Module 3.1 - Sensory receptors

Types of proprioceptors

MUSCLE SPINDLES • In skeletal muscle • Sets muscle tone (degree of contraction while muscle at rest) MUSCLE SPINDLES are plentiful in muscles that produce fine motor skills and less so in larger muscles involved in forceful, coarse movement (i.e., quadriceps femoris) TENDON ORGANS • Located at junction of tendon and muscle • When tension applied to muscle, tendon organs generate nerve impulse to CNS resulting in muscle relaxation JOINT KINESTHETIC RECEPTORS • Present within and around articular capsules of synovial joints

Types of tactile receptors

TOUCH • Rapidly adapting touch receptors • Meissner corpuscles (encapsulated) • Hair root plexuses (free nerve endings) • Slowly adapting touch receptors • Merkel discs (free nerve endings) • Ruffini corpuscles (encapsulated) PRESSURE • sustained touch over larger area • Meissner corpuscles • Merkel discs • Pacinian corpuscle (encapsulated) VIBRATION • Meissner corpuscle • Pacinian corpuscles ITCH • free nerve endings TICKLE cannot arise when you touch yourself! • free nerve endings

3. Type of stimulus detected

a) MECHANORECEPTORS Detect mechanical stimuli; provide sensation of touch, pressure, vibration, proprioception, and hearing and equilibrium; also monitor stretching of blood vessels and internal organs b) THERMORECEPTORS Detect changes in temperature c) NOCICEPTORS Respond to painful stimuli resulting from physical or chemical damage to tissue d) PHOTORECEPTORS Detect light that strikes the retina of the eye e) CHEMORECEPTORS Detect chemicals in the mouth (taste), nose (smell), and body fluids f) OSMORECEPTORS Sense osmotic pressure of body fluids

2. Receptor location

a) EXTERORECEPTORS Located at or near body surface; sensitive to stimuli origination outside the body; provide information about external environment; convey visual, smell, taste, pressure, vibration, thermal and pain sensations b) INTERORECEPTORS Located in blood vessels, visceral organs, and nervous system; provide information about internal environment; impulses are not consciously perceived but occasionally may be felt as pain or pressure c) PROPRIOCEPTORS Located in muscles, tendons, joints, and inner ear; provide information about body position, muscle length and tension, position and motion of joints, and equilibrium (balance)

1. Microscopic structure

a) FREE NERVE ENDINGS - bare dendrites (receptors for pain, temperature, tickle, itch, some touch) b) ENCAPSULATED NERVE ENDINGS - dendrites enclosed in connective tissue (receptors for pressure, vibration, some touch) c) SEPERATE CELLS (that then synapse with first-order neurons) - usually for the special senses (hair cells, taste buds, photoreceptors)

Primary sensory cortices

• All sensory information (except olfaction) goes to the thalamus and is relayed from there to the cortex. • Also, all sensations from one side of the body goes to the thalamus and cerebral cortex on the opposite side of the brain • Contralateral control/awareness

Learning Objectives

• Describe different ways to classify sensory receptors • Describe the location and function of the somatic sensory receptors for tactile, thermal, and pain sensations • Identify the receptors for proprioception and describe their functions

Localisation of pain

• Fast pain easily localized • Slow somatic pain and some visceral pain is well localized but diffuse • Visceral pain can cause referred pain • Pain is felt in or just deep to the skin that overlies the stimulated organ OR in a surface area FAR from the stimulated organ • Because the visceral organ involved and the area to which the pain is referred are served by the same segment of the spinal cord

Process of Sensation

• Four events typically occur: 1. STIMULATION of sensory receptor - stimulus activates sensory receptor 2. TRANSDUCTION - energy converted to graded potential 3. GENERATION of nerve impulse - AP propagated along axon (first-order neurons - from PNS to CNS) 4. INTEGRATION - region of CNS receives nerve impulse and conscious perception is integrated with other modalities (i.e., vision of car, but also red, and moving)

2. Thermal Sensation

• Free nerve endings (1mm receptor fields) • Different receptors for cold and warm COLD RECEPTORS • Activate at temps 10º and 40º C WARM RECEPTORS • Activate at temps 32º and 48º C • Below 10º and above 48º C? • PAIN RECEPTORS!

Sensory Modalities

• Modalities are touch, pain, vision, audition, etc. • Neurons are modality-specific (i.e., neuron for touch does not transmit info for pain) • Sensory modalities grouped into.... General senses: 1 - Somatic senses: tactile, thermal, pain, proprioception 2 - Visceral senses: conditions within internal organs (pressure, stretch, nausea, hunger, temperature) 3 - Special senses: smell, taste, vision, audition, equilibrium/balance

3. Pain Sensation

• Pain serves as a protective function, signaling noxious, tissue-damaging conditions • Nociceptors - free nerve endings - found in every tissue except the brain Types of pain..... FAST PAIN - within 0.1 second - acute, sharp, pricking pain (only felt on surface/skin - superficial somatic pain) SLOW PAIN - after one second - chronic, burning, aching, throbbing pain (felt in skin and deeper tissues/internal organs - deep somatic pain and visceral pain)

4. Proprioceptive Sensation

• Proprioceptive sensation allows us to know where our head and limbs are located and how they are moving without looking at them • Kinesthesia (kin-es-THE-ze-a) is the perception of body movt • Proprioceptors are embedded in muscles and tendons (and hair cells in inner ear - CN Lect 3) • Weight discrimination - ability to assess weight of an object and determine muscular effort

Overview

• Receptors • Somatic sensations • Tactile • Thermal • Pain • Proprioceptive - Sensation supplying Skin, muscles & joints

Somatic Sensations

• Sensory receptors distributed unevenly • Some parts of body densely populated (tip of tongue, lips, fingertips), others not (back, leg) • Cutaneous sensation (from the skin) 1 - Tactile 2 - Thermal 3 - Pain 4 - Proprioceptive

Introduction to Somatosensation

• Somatosensation: sensory information from the skin and muscles • Receptors in periphery encode mechanical, chemical, or thermal stimulation into receptor potentials (exceeds threshold -> AP generated) • AP conducted along peripheral axon, to soma in dorsal root ganglion, then along proximal axon to spinal cord • Ascends via axons (white matter) to brain/cortex • Perception is the conscious awareness of a sensation and occurs once impulses reach the cortex • Some sensations are never consciously perceived because they never propagate to the cortex (e.g., blood pressure information is dealt with in the medulla oblongata)

Sensory Receptors

• There are three ways sensory receptors can be classified (tables of all three provided): 1. Microscopic structure 2. Receptor location 3. Type of stimulus detected

1. Tactile Sensation

• Touch, pressure, vibration, itch, tickle • Although perceived as different, arise from same types of receptors - encapsulated mechanoreceptors and free nerve endings