Somatic Sensations

Somatic Sensations

Sensations detected by receptors in the skin, subcutaneous tissue, mucous membranes, muscles, tendons, and joints (e.g., tactile, thermal, pain, and proprioceptive).

Somatic Sensations: Distribution

Somatic sensory receptors are distributed unevenly; some areas are densely populated with receptors, and others contain only a few (e.g., the tip of the tongue, the lips, and the fingertips have the highest density of somatic sensory receptors)

Cutaneous Sensations

Somatic sensations that arise from stimulating the skin surface

Tactile Sensations

Sensations include touch, pressure, vibration, itch, and tickle.

Tactile Sensation: Touch, Pressure, and Vibration

Encapsulated mechanoreceptors attached to large, myelinated A fibers detect touch, pressure, and vibration.

Tactile Sensation: Other Sensations

Free nerve endings attached to small-diameter, unmyelinated C fibers detect sensations, such as itch and tickle sensations.

Tactile Sensation: Recall

Recall that larger-diameter, myelinated axons propagate nerve impulses more rapidly than do smaller-diameter, unmyelinated axons.

Tactile Sensation: Example

Tactile receptors in the skin or subcutaneous tissue include tactile corpuscles, hair root plexuses, nonencapsulated sensory corpuscles, lamellar corpuscles, bulbous corpuscles, and free nerve endings

Touch

Sensations of touch generally result from stimulation of tactile receptors in the skin or subcutaneous tissue.

Touch: Types of Rapidly Adapting Receptors

There are two types of rapidly adapting touch receptors: tactile corpuscles (Meissner corpuscles) and hair root plexuses

Touch: Tactile Corpuscles

Rapidly adapting touch receptors are found in the dermal papillae of hairless skin, especially in the fingertips, hands, eyelids, lips, and soles, which detect light touch and low-frequency vibrations mainly when they first occur.

Touch: Tactile Corpuscles: Characteristics

Each corpuscle is an egg-shaped mass of modified Schwann cells and nerve endings enclosed by a capsule of connective tissue

Touch: Hair Root Plexuses

Rapidly adapting touch receptors are found in hairy skin and detect hair movement caused by touch or skin movement

Touch: Hair Root Plexuses: Characteristics

Receptors that consist of free nerve endings wrapped around hair follicles

Touch: Types of Slowly Adapting Receptors

There are two types of rapidly adapting touch receptors: nonencapsulated sensory corpuscles (Merkel discs) and bulbous corpuscles (Ruffini corpuscle)

Touch: Merkel Discs

Slowly adapting touch receptors are found in the fingertips, hands, lips, and external genitals, which detect continuous touch

Touch: Merkel Discs: Characteristics

Saucer-shaped, flattened free nerve endings that make contact with tactile epithelial cells (Merkel cells) of the stratum basale

Touch: Ruffini Corpuscles

Slowly adapting touch receptors are found in the dermis, subcutaneous tissue, and other tissues of the body, and detect skin stretch, continuous pressure, and joint position

Touch: Ruffini Corpuscles: Characteristics

Elongated, encapsulated receptors that consist of branched nerve endings and Schwann cells

Pressure

A sustained sensation caused by deeper deformation of the skin and subcutaneous tissue, detected by lamellar and bulbous corpuscles, which are slowly adapting receptors.

Vibration

A sensation caused by rapidly repetitive sensory signals from tactile receptors (e.g., lamellar corpuscles and tactile corpuscles)

Vibration: Lamellar Corpuscle (Pacinian Corpuscle)

Rapidly adapting receptors are found in the dermis and subcutaneous tissue and detect high-frequency vibrations, such as those from power tools.

Vibration: Lamellar Corpuscle: Characteristics

Receptors that consists of a nerve endings surrounded by a multilayered connective tissue capsule

Itch

A sensation results from stimulation of free nerve endings by certain chemicals, such as bradykinin (a kinin, a potent vasodilator), histamine, or antigens, often because of a local inflammatory response.

Itch: Scratching

Scratching usually alleviates itching by activating a pathway that blocks transmission of the itch signal through the spinal cord.

Tickle

Free nerve endings can detect tickle sensations, which usually occur when someone else touches you because the cerebellum predicts the sensations when you touch yourself, reducing the tickle response.

Thermal Sensations

Free nerve endings have receptive fields about 1 mm in diameter on the skin surface with two distinct receptors for heat and cold.

Thermal Sensations: Cold Receptors

Sensory receptors located in the stratum basale that detect temperatures between 10°C and 35°C (50°F–95°F)

Thermal Sensations: Cold Receptors: Characteristics

They are mainly attached to medium-diameter, myelinated A fibers, although a few are connected to small-diameter, unmyelinated C fibers.

Thermal Sensations: Warm Receptors

Sensory receptors located in the dermis that detect temperatures between 30° and 45°C (86–113°F)

Thermal Sensations: Warm Receptors: Characteristics

They are mainly attached to small-diameter, unmyelinated C fibers; however, warm receptors are not as abundant as cold receptors

Thermal Sensations: Nerve Impulse

Cold and warm receptors adapt rapidly at first, then continue sending nerve impulses at a lower frequency during a prolonged stimulus.

Thermal Sensations: Pain

Temperatures below 10°C and above 45°C primarily stimulate pain receptors, rather than thermoreceptors

Pain Sensation

A protective bodily signal caused by actual or potential tissue damage that the nervous system detects and processes.

Pain Sensation: Nociceptors

Free nerve endings, found in every tissue of the body except the brain, that respond to intense thermal, mechanical, or chemical stimuli.

