Segmental Control of Lower Motor and Local Circuit Neruons

Question 1

Upper Motor Neuron

Answer 1

• contained entirely within the central nervous system (no part of the neuron leaves the central nervous system)
• sends its axon which is known as the corticospinal tract or also called the pyramidal tract, through the brainstem to cross over to the contralateral side at the juncture b/w the brianstem & spinal cord (pyramidal decussation) to synapse in the lower part of the spinal cord on an anterior horn neuron, the lower motor neuron

Question 2

Lower Motor Neuron

Answer 2

• neuron with the cell body in the central nervous system projecting its axon into the peripheral nervous system to innervate something such as a skeletal muscle fiber
• sends its axon out through the spinal peripheral nerve to reach its target organ, skeletal muscle
• located in ventral horn of gray matter

Question 3

Rexed’s Laminae

Answer 3

Layer I - VI Intermediate sensory neurons
Layers VII - VIII Local circuit, autonomic, & commisural neurons
Layer IX Lower Motor Neurons

Question 4

Corticospinal Tracts

Answer 4

• major motor fibers carrying motor instructions from higher brain centers to the local circuit and lower motor neurons
• lateral tract synapses only with ipsilateral motor neurons serving the distal extremities
• ventral tract synapses bilaterally on the medial motor neurons serving the trunk muscles

Question 5

Lower motor neurons relationship with muscle

Answer 5

1) Size of lower motor neuron bears a proportional relationship to the # of muscle fibers that it innervates (larger cell body, greater # of fibers it synapses with)
2) Trophic factor relationship with its muscle fibers, determines which of 3 types of muscle fibers it innervates

Question 6

3 Muscle Fiber Types

Answer 6

1) Slow Fatigue Resistant - smaller force, long period, small in size, rich in myoglobin & capillary beds, many mitochondria, aerobic metabolism (red meat) (TYPE I)
2) Fast Fatigue Resistant - b/w other 2
3) Fast Fatigable - large contraction, fatigue quick, lesser amounts of myoglobin, fewer mitochondria, & less dense capillary supply (white meat), few motor neuron, muscle fiber to neuron ration is very high (TYPE II)

Question 7

Alpha Motor Neurons

Answer 7

• innervate skeletal muscle fibers to form motor neurons
• in ventral horn of spinal cord: receive feedback from sensory organs within the bed of the muscle
• the afferent neurons from these sensory organs have cell bodies in the dorsal root ganglion, if they provide excitatory information to the motor neuron, the nerve has a monosynaptic connection with the motor neuron

Question 8

When afferent nerve arising from a sensory organ within a muscle bed provides inhibitory feedback to the motor neuron?

Answer 8

• an inhibitory interneuron is necessary to form an inhibitory synapse on the motor neuron
• allows an afferent nerve to provide excitatory info to some motor neurons & inhibitory info to others
• Gamma-aminobutyric acid and glycine are inhibitory

Question 9

Inhibitory connections on motor neurons all occur through?

Answer 9

-inhibitory interneurons

Question 10

Muscle Spindle

Answer 10

• sensory organ
• may be associated with a particular motor unit or several motor units
• give rise to type Ia and type II afferent nerves
• innervated by a gamma-motor neuron

Question 11

Golgi Tendon Organs

Answer 11

• give rise to type Ib afferent nerves

- sensory organs located at the semitendonous junction where muscle fibers terminate to form a tendon

Question 12

Muscle Spindle Structure

Answer 12

• situated among a group of muscle fibers
• sensory structure which generates the sensory information that triggers the muscle reflex arc
• comprised of small, intrafusal muscle fibers, contained in connective tissue sheath
• intrafusal spindle fibers lie in parallel with the extrafusal muscle fibers that make up the bulk of the muscle

Question 13

Type II Afferent Nerve Activity

Answer 13

• encodes muscle fiber length information in the frequency of its action potentials
• inc. length inc. the frequency of APs, dec. length dec. frequency of APs

Question 14

Type Ia Afferent Nerve Activity

Answer 14

• encodes length & velocity info in the frequency of its action potentials
• rapid change in muscle fiber length, the frequency of AP inc. dramatically during a rapid inc. in muscle fiber length, and dec. dramatically during a rapid dec. in muscle fiber length
• length is static: freq. of AP in the nerve reflects the length of the muscle fiber