Somatic Senses: General Organization, Tactile and Position Senses

Classification of Somatic Senses

Somatic senses collect sensory information from all over the body.
They are distinct from special senses (vision, hearing, smell, taste, equilibrium).
Somatic senses are classified into three physiological types:
1. Mechanoreceptive somatic senses: Tactile and position sensations stimulated by mechanical displacement of body tissues.
2. Thermoreceptive senses: Detect heat and cold.
3. Pain sense: Activated by factors that damage tissues.
Tactile senses include touch, pressure, vibration, and tickle.
Position senses include static position and rate of movement.

Other Classifications of Somatic Sensations

Exteroreceptive sensations: From the body surface.
Proprioceptive sensations: Relating to the physical state of the body, including position, tendon and muscle sensations, pressure from the feet, and equilibrium.
Visceral sensations: From the viscera (internal organs).
Deep sensations: From deep tissues like fasciae, muscles, and bone; mainly deep pressure, pain, and vibration.

Detection and Transmission of Tactile Sensations

Interrelations Among Touch, Pressure, and Vibration

Touch, pressure, and vibration are detected by the same types of receptors but differ in:
1. Touch: Stimulation of tactile receptors in or immediately beneath the skin.
2. Pressure: Deformation of deeper tissues.
3. Vibration: Rapidly repetitive sensory signals.

Tactile Receptors

At least six different types of tactile receptors exist:
1. Free Nerve Endings:
  - Found everywhere in the skin and other tissues.
  - Detect touch and pressure; even light contact with the cornea elicits these sensations.
2. Meissner's Corpuscles:
  - Elongated encapsulated nerve endings of large (type Aβ) myelinated sensory nerve fibers.
  - Abundant in nonhairy skin, especially fingertips and lips.
  - Sensitive to movement of objects over the skin and low-frequency vibration.
  - Adapt quickly after stimulation.
3. Expanded Tip Tactile Receptors (Merkel's Discs):
  - Found in fingertips and hairy skin.
  - Transmit an initially strong but partially adapting signal, followed by a weaker, slowly adapting signal.
  - Important for determining continuous touch of objects against the skin.
  - Often grouped in touch domes, which are extremely sensitive receptors.
  - Innervated by a single large myelinated nerve fiber (type Aβ).
  - Play roles in localizing touch sensations and determining texture.
4. Hair End-Organs:
  - Nerve fibers entwining the base of each hair.
  - Detect movement of objects on the body surface and initial contact.
  - Adapt readily.
5. Ruffini's Endings:
  - Multibranched encapsulated endings located in deeper skin layers and internal tissues.
  - Adapt very slowly.
  - Signal continuous states of deformation, such as heavy prolonged touch and pressure.
  - Found in joint capsules, signaling joint rotation.
6. Pacinian Corpuscles:
  - Lie beneath the skin and deep in fascial tissues.
  - Stimulated by rapid local compression.
  - Adapt in a few hundredths of a second.
  - Important for detecting tissue vibration and rapid changes in mechanical state.

Transmission of Tactile Signals in Peripheral Nerve Fibers

Specialized sensory receptors transmit signals in type Aβ nerve fibers (30-70 m/sec).
Free nerve endings transmit signals mainly via small type Aδ myelinated fibers (5-30 m/sec).
Some tactile free nerve endings transmit via type C unmyelinated fibers (fraction of a meter up to 2 m/sec), mainly subserving the sensation of tickle.
Critical sensory signals are transmitted in more rapidly conducting nerve fibers.
Cruder signals are transmitted via slower, smaller nerve fibers.

Detection of Vibration

All tactile receptors are involved in detecting vibration.
Pacinian corpuscles detect vibrations from 30 to 800 cycles/sec, transmitting signals over type Aβ nerve fibers.
Low-frequency vibrations (2-80 cycles/sec) stimulate other tactile receptors, especially Meissner's corpuscles.

Detection of Tickle and Itch by Mechanoreceptive Free Nerve Endings

Sensitive, rapidly adapting mechanoreceptive free nerve endings elicit tickle and itch sensations.
Found in superficial skin layers.
Transmitted by small type C unmyelinated fibers.
Itch sensation alerts to mild surface stimuli, activating the scratch reflex.
Scratching relieves itch by removing the irritant or eliciting pain, which suppresses itch signals in the cord via lateral inhibition.

