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

primary processing of motor output by CNS

Question 2

what are spinal nerves?

Answer 2

highways for both somatosensory information traffic to the spinal cord (afferent) and motor information from the spinal cord to muscles (effect)

Question 3

what is the difference between sensation and perception?

Answer 3

sensation = detection by receptors
perception = interpretation by spinal cord and brain circuits

Question 4

what are receptors?

Answer 4

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

Question 5

what do neurons in sensory signal systems signal events by?

Answer 5

spatial
rate
temporal codes

Question 6

impact of convergence and divergence

Answer 6

high spatial resolution required: low convergence
detection of weak signals: high convergence
input used for complex/multiple functions: divergence

Question 7

2 main kinds of 1st order sensory neurons

Answer 7

bare nerve endings - neuron detects stimulus (most common)

mechanoreceptors - detect stimulus then transmit signal to sensory neuron (found in sense organs)

Question 8

A beta fibre 1st order neurons

Answer 8

for touch, pressure and vibration
wide diameter, fast
receptor type: mecanoreceptors e.g. Merkell’s cells, Ruffini end-organs, Pacinian corpsucles

Question 9

A delta fibre 1st order neurons

Answer 9

for touch, pressure, vibration and pain
medium diameter and speed
bare nerve endings

Question 10

C fibre 1st order neurons

Answer 10

for pain
thin diameter, slow
bare nerve endings

Question 11

cutaneous mechanosensory receptors

Answer 11

Meissner’s corpuscle
Merkel disc
Ruffini’s corpuscle
Pacinian corpuscle

Question 12

what do Meissner’s corpuscles detect?

Answer 12

light tough

Question 13

what do Merkel discs detect?

Answer 13

mechanical pressure and position

Question 14

what do Ruffini’s corpuscles detect?

Answer 14

tension, folding

Question 15

what to Pacinian corpuscles detect?

Answer 15

vibration, gross pressure changes

Question 16

how can sensory stimulation be discriminated?

Answer 16

different types of receptors for same modality
spatial distribution of receptors
windows of response intensity

Question 17

what is 2 point discrimination?

Answer 17

measure variation in sensitivity of tactile discrimination as a function of location on the body surface
if there is a higher density of mechanoreceptors, smaller distance at which 2 tactile stimuli can be discriminated

Question 18

where is mechanoreceptor density greatest?

Answer 18

on hands on face

allows detection of stimuli at greater spatial resolution

Question 19

what is the motor system?

Answer 19

our muscles and neurons that command them

system that gives rise to behaviour

Question 20

what is the simplest motor system?

Answer 20

spinal cord can generate complex motor patterns independently of the brain

Question 21

what are lower motor neurons?

Answer 21

final common path for all signals from CNS to skeletal muscles
collected in longitudinally organised columns
each neuron receives motor fibres through more than one ventral root

Question 22

each (lower motor neuron) column …

Answer 22

• contains larger alpha and smaller gamma motor neurons to one muscle
• extends through more than one segment of the cord

Question 23

what is the difference between alpha and gamma motor neurons?

Answer 23

alpha: thick axon, high conductance velocity
gamma: thin axon, low conductance velocity

Question 24

what can destrution of a single ventral root/spinal nerve cause?

Answer 24

muscle weakness = paresis

not paralysis

Question 25

what is a motor unit?

Answer 25

a single motor neuron and muscle fibre it innervates

Question 26

what are the simplest reflex responses based on?

Answer 26

an interaction between proprioceptive sensory input and a motor unit

Question 27

what do proprioceptive sensory neurons detect?

Answer 27

muscle spindles - negative feedback to regulate muscle length
golgi tendon organs - negative feedback to regulate muscle tension

Question 28

an example of a monosynaptic reflex pathway

Answer 28

stretch reflex - negative feedback loop to regulate muscle length

Question 29

how does the monosynaptic reflex pathway work?

Answer 29

muscle spindle (sensory receptor) initiates reflex
stretch increases
afferent activity increases
alpha motor neuron activity increases: muscle contracts

Question 30

an example of a polysynaptic reflex pathway

Answer 30

flexion reflex

Question 31

how does the polysynaptic reflex pathway work?

Answer 31

stimulation of cutaneous pain receptors in foot
activation of spinal cord local circuits
stimulated extremity is flexed
other extremity extends to provide compensatory support