Circuits of the spinal cord

Question 1

What are the functions of spinal cord circuits?

Answer 1

• Initial processing of somatosensory afferent input by the CNS
• Final processing of motor efferent output by CNS

Question 2

What is the definition of sensation?

Answer 2

Detection by receptors

Question 3

What is the definition of perception?

Answer 3

Interpretation of spinal cord and brain circuits

Question 4

With what codes do neurones in sensory systems signal events?

Answer 4

• rate (firing frequency)
• spatial codes (different parts of the body send signals to different parts of the brain)
• temporal codes (relationship of firing pattern of different neurones (duration, latency etc)

Question 5

What are the two types of first order neurone?

Answer 5

Those with a sensory cell that transmits to the neurone, and those where the neurone is what detects the stimulus.

Question 6

What is the purpose of an energy filter between the stimulus and the receptor?

Answer 6

To give rise to specificity. Prevents all types of stimulus activating all types of receptors. Gives rise to the notion of ‘adequate stimulus’ (stimulus producing a response).

Question 7

What is a modality?

Answer 7

An aspect / type of stimulus. For example, mechanosensory or olfactory. There can be multiple receptors for the same modality.

Question 8

What are the 4 parts to sensory stimulus discrimination?

Answer 8

1. Different types of receptors for the same modality.
2. Spatial distribution of receptors (spatial resolution)
3. Windows of response intensity.
4. Lateral inhibiton
   They all mostly rely on anatomical / functional differences.

Question 9

What are some examples of mechanosensory receptors?

Answer 9

1. Ruffini’s corpuscle = tension/ folding
2. Parcinian corpuscle = vibration/ pressure.
3. Merkel’s disk = contact.

Question 10

What is lateral inhibition?

Answer 10

The capacity of excited neurones to reduce the activity of its neighbours. Allows brain to extract specific information about an object (resolution).

Question 11

How does lateral inhibition work?

Answer 11

Excitation of a neurone excites other excitatory neurones but also excites inhibitory neurones. This decreases / prevents the activity of more peripheral neurones.

Question 12

From the brain to the bottom of the spinal cord, what are the 4 sections?

Answer 12

Cervical (arms)
Thoracic
Lumbar (legs)
Sacral

Question 13

Describe the pathway of mechanosensory information reaching the brain.

Answer 13

Reaches by 3 neurone relay. Fast and precise info about type of stimulus, site, start and stop.

1. 1st order neurones synapse onto second order neurones in dorsal column
2. Info travels up dorsal column into the medulla, where it crosses the other side.
3. Projects to somatosensory cortex by 3rd order neurones.

Question 14

Describe the pathway of pain and temperature (anteriolateral pathway).

Answer 14

3 neurone relay. Info about pain, location and physical/ emotional discomfort.

1. Sensory info from 1st order neurone immediately sent to the other side of the dorsal column.
2. Info ascends to medulla by 2nd order neurones.
3. Projects to cortex by 3rd order neurones.

Question 15

How are lower motor neurones organised in the spinal cord?

Answer 15

Longitudinally organised columns, that each extend through more than one segment of the cord.
Contain both alpha and gamma motor neurones for one muscle (or a few functionally similar muscles).

Question 16

What are alpha and gamma neurones?

Answer 16

alpha - larger. thick axon, high conductance velocity.

gamma - smaller. Thin axon, low conductance velocity.

Question 17

Describe the command of muscles

Answer 17

Each muscle recieves motor fibres from more than one ventral root and spinal nerve (convergence). Destruction of a single spinal nerve will not paralyse, just weaken.

Question 18

Describe the somatotopic organisation of lower motor neurones in the spinal cord.

Answer 18

Motor neurones innervating axial musculature are located more medially.
Motor neurones innervating distal musculature (eg hands) are located more laterally.

Question 19

What is a motor unit?

Answer 19

A single alpha motor neurone and the muscle fibres it innervates.

Question 20

what is proprioception?

Answer 20

Self detection of body’s location in the environment relative to objects and surroundings.

Question 21

What are the proprioceptive organs?

Answer 21

Muscle spindles - negative feedback regulation of muscle length. (Knee jerk reflex)
Golgi tendon organs - negative feedback regulation of muscle tension. (Arm wrestling with stronger person).