Sensory Pathways for Transmitting Somatic Signals into the Central Nervous System

Sensory information enters the spinal cord through dorsal roots of spinal nerves.
Signals are carried to the brain via two alternative sensory pathways:
1. Dorsal Column-Medial Lemniscal System: Carries signals upward to the medulla in the dorsal columns; signals synapse and cross to the opposite side, continuing to the thalamus via the medial lemniscus.
2. Anterolateral System: Signals synapse in the dorsal horns, cross to the opposite side, and ascend through the anterior and lateral white columns, terminating at levels of the lower brain stem and in the thalamus.
The two systems partially come back together at the level of the thalamus.

Differences Between the Two Systems

Dorsal Column-Medial Lemniscal System:
- Large myelinated nerve fibers (30-110 m/sec).
- High degree of spatial orientation.
- Transmits rapidly with temporal and spatial fidelity.
- Limited to discrete mechanoreceptive sensations.
Anterolateral System:
- Smaller myelinated fibers (few meters per second up to 40 m/sec).
- Less spatial orientation.
- Transmits a broad spectrum of sensory modalities (pain, warmth, cold, crude tactile sensations).

Sensations Transmitted in Each System

Dorsal Column-Medial Lemniscal System:
1. Touch sensations requiring high localization.
2. Touch sensations requiring fine intensity gradations.
3. Phasic sensations like vibratory sensations.
4. Sensations that signal movement against the skin.
5. Position sensations from the joints.
6. Pressure sensations related to fine degrees of judgment.
Anterolateral System:
1. Pain.
2. Thermal sensations.
3. Crude touch and pressure sensations with limited localizing ability.
4. Tickle and itch sensations.
5. Sexual sensations.

Transmission in the Dorsal Column-Medial Lemniscal System

Anatomy

Large myelinated fibers enter the spinal cord via dorsal roots, forming medial and lateral branches.
The medial branch ascends in the dorsal column to the brain.
The lateral branch synapses with local neurons in the cord gray matter, serving three functions:
1. Fibers enter the dorsal columns and travel to the brain.
2. Short fibers terminate locally, eliciting spinal cord reflexes.
3. Others give rise to the spinocerebellar tracts.

Dorsal Column-Medial Lemniscal Pathway

Nerve fibers pass uninterrupted to the dorsal medulla, synapsing in the cuneate and gracile nuclei.
Second-order neurons decussate and ascend via the medial lemnisci to the thalamus.
The medial lemniscus is joined by fibers from the trigeminal nerve for the head.
Fibers terminate in the ventrobasal complex of the thalamus.
Third-order nerve fibers project to the postcentral gyrus of the cerebral cortex (somatic sensory area I) and a smaller area in the lateral parietal cortex (somatic sensory area II).

Spatial Orientation

Distinct spatial orientation of nerve fibers is maintained throughout the system.
In the spinal cord, fibers from lower body parts lie toward the center, with higher segments forming lateral layers.
In the thalamus, the tail end of the body is represented laterally, and the head and face medially.
The left side of the body is represented in the right thalamus, and vice versa.

Somatosensory Cortex

The cerebral cortex is divided into about 50 areas (Brodmann's areas) based on structural differences.
Sensory signals terminate in the cortex posterior to the central fissure.
The anterior parietal lobe is concerned with somatosensory signals, and the posterior half provides higher-level interpretation.
Visual signals terminate in the occipital lobe, and auditory signals in the temporal lobe.
The motor cortex (posterior frontal lobe) controls muscle contractions in response to somatosensory signals.

Somatosensory Areas I and II

Two separate sensory areas in the anterior parietal lobe exist: somatosensory area I and area II.
Somatosensory area I is more extensive and important.
Area I has a high degree of localization; area II has poor localization.
Area II receives signals from the brain stem, somatosensory area I, and other sensory areas; projections from area I are required for its function.

Spatial Orientation in Somatosensory Area I

Area I lies immediately behind the central fissure in the postcentral gyrus (Brodmann's areas 3, 1, and 2).
Each lateral side receives sensory information from the opposite side of the body.
The sizes of cortical areas are proportional to the number of specialized sensory receptors in the corresponding body area (e.g., lips and thumb have large areas).
The nose, lips, mouth, and face are represented laterally, and the head, neck, shoulders, and lower body are represented medially.

