Intro to Senses

Exam 12/2

General Potential

is a type of graded potential that occurs in sensory receptors when they are stimulated

• When a stimulus (such as pressure) activates a sensory receptor, ion channels in the receptor membrane open causing a local depolarization

> if the generator potential is strong enough to reach threshold, it triggers an action potential in the associated sensory neuron

Unipolar neurons ____

are the primary sensory neurons for general senses (touch, pressure, pain, temp, proprioception) that will carry the information up to the brain

Receptive Field

the distribution area of the endings of a sensory neuron

• smaller receptive fields allow more precise stimulus localization

Sensation

a stimulus we are consciously aware of

• to enter consciousness, signals must reach cerebral cortex

• only a fraction of stimuli result in sensations

• a lot of sensory input goes to other areas of the brain

• intensity of stimulus determined by frequency of nerve signals to CNS

Receptors provide ____________-

CNS information about stimulus modality (light, sound, temp, taste, pressure or smell), location, intensity and duration 

Receptor Adaptation

helps determine stimulus duration

Adaptation- decreased sensitivity to continuous stimulus (think wearing a watch or necklace)

Tonic Receptors 

-show limited adaptation: respond continuously

–For example, head position receptors in inner ear; all pain receptors

Phasic Receptors

adapt rapidly: only respond to new stimuli

–For example, pressure receptors

Sensory Receptor Classification are catergorized by _____

receptor distribution

General Sense Receptors

–Simple structures distributed throughout the body

–Somatic sensory receptors: tactile receptors of skin and mucous membranes; proprioceptors of joints, muscles, and tendons

–Visceral sensory receptors: found in walls of internal organs, they monitor stretch, chemical environment, temperature, pain

Special sense receptors

–Specialized receptors in complex sense organs of the head

–5 special senses: olfaction, gustation, vision, audition, equilibrium

Can also be categorized by ______

origin

Exteroceptors

detect stimuli from external environment

–Skin and mucus membranes; special sense receptors

Interoceptors

detect stimuli from internal organs

–Visceral sensory receptors monitoring internal environment

Proprioceptors

detect body and limb movements

–Somatosensory receptors of muscles, tendons, and joints

We can also categorized by ________

modality of stimulus (stimulating agent)

Five types of stimulating agents

• chemoreceptors

• thermoreceptors

• photoreceptors

• mechanoreceptors

• nociceptors

1. Chemoreceptors

detect chemicals dissolved in fluid

• Include receptors for external environment (for example, smell of food) or internal environment (for example, oxygen levels in blood)

2. Thermoreceptors

.detect changes in temperature

• Include receptors in skin, hypothalamus

3. Photoreceptors

.detect changes in light intensity, color, movement

• in the retina of the eye

4. Mechanoreceptors

detect distortion of cell membrane

• Include touch, pressure, vibration, and stretch receptors

• Function as baroreceptors, proprioceptors, tactile receptors, and specialized receptors in the inner ear

5. Nociceptors

detect painful stimuli

• Somatic nociceptors detect chemical, heat or mechanical damage to the body surface or skeletal muscles

• Visceral nociceptors detect internal organ damage

Tacile Receptors

Abundant mechanoreceptors of skin and mucous membranes

• Endings can be encapsulated or unencapsulated

Unencapsulated tactile receptors

–Dendritic ends of sensory neurons with no protective cover

Free nerve endings

terminal ends of sensory neuron dendrites

• Simplest tactile receptors

• Reside close to skin surface and in mucous membranes

• Mainly for pain and temperature but also light touch and pressure

Root hair plexuses

wrap around hair follicle

• Located in deeper layer of dermis

• Detect hair displacement

Tactile discs

flattened endings of sensory neurons extending to tactile Merkel cells

• Tactile cells are specialized epithelial cells in basal layer of epidermis

• Respond to light touch

Encapsulated tactile receptors

–Neuron endings wrapped by connective tissue or covered by connective tissue and glial cells (neurolemmocytes)

End (Krause) bulbs

–Located in dermis and mucus membranes

–Detect pressure and low-frequency vibration

Lamellated (Pacinian) corpuscles

are wrapped in neurolemmocytes and concentric layers of connective tissue

–Located deep in dermis, hypodermis, some organ walls

–Detect deep pressure, course touch, high-frequency vibration

Bulbous (Ruffini) corpuscles

are wrapped in CT

–Within dermis and subcutaneous layer

–Detect deep pressure and skin distortion

Tactile (Meissner) corpuscles

are intertwined endings wrapped in modified neurolemmocytes, covered in connective tissue

• In dermal papillae (especially in sensitive regions of the body)

• Discriminative light touch—allow recognition of texture, shape

Proprioceptors

are general sensory receptors located in muscles, tendons, and joints

• Specialized mechanoreceptors relay sensory info regarding body position and movement

• Proprioceptors are all tonic receptors (adapt slowly)

• Proprioception (the “sixth sense”) – sense of body position and movement

Three types of proprioceptors:

–Muscle spindle – detect stretch in skeletal muscle

–Golgi tendon organ – detect stretch in tendon

–Joint kinesthetic receptor – detect stretch in articular capsule

Referred Pain 

inaccurate localization of sensory signals

• Signals from viscera perceived as originating from skin, muscle

• Many somatic and visceral sensory neurons send signals via the same ascending tracts within spinal cord

• Somatosensory cortex unable to determine true source

Phantom Pain

•Sensation associated with removed body part

•Occurs following amputation of a limb

•Experience of pain from removed part due to stimulation of sensory neuron pathway on remaining portion

•Cell body of sensory neuron still alive, sometimes “lack of information” is perceived as pain

•Pain sometimes quite severe

•Medications and treating the other limb that’s still there can help relieve phantom pain