clinical connection: Myasthenia gravis is an autoimmune disorder in which
serum antibodies target acetylcholine receptors on the postsynaptic
membrane of theneuromuscular junction resulting in fluctuating and
fatigable muscle weakness that improves with rest
Lower motor neurons are part of the motor unit
Motor unit (definition): BLANK
- large motor units: BLANK muscle fibers per motor neuron axon; BLANK levels of force
- small motor units: BLANK muscle fibers per motor neuron axon; BLANK levels of force, BLANK contraction and BLANK motor control
- BLANK: motor units are recruited sequentially by size (small to large) during muscle contraction
Motor unit (definition): one lower motor neuron and the muscle fibers it innervates
- large motor units: 600-1000 muscle fibers per motor neuron axon; high levels of force
- small motor units: 10-100 muscle fibers per motor neuron axon; low levels of force, rapid contraction and fine motor control
- size principle: motor units are recruited sequentially by size (small to large) during muscle contraction
Lower motor neurons are part of the motor unit
Clinical connection: muscles have more than one motor unit and most
muscles are BLANK innervated; radiculopathies typically produce
BLANK rather that BLANK, depending on the proportion of motor
units lost
Clinical connection: muscles have more than one motor unit and most
muscles are multi-segmentally innervated; radiculopathies typically produce
paresis rather that total paralysis, depending on the proportion of motor
units lost
Lower motor neurons are in the spinal cord
- cell bodies in BLANK horn
- axons exit via BLANK; travel in spinal nerve, BLANK, BLANK nerves → neuromuscular junction
- cell bodies in anterior horn
- axons exit via anterior root; travel in spinal nerve, rami, peripheral nerves → neuromuscular junction
Lower motor neurons are in the spinal cord
Lower motor neurons are in the spinal cord
Lower motor neurons are in the spinal cord
- somatotopic organization of the anterior
horn:
Lower Motor Neurons(LMN) of flexor muscles posterior to
Lower Motor Neurons(LMN) of extensor muscles;
axial muscle LMNs
most medially located with distal muscle
LMNs more laterally located
- somatotopic organization of the anterior
horn:
Lower Motor Neurons(LMN) of flexor muscles posterior to
Lower Motor Neurons(LMN) of extensor muscles;
axial muscle LMNs
most medially located with distal muscle
LMNs more laterally located
Lower motor neurons_regulation of activity
large alpha motor neurons: innervate BLANK skeletal muscle fibers
- activity regulated by:
1) BLANK motor neurons (descending systems)
2) sensory feedback from muscles (type Ia sensory fibers associated with BLANK;
type Ib fibers associated with BLANK
tendon organs);
sensory fibers may synapse directly on BLANK or may influence them BLANK via interneurons
large alpha motor neurons: innervate extrafusal (force-producing) skeletal muscle fibers
- activity regulated by:
1) upper motor neurons (descending systems)
2) sensory feedback from muscles (type Ia sensory fibers associated with muscle spindles;
type Ib fibers associated with Golgi
tendon organs);
sensory fibers may synapse directly on ALPHA LMNs or may influence them indirectly via interneurons
Lower motor neurons_regulation of activity
small gamma (γ) motor neurons: innervate BLANK muscle fibers of BLAKN; regulate spindle BLANK
- distributed near BLANK motor neurons; regulated primarily by inputs from BLANK systems
small gamma (γ) motor neurons: innervate intrafusal muscle fibers of muscle spindles; regulate spindle length
- distributed near ALPHA motor neurons; regulated primarily by inputs from descending systems
BLUE:
lateral tracts primarily influence LMNs that innervate distal (mainly) BLANK muscles
lateral tracts primarily influence LMNs that innervate distal (mainly) Flexors muscles
RED: medial tracts primarily influence LMNs that innervate paravertebral, proximal limb (mainly) BLANK muscles
RED: medial tracts primarily influence LMNs that innervate paravertebral, proximal limb (mainly) extensor muscles
Two types of proprioceptors: muscle spindles and Golgi tendon organs (GTOs)
Regulation of -LMN activity by sensory feedback
large muscles with BLANK movements have BLANK spindles; small muscles with BLANK motor control
have BLANK spindles;
spindles are anchored in BLANK with the BLANK muscle fibers
