How can we go about studying the neuronal processes involved in sensation and perception?

Transcription

1 How can we go about studying the neuronal processes involved in sensation and perception? quantified sensory input sensory coding psychophysi cs neuronal activity decoding, decision making percept, behavior

2 So what is sensory transduction? We know the world by our physical senses, whether nearby objects by touch or distant objects by sight. Sensory receptors are the gateway through which all signal must pass to enter the nervous system. Sensory transduction means getting the outside in

3 Sensory neuron ( receptor ) Captures energy of xx-modality and converts it to a variation of membrane potential ( transduction ). This introduces the external world into the nervous system. xx-modality is the definition of the sensory system. Receptor cell has resting membrane potential which is perturbed by absorption of some quantity of energy >> opening of membrane channels, etc.

4 For sensory signals there is a great deal of amplification at the receptor level, so that very small external stimuli provide a trigger to release stored charges that appear as electrical potentials and lead to sensations. For example odors from only a few molecules of specific odorant substances (pheromones) are able to act as sex attractants for moths. a few quanta of light trapped by receptors in the retina are sufficient to produce a visual sensation. In the inner ear, mechanical displacements of m are detectable.

5 A bit of history

6 1830. Johannes Muller (Berlino) wrote law of specific nerve energy : we sense not the world itself but information correlated with objects in the world as transmitted by sensory nerves. Muller: every nerve can be excited by a variety of stimuli, but is highly sensitive to just one type of stimulation. If excited by energy of another modality, the sensation is nevertheless correlated with the nerve, not the inappropriate modality (just rub your eye to find out). The difference between seeing and hearing, between hearing and touch, and so on - are not caused by differences in the stimuli themselves but by the different nervous structures that these stimuli excite.

7 1906. sir Charles Sherrington: The sensory organ is an apparatus that renders the afferent fiber particularly sensitive to one physical agent and, at the same time, makes the fiber insensitive to other agents. Filtering, selectivity. Physiologists: how?

8 Adrian, 1928 Neuronal coding Nerve is labelled line, physiological update of Muller. Modality or sensation is not by characteristics of firing: Action potentials same in every modality (if you hear a neuron firing, you have no idea what part of the brain it is in) yet optic nerve = light Intensity of firing (firing rate) encodes stimulus intensity, but also many other things. Firing patterns are also critical in some sensory systems.

9 Example muscle spindle distance Receptor potentials recorded extracellularly from a sensory nerve fiber supplying a muscle spindle. The recording electrode is placed as close as possible to the receptor. Downward deflection of the voltage record (lower trace) indicates receptor depolarization. Stretching the muscle (upper trace) produces a receptor potential, upon which is superimposed a series of action potentials (lower trace). Relationship between receptor potential amplitude and stretch. The slope flattens out at higher levels. Many sensory receptors take advantage of this nonlinear relationship to provide amplitude coding over a wide range of stimulus intensities: response amplitude continues to increase, but in proportion to the logarithm of stimulus intensity. Nicholls et al. (After Katz, 1950)

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11 frequenza alle lezioni

12 How can we go about studying the neuronal processes involved in sensation and perception? quantified sensory input sensory coding psychophysi cs neuronal activity decoding, decision making percept, behavior

13 So what is sensory transduction? We know the world by our physical senses, whether nearby objects by touch or distant objects by sight. Sensory receptors are the gateway through which all signal must pass to enter the nervous system. Sensory transduction means getting the outside in

14 Sensory neuron ( receptor ) Captures energy of xx-modality and converts it to a variation of membrane potential ( transduction ). This introduces the external world into the nervous system. xx-modality is the definition of the sensory system. Receptor cell has resting membrane potential which is perturbed by absorption of some quantity of energy >> opening of membrane channels, etc.

15 For sensory signals there is a great deal of amplification at the receptor level, so that very small external stimuli provide a trigger to release stored charges that appear as electrical potentials and lead to sensations. For example odors from only a few molecules of specific odorant substances (pheromones) are able to act as sex attractants for moths. a few quanta of light trapped by receptors in the retina are sufficient to produce a visual sensation. In the inner ear, mechanical displacements of m are detectable.

16 1830. Johannes Muller (Berlino) wrote law of specific nerve energy : we sense not the world itself but information correlated with objects in the world as transmitted by sensory nerves. Muller: every nerve can be excited by a variety of stimuli, but is highly sensitive to just one type of stimulation. If excited by energy of another modality, the sensation is nevertheless correlated with the nerve, not the inappropriate modality (just rub your eye to find out). The difference between seeing and hearing, between hearing and touch, and so on - are not caused by differences in the stimuli themselves but by the different nervous structures that these stimuli excite.

