Organisation of the Spinal Cord

Question 1

What is the usual site for a lumbar puncture?

Answer 1

L3-L4

Question 2

Where does the spinal cord end?

Answer 2

L1/L2

Question 3

How is the grey matter of the ventral horn organised?

Answer 3

Neurons innervating more proximal muscles are located more medially, and neurons innervating more distal muscles are located more lateral
There is also dorsal-ventral localisation (ventral muscles located ventrally, dorsal muscles dorsally)

Question 4

List the 5 general input areas to the final common motor pathway (motoneurons)

Answer 4

Motor areas
Non-motor areas
Basal ganglia
Cerebellum
Brain stem nuclei

Question 5

What is a motor unit?

Answer 5

Composed of a single motor neuron and all the muscle fibres it innervates

Question 6

What is a motor neuron pool?

Answer 6

Composed of all the neurons innervating a single muscle completely

Question 7

What is a muscle spindle?

Answer 7

A sensory apparatus composed of different types of intrafusal muscle fibres, and which responds to muscle stretch to communicate information about muscle length

Question 8

What is the afferent and efferent supply to a muscle spindle?

Answer 8

Group I and II afferent nerve axons

Y motor neuron axons

Question 9

What is the role of Y motor neurons in a muscle spindle?

Answer 9

Allows the spindle to contract with the muscle

Question 10

What is a Golgi tendon organ?

Answer 10

A secondary sensory structure located at the tendon where muscle joins bone, and which responds to force transmitted through the tendon

Question 11

Describe the basic composition of a Golgi tendon organ

Answer 11

A collagen elastin mesh in series with muscle fibres, innervated by an Ib afferent neuron

Question 12

Describe the neuron arc involved in a monosynaptic reflex

Answer 12

Muscle spindle is stretched and conveys afferent signals through the dorsal root to the spinal cord
Afferent neuron makes an excitatory connection with motor neurons for the muscle, and an excitatory connection with an inhibitory interneuron, which then acts on the motor neuron for the antagonist muscle

Question 13

Describe the neuron arc involved in a Golgi tendon organ reflex

Answer 13

Golgi tendon organ responds to force transmitted through the tendon and conveys afferent signals through the dorsal root to the spinal cord
Afferent neuron makes an excitatory connection with an inhibitory interneuron, which then acts on the motor neuron for the muscle; it also makes an excitatory connection with an excitatory interneuron, which acts on the motor neuron for the antagonist muscle

Question 14

Describe the neuron arc involved in a cross-extensor (bilateral) reflex

Answer 14

Nociceptive input sets off multiple interneuronal connections
A chain of excitatory interneurons activate flexors and ultimately inhibit extensors on the side of the input
A chain of excitatory interneurons activate extensors and ultimately inhibit flexors on the side contralateral to the input

Question 15

What are the propriospinal tracts?

Answer 15

Collections of nerve fibres that interconnect various levels of the spinal cord

Question 16

What is the difference between a long and a short propriospinal tract?

Answer 16

Short propriospinal tracts tend to be lateral and communicate with other spinal cord levels ipsilaterally; they control complex, dextrous movements
Long propriospinal tracts tend be closer to the midline and have lots of intersegmental interactions bilaterally; they control postural mechanisms

Question 17

How is excitation of motor neurons increased?

Answer 17

By decreasing the inhibitory input

Question 18

Do stretch reflexes test the spinal cord or the brain?

Answer 18

Both