Sensory receptors and neural circuits Haytham Eloqayli

Sensory receptors

Sensory receptors: organs which detect sensory stimuli .

Sensory stimuli: sensation modality (touch, sound, light, pain, cold, and warmth..etc)

Sensory receptors

Mechanoreceptors: detect mechanical compression or stretching of tissues Thermoreceptors: detect changes in temperature

Nociceptors (pain receptors): detect tissue damage ( physical or chemical damage)

Chemoreceptors: detect taste in the mouth, smell in the nose, oxygen level ..etc.electromagnetic receptors: detect light on the retina

Sensory receptors

Sensory receptors

Sensory stimulus change in receptor membrane permeability open ion channels allows ions to flow through membrane channels change in transmembrane potential receptor potential.

When the receptor potential rises above the threshold for eliciting action potentials in the nerve fiber attached to the receptor starts action potentials in the nerveThe nerve transmit the action potentials to the CNS.The sensations are transmitted from the receptor to the CNS as impulses (action potentials)

Sensory receptors

How do two types of sensory receptors detect different types of sensory stimuli?

and

If the sensory modalities is transmitted as action potentials, how do we experience these different modalities of sensations?

Sensory receptors

-differential sensitivities of the receptor: each type of receptor is highly sensitive to one type of stimulus for which it is designed and nonresponsive to other types of sensory stimuli.

-labeled line principle: each type of nerve fibers transmitt only one modality of sensation -each nerve fiber (or tract) terminates at a specific point in the CNS, and the type of sensation felt when a nerve fiber is stimulated is determined by the point in the nervous system where the nerve fiber terminate.

Sensory receptors

Receptors characters:

Range of response

Adaptation

Sensory receptors

progressive stronger stimulus progressive increase in the receptor potential progressive increase frequency of repetitive action potentials (if receptor potential above the threshold)

In another word: frequency of action potentials transmitted from sensory receptors increases approximately in proportion to the increase in receptor potential.

This allows the receptor to have an extreme range of response, from very weak to very intense according to the stimulus intensity and change in stimulus intensity.

Sensory receptors

Sensory receptors

Adaptation of Receptors Rapidly adapting Rc (Rated receptors): pacinian and hair receptors detect the change in stimulus strength (detect movement)Slowly adapting Rc (Tonic receptors): joint capsule, muscle spindledetect continuous stimulus strength (give report to the brain about the status of the body).

Non adapting Rc: pain receptors and chemoreceptor

Sensory receptors

Sensory receptors

Nerve Fibers which transmit the signal:

Types

Modality of sensation each one transmit

Nerve different intensity transmission of Signals

Sensory receptors

Types:1st classification: A (large myelinated): fast transmissionC (small un myelinated): slow transmission.

2nd classificationGroup I (a,b), II, III, IV.

Sensory receptors

A,,: muscle spindle, muscle tendon (Golgi organ)

A: fast pain, temperature, crude touch

C: slow pain, crude touch.

Sensory receptors

Spatial Summation: increasing signal strength is transmitted by using progressively greater numbers of fibers.

Temporal Summation: increasing signal strength is transmitted by increasing the frequency of nerve impulses in each fiber.

Sensory receptors

CNS is composed of neuronal pools with different mechanisms of signal processing.

ExcitationFacilitationInhibitionConvergenceDivergence

Sensory receptors

discharge of a single excitatory presynaptic terminal almost never causes an action potential in a postsynaptic neuron.

large numbers of input terminals (presyneptic) must stimulate the postsynaptic neuron simultaneously or in rapid succession to cause excitation

Sensory receptors

Excitatory stimulus(suprathreshold stimulus): enough presyneptic terminals which discharge to cause an action potential in the postsynapse.

Facilitated (subthreshold): presyneptic terminals which discharge is not enough to cause postsyneptic action potential but it makes the post synepse easy to stimulate.Inhibition: some preseyneptic action potentials cause inhibitory postsyneptic signal (inhibit neurons, rather than exciting them)

Sensory receptors

Sensory receptors

Convergence of Signals: multiple inputs uniting to excite a single neuron

Divergence of Signals: the input neurons divided to 2 or more divisions.amplifying type: input signal spreads to an increasing number of neurons divergence into multiple tracts: input signal is transmitted in two or more directions

Sensory receptors

Sensory receptors