Lower Motor Neurons

Which neurons are responsible for innervating skeletal muscle, and what part of the spinal cord do they exit?

Alpha motor neurons, which are lower motor neurons. They exit through the ventral root.

What lower motor neurons are discussed in this section?

Alpha and gamma motor neurons.

List the inputs to the alpha motor neuron.

Spinal interneurons, sensory input, and input from upper motor neurons.

What is the difference between extrafusal and intrafusal muscle?

Extrafusal is typical muscle, like sarcomeres. Intrafusal consists sensory organs, including the muscle spindle.

Describe the key details of sensory 1a neurons including the following: myelination, where they enter the spinal cord, what they synapse with, and what their channels are.

They have the thickest myelin which makes it one of the fastest neurons. They enter through the dorsal root. They synapse with interneurons and alpha motor neurons. They have mechanosensitive channels.

How does stretching lead to the depolarization of the 1a neuron?

Stretch opens the mechanosensitive channels. Sodium and calcium ions enter the cell through these channels.

Describe the knee-jerk reflex.

Tap on the patellar tendon. The quadriceps muscle briefly stretches. 1a neuron fires and synapses on an alpha motor neuron. The alpha motor neuron leads to the contraction of the quadricep.

Where are gamma motor neurons located?

Gamma motor neurons are located on both sides of the muscle spindle.

Describe the sequence that leads to the firing of the 1a sensory neuron, alpha motor neuron, and gamma motor neuron.

The muscle stretches. The 1a sensory neuron fires and synapses on the alpha motor neuron of that muscle. The muscle contracts and slack builds in the intrafusal muscle, specifically in the muscle spindle. Now that the muscle is not stretched, the 1a sensory neuron stops firing. The gamma motor neuron fires to relax the muscle and intrafusal muscle.

What is gain?

Gain is the feedback or correction of the gamma motor neuron. During normal movements, alpha and gamma motor neurons fire simultaneously. The level of gamma motor neuron activation is called gain.

Where are Golgi tendon organs located and when does it fire action potentials?

They are located in muscle tendons. When the muscle contracts, the 1b sensory neurons of the organ fires action potentials. This prevents the muscle from over contracting.

Describe the sequence of the muscle contractions between the alpha motor neurons, interneurons, and sensory 1b neurons.

Alpha motor neuron causes the muscle to contract. Sensory 1b neuron fires and synapses on an inhibitory interneuron in the spinal cord. The inhibitory interneuron fires on the alpha motor neuron ending the muscle contraction.

Describe what tetanus does to the sensory 1b neuron, and what is the consequence?

Tetanus inhibits interneuron inhibiting the spinal cord. This means the sensory 1b neuron firing does not lead to the relaxation of the muscle. This causes constant contraction.

Describe reciprocal inhibition with the following example: weight added to the hand.

Weight added to the hand leading to the stretch of the bicep. Sensory 1a neurons pick up on this stretch and fires. The sensory 1a neuron synapses on two neurons. The first is an alpha motor neuron of the bicep which causes the bicep to contract. The second is on an inhibitory interneuron. This inhibitory interneuron will synapse on the alpha motor neuron of the tricep, which is the bicep’s antagonist.

What stimulation does A-delta sensory neurons sense?

A-delta sensory neurons are nociceptive, which pick up pain.

Describe the flexor withdrawal reflex in your leg. You stepped on a pin with your right foot.

A-delta sensory neuron picks up the pain signal and relays this to the spinal cord. This synapses on spinal interneurons.

In the right (painful) leg, the sensory neuron fires on an excitatory interneuron that then fires on the alpha motor neuron of a leg flexor. It also synapses on an inhibitory interneuron that then fires on the alpha motor neuron of a leg extensor. This withdraws the pained leg.

For the left (non-painful) leg, the sensory neuron fires on an inhibitory interneuron that then fires on the alpha motor neuron of the left leg’s flexor. It also synapses on an excitatory interneuron that then fires on the alpha motor neuron of a leg extensor. This ensures the non-painful leg recovers your body weight.

What two primitive reflexes were described in class?

The root reflex at the corner of the mouth is used for breast feeding. It disappears with age.

The Babinski reflex causes a baby’s toes to fan out when external stimulation is applied to the bottom of the foot. In adults, this is replaced by the plantar reflex.