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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 [email protected]