3.1.1 Lower motor neurons

What is the role of upper motor neurons?

Inhibitory effect on lower motor neurons (e.g. to override a reflex)

What do alpha motor neurons innervate?

Extrafusal (regular) muscle fibres

What do gamma motor neurons innervate?

Intrafusal muscle fibres- specialised striated muscle spindles in the middle of extrafusal muscles

Where do muscle spindles of the intrafusal fibres innervate?

Middle non striated segment (detects pulling of intrafusal fibres)

What structures facilitate coactivation of both alpha and gamma motor neurons?

Spinal cord relay neurons

Name the 2 arrangements of muscle spindles at the middle of the muscle spindle.

Nuclear bag and nuclear chain

Describe how the muscle spindles maintain the ability to sense length change during extrafusal muscle contraction. (3)

• Muscle spindle bends due to reduced length —> reduced tension

• Signal sent to spinal cord

• Activation of gamma motor neurons triggers contraction of intrafusal fibres to increase tension- spindle is not slack and can detect contractile length

Where are Golgi tendon organs found and what are they responsible for? (3)

• Tendon connecting muscle to bone (usually postural muscles)

• Detect stretch in muscle (connected to tendon)

• Transmit inhibitory signals to alpha motor neurons to prevent overstretching

Using the quadriceps in the patellar reflex as an example, describe a monosynaptic reflex. (5)

• Patellar ligament stretches after being hit

• Intrafusal fibres connected to tendon sense stretch of tendon

• Intrafusal fibres send excitatory signal to spinal cord via sensory neuron

• Sensory neuron synapses with alpha motor neuron- excitatory signal triggers quadriceps contraction

• Quadriceps contraction stops further muscle/tendon stretch

Describe a disynaptic reflex. (5)

• Overstretching on muscle stretches tendon

• Golgi tendon organ sends sensory signal to dorsal spinal cord via sensory neuron

• Sensory neuron synapses with spinal cord relay neuron

• Relay neuron synapses with alpha motor neuron- inhibitory signal on alpha motor neuron

• Muscle relaxes, tension on tendon decreases

Describe a multisynaptic reflex. (3)

• Muscle spindle stimulated due to stretch

• Monosynaptic reflex triggered to excite alpha neurons to reduce stretch

• Disynaptic reflex triggered on antagonist muscle to inhibit alpha neurons to facilitate stretch of other muscle

Describe the role of long spinal reflexes.

Different segments of spinal cord are involved in reflex/postural control (e.g. if vestibular system detects off balance, feet are used to correct balance)

Describe how pain in a left limb affects posture control via double reciprocal innervation. (2)

• Ipsilateral (left)- extensor relaxes and flexor contracts to put less weight on left limb

• Contralateral (right)- extensor contracts and flexor relaxes to put more weight on right limb

Describe how poliomyelitis leads to muscle paralysis.

Selective destruction of alpha motor neurons