sensing the world

Sensing the world • Sensory coding: sensory systems code for modality, intensity, location, and duration of external stimuli.

• Transduction: the conversion of a physical stimulus into a change in membrane potential (electrochemical signal) – Signals are transmitted in the form of

graded potentials, action potentials, and synaptic interaction

• Receptors: cells or regions of cells that respond to specific stimuli and perform transduction– The process of sensory coding starts

here– Specificity: receptors are often

sensitive to specific stimuli; varies with receptor type

S 1

Changes of membrane potential depend on ion channels

Reminder: leak channels are not gated.

S 2

Receptors for gustation

Receptor proteins

Receptor cell

1st order sensory neuron

S 3

5 different types of receptor proteins (but not all in the same cell)

Hair cells = specialized receptor cell with

mechanically-gated K+ channels.

Receptors for audition (hearing)and

equilibrium(movement and orientation to gravity)

Receptor cell

1st order sensory neuron

S 4

Scanning electron micrograph of hair cells from the bullfrog inner ear, which contain the mechanically-gated ion channel TRPA1. (Image courtesy of Howard Hughes Medical Institute)

Receptor cells withreceptor proteins.

Encapsulated

S 6

TZ

TZTZ

Somatosensory and the sensation of touch

Skin is largest sense organ: up to 2 million receptors

Sensory system must code for where (location), how hard (intensity), how long (duration), and modality (sense of touch, temperature, vibration, wet/dry, damage (pain; noxious)).

S 7

Somatosensory: sensation of touch, vibration, pain, and temperature

Deep

Superficial

Sustained stimulus

Fluctuating stimulus Encapsulated nerve endings

S 8

“Adequate” stimulus: the stimulus type to which areceptor responds best (lowest threshold.)

This diagram is misleading: Different types of receptors are NOTpart of the same sensory neuron!

S 9

Labeled Lines:Different sensory modalities are transmitted separately along distinct pathways.

S 10

Figure 7.15Must Code for 1) Stimulus Localization& 2) Modality & 3) Intensity& 4) Duration

Three neurons to the cortex!

Labeled Line

S 11

Anterolateral (spinothalamic) tract1st order synapses onto 2nd order in spinal cord, 2nd order axon decussates in spinal cord, travels to thalamus in contralateral anterolateral tract, synpases onto 3rd order neuron in thalamus, which sends its axon to SSC.

Dorsal column-medial lemniscal tract1st order axons ascend in ipsilateral dorsal columns to synapses onto 2nd order in dorsal column nuclei of brainstem, 2nd order axon decussates in brainstem, travel to thalamus to synpases onto 3rd order neuron, which sends it axons to SSC.

Right side

Right sideRight side

Right side

Proprioception

1

2

3

S 12

Regions more densely innervated by sensory receptors occupy more cortical tissue.

Somatotopy in the Somatosensory Cortext (post-central gyrus.)S 13

Referred pain

Who cares?

S 14

What about referred pain from the heart?

S 16

Referred PainFig 7-18 p. 200 of text

Receptive field = the area of the body that, when stimulated, leads to activity in a neuron.

= 1st order sensory neuron

Could be encapsulated

S 17

Dorsal column-medial lemniscal tract1st order axons ascend in ipsilateral dorsal columns to synapses onto 2nd order in dorsal column nuclei of brainstem, 2nd order axon decussates in brainstem, travel to thalamus to synpases onto 3rd order neuron, which sends it axons to SSC.

Right side

Right sideRight side

Right side

Proprioception

Receptive field = the area of the bodythat, when stimulated, leads to activity in a neuron.

1st, 2nd, and 3rd order sensory neurons each have receptive fields.

S 18

Receptor potentials

• Graded potentials are the result of transduction within a receptor. Transduction produces a receptor potential

• Amplitude is usually in proportion to the stimulus intensity

• Specialty receptor cells with no axon (visual, gustatory, auditory, and vestibular systems). The graded receptor potentials will directly change amount of NT secretion onto 1st order sensory neuron.

• Receptors with axons (somatosensory and olfactory systems) have a trigger zone and receptor potentials generate action potentials

e.g. somatosensory

e.g. gustatory

S 19

Activation of mechanically gated channels

Receptors and transduction

Could be encapsulated

S 20

Sensations to touchMechanoreceptors contain receptor proteins that respond to

stretching, distortion, or pressure on the peripheral plasma membrane

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Summary to now:

Modality encoded by _______.Intensity encoded by _______.

What’s left?Location encoded by _______.Duration encoded by _______.

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Figure 7.06

The size of receptive fields varies dramatically in different regions of skin (i.e. lips, palm, fingertip, calf).

For touch discrimination, small receptive fields allow greater accuracy in “two point discrimination” test (in lab!)

Stimulus LocalizationS 24

Receptive fields of different neurons often overlap such that any patch of skin may have several receptors of the same type (modality) and receptors of different types (different modalities I.e. touch, pain temperature, etc.)

Overlapping receptive fields of touch receptors (Meissners, Merkels) allow for more precise localization of a stimulus via the mechanism of lateral inhibition (next slide.)

Stimulus Localization& Intensity

S 25

Lateral inhibition exaggerates the difference in stimulus intensity detected by adjacent neurons, aids with LOCALIZATION.

Stimulus Localization & Intensity

PresynpaticInhibition

Mildinhibition

Massiveinhibition

S 26

Lateral inhibition improves stimulus localization.

Stimulus Localization& Intensity

S 27

Which is easier to localize and why: a light touch or a moderate touch?

Right sideRight side

Right side

Proprioception

1

2

3

1QQ # 12 Answer one1. Which of the following are correct statements:

a) The intensity of a stimulus is proportional to the size of the graded potential in the receptive membrane.b) The modality of a stimulus is encoded by which type or types of sensory receptors are activated.c) The intensity of a stimulus is encoded by the frequency of action potentials.d) Some ascending spinal neurons receive synaptic inputs from more than one part of the body.e) Each taste receptor cell has at least two different categories of tastant receptor proteins.

2. What is the meaning of labeled line in the context of sensory physiology?

Adaptation = decrease in frequency of APs with sustained stimulus

S 28

Coding for stimulus duration.

Figure 7.11

Examples: waistband of underwear, top of socks, earrings, mechanoreceptors in carotid arteries for blood pressure

YouTube

S 29

“Flutter” receptors are rapidly adapting.