PHYSIOLOGY OF SENSORY SYSTEMS

Dr Serge Eddy MbaNeurosurgery Registrar UZ- CHS

OUTLINE

•SENSORY RECEPTOR PHYSIOLOGY•ANATOMY OF SOMATIC SENSORY SYSTEM•CORTICAL INTEGRATION OF SENSORY PERCEPTION

SENSORY RECEPTOR PHYSIOLOGY

• Information about the internal and external environment activates the CNS via a variety of sensory receptors• Receptors are transducers that convert different type of energy into action

potentials in neurons• CLASSIFICATION• Numerous, none is perfect• Mechanoreceptors: for touch and pressure• Nociceptors codes for potentially harmful stimuli such as extreme heat and

extreme cold• Chemoreceptors: mediates changes in chemical composition of the

environment in which they are localized• Photoreceptors like the cones and rods responds to light

Sensory system Modality Stimulus energy Receptor class Receptor cell types

somatosensory touch Tap, flutter 5-40Hz C* mechanoreceptor Meissner corpuscles

somatosensory touch motion C* mechanoreceptor Hair follicle receptors

somatosensory touch Deep pressure, vibration 60-300Hz C* mechanoreceptor Pacinian corpuscles

somatosensory touch Touch, pressure C* mechanoreceptor Merkel cells

somatosensory touch Sustained pressure C* mechanoreceptor Ruffini corpuscles

somatosensory proprioception stretch mechanoreceptor Muscle spindle

somatosensory proprioception tension mechanoreceptor Golgi tendon organ

somatosensory temperature thermal thermoreceptor Cold n warm receptors

somatosensory pain Chemical, thermal, mechanical Chemo, thermo, mechanoreceptor polymodal

somatosensory itch chemical chemoreceptor Chemical nociceptors

visual vision light photoreceptor Rods and cones

auditory hearing sound mechanoreceptor Hair cells (cochlea)

vestibular balance Angular acceleration mechanoreceptor Hair cells (semicircular canals)

vestibular balance Linear acceleration, gravity mechanoreceptor Hair cell(Otolith organs)

olfactory smell chemical chemoreceptor Olfactory sensory neuron

gustatory taste chemical chemoreceptor Taste buds

Transduction of stimulus into neural energy

Sensory coding• MODALITY: type of energy

transmitted by the stimulus

• LOCATION: site of the body where the stimulus originated

• INTENSITY: response amplitude or frequency of action potential generation

• DURATION: time from start to end of a response in the receptor

• MODALITY: The unique stimulus that activates a specific receptor at a low energy level is called an adequate stimulus. The specificity of response in receptors underlies the labeled line code, the most important coding mechanism for stimulus modality• LOCATION: sensory unit is a single sensory axon and all its peripheral branches.Receptive field: spatial distribution from which a stimulus produces a response. lateral inhibition:two point discrimination

• INTENSITY: Weber-Fechner law R= KSA

• DURATION: adaptation and desensitization, phasic vs tonic receptors

receptor morphology influences adaptationpacinian corpuscle is rapidly adapting receptorcomposed of concentrically fluid filled lamellaecapsule deflects steady pressureeach vibratory cycle generate an action potential

LAW OF SPECIFIC NERVE ENERGIES• First enunciated by Mϋller in 1835• One of the cornerstones of sensory physiology• The specific sensory pathways are discrete from sense organ to cortex• The modality of sensation is not determined by the proximal stimulus• “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 initiated”

LAW OF PROJECTION• “no matter where a particular sensory pathway is stimulated along

its course to the cortex, the conscious sensation produced is referred to the location of the receptor”

e.g: cortical experiment during neurosurgical procedures on conscious patients

CLINICAL EXAMINATION• test for vibratory sensibility is done by applying a vibrating(128Hz) tuning

fork to the skin of the fingertip• Pallesthesia is the ability to feel mechanical vibration• Receptors involved are Pacinian corpuscles• Elevation of the threshold for vibratory sense is an early sign post column

degeneration e.g in Pernicious Anaemia• Stereognosis: perception of the form and nature of an object without

looking at it. Depends on intact touch and pressure sensation carried by dorsal colum• stereoagnosia: defect along the sensory pathway, even lesion in parietal

lobe posterior to postcentral gyrus• Visual, tactile, auditory, color, position ….agnosia

ANATOMY OF SOMATOSENSORY SYSTEM

• The sodium channel BNC1 is closely associated with touch receptors• DEGENERINS

CORTICAL INTEGRATION OF SENSORY PERCEPTION

references

Dr S. E Mba

THANK YOU

Contact: Dr Mba mbasedy@yahoo.fr