Dr Carlos Ballario, Dra Laura Davidow

If an object touches the hand, it can accurately say the place, the pressure, the texture and the permanence of the contact. If that object moves from the hand to the wrist, it can be said its speed and position, even if  there is not looking at all.

The somatic sensory system allows to feel, experience pain, and know the position or action of different parts of the body.

Somatic sensitivity can be differentiated into exteroceptive or superficial, which includes pain, temperature and touch-pressure, and proprioceptive or deep, consisting of movement, position, vibration and deep pain.

Anatomy of sensitivity

It is important to know the anatomy of the somatic sensory system to be able to interpret the sensory alterations, to recognize its probable site of origin and its implications.

Somatic sensitivity has receptors throughout the body responsible for recognizing different types of information: mechanoreceptors, nociceptors, thermoreceptors, proprioceptors, and chemoreceptors.

The information collected by these sensory receptors is transmitted to the central nervous system through primary afferent axons. In the ganglion of the dorsal root of the medulla is the cell body of the afferent axon. These cells are also called first-order neurons. In addition, they give rise to the central axons that penetrate the medulla through the dorsal roots of each medullary segment. Once they enter the medulla, according to the information they transmit, they will form the different cortical afferent pathways.

The spinothalamic pathway or anterolateral system transmits prostatic nociceptive, thermal, and tactile sensitivity. The fibers, when penetrating the spinal cord, synapse in the second order neurons located in the gelatinous substance of the dorsal horn, from there they cross the midline and run in the contralateral anterior and lateral cords of the spinal cord. Through the anterior cord the fibers of the prostatic tactile sensitivity and pressure pass, along the lateral are the fibers of sensitivity to pain and temperature. Then both groups continue through the brain stem until they make synapses in the thalamus, where the third-order neuron is located and from there to the parietal cortex.

The lemniscal route transmits discriminative proprioceptive and tactile sensitivity , running through the homolateral posterior cord of the spinal cord, until it synapses in the graceful and wedge-shaped nuclei of the medulla oblongata (second-order neuron). From there they decuse and ascend through the internal lemniscus towards the thalamus (third order neuron) and then to the parietal cortex.

Sensitivity from the face is transmitted by the trigeminal-thalamic tract, where the first order neurons are found in the trigeminal ganglion and their peripheral extensions make up the trigeminal nerve with its three branches, ophthalmic, maxillary and mandibular. The central extensions synapse in the trigeminal nuclei in the brain stem and from there they go to the thalamus and then to the parietal cortex.

Segmental and Neural Sensory Innervation

Dermatoma refers to the area of ​​the skin innervated by the sensory axons of a single dorsal root. The different dermatomes usually overlap with each other, therefore the lesion of a dorsal root does not produce a complete loss of sensation in that region of the skin. The overlap is greater for tactile sensitivity and less for pain and temperature.

Knowledge of dermatomes and the distribution of cutaneous innervation of peripheral nerves is very important for neurology because it allows defining and locating spinal or nerve lesions.

It is important to know the anatomy of the somatic sensory system to be able to interpret the sensory alterations, to recognize its probable site of origin and its implications.

Somatic sensitivity has receptors throughout the body responsible for recognizing different types of information: mechanoreceptors, nociceptors, thermoreceptors, proprioceptors, and chemoreceptors.

The information collected by these sensory receptors is transmitted to the central nervous system through primary afferent axons. In the ganglion of the dorsal root of the medulla is the cell body of the afferent axon. These cells are also called first-order neurons. In addition, they give rise to the central axons that penetrate the medulla through the dorsal roots of each medullary segment. Once they enter the medulla, according to the information they transmit, they will form the different cortical afferent pathways.

The spinothalamic pathway or anterolateral system transmits prostatic nociceptive, thermal, and tactile sensitivity. The fibers, when penetrating the spinal cord, synapse in the second order neurons located in the gelatinous substance of the dorsal horn, from there they cross the midline and run in the contralateral anterior and lateral cords of the spinal cord. Through the anterior cord the fibers of the prostatic tactile sensitivity and pressure pass, along the lateral are the fibers of sensitivity to pain and temperature. Then both groups continue through the brain stem until they make synapses in the thalamus, where the third-order neuron is located and from there to the parietal cortex.

The lemniscal route transmits discriminative proprioceptive and tactile sensitivity , running through the homolateral posterior cord of the spinal cord, until it synapses in the graceful and wedge-shaped nuclei of the medulla oblongata (second-order neuron). From there they decuse and ascend through the internal lemniscus towards the thalamus (third order neuron) and then to the parietal cortex.

