L18 - Physiology of the Spinal Cord

Question 1

what are the 2 main 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

what is the difference between sensation and perception

Answer 2

• Sensation: Detection by receptors

Perception: Interpretation by spinal cord and brain circuits

Question 3

what are receptors

Answer 3

-Receptors are neurons specialized in the transduction of energy generated by external stimuli

-

Question 4

how do neurones in sensory systems signal events

Answer 4

a combination of:

• Rate
• Spatial
• Temporal

Question 5

describe when convergence and divergence is used in the body

Answer 5

1) Convergence and divergence.
- High spatial resolution required: low convergence (e.g., cones and bipolar cells in retina).

2) - Detection of weak signals required: high convergence
(e. g., rods and bipolar cells in the retina).

3)Input is used for complex or multiple functions: high divergence (e.g., vestibulo-ocular reflex).

Question 6

what are the 2 main types of primary sensory receptors

Answer 6

slide 6

Question 7

describe stimulus specificity

Answer 7

• ‘Adequate stimulus’: lowest response threshold

Question 8

describe the Main cutaneous receptor types and modalities of the somatosensory system

Answer 8

Modality Receptor type Afferent fibre type and
conduction velocity

Touch, pressure Mechanoreceptors Aβ
and vibration (e.g., Merkell’s cells, Wide diameter, fast
Ruffini end-organs,
Pacinian corpuscles)

					Bare nerve endings					A
												Medium diameter and speed

Pain Bare nerve endings A
- fast ‘pricking’ pain Medium diameter and speed

					Bare nerve endings					C-fibres
					- slow burning pain, itch			Thin diameter, slow

(slide 7)

Question 9

state the 2 Mechanisms of sensory stimulus discrimination

Answer 9

I. Different types of receptors for the same modality.

II. Spatial distribution of receptors.

Question 10

describe mechanism 1

Answer 10

For pain there are:

• Pacinian corpuscles=deep pressure/vibration
• merkel discs=contact
• Meissner’s corpuscle =shear

Ruffini’s corpuscle=Tension/folding

Question 11

where do Pacinian corpuscles send their signals to

Answer 11

-Propagation of afferent signal to dorsal root ganglion

Question 12

describe mechanism 2:spatial distribution of receptors

Answer 12

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.

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 13

describe the Mechanisms of sensory stimulus discrimination. III. Windows of response intensity.

Answer 13

look at slide 13

Question 14

why is the spinal cord called the simplest motor system and give an example

Answer 14

• The spinal cord can generate complex motor patterns independently of the brain

ie: After cord transection in a dog:
The hindlimbs are still able to walk on a treadmill

-Patterned electrical activity can be recorded from the muscles

Question 15

what are the motor neurons present in a spinal cord called

Answer 15

• Lower motor neurones

Question 16

what is a lower motor neuron

Answer 16

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

Question 17

how are LMNs collected (5 marks)

Answer 17

neurons are collected in longitudinally organized 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 fibers through more than one ventral root and spinal nerve.

Question 18

what does destruction of a ventral root lead too

Answer 18

–Destruction of a single ventral root or a single spinal nerve will not produce paralysis, only weakness (paresis).

Question 19

what is a motor unit

Answer 19

: a single  motor neuron and the muscle fibres it innervates

-

Question 20

what does a simple reflex involve

Answer 20

The simplest reflex responses are based on an interaction between a proprioceptive sensory input and a motor unit.

Question 21

name 2 types of proprioceptive sensory organs

Answer 21

Muscle spindles – negative feedback regulation of muscle length

Golgi tendon organs – negative feedback regulation of muscle tension

Question 22

what is the difference between the role of the tendon organ and muscle spindle in two different simple reflexes (ie- clasp-knife & knee-jerk reflex)

Answer 22

1- clasp…. has an inhibitory interneuron

2.no interneuron

slide 20

Question 23

what do tendon organs respond to

Answer 23

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 24

what is the Schematic representation of contractile and stretch-sensitive (non-contractile) elements in muscle

Answer 24

• serial connection -contractile unit
• parallel connection - stretch-sensitive unit

slide 21

Question 25

describe a Stretch reflex circuitry: a monosynaptic reflex pathway

Answer 25

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 26

describe an example of a polysynaptic reflex pathway

Answer 26

Spinal cord circuitry responsible for the flexion reflex:

-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 27

describe the location of motor neurons innervating axial musculature (postural trunk muscles)

Answer 27

• are located most medially

Question 28

describe motor neurones innervating distal musculature (in the hands)

Answer 28

• are located most laterally.

Question 29

describe the Somatotopic order in the human primary somatic sensory cortex.

Answer 29

A) Electrical activity is recorded following mechanosensory stimulation of different parts of the body.

(B) Somatotopic representation of body parts from medial to lateral.

(C) Cartoon of the homunculus constructed on the basis of such mapping.

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 30

describe the anterior cord syndrome (lesions in spinal cord)

Answer 30

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).

But: Tactile discrimination and vibratory and proprioceptive sensations are preserved because the posterior (dorsal) white columns on both sides are undamaged.

Question 31

describe . Brown – Séquard or cord hemisection syndrome (lesions in spinal cord)

Answer 31

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 32

describe . Complete cord transection syndrome(lesions in spinal cord)

Answer 32

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)