17 1906. sir Charles Sherrington: The sensory organ is an apparatus that renders the afferent fiber particularly sensitive to one physical agent and, at the same time, makes the fiber insensitive to other agents. Filtering, selectivity. Physiologists: how?

18 Adrian, 1928 Neuronal coding Nerve is labelled line, physiological update of Muller. Modality or sensation is not by characteristics of firing: Action potentials same in every modality (if you hear a neuron firing, you have no idea what part of the brain it is in) yet optic nerve = light Intensity of firing (firing rate) encodes stimulus intensity, but also many other things. Firing patterns are also critical in some sensory systems.

19 Example muscle spindle distance Receptor potentials recorded extracellularly from a sensory nerve fiber supplying a muscle spindle. The recording electrode is placed as close as possible to the receptor. Downward deflection of the voltage record (lower trace) indicates receptor depolarization. Stretching the muscle (upper trace) produces a receptor potential, upon which is superimposed a series of action potentials (lower trace). Relationship between receptor potential amplitude and stretch. The slope flattens out at higher levels. Many sensory receptors take advantage of this nonlinear relationship to provide amplitude coding over a wide range of stimulus intensities: response amplitude continues to increase, but in proportion to the logarithm of stimulus intensity. Nicholls et al. (After Katz, 1950)

20 Classification schemes (continued) by the nature of the receptor potential and the synaptic transmission short-distance receptors / usually graded potential / graded transmitter release long-distance receptors / usually action potential / binary transmitter release by source of energy exteroceptors interoceptors by the rate of adaptation rapidly adapting slowly adapting by selectivity chemical taste, olfaction mechanical thermal pain photo sound wave

21 Classification schemes Short versus long receptors The distinction is not as silly as it sounds. Short can transmit analog signals by receptor potential, but long can only transmit digital signals

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23 Classification schemes (continued) by the nature of the receptor potential and the synaptic transmission short-distance receptors / usually graded potential / graded transmitter release long-distance receptors / usually action potential / binary transmitter release by source of energy exteroceptors interoceptors by the rate of adaptation rapidly adapting slowly adapting by the nature of conversion direct indirect by selectivity chemical taste, olfaction mechanical thermal pain photo sound wave

24 Classification schemes (continued) by the nature of the receptor potential and the synaptic transmission short-distance receptors / usually graded potential / graded transmitter release long-distance receptors / usually action potential / binary transmitter release by source of energy exteroceptors interoceptors by the rate of adaptation rapidly adapting slowly adapting by the nature of conversion direct indirect by selectivity chemical taste, olfaction mechanical thermal pain photo sound wave

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26 Adaptation

27 The Pacinian receptor illustrates the principle of mechanoreceptor filtering. ANATOMY The primary afferent sensory ending lies within a 1-mm long, multilamellar, fluid-filled capsule deep in the skin. The nerve fiber is myelinated up to its entry in the capsule, where it loses the myelin sheath. Its membrane contains ion channels sensitive to compression.

28 Pacinian corpuscle, rapidly adapting receptor If the skin vibrates at frequencies of more than about 80 Hz, the stiffness of the lamellar structure of the capsule prevents it from changing shape as quickly as the probe indentation. All the lamellae move up and down together in phase with the probe, compressing and decompressing the termination. Each vibratory cycle, even as small as 1 micron in magnitude, evokes a depolarization in the sensory nerve that leads to an action potential. If the vibratory frequency is slowed to less than Hz, each indentation squeezes together the upper layers of the capsule, and displaces the fluid laterally. At the same time the bottom layers close to the nerve remain rigid: mechanical energy is dissipated before reaching the nerve s mechanosensory channels. Thus, activity in the nerve fiber from the Pacinian corpuscle transmits information only about high-frequency vibrations.

29 FUNCTION Although they are found generally in subcutaneous tissue, they are particularly common around footpads and claws of mammals, and in the interosseus membranes bridging the bones of the leg and forearm, where they act as sensitive detectors of ground vibration. A similar structure, the Herbst corpuscle, is found in the legs, bills, and cutaneous tissue of birds (and in the tongues of woodpeckers!). Speculation about their physiological function includes detection by the duck s bill of aquatic vibrations due to small prey, and in soaring birds, detection of vibration of flight feathers due to improper aerodynamic trim. To check whether the fan in your computer is rotating, you might place your hand on the case to detect humming vibrations transduced by Pacinian receptors. Pacinian role in TEXTURE.