Sensitivity from the face is transmitted by the trigeminal-thalamic tract, where the first order neurons are found in the trigeminal ganglion and their peripheral extensions make up the trigeminal nerve with its three branches, ophthalmic, maxillary and mandibular. The central extensions synapse in the trigeminal nuclei in the brain stem and from there they go to the thalamus and then to the parietal cortex.

Segmental and Neural Sensory Innervation

Dermatoma refers to the area of ​​the skin innervated by the sensory axons of a single dorsal root. The different dermatomes usually overlap with each other, therefore the lesion of a dorsal root does not produce a complete loss of sensation in that region of the skin. The overlap is greater for tactile sensitivity and less for pain and temperature.

Knowledge of dermatomes and the distribution of cutaneous innervation of peripheral nerves is very important for neurology because it allows defining and locating spinal or nerve lesions.

It is important to know the anatomy of the somatic sensory system to be able to interpret the sensory alterations, to recognize its probable site of origin and its implications.

Somatic sensitivity has receptors throughout the body responsible for recognizing different types of information: mechanoreceptors, nociceptors, thermoreceptors, proprioceptors, and chemoreceptors.

The information collected by these sensory receptors is transmitted to the central nervous system through primary afferent axons. In the ganglion of the dorsal root of the medulla is the cell body of the afferent axon. These cells are also called first-order neurons. In addition, they give rise to the central axons that penetrate the medulla through the dorsal roots of each medullary segment. Once they enter the medulla, according to the information they transmit, they will form the different cortical afferent pathways.

The spinothalamic pathway or anterolateral system transmits prostatic nociceptive, thermal, and tactile sensitivity. The fibers, when penetrating the spinal cord, synapse in the second order neurons located in the gelatinous substance of the dorsal horn, from there they cross the midline and run in the contralateral anterior and lateral cords of the spinal cord. Through the anterior cord the fibers of the prostatic tactile sensitivity and pressure pass, along the lateral are the fibers of sensitivity to pain and temperature. Then both groups continue through the brain stem until they make synapses in the thalamus, where the third-order neuron is located and from there to the parietal cortex.

The lemniscal route transmits discriminative proprioceptive and tactile sensitivity , running through the homolateral posterior cord of the spinal cord, until it synapses in the graceful and wedge-shaped nuclei of the medulla oblongata (second-order neuron). From there they decuse and ascend through the internal lemniscus towards the thalamus (third order neuron) and then to the parietal cortex.

Sensitivity from the face is transmitted by the trigeminal-thalamic tract, where the first order neurons are found in the trigeminal ganglion and their peripheral extensions make up the trigeminal nerve with its three branches, ophthalmic, maxillary and mandibular. The central extensions synapse in the trigeminal nuclei in the brain stem and from there they go to the thalamus and then to the parietal cortex.

Segmental and Neural Sensory Innervation

Dermatoma refers to the area of ​​the skin innervated by the sensory axons of a single dorsal root. The different dermatomes usually overlap with each other, therefore the lesion of a dorsal root does not produce a complete loss of sensation in that region of the skin. The overlap is greater for tactile sensitivity and less for pain and temperature.

Knowledge of dermatomes and the distribution of cutaneous innervation of peripheral nerves is very important for neurology because it allows defining and locating spinal or nerve lesions.

Sensitivity alterations

Sensitivity alterations manifest with loss or reduction of the different sensory modalities and with so-called "positive" phenomena caused by dysfunction of the sensory system. The most frequent positive phenomena are abnormal sensations described as pins and needles, called paresthesias.

The terms used to describe sensory disturbances are:

• Hypoaesthesia: decreased sensitivity
• Anesthesia: complete loss of sensation
• Hypoalgesia: decreased perception of pain
• Paresthesias: spontaneous abnormal sensations (tingling, pricking, burning)
• Dysesthesias: abnormal painful sensation related or not to a stimulus
• Hyperesthesia: exaggerated perception of a tactile stimulus
• Allodynia: pain in response to a normal non-painful stimulus such as touch
• Hyperalgesia: exaggerated pain in response to a painful stimulus

Location of sensory disturbances

The location of the sensory alterations allows in most cases to define where the problem originates.

Nerves and nerve roots:

Mononeuropathy: This is the name given to the involvement of a peripheral nerve individually. The sensory alteration is located in the territory corresponding to the affected nerve. (fig 2)

Polyneuropathy: It is the involvement of multiple peripheral nerves. They usually present a characteristic distal and symmetrical pattern of involvement called "stockings and gloves". They may be accompanied by decreased or loss of reflexes and symmetrical muscle weakness.

Radiculopathy: the alteration is located in the corresponding dermatome. It is usually accompanied by motor and reflex disturbances.

