Sensations and Receptors

By

Dr. D. Fisher

SENSATION• 4 events must occur for a sensation to take place:

1. Sensory receptors or sense organs must respond to stimulus (Stimulus is capable of activating specific sensory neurons Specific to that stimulus)

1. Information is then transduced into nerve impulses

1. Sensory neurons conduct nerve impulses to CNS by first-order neurons translation

1. A region of CNS must translate nerve impulses into sensation (mostly in cerebral cortex)

CLASSIFICATION OF RECEPTORS

• Sensory receptors have selectivity (i.e. they respond vigorously to one particular type of stimulus OR weakly or not at all others)

• Different classifications of peripheral sensory receptors

CLASSIFICATION OF RECEPTORS

• Simple receptors – associated w/ general (or somatic) senses Vs. Complex receptors – associated w/ special senses

• Location: near or at surface of body, deep w/in blood vessels, viscera, or musculoskeletal organs

• Type of Stimulus Detected: mechanical pressure or stretching, temperature changes, light intensity, chemical changes, damage to tissues

• Adaptation (i.e. change in sensitivity to long lasting stimuli)

• Structure – encapsulated vs. free nerve endings

LOCATION

• Exteroceptors external stimuli

• Interoceptors internal stimuli

• Proprioceptors position & movement stimuli

TYPE OF STIMULUS

Receptor Type Modality

• Mechanoreceptor mechanical

• Thermoreceptor temperature

• Photoreceptor light

• Chemoreceptor chemical

• Nociceptors pain/tissue damage

STRUCTURE

• Unencapsulated dendritic endings– Free or naked nerve endings – Abundant in epithelial tissue and connective

tissue underlying epithelia

• Encapsulated – Nerve fibers enclosed in capsule of

connective tissue– Widely shaped, sized, and found throughout

the body

CUTANEOUS RECEPTORS• Tactile sensation (i.e. touch,

pressure, vibration, itch, tickle)• Thermal sensations (cold & warmth)

• Cutaneous receptors densely populate tip of tongue & lips, but few are found on back of neck

• Dendrites of cutaneous receptors may or may not be enclosed in capsule of epithelial or connective tissue

• Nerve impulses of cutaneous receptors are transmitted along somatic afferent neuron in spinal cord or cranial nerves thru thalamus to somatosensory are of parietal lobe of cerebral cortex

CUTANEOUS RECEPTORS

• End organ of Ruffini crude touch

• Merkel discs discriminative touch

• Root hair plexus hair movement

CUTANEOUS RECEPTOR(FREE NERVE ENDINGS)

• Itch & tickle

• Thermoreceptors Heat loss & gain

• Nociceptors pain

NOCICEPTORS• Free nerve endings• Tonic receptors• Detect pain or tissue damage• Somatic or visceral pain

• Referred pain is a phenomenon in which pain is felt in a surface area far from a stimulated organ (brain freeze or pain in arm during heart attack)

• Phantom pain (phantom limb sensation) is phenomenon in which pain is felt in an amputated (nonexistent, phantom) limb may feel itch on toes or fingers/actual pain of lost part

PROPRIOCEPTORS

• Position or movement of body

• Tonic

• Somatosensory area of cerebrum

• Cerebellum via spinocerebellar tracts

• Muscle spindle fibers

• Golgi tendon organs

• Joint kinesthetic receptors

TOUCH

Types of classification

1. Capsulated/ Non-capsulated

2. Fast adapting/ slow adapting-myelin wrapping amount

3. Superficial/ deep layer

Merkel= Slow adapting, superficial layer, non-capsulated

Meissner= Fast adapting, superficial layer, capsulated

Pacinian= Fast adapting, Deep layer, capsulated

Ruffini= Slow adapting, Deep layer, non-capsulated

Nociceptors= Non-capsulated

Thermal= Non capsulated

ADAPTATION TO STIMULI

• Phasic rapidly adapting role in signaling changes in intensity

• Tonic adapt slowly or not at all role in signaling information regarding steady state

Effects of Stimulus Strength on AP transmission

MUSCEL SPINDLE FIBERS

• Monitor changes in length of skeletal muscle

• Degree of muscle stretch

• Aid in coordination & efficiency of muscle contraction

TENDON ORGANS (GOLGI TENDON ORGANS)

• Junction of tendon & muscle

• Monitors force of muscle contraction

• Detect tension applied to tendon

• Protects tendon & muscle from excessive tension

JOINT KINESTHETIC RECEPOTRS

• Located in articular capsules & synovial joints

• Acceleration & deceleration of joint movement

• Pressure in joint

• Excessive joint strain