large muscles with coarse movements have few spindles; small muscles with fine motor control
have many spindles;
spindles are anchored in series with the extrafusal muscle fibers
Muscle spindles are encapsulated sensory organs within muscles
- muscle spindles are composed of 6-7 small skeletal muscle (intrafusal) fibers enclosed in a connective tissue capsule anchored to the
surrounding extrafusal skeletal muscle fibers - intrafusal fibers of the spindle have a central noncontractile region innervated by sensory afferent (Ia and II) fibers and polar contractile regions innervated by gamma motor neurons
- activation of GAMAM motor neurons result in shortening of the intrafusal muscle spindle fibers in concert with the extrafusal fibers during
muscle contraction - two types of intrafusal fibers: nuclear bag and nuclear chain
Muscle spindles_nuclear bag intrafusal fiber
- nuclei clustered in BLANK region of fiber
- dynamic nuclear bag fibers innervated by
BLANK sensory neurons (central region); BLANK respond phasically to rate of change in muscle length
- innervated by dynamic gamma motor
neurons (at poles) that are activated
by BLANK (from BLANK
chain fibers)
- provide information about BLANK
dynamics, including the BLANK and
BLANK of movement
- static nuclear bag (not shown) innervated
by BLANK
- nuclei clustered in central region of fiber
- dynamic nuclear bag fibers innervated by
type Ia sensory neurons (central region); mechanoreceptors respond phasically to rate of change in muscle length - innervated by dynamic gamma motor
neurons (at poles) that are activated
by type II afferents (from nuclear
chain fibers) - provide information about limb
dynamics, including the velocity and
direction of movement - static nuclear bag (not shown) innervated
by type II sensory neurons
Ia sensory neurons directly excite BLANK motor neurons that innervate the BLANK muscle fibers of the BLANK muscle (important role in BLANK reflexes)
Ia sensory neurons directly excite alpha motor neurons that innervate the extrafusal muscle fibers of the homonymous muscle (important role in myotatic stretch reflexes)
Muscle spindles_nuclear chain intrafusal fibers
- most numerous type of intrafusal fiber
- nuclei arranged in a BLANK row
- innervated by BLANK (central region); mechanoreceptors respond BLANK to changes in muscle BLANK
- innervated by BLANK motor neurons (at poles) that are activated by BLANK fibers
-provide information about BLANK position of limbs; fire tonically in proportion to the degree of BLANK
- most numerous type of intrafusal fiber
- nuclei arranged in a single row
- innervated by type II sensory neurons (central region); mechanoreceptors respond tonically to changes in muscle length
- innervated by static gamma motor neurons (at poles) that are activated by descending fibers
-provide information about static position of limbs; fire tonically in proportion to the degree of stretch
Muscle spindles_nuclear chain intrafusal fibers
Gamma loop: BLANK sensory fibers directly excite BLANK gamma motor neurons that innervate the BLANK bag BLANK fibers of the BLANK muscle spindle; regulates BLANK of muscle spindle thus maintaining BLANK(allows continuous
monitoring of muscle activity by CNS)
Gamma loop:type II sensory fibers directly excite dynamic gamma motor neurons that innervate the nuclear bag intrafusal fibers of the homonymous muscle spindle; regulates length of muscle spindle thus maintaining spindle sensitivity (allows continuous
monitoring of muscle activity by CNS)
Gamma () motor neurons maintain spindle sensitivity
- if alpha motor neuron is activated BLANK, there is contraction of
BLANK fibers but not BLANK fibers (“unloads the spindle”; i.e., produces a BLANK spindle); in a BLANK configuration, no change in muscle BLANK can be signaled by BLANK
- if alpha motor neuron is activated alone, there is contraction of
extrafusal fibers but not intrafusal fibers (“unloads the spindle”; i.e., produces a slack spindle); in a slack configuration, no change in muscle length can be signaled by muscle spindles
- alpha-gamma coactivation is required to prevent spindles from becoming BLANK during BLANK movements; ensures
that intrafusal and extrafusal fibers are always the same BLANK; maintains spindle BLANK throughout
dynamic range
- alpha-gamma coactivation is required to prevent spindles from becoming slack during voluntary movements; ensures
