(8) General principles of sensory processing, touch and pain

Sensory receptor organ

An organ (such as the eye or ear) specialized to receive particular stimuli.

Stimulus

A physical event that triggers a sensory response. E.g. soundwaves reaching the ear, light entering the eye, or food touching the tongue.

Receptor cell

A specialized cell that responds to a particular energy or substance in the internal or external environment and converts energy into a change in the electrical potential across its membrane (electrical signals).

Adequate stimulus

The type of stimulus for which a given sensory organ is particularly adapted. E.g. the adequate stimulus for the eye is photic (light) energy; an electrical shock or pressure on your eye can create an illusory sensation of light (called a phosphene), but neither electricity nor mechanical pressure is considered an adequate stimulus for the eye.

Specific nerve energies

The doctrine that the receptors and neural channels for the different senses are independent and operate in their own special ways and can produce only one particular sensation each. E.g. however the eye is stimulated, the resulting sensation is always visual.

Labeled lines

 The concept that each nerve input to the brain reports only a particular type of information. (Particular neurons are, at the outset, labeled for distinctive sensory experiences). Neural activity in one line signals a sound, activity in another line signals a smell, and activity in other lines signal touch. We can even distinguish different types of touch because some lines signal light touch, other indicate vibration and yet other lines report stretching of the skin.

Sensory transduction

The process in which a receptor cell converts the energy in a stimulus into a change in the electrical potential across its membrane. (Receptor cells are transducers that convert energy around us into neural activity that leads to sensory perception.)

Receptor potential

Also called generator potential. A local change in the resting potential of a receptor cell (membrane potential) that mediates between the impact of stimuli and the initiation of action potentials. Resembles, in most instances, the excitatory postsynaptic potentials.

Pacinian corpuscle

Also called lamellated corpuscle. A skin receptor cell type that detects vibration. Is found throughout the body in skin and muscles. Consists of an axon surrounded by a structure that resembles a tiny onion because it has concentric layers of tissue. Mechanical stimuli (in this case vibration) delivered to the corpuscle produce a graded electrical potential with an amplitude that is directly proportional to the strength of the stimulus. When this receptor potential gets big enough, an action potential is generated. The sequence of excitatory events: mechanical stimulation deforms the corpuscle --> Deformation of the corpuscle stretches the tip of the axon --> stretching the axon opens mechanically gated ion channels in the membrane, allowing positively charged ions to enter --> when the receptor potential reaches threshold amplitude, the axon produces one or more action potentials. This is an example of the generator potential (/receptor potential). This type of skin receptor has a large, but vague, border regarding its receptive fields.

Threshold

The stimulus intensity that is just adequate to trigger an action potential.

Coding

The rules by which action potentials in a sensory system reflect a physical/original stimulus. Neural codes are limited in that each action potential is always the same size and duration, so sensory information is encoded by other features of neural activity, such as the number and frequency of the action potentials, the rhythm in which clusters of action potentials occur, and so on. In general, more intense stimuli generate more rapid action potentials, but only up to some limit because a given neuron can only fire so fast.

Range fractionation

A hypothesis of stimulus intensity perception stating that a wide range of intensity values can be encoded by a group of cells, each of which is a specialist for a particular range of stimulus intensities.

Somatosensory

Referring to body sensation, particularly touch and pain sensation.

Adaptation

Here, the progressive loss of receptor sensitivity as stimulation is maintained. In terms of adaption, there are two kind of receptors: Tonic receptors and phasic receptors. Adaption means that there is a progressive shift in neural activity away form accurate portrayal of physical events. Thus, the sensory system may fail to register neural activity even though the stimulus continues. Sensory systems emphasize change in stimuli because changes are more likely to be significant for survival. Also sensory adaption prevents the nervous system form becoming overwhelmed by stimuli that offer very little "news" about the world. The basis of adaption includes both neural and nonneural events.

Tonic receptor

A receptor in which the frequency of action potentials declines slowly or not at all as stimulation is maintained. In other words, these receptors show relatively little adaption.

