The Somatic Senses

The Somatosensory System is a complex system involving many processing units and receptor cells in order to produce sensations of touch, pain, temperature sense of global position (vestibular), and body-position-awareness (kinesthesia).

The first three are skin systems, the latter two are proprioception (relative positioning).

The Skin

Skin is the largest and heaviest organ of the body. Its role is to prevent damaging agents from entering the body, as well as provide temperature regulation and enable physical comfort/pleasure (relationship between touch and emotions).

Touch is pressure-based, and the receptors are mechanoreceptors. This is in contrast with temperature which is molecular vibrations, and has free nerve cells as receptors.

Pain on the other hand, has no direct physical stimulus - there are no 'pain waves' or 'pain molecules'.

Basic Anatomy

The mechanoreceptors are embedded in the top two layers of skin; the epidermis and the dermis. We divide the receptors by layer, and by speed of adaptation.


Slow Adapting Fibres (SA)

Slow Adapting Fibres fire continuously as long as pressure is applied. Their primary functions are detection and perception of fine detail and texture, as well as finger positioning and stable grasping.


Rapidly Adapting Fibres (RA)

Rapidly Adapting Fibres fire at the onset and offset of pressure. Their primary functions are low frequency and high frequency vibration detection.


Receptor Path


Homunculus; Size Representation

Ahomunculus is a representation of a human. We can use a homunculus to show the relative sizes of the areas of the brain that correspond to body parts. Kind of like the size of things on the brain!Map.



Basic/Complex Tactile Sensations

Basic Sensations

Basic sensations include:

  • Pressure
  • Flutter
  • Vibration

Complex Sensations

Complex sensations include:

  • Roughness-Smoothness
  • Hardness-Softness
  • Viscosity (Friction)
  • Elasticity
  • Stickiness

"…at suprathreshold levels, fundamental qualities like 'pressure', 'flutter' and
'vibration' may combine to form the many sensory attributes ascribed to the
somatosensory system 'roughness', "softness', or 'intensity' (among myriad other
- Bolanowski et al. 1988

Tactile Sensitivity


Tactile Acuity/Discrimination Thresholds

There is a high correlation between density of Merkel receptors/SA1 fibre density and
tactile acuity. (From Craig & Lyte, 2002.)


There is a slight variation between sensitivity on each side of the body.

Shape Coding

SA1 fibres fire at different rates depending on the curvature of the thing its touching, bsed on A. W. Goodwin (1998)


Haptic Perception

Haptic perception (the exploration of 3D objects) is the interaction and integration of three distinct systems:

  • Sensory System
  • Motor System
  • Cognitive System

Pain Perception

Pain is an unpleasant sensory and emotional experience associated with actual/potential tissue damage, or described in terms of such damage". It's adaptive to some degree - it can motivate action to alter the experience.

Pain Type Source
Nociceptive Pain Signals upcoming damage to the skin, different types of nociceptors respond to heat, chemicals, sever pressure, and cold.
Inflammatory Pain Damage to tissues and joints releases chemicals that activate nociceptors.
Neuropathic Pain Damage to the nervous system; brain damage caused by stroke, repetivie movements causing carpal tunnel/RSI.

Pathway of Pain



Unlike mechanoreceptors, the signals from nociceptors are delivered to the cortex and the nerve fibres connected to the spinal cord muscles.

Nerve Fibres

There are at least two types of nerve fibres that we look at; large a-delta fibres (rapid, sharp pain) and small C fibres (slower, duller, burning pain).

Cognitive and Experiential Aspects of Pain

Factors Affecting Pain Experience

  • Expectation
    • When surgical patients are told what to expect, they request less pain medication and leave the hospital earlier
  • Shifting attention
    • Attention on stimuli other than the pain-inducing stimulation reduce the level of experienced pain
  • Emotional distraction
    • Participants could keep their hands in cold water longer when pictures they were shown were positive (deWied and Verbaten, 2001).
    • Participants could tolerate the pain of cold water (whilst rating it the same level), when a sweet-smelling odour was present (Prescott & Wilkie (2007)).

Gate-Control Theory

A superseded theory, proposed by Melzak & Wall (1965; 1983), that the spinal cord contains a neurological "gate" that either blocks/allows pain signals to the brain. The gate could be opened by pain signals travelling up thick A-delta fibres, and closed by other A fibres with other information (e.g. pressure/vibration). This was thought to explain why rubbing an injury can ease pain, and how we psychologically modulate pain.

Temperature Detection

The neural processing of temperature is closely related to that of pain; The sensation of cold can inhibit the sensation of pain, and it seems to be gated similarly to pain too (stimulating nearby cold+warm spots produces a feeling of intense heat).

The cold receptors code for extreme temperatures, not just cold, so when we feel cold and warmth we interpret it as extreme and warmth, and feel extreme heat - (Craig, Reiman, Evans & Bushnell, 1996).


The Vestibular System

Vestibular system is responsible for our sense of balance an awareness of gravity, movement and acceleration.

Stimulus Organ
Rotation, especially movement of the head The vestibular labyrinths, or vestibular organs
Linear acceleration, including gravity i.e. head tilt

saccule and utricle (in the ear)||



Fluid in the Crista

During a rapid head turn, the fluid in the canals lags behind the head motion because of inertia. As the fluid catches up, it stimulates clumps of hair cells (in the crista) in the


After a rapid stop, the fluid continues to flow, even though the head is
static; this continued flow of fluid stimulates the crista, and hence we experience dizziness; the “oculogyral illusion”.


Ingestion of alcohol causes the crista to become buoyant (as alcohol in the blood is relatively light). The buoyancy causes the crista to wobble; hence the dizziness associated with alcohol consumption.

Saccules and Utricles Crystals

The otolith organs contain crytals that shift under gravity as the head tilts.



Kinesthesia is the encoding of body position. It provides information about the movement and
position of muscles, joints and the distribution of weight.

We use it constantly without being aware of it; Kinesthetic receptors are present in the muscle, skin and joints. Loss of this system has devastating consequences; without it, the body doesn’t have signals indicating the position of the limbs.