that intrafusal and extrafusal fibers are always the same relative length; maintains spindle sensitivity throughout
dynamic range
Golgi tendon organs
- located in BLANK junctions; fibers intermingle with
BLANK fibers of BLANK -
BLANK in the capsule along its BLANK axis BLANK the type
BLANK sensory fibers and generates receptor BLANK
Active versus passive muscle contraction:
- Golgi tendon organs increase firing rate when BLANK is generated in the tendon by active BLANK
- passive stretching of the muscle activates BLANK, but has
little effect on BLANK
- located in myotendinous junctions; fibers intermingle with
collagen fibers of tendons -
tension in the capsule along its long axis compresses the type
Ib sensory fibers and generates receptor potentials
Active versus passive muscle contraction:
- Golgi tendon organs increase firing rate when tension is generated in the tendon by active muscle contraction
- passive stretching of the muscle activates muscle spindles, but has
little effect on Golgi tendon organs
Golgi tendon organs_the Ib reflex
- innervated by BLANK; mechanoreceptors respond to change in muscle or tendon BLANK
- synapse on BLANK interneurons in BLANK that inhibit BLANK to the BLANK muscle (feedback inhibition)
- function: modulation of BLANK
- when increased force is required, Ib afferent-mediated BLANK is turned BLANK
- allows large increases in muscle BLANK to occur without BLANK
- innervated by type Ib sensory neurons; mechanoreceptors respond to change in muscle or tendon tension
- synapse on inhibitory interneurons in anterior horn that inhibit alpha motor neurons to the homonymous muscle (feedback inhibition)
- function: modulation of force
- when increased force is required, Ib afferent-mediated inhibition is turned down, turned off , or even reversed
- allows large increases in muscle tension to occur without feedback inhibition
Reflexes_integration of sensory and motor systems
- a BLANK is an involuntary , stereotyped response to a sensory stimulus
- does not involve BLANK
- may involve as few as BLANK
two main types:
- stretch reflexes BLANK
- cutaneous reflexes BLANK
- a reflex is an involuntary , stereotyped response to a sensory stimulus
- does not involve cerebral cortex
- may involve as few as two neurons and one synapse (monosynaptic)
two main types:
- stretch reflexes (DTRs; PCMLS pathway)
- cutaneous reflexes (ALS pathway)
Stretch reflexes involve muscle spindles and GTOs
- setting of muscle spindles and the state of BLANK of BLANK and BLANK motor neurons determine the level of activity of tendon reflexes and muscle tone (responsiveness of muscle to passive stretch)
- muscle spindles are active at rest (BLANK, static GAMMA-MNs tonically BLANK); sensitive to BLANK stretch
- Golgi tendon organs are silent in BLANK and during BLANK; activated by internally generated BLANK
- muscle spindles and GTOs function collectively to monitor or calibrate BLANK
- setting of muscle spindles and the state of excitability of alpha and gamma motor neurons determine the level of activity of tendon reflexes and muscle tone (responsiveness of muscle to passive stretch)
- muscle spindles are active at rest (type II afferents, static GAMMA-MNs tonically active); sensitive to passive stretch
- Golgi tendon organs are silent in relaxed muscle and during passive stretch; activated by internally generated muscle tension
- muscle spindles and GTOs function collectively to monitor or calibrate the length and force of muscle contraction under different conditions
Spinal stretch reflex circuits_role of muscle spindles
Myotatic stretch reflex (DTR)
1. tap on tendon causes change in muscle BLANK (passive stretch)
2. stretch detected by BLANK sensory neuron → BLANK action potential firing rate
3. BLANK sensory neuron forms BLANK synapse directly on BLANK motor neuron that innervates the muscle being stretched
4. BLANK
Myotatic stretch reflex (DTR)
1. tap on tendon causes change in muscle length (passive stretch)
2. stretch detected by Ia sensory neuron → increased action potential firing rate
3. Ia sensory neuron forms excitatory synapse directly on alpha motor neuron that innervates the muscle being stretched
4. muscle contracts
Spinal stretch reflex circuits_role of muscle spindles
Reciprocal inhibition
1.BLANK sensory neuron forms BLANK synapse on BLANK interneuron
2. BLANK interneuron synapses on BLANK motor neuron to BLANK of muscle being stretched