Phasic receptor

A receptor in which the frequency of action potentials drops rapidly as stimulation is maintained.

Top-down process

A process in which higher-order cognitive processes control lower-order systems, often reflecting conscious control.

Sensory pathway

The chain of neural connections from sensory receptor cells to the cortex. Each sensory modality (such as touch, vision or hearing) has a distinct hierarchy of tracts and stations in the brain that are collectively known as the sensory pathway for that modality. Each station in the pathway accomplishes a basic aspect of information processing.

Thalamus

The brain regions at the top of the brainstem that trade information with the cortex. For most senses, information reaches the thalamus before being relayed to the cortex. Information about each sensory modality is sent to a separate division of the thalamus. One way for the brain to suppress particular stimuli is for the cortex to direct the thalamus to emphasize some sensory information and suppress other information - another example of top-down processing.

Receptive field

The stimulus region and features that affect the activity of a cell in a sensory system. The receptive field of a sensory neuron consists of a region of space in which a stimulus will alter that neuron's firing rate. To determine this receptive field, investigators record the neuron's electrical responses to a variety of stimuli to see what makes the activity of the cell change from its resting rate. E.g. which patch of skin must we vibrate to change the activity of a particular Pacinian corpuscle? Receptive fields make it easier to detect edges and discontinuities on the objects we feel. Receptive fields differ in size, shape and in the quality of stimulation that activates them, e.g. some neurons respond preferentially to light touch, while others fire most rapidly in response to painful stimuli and still others respond to cooling.

Primary sensory cortex

For a given sensory modality, the region of cortex that receives most of the information about that modality from the thalamus or, in the case of olfaction, directly from the secondary sensory neurons. Information is sent back and forth between the primary and nonprimary sensory cortex through subcortical loops.

Secondary sensory cortex

 Also called nonprimary sensory cortex. For a given sensory modality, the cortical regions receiving direct projections from primary sensory cortex for that modality. Information is sent back and forth between the primary and nonprimary sensory cortex through subcortical loops.

Primary somatosensory cortex (S1)

Also called somatosensory 1. The gyrus just posterior to the central sulcus, in the parietal lobe, where sensory receptors on the body surface are mapped; primary cortex for receiving touch and pain information. Each S1 receives touch information from the opposite side of the body. The cells in S1 are arranged according to the plan of the body surface. Each region is a map of the body in which the relative areas devoted to body regions reflect the density of body innervations. Thus, parts of the body where we are especially sensitive to touch (like the hand and the fingers) send information to a larger area of S1 than do less sensitive body regions (like the shoulder).

Secondary somatosensory cortex (S2)

Also called somatosensory 2. The region of cortex that receives direct projections from primary somatosensory cortex. Maps both sides of the body in registered overlay; that is, the left-arm and right-arm representations occupy the same part of the map, and so forth.

Attention

A state or condition of selective awareness or perceptual receptivity, by which specific stimuli are selected for enhanced processing. (The process by which we select or focus on one or more specific stimuli for enhanced processing and analysis). Sometimes it is a top-down process, as when we decide to concentrate on a particular task. In other cases it is a bottom-up process, as when a sudden loud noise may pull our attention to some dramatic event.

Cingulate cortex

Also called cingulum. A region of medial cerebral cortex that lies dorsal to the corpus callosum. Has been implicated in attention. The anterior part of this cortex seems to mediate the emotional, discomforting aspect of pain.

Polymodal

Involving several sensory modalities.

Synesthesia

A condition in which stimuli in one modality evoke the involuntary experience of an additional sensation in other modality.

Epidermis

The outermost layer of skin, over the dermis. The thinnest layer.

Dermis

The middle layer of skin, between the epidermis and the hypodermis. Contains a rich web of nerve fibers in a network of connective tissue and blood vessels.

Hypodermis

Also called subcutaneous tissue. The innermost layer of skin, under the dermis. Provides an anchor for muscles, contains Pacinian corpuscles, and helps shape the body.

Tactile

Of or relating to touch.

