Sensory Systems 1

Four basic attributes that sensory systems mediate

1. Modality: type of sensation/stimulus

2. Location

3. Intensity

4. Timing

Stimulus Transduction

The process by which environmental energy is transformed by a receptor into an electrical signal

Receptor

Specialized neuron or cell that responds to a specific type or environmental energy/stimulus

Receptive Field

Area on sensory surface where adequate stimulus increases or decreases activity in the sensory system

Adequate (appropriate Stimulus

Type of stimulus energy that activates receptor at the loweset relative intensity

Receptor Potential

evoked by adequate stimulus

• graded (like epsp)

• increases with stimulus intensity

• can be evoked directly by opening (or closing) ion channels, or indirectly via second messenger pathways

Tonic Receptor

Doesn’t adapt or adapts slowly to a sustained stimulus; provides continuous information about the stimulus

Phasic Receptor

Adapts rapidly to a sustained stimulus

• frequently exhibits an off response when the stimulus is removed

• receptor signals changes in stimulus intensity

Adaptation

A property of the receptor that determines how long it continues to fire in presence of maintained stimulus

• very common in all sensory systems

Labeled Line Coding

The brain associates a signal from a specific group of receptors with a specific modality

• simulus location is coded by the site of a stimulated receptor

• APs from each receptor travel unique pathways to specific CNS regions associated only with that modality

Receptive field

The stimulus region within which a stimulus evokes a sensory response (frequency)

• receptors transduce specific types of energy into an electrical signal (action potentials)

Somatosensory system

Touch, pain, and temperature

What are mechanoreceptors

highly specialized neuron endings encapsulated in elaborate cellular structures that are sensitive to physical distortions such as bending or stretching

Thermoreceptors

Respond to changes in temperature

Chemoreceptors

Sensitive to various chemical alterations

Photoreceptors

Sensitive to light

Nociceptors

Transduce harmful stimuli into pain perception

What is the primary function of a mechanoreceptor’s capsule?

To transmit the mechanical tension in the capsule to ion channels in the neuron endings, activating them

How do stretch-activated ion channels work in mechanoreceptors?

Open when forces in the lipid bilayer change (e.g., tension or curvature)

How do mechanoreceptors differ in their rate of adaptation?

Some respond only when the stimulus is changing (touch, movement, and vibration)

• Others adapt more slowly (e.g., sensation of pressure)

Tethered in mechano-transduction

Some mechanically gated channels require links to extracellular or cytoskeletal proteins

• displacements that change tension on these links open the ion channel

Indirect gated channel in mechano-transduction

Mechanosensitive protein regulates ion-channel opening through an intracellular second messenger

Touch receptors in the skin

Different types of touch receptors are scattered throughout the superficial (near the surface) and deep layers of the skin

• respond to different forms of physical contact: stretch, steady, pressure, vibration, texture

Touch receptors receptive fields

The firing pattern of mechanoreceptors in teh superficial layers of the skin encode the texture of object

• Merkel receptors can resolve the patterns because the fields are smaller than dot spacing

• Pacinian Corpulsces can not resolve the texture patterns because fields are larger than dot spacing

Chemoreceptors

Receptors respond to various stimuli that may pass directly through ion channels or bind to and block ion channels

• basis for taste, olfaction and numerous feedback systems in the body

Lateral inhibition

A mechanism that enables the localization of a stimulus site

• information from receptors at the edge of a stimulus are inhibited compared to the receptors at the stimulus center

• enhances the contrast between the center and the edge of the stimulated region

Somatosensory pathways

Sensations from one side of the body goes to the cerebral cortex on the opposite side

Dorsal Column Pathway

Transmits information from mechanoreceptors and proprioceptors

• crosses to the contralateral side in the brainstem

Spinothalamic Pathway

Transmits information from thermoreceptors and nociceptors

• crosses to the contralateral side in spinal cord

Primary Somatosensory Cortex (S1) location

Located in the Parietal Lobe within the central sulcus of the brain

• highly organized in a columnar fashion

Primary Somatosensory Cortex (S1) function

Maintains the association between receptors and the modality they transmit

Place specificity

Receptors innervating a specific site on the body project to a specific columnar area at the appropriate site on somatopic map

Modality Specificity

A given receptor is represented in a specific columnar region at the appropriate site on the somatotopic map

• sensations for a given body part are processed in close proximitity

• e.g., termination of cold receptors and pressure receptors for left hand are next to each other

Receptor density

Areas with higher receptor density have larger cortical representation

• cortical magnify-cation