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181

- 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

182

Somatic sensation

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

183

- 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

Touch-Pressure

184

- 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

185

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

186

- 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

Temperature

187

- 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

Thermoreceptors

188

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

Pain: Nociceptors

189

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

190

- 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

Nociceptors

191

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

Tissue ischemia

192

Effect of mechanoreceptive pain receptors, ischemia

Muscle spasm

193

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

Headache

194

Types of Intracranial headache

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

195

Severe headache from the inflammation of meninges

Headache of meningitis

196

- 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

197

headache from absorbed toxic products or fluid loss in the gut

Headache caused by constipation

198

headache due to Alcohol- toxic to tissues

Alcoholic headache

199

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

200

- 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

201

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

202

- 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

203

Sensory Transduction

Action potentials in nerve fibers

Receptor potentials

Transformation of stimulus energy

204

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

205

-- 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

206

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

sensory transduction

207

- A single afferent neuron with all its receptor endings

Sensory unit

208

- 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

209

- 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

210

Characteristics of the stimuli

- Type (Modality)
- Intensity
- Location
- Duration