Meissner’s corpuscle

A skin receptor cell type that detects light touch. Is fast-adapting. Is densely distributed in skin regions where we can discriminate fine details by touch (fingertips, tongue and lips). These skin receptors are more numerous than Merkel's disc, but offer less spatial resolution. Seems specialized to respond to changes in stimuli to detect localized movement between the skin and a surface. This sensitivity to change in stimuli provides detailed information about texture. These skin receptors, as well as Merkel's disc preferentially respond to edges on a surface. This type of skin receptor has a small receptive field with sharp borders.

Merkel’s disc

A skin receptor cell type that detects fine touch. Is slow-adapting. Is  densely distributed in skin regions where we can discriminate fine details by touch (fingertips, tongue and lips). These skin receptors are less numerous than Meissner's corpuscles, but offer more spatial resolution. These skin receptors, as well as Meissner's corpuscle preferentially respond to edges on a surface. This type of skin receptor has a small receptive field with sharp borders.

Piezo

 A family of two proteins that respond to mechanical stretch by opening channels to let cations in to depolarize the cell.

Ruffini’s ending

A skin receptor cell type that detects stretching of the skin when we move fingers of limbs. Is slow-adapting. There is only a few of this type of touch receptor (compared to the other three types), however they have large receptive fields.

Dorsal column system

A somatosensory system that delivers most touch stimuli via the dorsal columns of spinal white matter to the brain. Is made up of the touch receptors Pacinian corpuscles, Merkel's discs, Meissner's corpuscles and Ruffini's endings that send their axon to the spinal cord, where they enter the dorsal horn and turn upward, travelling to the brain along the spinal cord's dorsal column of white matter.

Dorsal column nuclei

Collection of neurons in the medulla that receive somatosensory information via the dorsal columns of the spinal cord. These neurons send their axons across the midline and to the thalamus.

Dermatome

A strip of skin innervated by a particular spinal nerve.

Pain

The discomfort normally associated with tissue damage. "An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or descried in terms of such damage"

Congenital insensitivity to pain

The condition of being born without the ability to perceive pain.

Nociceptor

 A receptor that responds to stimuli that produce tissue damage or pose the threat of damage. Thus they are specialized to respond to pain.

Free nerve ending

An axon that terminates in the skin without any specialized cell associated with it and that detects pain and/or changes in temperature. Display no specialized structure (they look like naked nerve endings), but they have specialized receptor proteins on the cell membrane that respond to various signals. Thus, different nerve endings produce different receptor protein, so they report different stimuli, such as pain and/or changes in temperature.

Capsaicin

 A compound synthesized by various plants to deter predators by mimicking the experience of burning. Is what makes chili peppers spicy hot.

Transient receptor potential vanilloid type 1 (TRPV1)

Also called vanilloid receptor 1. A receptor that binds capsaicin to transmit the burning sensation from chili peppers and normally detects sudden increases in temperature. The receptors normal job is to report drastic rise in temperature to signal burning.  Because the brain interprets action potentials from that nerve/receptor as signaling painful heat, we experience painful heat when substances such as capsaicin (found in chili peppers) binds to the receptor. This receptor consists of thin, unmyelinated fibers called C fibers, thus, this receptor adapts slowly.

Transient receptor potential type M3 (TRPM3)

A receptor, found in some free nerve endings, that opens its channel in response to rising temperatures. This receptor detects even higher temperatures than does TRPV1. The receptor differs from TRPV1 in that, this receptor doesn't respond to capsaicin and in that these receptors are found on nerve fibers larger than those carrying TRPV1, meaning, that this receptor reports to the spinal cord more rapidly than do TRPV1. This receptor is found on relatively large axons known as Aδ fiber (A delta fibers).

Aδ fiber

A moderately large, myelinated, and therefore fast-conducting axon, usually transmitting acute pain information.

C fiber

A small, unmyelinated axon that conducts pain information slowly and adapts slowly.

TRPM8

Also called cool-menthol receptor 1 (CMR1). A sensory receptor, found in some free nerve endings, that opens an ion channel in response to a mild temperature drop or exposure to menthol. The receptor  is found on small C fibers, so it transmits information about cool temperatures rather slowly.

