Physiology of the spinal cord

Question 1

List the physiological functions of the spinal cord

Answer 1

Initial processing of somatosensory input by the CNS

Final processing of the motor output in the CNS

Question 2

What is sensation?

Answer 2

Detection by receptors

Question 3

What is perception?

Answer 3

Interpretation by spinal cord and brain circuits

Question 4

What are receptors?

Answer 4

Neurons specialised in the transduction of energy generated by external stimuli

Question 5

How do neurons in the sensory system signal events?

Answer 5

By rate, spatial and temporal codes

Question 6

Receptors are specific for…

Answer 6

A narrow range of input

Question 7

What receptor type detects touch, pressure and vibration?

Answer 7

Mechanoreceptors

Bare nerve endings

Question 8

What is the afferent fibre type and conduction velocity of mechanoreceptors?

Answer 8

Abeta

Wide diameter and fast

Question 9

What is the afferent fibre type and conduction velocity of bare nerve endings?

Answer 9

Adelta

Medium diameter and speed

Question 10

What are the receptor types that detect pain?

Answer 10

Bare nerve endings - fast pricking pain

Bare nerve endings - slow burning pain and itch

Question 11

What is the afferent fibre type and conduction velocity of fast pricking pain bare nerve endings?

Answer 11

Adelta

Medium diameter and speed

Question 12

What is the afferent fibre type and conduction velocity of slow burning pain and itch bare nerve endings?

Answer 12

C fibres
Thin diameter
Slow

Question 13

Describe some mechanisms of sensory stimulus discrimination

Answer 13

Different types of receptors for the same modality
Spatial distribution of receptors
windows of response intensity

Question 14

Describe the spatial distribution of receptors

Answer 14

The higher the density of mechanoreceptors the smaller the distance at which two tactile stimuli can be discriminated

The density of mechanoreceptors is much greater on the hand and face than elsewhere, allowing the detection of stimuli at a much greater spatial resolution.

Question 15

Name a mechanoreceptor

Answer 15

Pacinian corpuscle

Question 16

What is the motor system?

Answer 16

Our muscles and the neurons that command them. This is the system that actually gives rise to behaviour.

Question 17

What are the lower motor neurons?

Answer 17

Final common path for all signals (neuronal information) from the CNS to skeletal muscles (“keys on a piano”)

Question 18

How are lower motor neurons organized?

Answer 18

Collected in longitudinally organized columns

Question 19

What does each column contain?

Answer 19

Each column contains the larger, alpha (thick axon, high conductance velocity) and smaller gamma (thin axon, low conductance velocity), motor neurons to one muscle (or a few functionally similar muscles.

Question 20

Where does each column extend?

Answer 20

Through more than one segment of the cord

Question 21

What does each muscle receive?

Answer 21

Motor fibres through more than one ventral root and spinal nerve

Question 22

What will destruction of a single ventral root or single spinal nerve lead to?

Answer 22

Will not produce paralysis, only weakness (paresis)

Question 23

What is the motor unit?

Answer 23

A single alpha motor neuron and the muscle fibres it innervates
Each motor neuron synapses with multiple motor fibres.

Question 24

What are the simplest reflexes based on?

Answer 24

Interaction between a proprioceptive sensory input and a motor unit

Question 25

Name some proprioceptive sensory organs

Answer 25

Muscle spindles - negative feedback regulation of muscle length Golgi tendon organs- negative feedback regulation of muscle tension

Question 26

Describe stretch reflex circuitry

Answer 26

(A) Muscle spindle: the sensory receptor that initiates the reflex. (B) Stretch  increased Ia afferent activity  increased α motor neuron activity leading to contraction of the same muscle. (Ia afferents also excite motor neurons that innervate synergistic muscles, and inhibit motor neurons of the antagonist muscles). (C) Stretch reflex: negative feedback loop to regulate muscle length. ‘Desired’ length is preset by descending pathways.

Question 27

What is the Stretch reflex circuitry an example of?

Answer 27

a monosynaptic reflex pathway

Question 28

What is spinal cord circuitry responsible for?

Answer 28

Flexion reflex

Question 29

What is the flexion reflex an example of?

Answer 29

Polysynaptic reflex pathway

Question 30

Describe the flexion reflex pathway?

Answer 30

Stimulation of cutaneous pain receptors in the foot leads to activation of spinal cord local circuits that withdraw (flex) the stimulated extremity and extend the other extremity to provide compensatory support.

Question 31

Where are motor neurons innervating axial musculature located?

Answer 31

Medially

Question 32

Where are motor neurons innervating distal musculature located?

Answer 32

Most laterally

Question 33

Describe anterior cord syndrome

Answer 33

Bilateral lower motor neuron paralysis and muscular atrophy in the segment of the lesion (due to damage to lower motor neurons). Bilateral spastic paralysis below the level of the lesion (due to loss of anterior descending tracts). Bilateral loss of pain, temperature and light touch sensations below the level of the lesion (due to loss of anterior and lateral spinothalamic tracts). Tactile discrimination and vibratory and proprioceptive sensations are preserved because the posterior (dorsal) white columns on both sides are undamaged.

Question 34

Describe Brown – Séquard or cord hemisection syndrome.

Answer 34

Ipsilateral lower motor neuron paralysis and muscular atrophy in the segment of the lesion (due to damage to lower motor neurons). Ipsilateral spastic paralysis below the level of the lesion (due to loss of anterior descending tracts). Ipsilateral band of cutaneous anesthesia in the segment of the lesion (due to loss of dorsal root). Ipsilateral loss of tactile discrimination and of vibratory and proprioceptive sensations below the level of the lesion (due to loss of ascending tracts in the dorsal white column on the side of the lesion). Contralateral loss of pain, temperature and light touch (due to loss of crossed lateral spinothalamic tracts on the side of the lesion). Discriminative touch pathways travelling in the ascending tracts in the contralateral dorsal white column remain intact.

Question 35

Describe complete cord transection syndrome

Answer 35

Complete loss of sensation and voluntary movement below the level of the lesion. Bilateral lower motor neuron paralysis and muscular atrophy in the segment of the lesion. Bilateral spastic paralysis below the level of the lesion (due to loss of descending tracts). Bilateral loss of all sensations below the level of the lesion (due to loss of ascending tracts). Bladder and bowel functions no longer under voluntary control (due to loss of descending autonomic fibres)