The Cutaneous Senses (Cutis: Latin for skin)

ELL788 Date:26/09/2016

Cutaneous System

•  Skin - Protects the organism by keeping damaging agents from penetrating the body

•  Weight? Is it the heaviest organ? •  Surface area? Is it the largest organ?

Epidermis is the outer layer of the skin, which is made up of dead skin cells Dermis is below the epidermis and contains mechanoreceptors that respond to stimuli such as pressure, stretching, and vibration

Merkel receptor – disk-shaped receptor located near the border between the epidermis and dermis

Meissner corpuscle - stack of flattened disks in the dermis just below epidermis

Ruffini cylinder - branched fibers inside a cylindrical capsule

Pacinian corpuscle - onion-like capsule located deep in the skin

Slowly adapting (SA) fibers continue to fire during a sustained stimulus.

Rapidly adapting (RA) fibers fire only at the onset and offset of a sustained stimulus.

Properties of mechanoreceptor nerve fibers Temporal properties - adaptation:

Properties of Mechanoreceptor Nerve Fibers

•  Spatial properties - detail resolution – SA1 fibers (Merkel receptor) respond to

patterns of grooves – RA2 fibers (Pacinian corpuscle) do not

respond to the details of these stimuli

(a) The firing of Merkel receptors/SA1 fiber signals the grooved stimulus pattern, but (b) the firing of the Pacinian corpuscle/RA2 fiber does not. Results such as these indicate that the Merkel receptor/SA1 fiber signals details

What happens beyond the skin?

peripheral nerves

proprioceptive and touch information

temperature and pain information

Travel to contralateral Thalamus/cortex

Pathways from Skin to Cortex

Maps of the Body on the Cortex

•  Signals travel from the thalamus to the somatosensory receiving area in the cortex

•  Body map (homunculus) on the cortex shows more cortical space allocated to parts of the body that are responsible for detail

•  Plasticity in neural functioning leads to changes in how cortical cells are allocated to body parts

Body map on the somatosensory cortex

The sensory homunculus of the somatosensory cortex

(a) Each numbered zone represents the area in the somatosensory cortex that represents one of the monkey’s five fingers. The shaded area on the zone for finger 2 is the part of the cortex that represents the small area on the tip of the finger shown in (b). (c) The shaded region shows how the area representing the fingertip increased in size after this area was heavily stimulated over a 2-month period.

Plasticity of the somatosensory map

Any evidence of this in humans?

Maps of the Body on the Cortex - continued

Focal hand dystonia in musicians (“musician’s cramp”), is a greatly feared condition that leads to reduced performance levels, very often resulting in the termination of a musician’s career. This neurological disorder is characterized by a loss of control over individual finger movements. The Symptoms usually only occur when patients perform certain tasks such as playing an instrument.

A B

Research examining the cortex has found that musicians with this disorder have “fused” cortical areas belonging to the affected hand Fortunately this only happens in about 1% of musicians

Why does focal dystonia happen?

Cross section of the cortex, showing where the five fingers (1-5) are represented in the left and right hemispheres in a musician with focal dystonia (musician’s cramp). The areas representing all the fingers of the left hand (on the right side of the brain) are separated, but the areas representing fingers 3, 4, and 5 on the right hand (left side of brain) are superimposed. This superimposition is associated with the dystonia of the right hand.

Maps of the Body on the Cortex

•  Signals travel from the thalamus to the somatosensory receiving area in the cortex

•  Body map (homunculus) on the cortex shows more cortical space allocated to parts of the body that are responsible for detail

•  Plasticity in neural functioning leads to changes in how cortical cells are allocated to body parts

How do we quantify tactitle acuity?

Perceiving Details •  Measuring tactile acuity

–  Two-point threshold - minimum separation needed between two points to perceive them as two units

–  Grating acuity - placing a grooved stimulus on the skin and asking the participant to indicate the orientation of the grating

Receptor Mechanisms for Tactile Acuity

•  There is a high density of Merkel receptor/SA1 fibers in the fingertips

•  Merkel receptors are densely packed on the fingertips - similar to cones in the fovea

•  Both two-point thresholds and grating acuity studies show these results

Correlation between density of Merkel receptors/SA1 fiber density and tactile acuity

Cortical Mechanisms for Tactile Acuity

•  Body areas with high acuity have larger areas of cortical tissue devoted to them

•  This parallels the “magnification factor” seen in the visual cortex for the cones in the fovea

•  Areas with higher acuity also have smaller receptive fields on the skin

Receptive fields of cortical neurons are smallest on the monkey’s fingers and become larger on the hand and the forearm.

Two-point thresholds across the male body

Putting tactile perception to use: Perceiving textures Perceiving objects

Perceiving Texture

•  Is texture perception just a matter of resolving the spatially separated bumps on a surface?

Perceiving Texture

•  Katz (1925) proposed that perception of texture depends on two cues: – Spatial cues are determined by the size,

shape, and distribution of surface elements – Temporal cues are determined by the rate

of vibration as skin is moved across finely textured surfaces

•  Two receptors may be responsible for this process - called the duplex theory of texture perception

Perceiving Texture - continued

•  Past research showed support for the role of spatial cues

•  Recent research by Hollins and Reisner shows support for the role of temporal cues –  In order to detect differences between fine

textures, participants needed to move their fingers across the surface

(a) Participants in Hollins and Reisner’s (2000) experiment perceived the roughness of two fine surfaces to be essentially the same when felt with stationary fingers, but (b) could perceive the difference between the two surfaces when they were allowed to move their fingers.

Experiment by Hollins et al.