Na_V1.7

Also called SCN9A. A voltagegated sodium channel used almost exclusively by nociceptors to initiate action potentials.

Anterolateral system

Also called spinothalamic system. A somatosensory system that carries most of the pain and temperature information from the body to the brain. Free nerve endings in the skin send their axons to synapse on neurons in the dorsal horn of the spinal cord. These spinal cord neurons send their axons across the midline to the opposite side and up the anterolateral column of the spinal cord to the thalamus.

Glutamate

An amino acid transmitter; the most common excitatory transmitter.

Substance P

A peptide transmitter implicated in pain transmission. (A neuropeptide).

Natriuretic polypeptide B (Nppb)

A peptide neurotransmitter used by neurons reporting itch to the spinal cord.

Neuropathic pain

Pain caused by damage to peripheral nerves. It is often difficult to treat. The pain seems to be due to inappropriate signaling of pain by neurons (rather than to tissue damage). An example of this kind of pain is phantom limb pain. The pain experienced may also be called neuralgia.

Migraines

Intense headaches, typically perceived from one half of the head, that recur regularly and can be difficult to treat. Some migraines are associated with a wave of neuronal hyperexcitation that spreads across the cortex, leaving it its wake a wave of inhibition or cortical spreading depression. The wave of activity stimulates trigeminal nerves that normally signal damage in blood vessels, causing the experience of pain. Another theory is that the trigeminal nerves release a neuropeptide called CFRP, which actually causes pain.

Cannabis

Also called marijuana. Dried leaves and flowers of the plant Cannabis sativa, typically smoked to obtain THC for a psychoactive effect. Has also been used to reduce pain. Reduces pain by stimulating endogenous cannabinoid receptors (CB1 receptors), both in the spinal cord and in the free nerve endings of the nociceptors themselves.

Analgesia

Absence of or reduction in pain.

Opiates

A class of compounds that exert an effect like that of opium, including reduced pain sensitivity.

Endogenous opioids

A class of peptides produced in various regions of the brain that bind to opioid receptors and act like opiates.

Endorphins, Enkephalins, Dynorphins

Three kinds of endogenous opioids, substances that reduce pain perception.

Opioid receptor

A receptor that responds to endogenous and/or exogenous opioids.

Periaqueductal gray

The neuronal body–rich region of the midbrain surrounding the cerebral aqueduct that connects the third and fourth ventricles. It is involved in pain perception.

Placebo

A substance that is known to be ineffective or inert but that, when administered like a drug, can sometimes bring relief. It's implied that placebo relieves pain by causing the release of endogenous opioids. Functional brain imaging indicated that opioids and placebos activate the same brain regions, and that both treatments reduce the activity of neural regions responding to pain, including cingulate cortex and spinal cord. Also, people who responds to placebo show a greater activation of brain regions with opioid receptors than do nonresponders, which further implicates endogenous opioids in the placebo effect. Placebo is effected by the expectation, the receiver of the treatment, has. If he/she expect the treatment to be effective, this contributes to that effectiveness. Thus, e.g. large pills are more effective than small pills, and medicines are more effective if they are thought to be expensive. Also, having a doctor/nurse in a white coat assert that the treatment will work also boosts the effectiveness of placebos.

Naloxone

A potent antagonist of opiates that is often administered to people who have taken drug overdoses. It blocks receptors for endogenous opioids.

Nocebo

An inert substance that causes discomfort due to the patient’s expectations. As with placebo, treatments that are perceived as expensive are more likely to elicit responses form participants.

TRPA1

TRP ion channel that responds to chemical irritants and is involved in inflammatory pain.

Allodynia

Pain from non-painful stimuli

Hyperalgesia

Increased sensitivity to painful stimuli

McGurk effect

An effect within speech perception. Seeing a person speak may effect what we hear.

Double flash illusion

Two identical beeps presented in quick succession together with a single visual flash may lead to the perception of two visual flashes.