Spinal cord

Central cord syndrome: The injury located in the central region of the cord produces the loss of pain and temperature sensation below the injury, with preservation of the other sensory modalities. This is due to the compromise of the fibers that cross the midline into the spinothalamic tract. Some examples of the most common injuries that cause this syndrome are syringomyelia, tumors, and trauma.

Anterior cord syndrome: When the anterior part of the spinal cord is injured, there is a loss of sensitivity for pain and temperature below the injury due to the involvement of the spinothalamic tract. Anterior spinal artery occlusion is usually the most common cause of this injury.

Medullary hemisection syndrome (Brown-Séquard): This syndrome is characterized by the loss of positional and vibratory sensation below the injury associated with the loss of pain and thermal sensation contralateral to the injury and beginning at various levels due to below it.

Posterior column syndrome:Injury to the posterior columns of the cord produces loss of positional and vibratory sensation below the injury with preservation of the other modalities.

Complete spinal section: In complete spinal section there is loss of all sensory modalities below the injury.

Brainstem: Lesions affecting the lower part of the medulla characteristically produce an alteration of sensation on the side of the face corresponding to the injury and on the opposite side of the body. This is due to injury to the ipsilateral trigeminal nucleus or tract and the contralateral spinothalamic tract.

High truncal lesions, that is, in the upper part of the brain bulb, pons and peduncles, produce altered sensation in the entire contralateral half body to the lesion, since at this level the trigeminal and spinothalamic tracts run together.

Thalamus: Thalamic injuries cause loss of all sensory modalities contralateral to the injury. It can also be accompanied by intense pain, spontaneous or triggered by non-painful and very unpleasant stimuli. In this case it is called Dejèrine-Roussy Syndrome.

Sensory cortex: Cortical involvement manifests on the opposite side of the lesion. It produces an inability to recognize objects by touch, estimate their size, consistency, weight and texture. This alteration is called astereognosia.

It can also be manifested by alteration of the discrimination between two points and the ability to locate touch and pain (topoagnosia).

Another characteristic is sensitive inattention or extinction. In this phenomenon, bilateral sensory stimulation, whether to touch or pain, is identified only on the healthy side. Likewise, the so-called sensitive negligence can occur, where the affected person ignores the side of the body and the extrapersonal space contralateral to the parietal injury.

The primary sensory modalities remain intact.

Psychogenic disorders . In some circumstances, conversion psychiatric disorders may present with sensory disturbances. These alterations do not usually respect the anatomical distribution or may manifest dissociated sensory alterations that are difficult to interpret, such as, for example, alteration of pain sensitivity with preservation of temperature.

Examination of primary sensitivity

Tactile sensitivity: Tactile sensitivity is explored with a cotton swab, a soft fingertip, or a fine-bristle brush. The patient with closed eyes should indicate if he feels the stimulus in the different parts of the body.

Pain sensitivity: This modality is explored with a pin. The relaxed patient with eyes closed indicates the sensation experienced. It must be clarified that he must concentrate on the unpleasant sensation of the stimulus and not on the perception of friction.

If an area of ​​hypoalgesia is detected, it must be delimited, from the most affected area to the periphery. In this way, it seeks to establish a pattern of affectation, of a nerve, root, trunk, etc.

Thermal sensitivity:It is done by applying test tubes with hot and cold water in contact with the skin. Both temperatures must be studied since the receptors for each are different.

Proprioceptive sensitivity: The exploration of the sensation of joint position and movement can be carried out as follows: the patient with the eyes closed is passively mobilized the big toe and fingers of the hands. You are then asked to identify the direction of movement. If mistakes are made, more proximal joints are studied.

Vibration sensitivity:The vibration is studied using a 128 Hz tuning fork that is placed over the bony prominences and started distally. It should be clarified that the vibration must be detected and not just the pressure of the tuning fork against the body.

Examination of cortical or complex sensitivity

Two-point discrimination: Scanned with a compass, both tips are placed at a previously prepared separation, eg 3mm, and applied to a patient's body surface. The distance at which the stimulus can be recognized as two separate points varies at different locations. Usually there is a separation of 3 mm at the fingertips, 1 mm at the tip of the tongue, and 8 to 15 mm at the palms of the hands.

Touch localization: by light pressure with the fingertip of the examiner, the patient is asked to identify the contact areas with his eyes closed.

Graphaesthesia: the ability to identify with closed eyes letters or numbers traced by the explorer in various parts of the body is explored.

Stereognosia: the ability to identify common objects by palpation is studied, recognizing their shape, texture, size.

Barognosia: the ability to recognize between different weights is explored.

Bilateral sensory discrimination: Sensitivity on two analogous sides of the body is stimulated simultaneously. When this is disturbed it is called sensory extinction or inattention, if the stimulation on one side is not recognized.