Systems sensory

Bio 225

What is modality encoded by

the neural pathway activated

What is location encoded by

receptive fields

What is intensity encoded by

action-potential frequency

What is duration encoded by

tonic vs. phasic responses/adaption

Generator potential

A graded potential that occurs directly in the sensory neuron. The receptor is part of the sensory neuron’s dendritic region.

Receptor potential

A graded potential occurs in a separate receptor/accessory cell, then communicates with afferent neuron.

Chemoreceptors

Detect chemicals

Mechanoreceptors

Detect pressure and movement, including proprioception.

Photoreceptors

detect light

Thermoreceptors

detect temperature

Electroreceptors

detect electric fields

Magnetoreceptors

Detect magnetic fields

Adequate stimulus

stimulus modality which receptor is most sensitive and preferentially responsive

Polymodal Receptors

respond to more than one stimulus modality

Example of Polymodal Receptors

nociceptors because they respond to thermal, mechanical, and chemical cues

What encodes stimulation location?

receptive fields

Receptive Fields

region of sensory surface that causes response when stimulated

Small Receptive Field = …

precise localization, higher spatial acuity

Large Receptive Field = …

less precise localization, low spatial acuity

Lateral Inhibition

signals from strongly stimulated center pathways inhibit neighboring pathways

Effects of Lateral Inhibition

enhances contrast, decrease noise, improve edge detection, improve boundary detection, improve spatial acuity

What is the relationship between a stimulus and action potential frequency?

stronger stimulus = higher action potential frequency

Dynamic Range

range of stimulus intensities over which receptor changes its response

Threshold

weakest stimulus that produces a response

Saturation

top of dynamic range, maximal response

Large Dynamic Range = …

large change in stimulus intensity causes small change in AP frequency, poor discrimination

Narrow Dynamic Range = …

small change in stimulus intensity causes large change in AP frequency, better discrimination

Range Fractionation

there are specific receptors dedicated to specific stimulus intensities

Logarithmic Encoding

compressing large intensity range so weak and strong stimuli can be represented

Tonic Receptors

respond for entire duration of stimulus, slow adapting

Phasic Receptors

respond to changes in stimulus, rapidly adapting

Sensory Adaptation

a prolonged constant stimulus can decrease response frequency

Sensory Adaptation - Tonic Receptor

continues firing during the stimulus, reduced rate

Sensory Adaptation - Phasic Receptor

responds strongly at beginning, stop when stimulus constant, responds when stimulus changes

Vision - Photoreceptors

convert light energy into changes in membrane potential

Opsins

7 transmembrane GPCRs located in outer segment membranes of photoreceptors

What do Opsins bind?

vitamin A derived chromophore (RETINAL)

Light-Induced Events Steps

opsin binds chromophores, photons isomerize chromophore, isomerization leads to changes in opsin, conformational change or dissociation of chromophore, G protein signaling causes causes change in membrane potential

Two Classes of Photoreceptors

Rhabdomeric and Ciliary

What do Rhabdomeric photoreceptor signal through?

Gq

Rhabdomeric signal pathway

Light → retinal isomerization of all trans→ opsin activated → Gq → PLC →PIP2 produces DAG and IP3 → DAG activates TRP cation channels → Na and Ca2 enter → depolarization.

Ciliary photoreceptors

Vertebrate rods and cones. They signal through Gi/transduction.

Sequence of ciliary photoreceptors

1. Light is absorbed by 11-cis retinal.

2. Retinal becomes all-trans.

3. Activated opsin activates transducin.

4. Transducin activates PDE.

5. PDE converts cGMP to GMP.

6. cGMP concentration falls.

7. Cyclic-nucleotide-gated Na+ channels close.

8. Na+ entry decreases.

9. The photoreceptor HYPERPOLARIZES.

How do photoreceptors communicate?

They do not fire action potentials. they communicate using graded changed in membrane potential and changes in NT release

Cornea

Major initial refraction of incoming light.

Lens

Fine-tunes focus onto the retina. Changes shape during accommodation.

Iris

contains smooth muscle that controls pupil diameter

Pupil

opening/hole in the iris

Retina

contains rods, cones, and retinal neural circuitry

Choroid

pigmented layer that absorbs stray light

Tapetum lucidum

Reflective layer in many nocturnal animals. Amplifies dim light and contributes to eye shine.

Accomodation

the eye’s ability to focus light coming from different distances by changing lens shape

How does accommodation work for a nearby object

Light rays are less parallel, the ciliary muscle contracts, suspensory ligaments slacken, the lends bulges/becomes more rounded, focus is rstored onto the retina.

Rods

Ciliary photoreceptors that are sensitive to dim light. Do not provide the high-acuity foveal pathway. High convergence, larger receptive fields, lower spatial acuity.

Cones

Ciliary photoreceptors that require brighter light. Important for color vision, Low convergence and small receptive fields in the fovea and higher spatial acuity.

Rod convergence

Many rods can synapse onto a single bipolar cell, multiple bipolar cells can then converge onto a single ganglion cell.

Consequences

Larger receptive field, greater sensitivity, and less precise spatial localization.

Foveal cone pathway

1 cone → 1 bipolar → 1 ganglion cell. this minimizes convergence. the result is very small receptive fields, high spatial acuity.

Fovea centralis

specialized for sharp central vision, exclusively cones, no obstructing capillaries, retinal layers are displaced so incoming light reaches cones with less obstruction.

Optic disc

Where the optic nerve exits, no photoreceptors, produces the blind spot.

Direction of light vs. direction of signal

Light passes through inner retinal layers before reaching rods and cones at the back of the retina. Neural information then travels in the opposite direction: Photoreceptor → bipolar cell → ganglion cell → CNS.

Disinhibition

Less inhibition can produce more downstream activity.

What happens when there is less inhibitory signaling to bipolar cell?

bipolar cell depolarizes, bipolar cell excites ganglion cell more, ganglion cell firing increases.