Physiology of the spinal cord

Question 1

What are the physiological functions of the spinal cord?

Answer 1

• Initial processing of somatosensory input by the CNS

- Final processing of motor output by the CNS

Question 2

Describe spinal nerves

Answer 2

Highways of both somatosensory information traffic to the spinal cord (afferent pathways) and motor information traffic from the spinal cord to the muscles (efferent pathways)

Question 3

Sensation

Answer 3

Detection by receptors

Question 4

Perception

Answer 4

Interpretation by spinal cord and brain circuits

Question 5

Receptors

Answer 5

neurons specialised in the transduction of energy generated by external stimuli
Receptors are specific for a narrow range of input

Question 6

Neurones in sensory systems signal events by

Answer 6

combination of rate, spatial and temporal codes

Question 7

When is low convergence needed?

Answer 7

for high spatial resolution

e.g. cones and bipolar cells in retina

Question 8

When is high convergence needed?

Answer 8

for detecting weak signals e.g. rods and bipolar cells in the retina

Question 9

When is input used for complex or multiple functions?

Answer 9

high divergence (e.g., vestibulo-ocular reflex).

Question 10

What determines the stimulus specificity of a receptor?

Answer 10

Adequate stimulus: lowest response threshold -> stimulus specificity e.g. light receptors have the lowest threshold to light, so makes them light receptor. Their stimulus specificity and adequate stimulus needed is determined by the lowest response threshold they have.

Question 11

Describe different types of sensory receptors in the skin

Answer 11

Meissner's corpuscles - sheer force
Merkel disk - contact
Ruffini's corpuscle - tension, folding
Pacinian corpuscle - deep pressure, vibration
Free nerve endings - pricking

Question 12

Describe Pacinian corpuscle

Answer 12

On-off response: action potential when pressure applied, action potential when pressure relieved
Fast adaptation
Afferent signal then propagated to dorsal root

Question 13

What is two-point discrimination?

Answer 13

Two-point discrimination: to measure variation in the sensitivity of tactile discrimination as a function of location on the body surface.
Principle: the higher the density of mechanoreceptors the smaller the distance at which two tactile stimuli can be discriminated.
e.g. Can’t feel closer than 5cm on calf, but can feel closer than 2mm on fingers

Question 14

Where is the density of mechanoreceptors greatest?

Answer 14

Hands and face

allowing the detection of stimuli at a much greater spatial resolution

Question 15

What is the motor system?

Answer 15

Our muscles and the neurons that command them. This is the system that actually gives rise to behaviour.
Anything we do is interaction between nervous system and muscle, and muscle and external environment

Question 16

What is the evidence that the spinal cord can generate complex motor patterns independently of the brain?

Answer 16

After cord transection:
The hindlimbs are still able to walk on a treadmill
Patterned electrical activity can be recorded from the muscles

Question 17

What are lower motor neurones?

Answer 17

Motor neurons present in the spinal cord

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 organised?

Answer 18

Lower motor neurons are collected in longitudinally organised columns

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).

Each column extends through more than one segment of the cord
Each muscle receives motor fibres through more than one ventral root and spinal nerve

Question 19

Alpha neurons

Answer 19

Larger
thick axon
high conductance velocity

Question 20

Gamma neuron

Answer 20

smaller
thin axon
low conductance velocity

Question 21

What is a motor unit?

Answer 21

The motor unit: a single motor neuron and the muscle fibres it innervates

Each motor neuron synapses with multiple muscle fibres. The motor neuron and the fibres it contacts defines the motor unit.

Question 22

Simplest reflex responses

Answer 22

based on an interaction between a proprioceptive sensory input and a motor unit

Question 23

Muscle spindles

Answer 23

negative feedback regulation of muscle length

Question 24

Golgi tendon organs

Answer 24

negative feedback regulation of muscle tension

Question 25

What type of afferent neurone is in the clasp-knife reflex?

Answer 25

Ib
synapses onto inhibitory interneuron which synapses onto motor neuron
in tendon organs

Question 26

What are Ia afferent neurons?

Answer 26

Synapses from stretch receptor in spindle
onto motor neuron
knee jerk reflex

Question 27

What do tendon organs and spindles respond to?

Answer 27

Tendon organs respond to muscle tension (due to contraction), spindles respond to length (due to passive stretch), but in a manner modified by the activity of their own contractile elements.

Question 28

Describe the stretch reflex circuitry

Answer 28

A monosynaptic reflex pathway

1. Muscle spindle is the sensory receptor which initiates the reflex
2. Stretch leads to increased Ia afferent activity -> increased alpha motor neuron activity -> contraction of the same muscle
   (Ia afferents also excite motor neurons that innervate synergistic muscles, and inhibit motor neurons of the antagonist muscles)
3. Stretch reflex: negative feedback loop to regulate muscle length - desired length is preset by descending pathways

Question 29

What is the flexion reflex?

Answer 29

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

Spinal cord circuitry is responsible for the flexion reflex

It is an example of a polysynaptic reflex pathway

Question 30

Describe the somatotopic organisation of lower motor neurons in the ventral horn at the cervical level of the spinal cord

Answer 30

Motor neurons innervating axial musculature (i.e. postural trunk) most medial

Motor neurons innervating distal musculature (e.g. hands) are most lateral

Question 31

What is notable about the somatic sensory cortex?

Answer 31

The amount of somatic sensory cortex devoted to the hands and face is much larger than the relative amount of body surface in these regions.

Question 32

Anterior cord syndrome

Answer 32

Bilateral lower motor neuron paralysis and muscle atrophy in segment of lesion (due to damage to lower motor neurons)

Bilateral spastic paralysis below the level of the lesion (loss of anterior descending tracts)
bilateral loss of pain, temperature and light touch sensations below level of the lesion (loss of anterior and lateral spinothalamic tracts)

But: tactile discrimination and vibratory and proprioceptive sensations are preserves because posterior (dorsal) white columns on both sides are undamaged

Question 33

Brown-Sequard or cord hemisection syndrome

Answer 33

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).

But: Discriminative touch pathways travelling in the ascending tracts in the contralateral dorsal white column remain intact.

Question 34

Complete cord transection

Answer 34

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)

Sadly, no buts.
(Or maybe?? Stem cell research….? Other new emerging treatments..? Brain-computer interface…?)