Layers of the Somatosensory Cortex and Their Function

The cerebral cortex contains six layers of neurons.
1. Layer IV receives incoming sensory signals.
2. Layers I and II receive diffuse input from lower brain centers, controlling excitability.
3. Layers II and III send axons to related areas on the opposite side of the brain (corpus callosum).
4. Layers V and VI send axons to deeper parts of the nervous system.
  - Layer V projects to the basal ganglia, brain stem, and spinal cord.
  - Layer VI projects to the thalamus, controlling incoming sensory signals.

Organization in Vertical Columns

The somatosensory cortex is organized in vertical columns, each serving a specific sensory modality.
Columns respond to stretch receptors, tactile hairs, pressure points, etc.
At layer IV, columns function separately; at other levels, interactions initiate analysis.
The anterior postcentral gyrus (Brodmann's area 3A) responds to muscle, tendon, and joint stretch receptors, projecting to the motor cortex.
Posteriorly, more columns respond to cutaneous receptors and deep pressure.
In the most posterior portion, some columns respond only to stimuli moving across the skin in a specific direction.

Functions of Somatosensory Area I

Excision causes loss of:
1. Discrete localization of sensations.
2. Judgment of critical degrees of pressure.
3. Judgment of the weights of objects.
4. Judgment of shapes or forms (astereognosis).
5. Judgment of texture.
Pain and temperature sense are preserved but poorly localized.

Somatosensory Association Areas

Brodmann's areas 5 and 7 (parietal cortex behind somatosensory area I) decipher deeper meanings of sensory information.
Electrical stimulation can cause complex body sensations.
These areas receive signals from:
1. Somatosensory area I.
2. Ventrobasal nuclei of the thalamus.
3. Other thalamic areas.
4. Visual cortex.
5. Auditory cortex.

Amorphosynthesis

Removal on one side causes loss of ability to recognize complex objects and forms on the opposite side.
Loss of the sense of form of the opposite side of the body.
Obliviousness to the opposite side, including forgetting to use it.
Tendency to recognize only one side of an object.
This sensory deficit is called amorphosynthesis.

Characteristics of Dorsal Column-Medial Lemniscal Signal Transmission and Analysis

Basic Neuronal Circuit

The neuronal circuit exhibits divergence at each synaptic stage.
Cortical neurons in the central part of the cortical field discharge to the greatest extent.
A weak stimulus affects only the most central neurons; a stronger stimulus affects more neurons, with the center discharging more rapidly.

Two-Point Discrimination

Tests tactile discrimination by determining the ability to distinguish two points.
Fingertips can distinguish points as close as 1-2 mm; the back requires 30-70 mm.
The dorsal column pathway transmits two-point discriminatory information via separate peaks of excitation in the somatosensory cortex.

Lateral Inhibition

Every sensory pathway gives rise to lateral inhibitory signals, which block lateral spread of excitatory signals.
Occurs at the dorsal column nuclei, thalamus, and cortex.
Increases the degree of contrast in the sensory pattern perceived in the cerebral cortex.
Enhances the separation of peaks in cortical excitation.

Transmission of Rapidly Changing and Repetitive Sensations

The dorsal column system can recognize changing stimuli in as little as \frac{1}{400} of a second.

Vibratory Sensation

Detected up to 700 cycles/sec.
Higher frequencies originate from Pacinian corpuscles, lower frequencies ($\leq$200 cycles/sec) from Meissner's corpuscles.
Transmitted only in the dorsal column pathway.
Vibration testing is used to assess the functional integrity of the dorsal columns.

Interpretation of Sensory Stimulus Intensity

The sensory system transmits experiences of varying intensities.
The auditory system can detect whispers and explosions, with intensity variations of more than 10^{10} times.
Eyes can see visual images with light intensities that vary as much as a half-million times.
Skin can detect pressure differences of 10,000 to 100,000 times.
The receptor potential of the Pacinian corpuscle increases markedly with slight changes in intensity at low stimulus levels but only slightly at high levels.

