Sensory Systems Biology Notes

Sensory Systems

Learning Objectives

• Understand how sensory information is received and processed.

• Differentiate between the different receptors.

Sensation

• Senses convert a stimulus (light, sound, body position) into an electrical signal in the nervous system.

• This process is called sensory transduction.

Types of Sensation

• Olfaction: Smell

• Gustation: Taste

• Equilibrium: Balance and body position

• Vision

• Hearing

• Electroreception

• Magnetoreception

• Somatosensation

  • Vestibular sensation: Spatial orientation and balance

  • Proprioception: Position of bones, joints, and muscles

  • Kinesthesia: Limb movement and tracking

Sensory Transduction Overview

1. Stimulus

2. Reception: Activation of sensory receptors

3. Ion Channels Affected: Changes in Electrical Potential

   • Positive change in membrane potential depolarizes the neuron.

   • Negative change hyperpolarizes the neuron.

4. If polarization change is sufficient (crossing a threshold), the neuron will fire.

Encoding and Transmission of Sensory Information

• Type of stimulus

• Location of the stimulus in the receptive field

• Duration of the stimulus

• Relative intensity of the stimulus

Intensity is encoded by:

• Rate of action potentials (frequency)

• Number of receptors activated (population)

Perception

• In humans, with the exception of olfaction, all sensory signals are routed from the thalamus to final processing regions in the cortex of the brain.

Mammalian Skin

• Mammalian skin has three layers: an epidermis, a dermis, and a hypodermis.

Receptor Types

• Thermoreceptors

• Pain receptors (Nociceptors)

• Chemoreceptors

• Mechanoreceptors

  • Tactile

    • Merkel’s disks, Meissner’s corpuscles, Ruffini endings, Pacinian corpuscles, Krause end bulbs

  • Proprioceptors

  • Baroreceptors

Receptor Types in Skin

• Merkel’s disks, which are unencapsulated, respond to light touch.

• Meissner’s corpuscles respond to touch and low-frequency vibration.

• Ruffini endings detect stretch, deformation within joints, and warmth.

• Pacinian corpuscles detect transient pressure and high-frequency vibration.

• Krause end bulbs detect cold.

Meissner’s Corpuscles

• Meissner’s corpuscles in the fingertips allow for touch discrimination of fine detail.

• Respond to touch and low-frequency vibration.

Pacinian Corpuscles

• Pacinian corpuscles detect transient pressure and high-frequency vibration.

Olfactory System

• In the human olfactory system, bipolar olfactory neurons extend from the olfactory epithelium, where olfactory receptors are located, to the olfactory bulb.

Taste

• Foliate, circumvallate, and fungiform papillae are located on different regions of the tongue.

Taste Types

• In humans, there are five primary tastes, and each taste has only one corresponding type of receptor.

• Like olfaction, each receptor is specific to its stimulus (tastant).

• Transduction of the five tastes happens through different mechanisms that reflect the molecular composition of the tastant.

  • Sweet

  • Salty

  • Sour

  • Bitter

  • Umami

Sound Waves

• For sound waves, wavelength corresponds to pitch.

• Amplitude of the wave corresponds to volume.

The Ear and Hearing

• Sound travels through the outer ear to the middle ear, which is bounded on its exterior by the tympanic membrane.

• The middle ear contains three bones called ossicles that transfer the sound wave to the oval window, the exterior boundary of the inner ear.

• The organ of Corti, which is the organ of sound transduction, lies inside the cochlea.

Sound Wave Transduction

• A sound wave causes the tympanic membrane to vibrate.

• This vibration is amplified as it moves across the malleus, incus, and stapes.

• The amplified vibration is picked up by the oval window causing pressure waves in the fluid of the scala vestibuli and scala tympani.

Hair Cells

• The hair cell is a mechanoreceptor with an array of stereocilia emerging from its apical surface.

• The stereocilia are tethered together by proteins that open ion channels when the array is bent toward the tallest member of their array, and closed when the array is bent toward the shortest member of their array.

Vestibular Information

• There are five vestibular receptor organs in the inner ear: the utricle, the saccule, and three semicircular canals.

• The roughly 30,000 hair cells in the utricle and 16,000 hair cells in the saccule lie below a gelatinous layer, with their stereocilia projecting into the gelatin.

• Embedded in this gelatin are calcium carbonate crystals—like tiny rocks.

The Electromagnetic Spectrum

• In the electromagnetic spectrum, visible light lies between $380 nm$ and $740 nm$.

Eye Anatomy

• The photoreceptors of the eye are located in the retina on the inner surface of the back of the eye.

• The cornea, the front transparent layer of the eye, and the crystalline lens, a transparent convex structure behind the cornea, both refract (bend) light to focus the image on the retina.

• The iris is a muscular ring that regulates the amount of light entering the eye.

Photoreceptors

• Rods

  • Strongly photosensitive and are located in the outer edges of the retina.

  • Detect dim light and are used primarily for peripheral and nighttime vision.

• Cones

  • Weakly photosensitive and are located near the center of the retina.

  • Respond to bright light, and their primary role is in daytime, color vision.

Transduction of Light

• Rhodopsin, the photoreceptor in vertebrates, has two parts: the transmembrane protein opsin, and retinal.

• When light strikes retinal, it changes shape from a cis to a trans form.

• The signal is passed to a G-protein called transducin, triggering a series of downstream events.

• When light strikes rhodopsin, the G-protein transducin is activated, which in turn activates phosphodiesterase.

• Phosphodiesterase converts cGMP to GMP, thereby closing sodium channels.

• As a result, the membrane becomes hyperpolarized.

• The hyperpolarized membrane does not release glutamate to the bipolar cell.