Somatosensitivity, Viscerosensitivity, Proprioception, and Pain

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*The anticipation factor of a potential nervous input or output is always done through the cortex. While the physical inputs and outputs preceding and following integration respectively are realized through sensors and effectors (sensory and motor neurons).


Sensory Division

Stimuli Characteristics

  1. Adequacy > A stimulus may be too small in voltage/amplitude in order to elicit a receptor potential.
  2. Qualitative:

The modality of the stimulus> What it is.

The localization of the stimulus.

  1. Quantitative:receptor-potential

Stimulus intensity.

It is transduced through the sensory neuron, at its axon hillock, into frequency as a number
of action potentials can arise at certain intervals. There is then a directly proportional relationship between stimulus intensity and action potential frequency, logarithmically.

  1. Duration.

2. Qualitative Characteristics of a Stimulus


  • Each modality is transmitted through an individual sensory pathway, with different receptor types at the start. These are e.g. pain, noxious (mechanical forces), and mild-temperature-change stimuli.
  • The quality of each stimulus is perceived by the use of labelled line coding: Meaning each sensory region on the body has a corresponding cortical area to it inside which sensory information from that region is integrated.

Receptive Fields

  • Both their sizes and their degree of overlap varies – with an inversely proportional relationship between size and resolution of the sensation.
  • Lateral inhibition is a phenomenon which amplifies the spatial resolving power of a sensory region, and works as follows:

A: When a stimulus is applied, the highest AP frequency is indeed generated in the sensory neuron directly associated with the small stimulated region, but receptor potentials followed by action potentials after relay are also elicited in neighboring neurons. This leads to the perception that a larger area was stimulated than in reality.

B: When lateral inhibition is implemented, both the directly-targeted neuron/receptor and the neighboring ones send collateral branches from their axons to inhibit all relay neurons except the one synapsing with the central receptor. This leads to an action potential being generated only in the neuron associated with the small area of skin being stimulated. Spatial resolving power is then heightened.

Receptor Adaptation

Feature Tonic Receptors Phasic Receptors
Adaptation speed Low High
Adaptation/decline in response to Location of the stimulus Constant stimuli


Receptors (Free nerve endings have the same function and mechanism)

  • Definition: Sensory neurons which act as energy converters, converting e.g. light energy into electrical energy – in the form of a receptor potential initially, followed by an action potential.
  • General types:
  1. Superficial > Somatosensors.
  2. Deep > Viscerosensors.
  3. Proprioceptors > In muscles, tendons, and joints.
  • Special types: Within sensory organs:
  1. Mechanoreceptors.
  2. Chemoreceptors.
  3. Thermoreceptors.
  4. Photoreceptors.
  • Types according to complexity:
  1. Complex > With the nerve ending being encapsuled in connective tissue.
  2. Simple > With a free nerve ending.

Sensory Signal Evolution

  1. Pain and temperature to aid immediate survival.
  2. Touch to aid long-term survival and with adaptive value.

Somatosensory Compartment

*The archispinothalamic system is the oldest and most rudimentary, representing intersegmental connection through the spinospinal tract.

Main Pathway/System Tract Evolutionary Age Resolution Information Transmitted Fibres
Paleospinothalamic Spinoreticular and spinotectal. Oldest; connected primarily to subcortical structures and secondarily to the cortex. Low. Dull and diffuse pain – Eliciting vegetative/reflex responses. Almost half cross the midline.
Neospinothalamic Spinothalamic. Younger; connected primarily to the neocortex. 1.High.


1. Sharp localized pain and temperature.

2.Crude touch.

Cross the midline at the entry segment’s level.
Dorsal Column Spinobulbar. Youngest (and with highest capacity). High. Touch, vibration and proprioception – Enables fine motor control and heightened object recognition. Cross the midline at the medulla oblongata’s level.

Other Systems

  • Dermatomes: Regions on the skin innervated by ending of a corresponding spinal sensory nerve.
  • The trigeminal system.
  • The thalamus in bilateral connection with the primary somatosensory cortex in the neocortex (with labelled line coding). The thalamus is the site of gating all afferent sensory information except for olfaction.


Definition: An unpleasant sensation, with subjective character, which is associated with current or potential damage.


  1. According to nature: Sensory or psychological.
  2. According to cause: Physiological or pathological.
  3. According to duration: Chronic when exceeding 6 months and acute when lasting for less than 6 months.

Ascending Circuit:

The corticolimbic pathway – Thalamus, parabrachial area, amygdala, and anterior cingulate cortex + Ventral tegmental area using dopamine (reward system) > Nucleus accumbens > Feedback on both the amygdala and the ventral tegmental area.


*RVM: Rostral ventromedial medulla.

Descending Circuit:

Somatomotoric cortex > Hypothalamus > Periaqueductal grey > Nuclei raphe in medulla > Spinal cord’s dorsal horn, where pain modulation is gate-controlled – enabling or disabling the pain sensation through the inhibition or activation of an inhibitory interneuron respectively.

Referred Pain:

A phenomenon which occurs when two afferent neurons from different sources e.g. intestine and skin converge onto a single sensory interneuron in the dorsal horn of the spinal cord. The information is then transmitted as one to a sensory neuron.

Notes by: Lina El Rifaie


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