Intensity Range

The tremendous intensity range of sensory reception is crucial for proper sensory function.

Judgment of Stimulus Intensity

Weber-Fechner Principle: Gradations of stimulus strength are discriminated in proportion to the logarithm of stimulus strength.
Expressed mathematically as: Interpreted signal strength = Log(Stimulus) + Constant
Accurate only for higher intensities of visual, auditory, and cutaneous sensory experiences.
Emphasizes that greater background sensory intensity requires a greater change for detection.

Power Law

Another mathematical relation is the power law:
Interpreted signal strength = K \times (Stimulus - k)^y
y, K, and k are constants that vary for each type of sensation.
A linear relation can be attained between interpreted and actual stimulus strength over a large range when using double logarithmic coordinates.

Position Senses

Also called proprioceptive senses.
Divided into:
1. Static position sense: Conscious perception of body part orientation.
2. Rate of movement sense (kinesthesia or dynamic proprioception).

Position Sensory Receptors

Knowledge of position depends on joint angulation and rates of change.
Multiple receptors help determine joint angulation, including skin tactile receptors and deep joint receptors.
Skin receptors are important in fingers, while deep receptors are more important for larger joints.

Muscle Spindles

Important for determining joint angulation in midranges of motion.
Control muscle movement.
Detect net stretch information.

Ligaments and Deep Tissues

At extremes of joint angulation, stretch of ligaments and deep tissues is an important factor.
Sensory endings used are Pacinian corpuscles, Ruffini's endings, and Golgi tendon receptor-like receptors.
Pacinian corpuscles and muscle spindles are adapted for detecting rapid rates of change.

Processing of Position Sense Information

Thalamic neurons respond to joint rotation, with some maximally stimulated at full rotation and others at minimal rotation.
Signals from receptors indicate how much each joint is rotated.

Transmission of Sensory Signals in the Anterolateral Pathway

Transmits sensory signals that do not require highly discrete localization or fine intensity gradations.
Includes pain, heat, cold, crude tactile, tickle, itch, and sexual sensations.

Anatomy

Fibers originate mainly in dorsal horn laminae I, IV, V, and VI.
Fibers cross immediately in the anterior commissure to the opposite anterior and lateral white columns, ascending via the anterior and lateral spinothalamic tracts.

Terminus

Terminates in:
1. Reticular nuclei of the brain stem.
2. Ventrobasal complex and intralaminar nuclei of the thalamus.
Tactile signals are transmitted mainly to the ventrobasal complex, then to the somatosensory cortex.
Only a small fraction of pain signals project directly to the ventrobasal complex; most terminate in the reticular nuclei and are relayed to the intralaminar nuclei.

Characteristics of Transmission in the Anterolateral Pathway

Velocities of transmission are a third to half those of the dorsal column-medial lemniscal system (8-40 m/sec).
Poor spatial localization of signals.
Less accurate gradations of intensities (10-20 gradations).
Poor ability to transmit rapidly changing or repetitive signals.
A cruder transmission system.
Transmits pain, temperature, tickle, itch, and sexual sensations, in addition to crude touch and pressure.

Some Special Aspects of Somatosensory Function

Function of the Thalamus

The thalamus has a slight ability to discriminate tactile sensation, even though it mainly relays information to the cortex.
Loss of the somatosensory cortex has little effect on pain perception and a moderate effect on temperature perception.
The lower brain stem, thalamus, and associated basal regions play dominant roles in discriminating these sensibilities.

Cortical Control of Sensory Sensitivity

Corticofugal signals are transmitted from the cerebral cortex to lower sensory relay stations, controlling the intensity of sensitivity.
Corticofugal signals are almost entirely inhibitory, decreasing transmission when sensory input intensity is too great.
This decreases lateral spread of signals and keeps the sensory system operating in an appropriate range of sensitivity.
This principle is used by all sensory systems.

Segmental Fields of Sensation-Dermatomes

Each spinal nerve innervates a segmental field of the skin called a dermatome.
Much overlap exists from segment to segment.
The anal region is in the dermatome of the most distal cord segment, dermatome S5.
The legs originate from lumbar and upper sacral segments (L2 to S3).
Dermatomal maps are used to determine the level of spinal cord injury.