Lecture: Lower motor neuron circuits and motor control
Learning goal: You are familiar with the anatomical organization of α-motor neurons and interneurons.
Answer:
• α-Motor neurons reside in the ventral horn of the spinal cord.
o Medial pools → innervate axial/proximal muscles (posture, trunk).
o Lateral pools → innervate distal muscles (fine hand/foot control).
o Cervical segments → arms; Lumbar segments → legs.
• Local circuit interneurons (in the grey matter) integrate sensory feedback and descending commands:
o Long-range interneurons (medial) coordinate posture and rhythmic patterns.
o Short-range interneurons (lateral) refine and shape precise movements.
Learning goal: You can provide a definition for the terms motor pool and motor unit.
Answer: Motor Pool = All α-motor neurons innervating a single muscle. Motor Unit = One α-motor neuron + all the muscle fibres it innervates.
Learning goal: You can reproduce the properties of different types of motor units (slow, fast fatigue-resistant and fast-fatigable) and can describe
their role in muscle contraction.
Answer: Recruitment follows the Size Principle (small → large) as synaptic drive increases, matching force to need.
Type Fibers innervated Force Fatigue resistant Typical activity
Slow (I and red) Few Low High Posture, standing
Fast-Fatigue-Resistant (IIa) Moderate Medium Moderate Walking, jogging
Fast-Fatigable (IIb and white) Many High Low Sprinting, jumping
Physiological traits:
• Slow: small motor neuron, slow conduction, high input resistance, oxidative metabolism.
• Fast-fatigable: large motor neuron, fast conduction, low input resistance, glycolytic metabolism.
Learning goal: You can describe the neurotransmission at the motor endplate and know how the discussed toxins can affect this signal.
Answer:
1. Arrival of AP at axon terminal → Ca²⁺ influx → vesicular ACh release.
2. ACh binds nicotinic receptors on the muscle endplate → Na⁺ influx → end-plate potential (EPP).
3. If EPP reaches threshold, voltage-gated Na⁺ channels trigger a muscle AP → contraction.
Toxins:
• Competitive antagonists (e.g., α-bungarotoxin from snake venom, cobra toxins) block AChRs → paralysis.
• Botulinum toxin prevents ACh release by cleaving SNARE proteins → flaccid paralysis.
Learning goal: You can describe the cross-bridge cycle.
Answer: Within each sarcomere (actin–myosin overlap zone):
1. ATP Binding: Myosin head detaches from actin.
2. ATP Hydrolysis: Myosin “cocked” into high-energy state.
3. Weak Attachment: Myosin loosely binds a new actin site.
4. Ca²⁺ Rise (from SR) exposes binding sites → Strong Binding.
5. Power Stroke: Pi release → myosin pulls actin ~10 nm toward M-line.
6. ADP Release: completes stroke; myosin remains bound until new ATP arrives.
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, Learning goal: You can describe the stretch reflex, the inverse myotatic reflex and the flexion reflex.
Answer:
Stretch Reflex (Myotatic Reflex):
This reflex maintains muscle length and posture when muscles are unexpectedly stretched. Example: Holding a cup while it’s being filled with liquid.
• When the muscle (e.g., biceps) stretches, muscle spindles detect the lengthening.
• This activates Ia afferent fibers, which synapse monosynaptically on α-motor neurons.
• The α-motor neurons contract the same (homonymous) muscle to resist the stretch.
• Simultaneously, inhibitory interneurons suppress the activity of the antagonistic muscle (e.g., triceps), ensuring coordinated motion.
Inverse Myotatic Reflex (Golgi Tendon Reflex):
This reflex protects muscles and tendons from damage due to excessive force. Example: Holding something too heavy.
• When tension increases in a muscle, Golgi tendon organs detect it.
• This activates Ib afferents, which synapse on inhibitory interneurons.
• These interneurons inhibit the α-motor neurons of the contracting muscle → muscle relaxation.
• At the same time, excitatory interneurons may activate antagonistic muscles.
• This negative feedback loop prevents muscle or tendon injury.
Flexion Reflex (Withdrawal Reflex):
This reflex is a protective response to pain, withdrawing a limb from harmful stimuli. Example: Stepping on a sharp object.
• Nociceptors in the skin activate Aδ or C afferent fibers.
• These afferents project to the spinal cord and activate interneurons, which excite flexor α-motor neurons of the affected limb.
• Flexor muscles contract → limb is withdrawn.
• Crossed extensor reflex: Extensors of the opposite leg are activated to maintain posture and balance.
Learning goal: You can explain the role of the afferents, efferents, muscle fibres, muscle spindles and golgi tendon bodies involved.
Answer:
Afferents (Sensory Fibers):
• Ia fibers: from muscle spindles, detect rapid changes in muscle length → involved in stretch reflex.
• Ib fibers: from Golgi tendon organs, detect tension → involved in inverse myotatic reflex.
• Aδ/C fibers: from nociceptors, detect pain → involved in flexion reflex.
• II fibers: slower, signal static muscle length, involved in muscle tone/posture.
Efferents (Motor Neurons):
• α-motor neurons: innervate extrafusal muscle fibers → cause contraction of the main muscle.
• γ-motor neurons: innervate intrafusal fibers within muscle spindles → adjust spindle sensitivity during movement.
Muscle Fibres:
• Extrafusal fibers: the contractile fibers that generate force → controlled by α-motor neurons.
• Intrafusal fibers: part of the muscle spindle, do not generate force but detect stretch → controlled by γ-motor neurons.
Muscle Spindles:
• Located within muscles.
• Detect muscle length and rate of stretch.
• Contain intrafusal fibers.
• Send information via Ia (phasic) and II (tonic) afferents.
