General Senses - Comprehensive Notes

General Senses

Learning Outcomes

• Describe the classification of general sensory receptors.

• Describe the locations and functions of receptors for tactile, thermal, and pain sensation and for proprioception.

Sensory Modalities

• Neurons only carry one type of sensation. Each sensory neuron is specialized to respond to a specific type of stimulus, ensuring that the brain receives distinct and accurate information about the environment.

• Two classes of sensations:

  • General senses:

  • Somatic – muscles, joints, skin: These senses include tactile sensations like touch, pressure, vibration, and temperature, as well as proprioception (body position) and pain. They originate from receptors located in the skin, muscles, and joints.

  • Visceral – internal organs: These senses provide information about the internal environment of the body, including sensations like pain, pressure, and distension in internal organs.

  • Special senses:

  • Sight: Vision is detected by photoreceptors in the eyes.

  • Hearing: Auditory information is detected by hair cells in the inner ear.

  • Taste/smell: Gustation (taste) and olfaction (smell) are detected by chemoreceptors in the taste buds and nasal cavity, respectively.

Classification of General Sensory Receptors

• Location:

  • Exteroceptors: Located near the external surface of the body, these receptors respond to stimuli from the external environment, such as touch, temperature, and pressure.

  • Interoceptors: Located in internal organs and blood vessels, these receptors detect internal stimuli, such as changes in blood pressure, pH, and oxygen levels.

  • Proprioceptors: Located in muscles, tendons, and joints, these receptors provide information about body position, muscle tension, and joint movement.

• Type of stimuli:

  • Mechanoreceptor: Respond to mechanical stimuli, such as touch, pressure, vibration, and stretch. Examples include tactile receptors in the skin and hair cells in the inner ear.

  • Thermoreceptor: Respond to changes in temperature. These receptors are located in the skin and detect both hot and cold stimuli.

  • Nocioceptor: Respond to painful stimuli. These receptors are free nerve endings found in almost every tissue of the body.

  • Baroreceptor: Respond to changes in pressure. These receptors are located in blood vessels and detect changes in blood pressure.

  • Osmoreceptor: Respond to changes in osmotic pressure of body fluids. These receptors are located in the hypothalamus and regulate fluid balance.

Types of Sensory Receptors

• Free nerve endings: These receptors are bare dendrites that are not enclosed in any specialized structures. They are primarily responsible for detecting pain, temperature, itch, and tickle.

• Encapsulated nerve endings: These receptors are enclosed in a connective tissue capsule, which enhances their sensitivity to specific types of stimuli, such as touch, pressure, and vibration.

• Separate receptor: These receptors are specialized cells that synapse with sensory neurons. Examples include hair cells in the inner ear (for hearing) and photoreceptors in the retina (for vision).

Tactile Receptors

• Free endings:

  • Pain, temp, itch, tickle: These sensations are detected by free nerve endings in the skin.

  • Plexus around hair – crude touch: Hair follicle receptors detect movement of hairs on the skin, providing a sense of touch.

• Encapsulated: Touch, pressure, vibration

  • Tactile disc – Merkel’s disc: These receptors are located in the epidermis and respond to light touch and pressure.

  • Messiner’s corpuscle: These receptors are located in the dermal papillae and are sensitive to fine touch and vibration.

  • Ruffini corpuscle: These receptors are located in the dermis and respond to sustained pressure and stretch.

  • Pacinian corpuscle: These receptors are located deep in the dermis and respond to deep pressure and high-frequency vibration.

Adaptation in Receptors

• Slowly adapting receptors continue to respond to a stimulus over time: These receptors provide continuous information about the presence and intensity of a stimulus. An example is the Ruffini corpuscle, which responds to sustained pressure.

• Rapidly adapting receptors quickly reduce their response even if the stimulus persists: These receptors are best suited for detecting changes in a stimulus. An example is the Pacinian corpuscle, which responds to high-frequency vibrations but quickly adapts if the vibration is constant.

Speed of Conduction

• Axon diameter and myelination affect conduction velocity. Larger diameter axons and myelinated axons conduct action potentials faster than smaller diameter and unmyelinated axons.

• Categories of axons:

  • A: Largest and fastest axons; Mechanoreceptor axons generally fall into category A. Further broken down into subgroups designated α (the fastest), β, and δ (the slowest).

  • B: These axons are myelinated and have intermediate conduction velocities. They transmit autonomic information.

  • C: Smallest and slowest axons: These axons are unmyelinated and transmit pain, temperature, and itch sensations.

• Muscle afferent axons are classified into four additional groups: I (the fastest), II, III, and IV (the slowest)—with subgroups designated by lowercase roman letters!

• Somatic Sensory Afferents:

  • Proprioception:

  • Receptor Type: Muscle spindle

  • Axon Diameter: 13-20 \mu m

  • Conduction Velocity: 80-120 m/s

  • Afferent Axon Type: Ia, II

  • Touch:

  • Receptor Type: Merkel, Meissner, Pacinian, and Ruffini cells

  • Axon Diameter: 6-12 \mu m

  • Conduction Velocity: 35-75 m/s

  • Afferent Axon Type: Aβ

  • Pain, temperature:

  • Receptor Type: Free nerve endings

  • Axon Diameter: 1-5 \mu m

  • Conduction Velocity: 5-30 m/s

  • Afferent Axon Type: Aδ

  • Pain, temperature, itch:

  • Receptor Type: Free nerve endings (unmyelinated)

  • Axon Diameter: 0.2-1.5 \mu m

  • Conduction Velocity: 0.5-2 m/s

  • Afferent Axon Type: C

Receptive Fields

• Area where a sensory neuron can detect stimuli. The size and density of receptive fields vary across the body. Areas with small, densely packed receptive fields, such as the fingertips, have high tactile acuity.

Pain

• Free nerve endings: Pain receptors are free nerve endings that are widely distributed throughout the body.

• Almost every tissue of the body: Nociceptors are found in the skin, muscles, joints, and internal organs, allowing the body to detect pain from a variety of sources.

• Intense stimuli: Pain receptors are activated by intense thermal, mechanical, or chemical stimuli.

• Chemicals released by the body: Tissue damage and inflammation can cause the release of chemicals, such as bradykinin, histamine, and prostaglandins, that stimulate nociceptors.

• Little adaptation: Pain receptors exhibit little adaptation, meaning that they continue to respond to painful stimuli over time. This lack of adaptation helps to ensure that the body is aware of potential tissue damage.

Fast Pain and Slow Pain

• Fast pain is transmitted via Aδ fibers (1-5 \mu m, 5-30 m/s): This type of pain is sharp and localized, allowing for quick responses to acute injuries.

• Slow pain is transmitted via C fibers (unmyelinated, 0.2-1.5 \mu m, 0.5-2 m/s): This type of pain is dull and diffuse, and it is often associated with chronic pain conditions.

Proprioceptors

• Muscle spindles: detect muscle length and changes in length. These receptors are located within muscles and provide information about muscle stretch and contraction.

• Tendon organs: detect muscle tension. These receptors are located in tendons and provide information about the force generated by muscle contractions.

• Joint kinaesthetic receptors: detect joint position and movement. These receptors are located in joint capsules and ligaments and provide information about joint angle and movement.