Physiology of spinal cord Flashcards
Functions of spinal cord
Initial processing of somatosensory input by the CNS.
Final processing of motor output by the CNS
Mechanoreceptors
- Modality
- Afferent fibre type
- Conduction velocity
- Examples
Detects touch, pressure and vibration.
A-beta fibre type
Wide diameter= fast conduction velocity
Examples:
- Merkell’s cells
- Ruffini end organs
- Pacinian corpuscles
Bare nerve ending for pain
Fast pricking pain
- A-delta fibres
- medium diameter
- Medium speed conduction velocity.
Slow burning pain/itch
- C-fibres
- Thin diameter
- Slow conduction velocity
Meissner’s corpuscle
Cutaneous mechano-sensory receptor
- Sensitive to shear forces/ light touch
- On smooth skin [hairless] like the hands
Merkel disk
Cutaneous mechanosensory receptors
- Sensitive to contact
- Appears on smooth skin
Ruffini’s corpuscle
Cutaneous mechanosensory receptor-
- Sensitive to tension, folding and stretching on smooth skin.
Pacinian corpuscle
Cutaneous mechanosensory receptor
- Sensitive to deep pressure and vibrations in the skin
- Transmits afferent signal to the dorsal root ganglion
Two point discrimination
Measurement in the variation of sensitivity to tactile discrimination.
- High density of mechanoreceptors = the smaller the distance tactile stimuli can be discrimination
Density of mechanoreceptors throughout the body
Greatest on the hand and face
Detection of stimuli here has a much greater spatial resolution than the rest of the body
Lower motor neurones
Final common path for all signals from CNS to skeletal muscles
Longitudinal organised columns
- Alpha and gamma axons
- Supply one muscle of functionally similar muscles
- Extends through more than one spinal cord segments
Muscles
- Receive motor inout from more than one ventral root and spinal nerve
Alpha and gamma axons
- Longitudinal organised columns
Alpha- thick axons with high conduction velocity
Gamma- thin axons with low conduction velocity
Organised in columns to supply a muscle or a group of functionally similar muscles
Paresis
- Organised columns
Results from the destruction of a single ventral root or single spinal nerve
- Each muscle receives motor input from more than one ventral root and spinal nerve.
A motor unit
A single alpha motor neurone and the muscle fibres it innervates.
Prioproceptive sensory organs
Muscle spindles
- Negative feedback regulation of muscle length
- Due to passive stretch
Golgi tendon organs
- Negative feedback regulation of muscle tension
- Due to contraction
Stretch reflex circuit
Monosynaptic
- Muscle spindle initiates the reflex
- Sensory receptor - Stretch increases la afferent activity
- Increases alpha motor neurone activity
- Contracts the same muscle - Stretch reflex
- Negative feedback loop
- Regulates muscle length via descending tracts
Flexion reflex
- Pain in foot example
Polysynaptic pathway
- Spinal cord circuit
- Stimulation of cutaneous pain receptors in foot [i.e stepping on sharp object]
- Activation of spinal cord circuits= flexion at stimulated extremity
- Extension at other extremities for support.
Anterior cord syndrome
Lesion to the anterior spinal cord
Damage of lower motor neurones
- Bilateral lower motor neurone paralysis
- Muscular atrophy in segment of lesion
Loss of anterior descending tract
- Bilateral spastic paralysis below the level of lesion
Loss of anterior and lateral spinothalmic tracts
- Bilateral loss of pain, temperature and light touch below level of lesion.
Dorsal column still preserved so:
- Tactile discrimination and proprioception still preserved
Brown- Sequard/ Cord hemisection syndrome
Lesion to the spinal cord
Damage to lower motor neurones
- Ipsilateral lower motor neurone paralysis and muscular atrophy in segment of lesion
Loss of anterior descending tract
- Ipsilateral spastic paralysis below level of lesion
Loss of dorsal root
- Ipsilateral band of cutaneous anesthesia in segment of lesion
Loss of ascending tract in the dorsal white column on the side of lesion
- Ipsilateral loss of tactile discrimination
- Ipsilateral loss of vibratory and proprioceptive sensations
- Below level of lesion
Loss of crossed lateral spinothalmic tracts on same side of lesions
- Contralateral loss of pain, temperature and light touch
Functions preserved in Brow-Sequard/ Cord hemisection syndrome
Ascending tracts in the contralateral dorsal white column intact
- Discriminative touch pathways remain
Functions preserved in the anterior cord syndrome
Dorsal white columns on both sides intact
- Tactile discrimination
- Proprioceptive sensations
- Both preserved
Complete cord transection syndrome
Complete loss of sensation and voluntary movement below level of lesion
Bilateral lower motor neurone paralysis and muscle atrophy in segment of lesion
Loss of descending tracts
- Bilateral spastic paralysis below level of lesion
Loss of ascending tracts
- Bilateral loss of all sensations below the level of lesion
Loss of descending autonomic fibres
- Bladder and bowel functions no longer under voluntary control,
3 mechanisms of sensory stimulus discrimination
- Include examples
- Different types of receptors for the same modality of stimulus
- Such as cutaneous mechanoreceptors - Different spatial distribution of receptors
- Seen with the distribution of cutaneous mechanoreceptors on skin - There are different windows of response intensity
- Example: cold receptors only response to specific temperatures, same with ‘hot receptors’
4.
Clasp-knife reflex
Reflex that prevents muscle damage [prevents tearing]
- Contraction of muscle detected by tendon organ— stimulates 1-beta afferent neurone
- Afferent neurone synapses with the inhibitory interneurone and excites it.
- This inhibits the alpha motor neurone, causing the same muscle to relax.
Somatrophic organisation of lower motor neurones
Proximal muscles are arranged more medially
- I.e trunk
Distal muscles for laterally
- I.e hands and feet
