Sensory Systems

Dr. DeBellow, Fall 2024, Lectures 11-12

Sensory Systems

• Detect external events

• The 3 discussed in class are organized according to a common anatomical plan

• Visual, auditory, somatosensory

Sensory Receptors/ Sensory Epithelium

• Specialized cell types/ parts of a cell that are in the periphery of the body- exposed to external events and stimuli

• These receptors are specialized to transduce environmental energy (modality) into chemical or electrical signals into the nervous system

Transduction

• The conversion of stimulus energy to a neural signal

• Accomplished by receptor cells

Receptor Cells

• A cell whose axon or dendrite is capable of transduction in a particular sensory modality

• Specialized to transduce a particular form of environmental energy (modality) into a change in membrane potential - receptor potential

• Grouped together in sheets referred to as ‘sensory epithelium’

Receptor Potential

• Change in membrane potential at the site of transduction

• Causes action potentials to be generated in the receptor cell or its downstream target

• The rate and timing of action potentials carry information about the stimulus to the brain

Relay Nuclei

• Groups of neurons located in the central nervous system that process signals from receptor neurons and transmit signals to the thalamus

Thalamus

• Obligatory relay of visual, auditory, and somatosensory information to primary cortices

• Groups of neurons organized into nuclei within the thalamus, that process signals from relay nuclei and transmit signals to the cerebral cortex

Primary Cerebral Cortex

• Anatomical target of the thalamus

• The first stop of sensory information on its way to cognitive processing

• Conscious perception occurs when information reaches

• Project to higher levels of cortex

Secondary Cerebral Cortex

• Anatomical area that processes signals from primary sensory cortex

• Transmits signals to association cortex, motor cortex, and subcortical structures

• Where multimodal and other perceptions are formed

Modality Specificity

• Category of stimuli to which receptor is sensitive

Receptive Field

• Location on the sensory surface within which a stimulus (of the appropriate modality) can influence the activity of a sensory neuron

• Range of locations on the sensory surface that, when stimulated, alter neurons activity

Lateral Inhibition

• Inhibition of adjacent neurons in a map, which facilitates localization of stimuli

• Sharpens the receptive field by inhibiting channels to its side

• Winner take all

Acuity

• Perceptual ability to discriminate between different parameter values

• Ability to discriminate 2 similar but not identical sensory stimuli

• Depends on the receptor density and receptive field

Spatial Organization of the Sensory Surface

• Maintained at higher levels of the brain

• Topographic maps

• Think of the topographic axonal projections as labelled lines

Pupil (visual system)

• Constriction and dilation allow less or more light to come through the eye

How/ where is light focused? (visual system)

• Light is focused by the lens on the back of the eye which houses the retina

• 2D camera trained on your visual field

Visual Field (visual system)

• The full range of what you can see

Retina (visual system)

• Site of transduction

Fovea (visual system)

• Center of the retina and sigh of highest acuity

Optic Nerve (visual system)

• Axons from output cells of the retina gather together and form the optic nerve, which heads towards the thalamus

• Blind spot

What is the path of light? (visual system)

• Light passes through the retinal circuitry and is absorbed by photoreceptors, rods and cones, they absorb photons and generate membrane potential

Rods (visual system)

• Sensitive to low light levels- scotopic

• Don’t distinguish between different wavelengths

• Low acuity

• Peripheral field vision

Cones (visual system)

• Sensitive to bright light levels- photopic

• Distinguish between different wavelengths- 3 types, red, green, blue

• High acuity

• Central filed of vision

• High spacial density in fovea

How is light absorbed in rods and cones? (visual system)

• Light is absorbed by photopigments which activate a G-protein cascade that enzymatically cleaves cGMP

• In the dark, cGMP holds open ligand gated Na+ channels which depolarizes the cell leading to the release of an inhibitory neurotransmitter that suppresses the downstream neuron

• In the light, cGMP concentration decreases, closing Na+ channels and stops the neurotransmitter release and the circuit is disinhibited

• The downstream cell is activated, causing the retinal ganglion cells to fire spikes

• Retinal ganglion cells gather together and leave the retina at the optic disc

Where to retinal ganglion cells project? (visual system)

• To the thalamus

• The medial axons cross the midline once

• Information content of the optic nerve, chiasm, and tract are different

Optic Nerve (visual system)

• Information across the visual field from one eye

• If severed, you lose all vision from that eye

Optic Chiasm (visual system)

• Information that crosses the midline

• Left visual field from the left eye

• Right visual field form the right eye

Optic Tract (visual system)

• Information from the contralateral visual field

• If severed you lose vision from the contralateral visual field on the side it was damaged

  • Left side damaged= right visual field lost

Retinotopic/ Visuotopic Map (visual system)

• The thalamus projects to the primary visual cortex which contains a topographic map of the retinal surface

• That is also a map of where the photons cam from, your visual scene

• Visual activity percolates out form the primary cortex along ‘what’ and ‘where’ pathways, involved in building complex perception (not covered in lecture)

Sound (auditory system)

• A wave with alternating cycles of compression and rarefaction of particles in a medium (air or water)

Pinna (auditory system)

• Part of the external ear

• Reflects sound into the ear canal

Tympanic Membrane (auditory system)

• Eardrum, compression waves (sound) causes it to vibrate

Ossicles (auditory system)

• Bones in the middle ear

• Mechanically efficient conduit of vibration

Oval Window (auditory system)

• Receives vibrations from the ossicles

• Set up fluid vibrations in the inner ear causing sound transduction

  • Fluid movement within the cochlea

Basilar Membrane (auditory system)

• There is a gradient in the physical properties that makes different locations resonate with different frequencies of sound

• Narrow, stiff end near the oval window best resonates in response to high frequencies

• Broad, compliant end near the helicotrema best resonates in response to low frequencies

• The entire length is populated by hair cells

  • Their apical stereocilia are embedded in the tectorial membrane, who’s pivot point is offset compared to the basilar membrane

Explain how the auditory system works (auditory system)

• Sound is reflected by the pinna into the ear canal, causing the tympanic membrane to vibrate

• Vibrations are conducted via the ossicles (mechanically efficient) to the oval window which causes fluid movement within the cochlea

• This causes the basilar membrane to move up and down

• The tectorial membrane also vibrates

• This creates a shearing force that bends the stereocilia forward and backward with each sound cycle

• The stereocilia membranes have mechanically gated ion channels that open with each cycle of sound and depolarize the hair cell (concentration gradients are flipped, K+ is higher outside the cell and causes the depolarization)- this is receptor potential

• It causes transmitter release form the hair cells to the primary afferent fibers which head towards the brain

• A map of the cochlear surface is maintained up through the primary cortex via labelled line projection- map of tones, not sound source locations

Pacinian Copuscle (somatosensory system)

• A type of touch receptor

• Their membranes contain stretch-activated channels that open in response to membrane deformation

Phasic Signaling (somatosensory system)

• In response to a sustained stimulus, encapsulated receptor types rapidly adapt, meaning they exhibit brief ‘on’ and ‘off’ response

• Rapidly adapting receptor types are partially responsible for percepts

Tonic Signaling (somatosensory system)

• Non-encapsulated receptor types exhibited sustained responses

Explain how pacinian corpuscle touch receptors work (somatosensory system)

• When the stretch-activated channels are opened in response to membrane deformation, the cell depolarizes, triggering spikes that propagate towards the spinal cord

• Rapid adaptation in pacinian corpuscles is due to slow mechanical separation of the overlying connective layers

• Sensory information crosses the midline exactly once on its journey to the cortex

• In the cortex there is a topographic map of the sensory surface/ hpmunculus