•  Adaptation experiment - participants’ skin was adapted with either: –  10-Hz stimulus for 6 minutes to adapt the RA1/Meissner corpuscle –  250-Hz stimulus for 6 minutes to adapt the RA2/Pacinian corpuscle

•  Results showed that only the adaptation to the 250-Hz stimulus affected the perception of fine textures

Perceiving Objects

•  Humans use active rather than passive touch to interact with the environment

(demo) •  Haptic perception is the active exploration of

3-D objects with the hand –  It uses three distinct systems:

• Sensory system • Motor system • Cognitive system

Perceiving Objects - continued

•  Psychophysical research shows that people can identify objects haptically in 1 to 2 sec

•  Klatzky et al. have shown that people use exploratory procedures (EPs) – Lateral motion – Pressure – Enclosure – Contour following

Some of the exploratory procedures (EPs) observed by Lederman and Klatzky as participants identified objects

(texture assessment) (material assessment)

(shape assessment)

The Physiology of Tactile Object Perception

•  The firing pattern of groups of mechanoreceptors signals shape, such as the curvature of an object

(a) Response of SA1 fibers in the fingertips to touching a high-curvature stimulus. The height of the profile indicates the firing rate at different places across the fingertip. (b) The profile of firing to touching a stimulus with more gentle curvature.

The Physiology of Tactile Object Perception

•  Neurons further upstream become more specialized –  Monkey’s thalamus shows

cells that respond to center-surround receptive fields

An excitatory-center, inhibitory-surround receptive field of a neuron in a monkey’s thalamus.

Receptive fields of neurons in the monkey’s somatosensory cortex. (a) This neuron responds best when a horizontally oriented edge is presented to the monkey’s hand. (b) This neuron responds best when a stimulus moves across the fingertip from right to left.

Somatosensory cortex shows cells that respond maximally to orientations and direction of movement

The Physiology of Tactile Object Perception

The Physiology of Tactile Object Perception - continued

•  Monkey’s somatosensory cortex also shows neurons that respond best to: – Grasping specific objects – Paying attention to the task

• Neurons may respond to stimulation of the receptors, but attending to the task increases the response

The response of a neuron in a monkey’s parietal cortex that fires when the monkey grasps a ruler but that does not fire when the monkey grasps a cylinder.

Pain Perception •  Pain results from exogenous factors (e.g. a pin prick)

and endogenous ones (e.g. emotions) •  There are three types of pain:

–  Nociceptive - signals impending damage to the skin

•  Nociceptors in the skin respond to heat, chemicals, severe pressure, and cold

•  The threshold at which nociceptors respond should be ‘just right’. Too low would cause normal activity to result in pain; too high would take away from the early warning aspect of pain (the skin would already be damaged before we sensed pain).

Types of Pain

–  Inflammatory pain - caused by damage to tissues and joints that releases chemicals that activate nociceptors

–  Neuropathic pain - caused by damage to the nervous system, such as:

•  Nerve damage due to amputation

•  Spinal cord injury

Pain can be created in a number of different ways.

Pain Perception - continued

•  Signals from nociceptors travel up the spinothalamic pathway and activate various subcortical and cortical areas (which are together are called the pain matrix)

PAIN MATRIX: The perception of pain is accompanied by activation of a number of different areas of the brain, including the somatosensory cortex, thalamus, anterior cingulate cortex, amygdala, hippocampus, and thalamus.

Experiment by Hoffauer et al.

•  Participants were presented with potentially painful stimuli and asked: – To rate pain intensity (sensory aspect) – To rate pain unpleasantness (affective aspect)

•  Brain activity was measured while they placed their hands into hot water

•  Hypnosis was used to increase or decrease the sensory and affective components

The perception of pain and corresponding brain responses can be affected by both sensory and cognitive factors.

Experiment by Hoffauer et al. - continued •  Results showed that:

–  Hypnotic suggestions to change the intensity led to changes in those ratings as well as those of unpleasantness and brain responses in somatosensory cortex S1

–  Hypnotic suggestions to change the unpleasantness of pain did not affect the intensity ratings but did change:

•  Ratings of unpleasantness •  Activation in the anterior cingulate cortex

Cognitive and Experiential Aspects of Pain (contd.)

•  Expectation - when surgical patients are told what to expect, they request less pain medication and leave the hospital earlier

•  Shifting attention - virtual reality technology has been used to keep patients’ attention on other stimuli than the pain-inducing stimulation

•  Content of emotional distraction - participants could keep their hands in cold water longer when pictures they were shown were positive

Pain in Social Situations

•  Experiment by Eisenberger et al. –  Participants watched a computer game –  Then were asked to play with two other “players”

who did not exist but were part of the program –  The “players” excluded the participant –  fMRI data showed increased activity in the anterior

cingulate cortex and participants reported feeling ignored and distressed

•  Singer’s expts on empathic and sensory pain

Cognitive and Experiential Aspects of Pain - continued

Given how debilitating pain can be, has the body evolved any mechanisms to overcome it?

Opioids and Pain

•  Brain tissue releases neurotransmitters called endorphins (one of the class of opioid chemicals) –  Evidence shows that

endorphins reduce pain •  Injecting naloxone blocks

the receptor sites causing more pain

•  Naloxone also decreases the effectiveness of placebos

•  People whose brains release more endorphins can withstand higher pain levels (a) Naloxone reduces the effect of heroin by occupying a

receptor site normally stimulated by heroin. (b) Stimulation-produced analgesia (SPA) may work by causing the release of endorphins, which stimulate opiate receptor sites. (c) Naloxone decreases the pain reduction caused by SPA and placebos by displacing endorphins.

Overview of Questions

•  Are there specialized receptors in the skin for sensing different tactile qualities?

•  What is the most sensitive part of the body? •  Is it possible to reduce pain with your

thoughts? •  Do all people experience pain in the same

way?