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- Receptors are particularly distinct to a specific type of environmental change and less sensitive to other forms of stimuli
- e.g. Vision receptors – contain pigment molecules that respond to light

Selective Response of Sensory Receptors


Somatic sensation

Tactile sensations - Touch, pressure, vibration, tickle, itch
Themoreceptive sensation - Heat and cold
Proprioception - Receptors from this sensations comes from the skin, muscles, bones, tendons, and joints


- Mechanoreceptors with nerve endings linked to networks of collagen fibers within a capsule
- Touch, movement, and vibration sensations - Rapid adapting receptors
- Pressure - Slow adapting receptors



- Muscle-spindle stretch receptors in skeletal muscles, mechanoreceptors in the joints, tendon organs (Golgi), ligaments, and skin
- Also supported by vision and the vestibular organs

Posture and Movement


Muscle spindle
- Activity depends on muscle length
- Annulospiral, flower-spray endings

Golgi tendon
- Passive stretch and active contraction increases the tension of the tendon that activate the tendon organ receptor

Stretch Receptors


- Sensitive to changes, not to absolute temperature
- Adapt only between 20° and 40° C
- Stimuli outside this range activate nocireceptors because of the high probability of tissue damage
- Skin thermoreceptors play a role in temperature regulation, which is controlled by centers in hypothalamus



- Gradiations of temperature: blue to red
(freezing cold > cold > cool > indifferent > warm > hot
> burning hot)
- Cold spots > warm spots: located beneath the skin at discrete “spots”
- Warm receptors- free nerve endings, transmitted thru type c fibers
- Cold receptors- type A delta nerve fibers, some type c



- free nerve endings that are stimulated when there is tissue damage

Pain: Nociceptors


Qualities of Pain

Cutaneous pricking pain: well localized and easily tolerated
Burning pain: poorly localized and poorly tolerated
Deep pain: arising from the viscera, musculature and joints, poorly localized, can be chronic and often associated with referred pain


- Sensitive to a stimuli causing tissue injury

- Chemical mediators include:
Histamine, bradykinin & prostaglandins from site of injury
ATP & 5-HT (serotonin) from platelets activated by injury
Substance P from the primary sensory neurons

- At 45°C, pain is perceived in the skin



- Lactic acid accumulates in the tissues
- Chemical agents formed
- Perception of pain

Tissue ischemia


Effect of mechanoreceptive pain receptors, ischemia

Muscle spasm


Pain from deep structures of the head referred to the surface

Areas that are pain sensitive:
- Venous sinuses
- Tentorium
- Dura at the brain base
- Meningeal blood vessels
- Middle meningeal artery



Types of Intracranial headache

- Headache of meningitis
- Low CSF pressure headache
- Migraine headache
- Alcoholic headache
- Headache cause by constipation


Severe headache from the inflammation of meninges

Headache of meningitis


- Unknown mechanism
- Starts with a prodrome lasting minutes to an hour

Theories of Migraine headache:
- Vasospasm of the arteries producing ischemia
- Spreading cortical depression
- Psychological abnormalities
- Vasospasm by excess potassium in the ECF

Migraine headache


headache from absorbed toxic products or fluid loss in the gut

Headache caused by constipation


headache due to Alcohol- toxic to tissues

Alcoholic headache


Types of Extracranial headache

- Headache from muscle spasm
- Headache from irritation of nasal and accessory nasal structures
- Headache caused by eye disorders
- Muscle contraction
- Excessive irradiation


- Pain of visceral origin is referred to sites on the skin and follows the dermatome rule
- Sites are innervated by nerves that arise from the same segment of the spinal cord
- E.g. ischemic heart pain is referred to the chest and shoulder

Referred Pain


Causes of true visceral pain (poorly localized pain)

- Ischemia of visceral tissue
- Chemical damage to the visceral surface
- Spasm of hollow viscus smooth muscle
- Overdistention of hollow viscus
- Stretching of tissues surrounding or within the viscera


- sharp, well localized pain
- Visceral disease spreads to parietal peritoneum, pleura or pericardium
- Parietal surface supplied with pain innervation – sharp pain
- Appendicitis - Inflamed appendix pass pain impulses into the spinal cord levels T10 or T11 – referred pain to the umbilicus

Parietal Pain


Sensory Transduction

Action potentials in nerve fibers

Receptor potentials

Transformation of stimulus energy


Mechanisms of Receptor Potentials

1. By mechanical deformation - Stretches the receptor membrane; Opens ion channels

2. By application of a chemical - Opens ion channels

3. By change of the temperature of the membrane
- Alters the permeability of the membrane


-- Central nerve fiber extending through its core.
-- Surrounding – multiple concentric capsule layers
--Compression anywhere on the outside of the corpuscle will Elongate, Indent or Deform the central fiber
-- Central fiber of the pacinian corpuscle
- The tip of the central fiber - unmyelinated
- The fiber - Myelinated
-- Sodium influx - a local circuit of current flow occurs
-- Node of Ranvier - action potentials are transmitted

Pacinian corpuscle: Eliciting an Action potential


- process by which an environmental stimulus activates a receptor and is converted to electrical energy

sensory transduction


- A single afferent neuron with all its receptor endings

Sensory unit


- area subserved by the sensory unit
- overlap so that when 1 point is stimulated it activates several sensory units
- E.g. ice cube on the skin give rise to sensations of touch and temperature simultaneously
- Area of the body when stimulated, changes the firing rate of a sensory neuron
- Large receptive field: less precise perception
- Small receptive field: more precise perception

Receptive field


- Conversion of receptor potentials into action potentials that conveys sensory information to the CNS
- Nature of a sensation and the type of reaction generated vary according to the destination of sensory impulses in the CNS

Sensory Coding


Characteristics of the stimuli

- Type (Modality)
- Intensity
- Location
- Duration