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Sensory Function

specialized cells that monitor internal and external conditions

Two types of senses



receptors are distributed throughout the body
1. Afferent impulses are sent to the somatosensory cortex


receptors are concentrated in sense organs
1. Afferent impulses are sent to special sense cortexes

Afferent (sensory) division

carries sensory info from receptors to the CNS

i. Somatic afferent fibers

carries impulses from skin, skeletal
muscles and joints

ii. Visceral afferent fibers

carries impulses from organs within
ventral body cavities

iii. Special sense afferent fibers

eyes, ears, taste, smell

Pseudounipolar neuron(sensory neuron) except special sensory

1. Cell body lies in the dorsal root ganglia
2. Action potential is initiated at the peripheral end of the

Sensory Receptors

-found on the peripheral end of sensory neurons.
-Act as transducers to change an incoming stimulus of one type into an electrical impulse

Adequate stimulus

receptors are specific for the type of stimulus it is
sensitive to an can transducer, ie cants shine light in ear and hear something.


Receptors can respond to other types of stimuli if strong enough. ie press on eye (mechainal stress) you will see flashes of light.

"Law of Specific Nerve Energies

regardless of the type of stimulus the sensation is perceived as what the receptor is specific for

Sensory Transduction

i. A stimulus alters the membrane permeability of the receptor. This leads to the production of a graded receptor potential

Sensory Transduction

ii. Receptors that are modified endings of the afferent neuron produce generator potentials

Sensory Transduction

iii. Receptors that are separate cells from the afferent neuron produce receptor potentials

Sensory Transduction

iv. Release chemicals that open ligand gated channels on the afferent neuron

i. Free nerve endings

1. Nociceptors
2. Themoreceptors

1. Nociceptors

activation causes the sensation of pain
a. Free nerve endings found in the skin, joints, bones,
and blood vessels
b. Sensitive to chemicals, tissue damage, and extreme

2. Themoreceptors

react to changes in temperature
a. Free nerve endings in the dermis,
hypothalamus, and liver
b. You have 3X more cold receptors than
warm receptors, cold vs colder= more action potiental high fequency, 10 degree or less you feel pain /45 then pain


stimulated by physical change such as pressure or movement

Merkel cells

cells in the stratum basale associated with
free nerve endings, detect fine touch

Hair root plexus

free nerve endings associated with hair follicles, detect movement of hair

Meissner's corpuscles

-detect touch, found in dermal papillae

Ruffini corpuscles

sensitive to stretch and distortion, found in the dermis

Pacinian corpuscles

deep pressure receptors found in the dermis


sensitive to internal pressures. Monitors
blood pressure in vessels (carotid sinus and aorta), lungs,
bladder, intestines


monitors position and stretch of muscles, tendons, ligaments, and joint capsules. Helps maintain posture and sense of body position


detect chemicals dissolved in solution. Olfactory, taste, osmolarity, pH, CO2, O2 alot of special senses

Ascending tracts

a bundle of axons transmitting impulses towards the
brain. Usually involves three successive neurons (three levels of processing) tend to be on dorsal and lateral sides of your spinal cord

i. Receptor level

Primary neuron running from the receptor to the
posterior horn of the spinal cord or medullary nuclei

ii. Circuit level

Secondary neuron synapses with the first and transmit impulses to the thalamus (all sensors go to thalamus but olfactory)

iii. Perception level

Tertiary neuron running from the thalamus to
the primary somatosensory cortex or a special senses cortex. The
cerebral cortex is responsible for

cerebral cortex is responsible for

1. Sensation - the awareness of the stimuli
2. Perception - the interpretation of the stimuli

Each sensory fiber is a "labeled line

the type, location, strength, and the intensity of the stimuli is encoded in the area of the cortex it travels to and the frequency of the impulse

Determining type of stimulus or receptor activated

1. Depends on the pathway it takes and the area of the
cerebral cortex it travels to

Determining type of stimulus or receptor activated

2. The area of the cortex devoted to each region is related to the regions sensitivity (# of receptors

Referred pain

many primary neruons sinape with the same secondary neruon, usually defalt to the most frequently stimulated ie skin not heart

Phantom pain

remodeling of the cortex, or a secondary neuron is irritated

Localization of the stimulus

Accuracy of localization depends on the size of the
receptor field. The smaller the receptor field the more

Localization of the stimulus

Accuracy also depends on lateral inhibition. Only the
most intensely stimulated pathway is excited. Less excited
pathways surrounding the stimulated area are inhibited

Number of receptors stimulated - stronger stimuli
usually affect larger areas

Number of receptors stimulated - stronger stimuli
usually affect larger areas

usually affect larger areas

Frequency of action potentials - stronger stimuli generate
larger receptor potential, therefore a greater frequency of
action potentials

Duration of the stimulus

Adaptation - receptors change their sensitivity in the
presence of constant stimulus ie a watch on your wrist

Phasic receptors

fast adapting
i. Report changes in the environment
ii. Burst of firing at the beginning and end of stimulus
iii. Examples: temperature, smell, touch

Tonic receptors

slow adapting or not at all
i. Constant firing rate
ii. For situations were continuous information about a stimulus is valuable
iii. Examples: proprioceptors pain, muscle stretch


is subjective , influenced by past adn present experiences, and is coupled with emotional responses

fast pain pathways

sharp pain, easy to localize, myleineated

slow pain pahtways

unmyleined, dull aching pain, persists long and hard to locialize

what stimulates the pain pathway

nociceptors are sensite to chemcials released by damaged tissues.
prostaglandins, bradykinins, histamine, capsaicin

pain pathway

nocicieptors stimulated and primary neurion relases suspstance P- secondary neuron then thalamus and to the reticular formation(brain stem- level of alertness) and hypothamlus( emotinal response)

analgesic pathway

endorphins bind to opiate recepotros on the axon termainal and block the release of susbstance P(example of presnapic inhibition)

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