Physiology of the spinal cord Flashcards
Sensation
Detection by receptors
Perception
Interpretation by spinal cord and brain circuits
Receptors
Neurones specialised in the transduction of energy generated by external stimuli
Specific for a narrow range of input
Mechanoreceptors
Touch, pressure and vibration
Aβ fibre type
Wide diameter, fast
Bare nerve endings
Aδ
Medium diameter and speed
Bare nerve endings
- fast ‘pricking’ pain
Aδ
Medium diameter and speed
Bare nerve endings
- slow burning pain, itch
C-fibres
Thin diameter, slow
Pacinian corpuscle
Fast adaption
On-off response
Two point discrimination
To measure variation in the sensitivity of tactile discrimination as a function of location on the body surface
Spatial distribution of receptors
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
Motor system
Our muscles and the neurones that command them
This is the system that actually gives rise to behaviour
Lower motor neurones
Final common path for all signals from the CNS to skeletal muscles
Collected in longitudinally organised columns
Lower motor neurone columns
Contains larger, alpha and smaller, gamma motor neurones to one muscle
Each column extends through more than one segment of the cord
The motor unit
A single α motor neurone and the muscle fibres it innervates
Proprioceptive sensory organs
Muscle spindles- negative feedback regulation of muscle length
Golgi tendon organs- negative feedback regulation of muscle tension
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
Stretch reflex circuit: a monosynaptic reflex pathway
Muscle spindle: sensory receptor that initiates the reflex
Stretch -> increased la afferent activity -> increased α motor neurone activity -> contraction of same muscle
Stretch reflex: negative feedback loop to regulate muscle length
Somatotopic organisation of lower motor neurones
Motor neurones innervating axial musculature (e.g. postural trunk muscles) located most medially
Those innervating distal muscles (e.g. in the hands) located most laterally
Somatotopic order in the human primary somatic sensory cortex
Electrical activity recorder following mechanosensory stimulation of different parts of the body
Somatotopic representation of body parts from medial to latleral
Amount of sensory cortex to hands and face much larger than relative amount of body surface
Anterior cord syndrome
Bilateral lower motor neurone paralysis and muscular atrophy in segment of lesion
Bilateral spastic paralysis below level of lesion
Bilateral loss of pain, temperature and light touch sensations below level of lesions
Tactile discrimination and vibratory and proprioceptive sensations preserved because posterior white column on both sides are undamaged
Brown- Sequard or cord hemisection syndrome
Ipsilateral lower motor neurone paralysis and muscular atrophy in segment of lesion
Ipsilateral spastic paralysis below level of lesion
Ipsilateral band of cutaneous anaesthesia in segment of lesion
Ipsilateral loss of tactile discrimination and vibratory and proprioceptive sensations below lesion
Contralateral loss of pain, temperature and light touch
Discriminative touch pathways travelling in ascending tracts in contralateral dorsal white column remain intact
Complete cord transection syndrome
Complete loss of sensation and voluntary movement below level of lesion
Bilateral lower motor neurone paralysis and muscular atrophy in segment of lesion
Bilateral spastic paralysis below level of lesion
Bilateral loss of all sensations below level of lesion
Bladder and bowel functions no longer under voluntary control