Pain Sensation: Nociceptors: Chemicals

Tissue irritation or injury releases chemicals such as prostaglandins, kinins, and potassium ions (K+ ) that stimulate nociceptors

Pain Sensation: Nociceptors: Adaptation

Nociceptors exhibit very little adaptation; pain may persist even after a stimulus is removed due to lingering pain-mediating chemicals.

Pain: Types of Pain

There are two types of pain: fast and slow.

Pain: Fast Pain

A rapid, sharp, or pricking pain felt within 0.1 seconds, carried by medium-diameter, myelinated A fibers, occurs mainly in the skin, not in deeper tissues.

Pain: Slow Pain

A dull, aching, or burning type of pain that travels along small-diameter, unmyelinated C fibers, which begins 1 second or more after a stimulus and gradually increases

Pain: Slow Pain: Sensation

This type of pain may be excruciating and is also referred to as chronic, burning, aching, or throbbing pain, which can occur both in the skin and in deeper tissues or internal organs

Pain: Superficial Somatic Pain

Pain that arises from stimulation of receptors in the skin

Pain: Deep Somatic Pain

Pain that arises from stimulation of receptors in the skeletal muscles, joints, tendons, and fascia

Pain: Visceral Pain

Pain that arises from stimulation of receptors in internal organs and can be severe when the organ is stretched (distension) or lacks blood flow (ischemia), such as from a kidney or gallstone blocking a duct.

Pain: Visceral Pain: Diffuse Stimulation

If stimulation is diffuse (involves large areas), visceral pain can be severe, which might result from distension or ischemia of an internal organ.

Pain: Localization

Fast pain is precisely localized, while somatic slow pain is less precise and diffuse over a larger area; visceral slow pain may be felt directly where the affected organ is located.

Pain: Referred Pain

A phenomenon in which n visceral pain is felt in a different area, because the organ and the area where pain is felt share the same spinal cord segment.

Proprioceptive Sensations

Sensation that help us know where our body parts are and how they are moving without looking, allowing us to walk, type, and dress without using our eyes.

Proprioceptive Sensations: Kinesthesia

The ability to sense the position, movement, and tension of your body parts using receptors in your muscles, joints, and tendons

Proprioceptive Sensations: Proprioceptors

Receptors in muscles and tendons detect muscle contraction, tendon tension, and joint position.

Proprioceptive Sensations: Proprioceptors: Hair Cells

Hair cells of the inner ear monitor the orientation of the head relative to the ground and head position during movements to provide information about balance and equilibrium.

Proprioceptive Sensations: Adaptation

Most proprioceptors adapt slowly, so the brain continuously receives information about body position and makes adjustments for coordination.

Proprioceptive Sensations: Weight Discrimination

The ability to assess the weight of an object that help determine the amount of muscular effort necessary to perform a task.

Muscle Spindles

Small proprioceptors inside skeletal muscles that detect changes in muscle length and stretch reflexes

Muscle Spindles: Muscle Tone

The brain adjusts muscle spindle activity to control muscle tone, the small amount of muscle contraction present at rest.

Muscle Spindles: Characteristics

Receptors that consists of slowly adapting sensory nerve endings wrapped around 3–10 specialized intrafusal muscle fibers, enclosed in a connective tissue capsule that anchors them to the endomysium and perimysium.

Muscle Spindles: Quantity

Muscles spindles are found among and parallel to most skeletal muscle fibers; they are numerous in muscles needing precise movements (fingers and eyes) and fewer in muscles used for powerful movements (thigh muscles)

Muscle Spindles: Function

Muscle spindles measure muscle length and stretching, sending signals to the CNS for awareness of limb position and to the cerebellum for coordinating muscle contractions.

Muscle Spindles: Gamma Motor Neurons

Muscle spindles also contain neurons that terminate near both ends of the intrafusal fibers and adjust the tension in a muscle spindle to variations in the length of the muscle

Muscle Spindles: Gamma Motor Neurons: Example

While your biceps muscle shortens in response to lifting something, gamma motor neurons stimulate the intrafusal fibers to contract slightly to keep spindle muscles tight and sensitive to stretching

Muscle Spindles: Extrafusal Muscle Fibers

Extrafusal muscle fibers are the standard skeletal muscle cells that make up the vast majority of muscle mass and contract to generate the force needed for body movements

Muscle Spindles: Alpha Motor Neurons

Large-diameter A fibers that supply the extrafusal muscle fibers

Muscle Spindles: Alpha Motor Neurons Function

During the stretch reflex, muscle spindle impulses travel to the spinal cord and brainstem, activating alpha motor neurons, which cause extrafusal muscle fibers to contract and relieve the stretch.

Muscle Spindles: Alpha and Gama Motor Neurons

The cell bodies of both gamma and alpha motor neurons are located in the anterior gray horn of the spinal cord (or in the brainstem for muscles in the head).

Tendon Organs

Slowly adapting receptors, located at the junction of a tendon and a muscle, protect tendons and their associated muscles from damage due to excessive tension

Tendon Organs: Function

When a muscle contracts, it pulls its attachment points toward each other, creating muscle tension (force).

Tendon Organs: Characteristics

Each tendon organ consists of a thin capsule of connective tissue that encloses a few tendon fascicles (bundles of collagen fibers)

Tendon Organs: Relaxation

When tension is applied to a muscle, the tendon organs send signals to the CNS, triggering muscle relaxation to reduce tension

Joint Kinesthetic Receptors

Mechanoreceptors located within and around the articular capsules of synovial joints that detect information about joint position and movement.

Joint Kinesthetic Receptors: Pressure

Free nerve endings and bulbous corpuscles detect pressure within joint capsules.

Joint Kinesthetic Receptors: Movement

Small lamellar corpuscles detect acceleration and deceleration of joints during movement.