30 Classification schemes (continued) by the nature of the receptor potential and the synaptic transmission short-distance receptors / usually graded potential / graded transmitter release long-distance receptors / usually action potential / binary transmitter release by source of energy exteroceptors interoceptors by the rate of adaptation rapidly adapting slowly adapting by selectivity chemical taste, olfaction mechanical thermal pain photo sound wave

31 forward lateral down

32 incus malleus stapes Oval window Round window scala vestibuli External acoustic meatus (ear and canal) Tympanic membrane scala timpani Basilar membrane Tympanic membrane captures the acoustic pressure wave and transforms it to a mechanical vibrations

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34 incus malleus stapes Oval window Round window scala vestibuli External acoustic meatus (ear and canal) Tympanic membrane scala timpani Basilar membrane

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36 organ of Corti

37 Hair cells

38 hudspeth hair bundle

39 bechara kach

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41 tip links can bend or break (loud noise)

42 The cilia are connected by TIP LINKS and the ion channels is found at the top of the cilium

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46 Somatosensory Aristotle listed the famous 5 senses, one of the touch. yet modern neuroscience recognizes at least 4 modalities within somatosensation. (1) proprioception (2) tactile or touch (light mechanical stimuli) (3) pain (damaging stimuli) (4) thermal

47 Are they really distinct? Well, yes and no. There can be selective deficits. For example tabe dorsale (syphilis) can cause a rather selective proprioceptive deficit. Specific spinal lesions of other sorts can also cause specific deficits Surgical lesion for chronic pain

48 Section of skin showing the morphology and position of tactile receptors. This figure refers to glabrous skin (meaning hairless, e.g. skin of the palms) in primates. The mechanoreceptive nerve membrane is sheathed in various accessory organs! The Meissner corpuscle, Pacinian corpuscle, Ruffini corpuscle, and Merkel disk are all specialized structures that filter the mechanical energy that excites the nerve termination. The free nerve endings are, as the name implies, uncovered, exposing them to substances released by the skin tissues. Touch, low-freq vibration High-freq vibration pressure Pressure, stretch Temperature, noxious stimuli

49 Hair follicles. Schematic view of a hair shaft and follicle from the skin of a primate. Nerve terminations are distributed throughout the follicle, and wrapped around the base of the hair, generating impulses for any hair movement. The muscle contracts to erect the hair in response to cold or arousal. Schematic view of a special hair follicle, that of the large whisker of the snout of a rat or mouse. Nerve terminations occupy many different locations within the follicle, and their positioning is likely to be closely related to type of hair movement that excites them (vibration, bending, pulling etc.).

50 you rat

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52 stimulus time (rapidly adapting, RA) (slowly adapting, SA) Receptor Sensitivity Pacini vibration, f > Hz RA Meissner vibration, f < 50Hz RA Merkel pressure, stretching (superficial) SA Ruffini pressure, stretching (deep) SA Hair follicle hair motion various Free termination thermal, noxious SA, but also sensitizing

53 Morphology of sensory receptor neurons. The cell body that lies in a dorsal root ganglion situated in the aperture between the vertebrae of the spine. A central branch that joins a dorsal root and projects into the spinal cord, and a peripheral branch that joins other fibers in a peripheral nerve and then terminates as a specialized cutaneous receptor.

54 What if we measure the signals of individual receptors as they travel towards the spinal cord in the peripheral nerve. What we find is the amazing relationship between single fibers and sensation!

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56 Vallbo e Johansson Receptive fields of Meissner receptors (sensitive to pressure and vibration)

57 1. Receptive field of a fiber in human subject is similar to experimental subject. 2. Tactile stimulation confirms submodality specificity. 3. Electrical stimulation of fiber confirms the equivalence between fiber activity and sensation. 4. The system has very low noise

58 Well, what about painful stimuli? Just a very strong activation of touch receptors?

59 The discovery of a separate population of nociceptors which correspond to C fibers together with the finding that low-threshold mechanoreceptors do not respond to painful stimuli, ruled out an earlier theory that pain results from the excessive mechanoreceptor stimulation. nociceptor pressure Tactile receptor Indeed there is growing evidence for nociceptor submodalities; for example, a population of itch-specific afferent neurons.

60 Temperature, noxious stimuli

61 Nociceptors are mechano-chemical receptors

62 Temperature changes are transduced by free nerve endings through the activation of Transient Receptor Potential (TRP) ion channels. Channel permeabilities vary with changes in skin temperature. Four different TRP channels, TRP-V1 to TRP-V4, are activated over different warm temperature ranges. TRP-V1 is activated by noxious heating, and is sensitive to capsaicin. TRP- M8 is a Ca2+ permeable channel activated by lowering temperature. Menthol and eucalyptol also activate this channel, which explains the cooling sensation evoked by these compounds.

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