• Crucial for the stretch reflex.
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Learning goal: You are familiar with the anatomical organization of α-motor neurons and interneurons.
Answer:
• α-Motor neurons reside in the ventral horn of the spinal cord.
o Medial pools → innervate axial/proximal muscles (posture, trunk).
o Lateral pools → innervate distal muscles (fine hand/foot control).
o Cervical segments → arms; Lumbar segments → legs.
• Local circuit interneurons (in the grey matter) integrate sensory feedback and descending commands:
o Long-range interneurons (medial) coordinate posture and rhythmic patterns.
o Short-range interneurons (lateral) refine and shape precise movements.
Learning goal: You can provide a definition for the terms motor pool and motor unit.
Answer: Motor Pool = All α-motor neurons innervating a single muscle. Motor Unit = One α-motor neuron + all the muscle fibres it innervates.
Learning goal: You can reproduce the properties of different types of motor units (slow, fast fatigue-resistant and fast-fatigable) and can describe
their role in muscle contraction.
Answer: Recruitment follows the Size Principle (small → large) as synaptic drive increases, matching force to need.
Type Fibers innervated Force Fatigue resistant Typical activity
Slow (I and red) Few Low High Posture, standing
Fast-Fatigue-Resistant (IIa) Moderate Medium Moderate Walking, jogging
Fast-Fatigable (IIb and white) Many High Low Sprinting, jumping
Physiological traits:
• Slow: small motor neuron, slow conduction, high input resistance, oxidative metabolism.
• Fast-fatigable: large motor neuron, fast conduction, low input resistance, glycolytic metabolism.
Learning goal: You can describe the neurotransmission at the motor endplate and know how the discussed toxins can affect this signal.
Answer:
1. Arrival of AP at axon terminal → Ca²⁺ influx → vesicular ACh release.
2. ACh binds nicotinic receptors on the muscle endplate → Na⁺ influx → end-plate potential (EPP).
3. If EPP reaches threshold, voltage-gated Na⁺ channels trigger a muscle AP → contraction.
Toxins:
• Competitive antagonists (e.g., α-bungarotoxin from snake venom, cobra toxins) block AChRs → paralysis.
• Botulinum toxin prevents ACh release by cleaving SNARE proteins → flaccid paralysis.
Learning goal: You can describe the cross-bridge cycle.
Answer: Within each sarcomere (actin–myosin overlap zone):
1. ATP Binding: Myosin head detaches from actin.
2. ATP Hydrolysis: Myosin “cocked” into high-energy state.
3. Weak Attachment: Myosin loosely binds a new actin site.
4. Ca²⁺ Rise (from SR) exposes binding sites → Strong Binding.
5. Power Stroke: Pi release → myosin pulls actin ~10 nm toward M-line.
6. ADP Release: completes stroke; myosin remains bound until new ATP arrives.
1|Page
, Learning goal: You can describe the stretch reflex, the inverse myotatic reflex and the flexion reflex.
Answer:
Stretch Reflex (Myotatic Reflex):
This reflex maintains muscle length and posture when muscles are unexpectedly stretched. Example: Holding a cup while it’s being filled with liquid.
• When the muscle (e.g., biceps) stretches, muscle spindles detect the lengthening.
• This activates Ia afferent fibers, which synapse monosynaptically on α-motor neurons.
• The α-motor neurons contract the same (homonymous) muscle to resist the stretch.
• Simultaneously, inhibitory interneurons suppress the activity of the antagonistic muscle (e.g., triceps), ensuring coordinated motion.
Inverse Myotatic Reflex (Golgi Tendon Reflex):
This reflex protects muscles and tendons from damage due to excessive force. Example: Holding something too heavy.
• When tension increases in a muscle, Golgi tendon organs detect it.
• This activates Ib afferents, which synapse on inhibitory interneurons.
• These interneurons inhibit the α-motor neurons of the contracting muscle → muscle relaxation.
• At the same time, excitatory interneurons may activate antagonistic muscles.
• This negative feedback loop prevents muscle or tendon injury.
Flexion Reflex (Withdrawal Reflex):
This reflex is a protective response to pain, withdrawing a limb from harmful stimuli. Example: Stepping on a sharp object.
• Nociceptors in the skin activate Aδ or C afferent fibers.
• These afferents project to the spinal cord and activate interneurons, which excite flexor α-motor neurons of the affected limb.
• Flexor muscles contract → limb is withdrawn.
• Crossed extensor reflex: Extensors of the opposite leg are activated to maintain posture and balance.
Learning goal: You can explain the role of the afferents, efferents, muscle fibres, muscle spindles and golgi tendon bodies involved.
Answer:
Afferents (Sensory Fibers):
• Ia fibers: from muscle spindles, detect rapid changes in muscle length → involved in stretch reflex.
• Ib fibers: from Golgi tendon organs, detect tension → involved in inverse myotatic reflex.
• Aδ/C fibers: from nociceptors, detect pain → involved in flexion reflex.
• II fibers: slower, signal static muscle length, involved in muscle tone/posture.
Efferents (Motor Neurons):
• α-motor neurons: innervate extrafusal muscle fibers → cause contraction of the main muscle.
• γ-motor neurons: innervate intrafusal fibers within muscle spindles → adjust spindle sensitivity during movement.
Muscle Fibres:
• Extrafusal fibers: the contractile fibers that generate force → controlled by α-motor neurons.
• Intrafusal fibers: part of the muscle spindle, do not generate force but detect stretch → controlled by γ-motor neurons.
Muscle Spindles:
• Located within muscles.
• Detect muscle length and rate of stretch.
• Contain intrafusal fibers.
• Send information via Ia (phasic) and II (tonic) afferents.
• Crucial for the stretch reflex.
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