Sensory system Flashcards
Functions of the somatosensory system
1 - allows to interpret sensory messages received from our body & consists of sensory receptors
2 - Regulate behaviour in accordance with external circumstances
3 - NB for coordinating internal activities directed at maintaining homeostasis
4 - evokes emotions
5 - stores info for future reference
Four major types of receptors
- Merkel’s Disc - light sustained touch, slow adapting
- Meissner’s corpsucles - light fluttering touch (low frequency vibrations & fine touch); rapidly adapting
- Ruffini endings - deep pressure & skin stretch, slow adapting
- Pacinian corpuscle - vibrations & deep pressure, rapidly adapting
Sensory Transduction
The process of converting the energy of a stimulus into an electrical signal.
Stimulus alters the permeability of cation channels in afferent nerve endings, generating depolarising current –> receptor/generator potential
If sufficient magnitude, the receptor potential reaches threshold for the generation of APs in the afferent fibre. The resulting rate of AP firing is proportional to the magnitude of the depolarization
Sensory Coding
Conversion of a receptor stimulus to a recognisable sensation
4 elementary attributes of a stimulus via:
1 - Modality: form of energy to which a receptor is most sensitive (chemical, mechanical, thermal, electromagnetic)
2 - Location: site where stimulus originated
3 - Intensity: response amplitude frequency of APs, increased firing rate & recruitment of more receptors; Weber Fechner’s Law
4 - Duration: start to end of a response in a receptor; doctrine of specific energies & law of projection (rapidly adapting phasic receptors - Pacinian & Meissner’s & slow adapting tonic receptors - Merkel cells & Ruffini corpuscles)
Lateral Inhibition
Info from neurons whose receptors are at the peripheral edge of the stimulus are inhibited, compared to info from the sensory neurons at the centre of a stimulus
Doctrine of specific energies
When the nerve pathways from a particular sense organ are stimulated, the sensation evoked is that for which the receptor is specialised no matter how or where along the pathway the activity is initated
The law of projection
If we stimulate a sensory pathway along its course to the sensory cortex, the conscious sensation produced is perceived to be from the location of the receptors (phantom limb pain)
Ascending Tracts of SSX
Dorsal Column Tract
Spinothalamic Tract
Spinocerebellar Tract
Dorsal Column-medial lemniscus pathway/ Posterior column pathway
Responsible for carrying info: touch, proprioception, pressure, vibration
Receptors for these in the neck, trunk & limbs
1. 1st order neuron cell body in dorsal root ganglion - ascends up ipsilateral side of spinal cord & terminates at medulla oblongata
2. Synapses onto 2nd order neuron in dorsal column nuclei –> decussation prior to entry into medial lemniscus (in medulla oblongata)
3. Posterior funiculus organised into two tracts:
- Fasciculus Gracilis - carries info from lower trunk
- Fasciculus Cuneatus - carries info from upper limbs & superior trunk
4. 2nd order neuron from the medial lemniscus synapse with thalamic neurons located in the ventral posterior lateral nucleus (VPL)
5. These third order neurons then terminate in the primary somatosensory cortex (postcentral gyrus)
Spinothalamic/ Anterolateral Tract Functions
Subserves pain & temperature sensibility
Provides for less precise, less well-localized, more emotion ladden type of somatic sensibility than the DC-ML system
Affective component of the pain experience - divergent projections of the spinothalamic system
Spinothalamic Tract Pathway
Enter spinal cord ascend up a segment(s) of SC - some descend down segment(S) of SC - some enter dorsal horn at same level
Decussation occurs over a no. of segments
Spinotectal & spinoreticular fibres go to reticular formation –> thalamic nucleus
Spinothalamic fibers go to the VPL thalamic nucleus & posterior thalamic nucleus before going to sensory cortexs.
Pain Stimuli from nociceptors transmitted via two fibre types:
1 - A delta: Thinly myelinated; transmit fast immediate pain; travels in the lateral spinothalamic tract
2 - C fibers: unmyelinated; transit slow aching pain that may result from inflammation
Nociceptors Classified:
1 - Mechanical - strong pressure
2 - Chemical - high acidity & environmental irritants
3 - Thermal - temperatures above 45 degrees & severe cold
4 - Polymodal - combinations of stimuli
Dorsal Column pathway for Visceral Pain
Afferents coming from visceral organs get to spinal cords & synapse on 2nd order neurons that carry pain & temp info.
Visceral afferents may form SS afferents & converge onto 2nd order neuros - info carried to SSX & become conscious that visceral organ is painful
Spinocerebellar Tract
Carries unconscious proprioceptive information - co-ordinate & refine movement
1 - Posterior Spinocerebellar tract - lower limbs to ipsilateral cerebellum
2 - Cuneocerebellar tract - from upper limbs to ipsilateral cerebellum
3 - Anterior spinocerebellar tract - from lower limbs - fibres decussate twice therefore terminate in the ipsilateral cerebellum
4 - Rostral Spinocerebellar tract - from upper limbs to ipsilateral cerebellum
Lesions of DCML pathway
A lesion in DCML pathway - loss of proprioception & fine touch
If lesion occurs in spinal cord sensory loss will be ipsilateral (decussation occurs in medulla oblongata)
Lesion of Anterolateral System (part of spinothalamic tract)
produce impairment of pain & temp
Sensory loss will be contralateral (spinothalamic tract decussates in spinal cord)
Brown-squared syndrome
Refers to a hemisection (one sided lesion) of spinal cord
Involves both anterolateral system & DCML pathway:
DCML - ipsilateral loss of touch, vibration & proprioception
Anterolateral pathway - contralateral loss of pain & temp sensation
Lesion of spinocerebellar tracts
Ipsilateral loss of muscle co-ordination
Unlikely to be damaged in isolation
Spinothalamic tract mediates what type of pain
Sensory-discriminative aspects of pain (first pain)
Affective-motivational (second pain) aspect of pain
Mediated by complex pathways in the anterolateral system
