Physiology Lab This Week Print out Powerpoints on Vision Part 2 and Vestibulo-cochlear. Sensory Physiology Part 3: Two point discrimination on back of hand, fingertip, cheek, and calf.

Sensing the world Sensory coding: sensory circuits 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 that will 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 (adequate stimulus)

300 types of olfactory protein receptors in the human system 5 major gustatory or taste receptors Chemosensory signals

Receptors for gustation Receptor proteins Receptor cell 1 st order sensory neuron S 3 5 different types of receptor proteins (but not all in the same cell)

Somatosensory and the sensation of touch Skin is largest sense organ: up to 2 million receptors What is occurring during transduction? What is the full repertoire of sensations from skin? Ok, something sweet

Somatosensory: sensation of touch, vibration, pain, and temperature Deep Superficial Sustained stimulus Fluctuating stimulus

Nerves not neurons! Concept of labeled lines.

This diagram is misleading: Different types of receptors are NOT part of the same sensory neuron! S 9

Labeled Lines: Different sensory modalities are transmitted separately along distinct pathways. S 10

Most mechanoreceptors (touch fibers) are similar – it is the environment around the neuron the varies

Activation of mechanically gated channels Receptors and transduction

Receptor potentials Def: the graded potentials that are the direct result of transduction within a receptor cell or receptive membrane. Transduction leads to a receptor potential. Amplitude of the receptor potential is usually in proportion to the stimulus intensity. Receptive membranes at distal tips of sensory axons (somatosensory and olfactory systems have a trigger zone and thus action potentials. Other receptors are short, specialty cells with no axon (visual, gustatory, auditory, and vestibular systems). The graded receptor potentials will directly change amount of NT secretion. e.g. somatosensory e.g. gustatory

Sensations to touch Mechanoreceptors contain receptor proteins that respond to stretching, distortion, or pressure on the peripheral plasma membrane

NT is released in “quanta” The amount of quanta released depends upon the frequency of APs across the axon terminal membrane

Fig Activation of mechanically gated channels and convergence of graded receptor potentials from different parts of the dendritic arbor of the receptor. Thus a receptor cell with a more extensive arbor will likely be more ____________. Why?

Receptive Fields An individual receptor will be activated by stimuli that fall within a specific area = receptive field (RF) RF size varies depending upon the dendritic arbor of the individual receptor and position of the receptor in the sensory organ RF is usually larger than arbor Where would you find small receptive fields and where would you find large fields? Dendritic Arbor Receptive Field

So where do you expect to see large amounts of convergence and where do expect to see low amounts? Receptive fields of secondary afferents depend upon amount of convergence in the circuit

RF?

Fig The size of receptive fields, amount of convergence (both within receptor and the circuit), and level of overlap will determine the resolution of the sensory modality and our ability to spatial discriminate sensations. Two-point discrimination test

Convergence and two-point discrimination test What about amount of overlap? How might that affect our ability to spatially resolve a stimulus?

Fig Receptive fields often overlap – how might the neural circuit enhance differences and thus spatial discrimination?

Spatial discrimination is enhanced by lateral inhibition Lateral inhibition involves near neighbors that inhibit each other Lateral projections inhibit via NTs and IPSPs

Fig

Relative timing of stimuli Receptors show different state of temporal adaptation (Shouldn’t that be “acclimation”, Dr Davis?) Tonic vs. phasic – Tonic receptors show little adaptation and continue to transmit signals as long as there is a stimulus – Phasic receptors show a high level of adaptation and will decrease their responsiveness to a steady stimulus