3. inhibition of antagonists facilitates BLANK
Reciprocal inhibition
1.Ia sensory neuron forms excitatory synapse on inhibitory interneuron
2. inhibitory interneuron synapses on alpha motor neuron to antagonists of muscle being stretched
3. inhibition of antagonists facilitates agonist stretch reflex
Spinal stretch reflex circuits_role of GTOs
Autogenic inhibition
1. engaged during BLANK muscle contraction
2. BLANK sensory neuron (golgi tendon organ) responds to increased muscle BLANK by BLANK action potential firing rate
3. Ib sensory neuron synapses on BLANK interneuron
4. BLANK interneuron inhibits BLANK motor neuron to
the muscle being BLANK
5. result is BLANK muscle tension
Function: regulates the level of BLANK in the neural network under varying conditions
Autogenic inhibition
1. engaged during active muscle contraction
2. Ib sensory neuron (golgi tendon organ) responds to increased muscle tension by increasing action potential firing rate
3. Ib sensory neuron synapses on inhibitory interneuron
4. inhibitory interneuron inhibits alpha motor neuron to
the muscle being contracted
5. result is decreased muscle tension
Function: regulates the level of excitability in the neural network under varying conditions
Spinal reflex circuits_cutaneous reflexes
Triple flexion reflex (stereotyped withdrawal response)
- BLANK-mediated
- activated by BLANK cutaneous input (stimulates nociceptor); BLANK- or BLANK fibers transmit pain signal into BLANK
- pain fibers synapse on BLANK and BLANK spinal interneurons at BLANK spinal cord segments
- excitatory interneurons synapse on BLANK motor neurons that innervate (BLANK) muscles required to withdraw limb from painful stimulus
- BLANK interneurons inhibit BLANK motor neurons that innervate (BLANK) muscles that would impede withdrawal from the painful stimulus
Triple flexion reflex (stereotyped withdrawal response)
- spinally-mediated
- activated by painful cutaneous input (stimulates nociceptor); AGAM- or C fibers transmit pain signal into spinal cord
- pain fibers synapse on excitatory and inhibitory spinal interneurons at multiple spinal cord segments
- excitatory interneurons synapse on alpha motor neurons that innervate (flexor) muscles required to withdraw limb from painful stimulus
- inhibitory interneurons inhibit alpha motor neurons that innervate (antagonist; extensor) muscles that would impede withdrawal from the painful stimulus
Clinical connection: application of noxious stimulus in comatose patients useful in
distinguishing BLANK(movement of limb away from stimulus indicates intact sensory and motor connections within spinal cord and supraspinal structures) from a
BLANK response (noxious stimulus to dorsum of foot produces flexion response toward the stimulus; indicative of supraspinal lesion)
Clinical connection: application of noxious stimulus in comatose patients useful in
distinguishing purposeful withdrawal (movement of limb away from stimulus indicates intact sensory and motor connections within spinal cord and supraspinal structures) from a
stereotyped withdrawal response (noxious stimulus to dorsum of foot produces flexion response toward the stimulus; indicative of supraspinal lesion)
Spinal reflex circuits_cutaneous reflexes
Crossed extension reflex
- activated in BLANK limb in response to BLANK
- BLANK or BLANK-fibers transmit pain signal into spinal cord and synapse on BLANK interneurons that cross the BLANK to activate BLANK and BLANK
interneurons in the BLANK spinal cord
- excitatory interneurons synapse on BLANK motor neurons that generally innervate BLANK extensor muscles required to stabilize the body during withdrawal of the limb from painful stimulus
- BLANK interneurons inhibit BLANK motor neurons that innervate BLANK
flexor muscles that would destabilize the body
Crossed extension reflex
- activated in contralateral limb in response to triple flexion reflex
- AGAM or C-fibers transmit pain signal into spinal cord and synapse on excitatory interneurons that cross the midline to activate excitatory and inhibitory
interneurons in the contralateral spinal cord
- excitatory interneurons synapse on alpha motor neurons that generally innervate contralateral extensor muscles required to stabilize the body during withdrawal of the limb from painful stimulus
- inhibitory interneurons inhibit alpha motor neurons that innervate contralateral
flexor muscles that would destabilize the body
