EXAM 2 - General Sensory Physiology

types of sensory receptors

mechanoreceptors, chemoreceptors, thermoreceptors, nociceptors, photoreceptors, propioreceptors

Mechanoreceptors detect

Sensory receptors responsible for sensing deformation in body tissues

Tactile Mechanoreceptors

touch, pressure, vibration, tickle and itch

thermoreceptors detect

detect heat and cold (temperature)

Nociceptors detect

detect tissue damage with pain receptor

what is pain

Tissue damage

photoreceptors detect

photon of light

chemoreceptors detect

chemicals

taste, smell, CO2, O2

proprioceptor

sense postition

Modalities of sensation

touch, pain, sigh, and sound, etc

each receptor is responsive to

one type of energy stimulus

How the sensation is perceived is determined by what?

1. the characteristics of the receptor

2. the central connections of the axon connected to the receptor

Receptor Excitation - Mechanical deformation

stretch of the membrane - causes ion channels to open- might lead to EPSP

Receptor Excitation - Application of chemicals

opens the ion channels - might lead to EPSP

Receptor Excitation - Change in temperature

alter permeability of membrane - might lead to EPSP

Receptor Excitation - Electromagnetic radiation

changes properties/characteristics of membrane - might lead to EPSP

Receptor potential

The membrane potential of the receptor

A local graded electrical potential produced by a receptor cell in response to a physical stimulus

Each receptor is responsive to

One type of stimulus energy: specificity is key

receptors are specific to one type of energy

Receptor potential rises above the threshold

makes action potential

The greater the intensity of the stimulus

the greater the receptor potential

the greater the rate that AP are created

larger stimulus=larger receptor potential=faster the AP created

Relationship Between Receptor Potential and Action Potentials

increase intensity: Increases the rate (of AP) and response

sensory system receptor potential threshold

WONKY

you only get an AP 50% of the time

Summary of Sensory Transduction

Stimulus opens ion channels on sensory neuron or sensory cell, which produces a graded potential called a: receptor potential

- Reach threshold and Leads to: Action potential 50% of time

Adaptation of Receptors

When a continuous stimulus is applied, receptors respond rapidly at first, but response declines until all receptors stop firing

Muscle spindle: Adaptation of Receptors

slow adaptation

Adaptation of Receptors: pacinian corpuscle

rapid adaptation

Adaptation

◻ Rate of adaptation varies with type of receptor

◻ Receptors respond when a change is taking place (i.e., think of the feel of clothing on your skin)

Slowly Adapting (Tonic) Receptors - definition

def: fire throughout stimulation (as long as stimulus is present then it is firing/transmitting signal)

Slowly Adapting (Tonic) Receptors function

gives information on DURATION of the stimulus

- keep brain aware of stimulus presence (how long is stimulus present)

Slowly Adapting (Tonic) Receptors - adaptation

Will adapt to extinction as long as the stimulus is

present; however, this may take hours or days

Slowly Adapting (Tonic) Receptors - examples

muscle spindles, GTOs, ruffini endings, chemoreceptors, barro receptors

Rapidly Adapting (Phasic) Receptors - definition

only fire/respond when a change is taking place

onset and offset of a stimulus

Rapidly Adapting (Phasic) Receptors are Important for what

predicting the future position or condition of the body

1. movement

2. balance

Rapidly Adapting (Phasic) Receptors - examples

semicircular canals and picinian corpuscles

Transmission of Receptor Information to the Brain - Velocity

• The larger the nerve fiber diameter the faster the

rate of transmission of the signal

• Velocity of transmission can be as fast as 120

m/sec or as slow as 0.5 m/sec

Nerve Fiber Classification

Type A or C

Type A nerve fiber

myelinated fibers of varying sizes, generally

fast transmission speed.

• subdivided into a, b, d, g (alpha, beta, delta, gamma)

Type C nerve fiber

unmyelinated fibers, small with slow

transmission speed.

Importance of Signal Intensity

Signal intensity is critical for interpretation of the

signal by the brain - best example: PAIN

Gradations in signal intensity can be achieved by

1) increasing the number of fibers stimulated - spatial summation

2) increasing the rate of firing in a limited number of

fibers - temporal summation

spatial summation

increasing the number of fibers stimulated

temporal summation

increasing the rate of firing in a limited number of

fibers

Sensory Modulation - filtering

used as a mechanism to increase sensitivity

EX: at a party trying to listen to one person, need to filter out background noise. The person will enter the CNS regular intervals. the background noise will enter through random intervals

Sensory Modulation - lateral inhibition

neuron influence input by nearby receptor cells

LOCALIZING THE AREA OF THE STIMULUS

Lateral Inhibition

The pattern of interaction among neurons in the visual system in which activity in one neuron inhibits adjacent neurons' responses. (can be in other systems too, not just visual)

- inhibit neighboring neurons -- inhibiting closest neurons the most and further neurons inhibited less

LOCALIZING THE AREA OF THE STIMULUS

receptive fields

2 point discrimination

we have 2 points poking and at what point do you feel that they are 2 separate pricks

Large Receptive Fields

have receptors spread far apart, which makes it difficult to localize a stimulus

larger receptors and farther apart so when 2 points are pricking, cannot tell that it is 2 until far apart

small receptive field

precise location and very sensitive

receptors closer together so when 2 points are pricking, can tell that it is 2 points close together

Weber-Fechner Relationship

Smallest change in a stimulus intensity that can be perceived as a different stimulus

humans- must be a 2.5% change difference for us to discriminate the difference

Classification of somatic sensations

Mechanoreceptive

Thermoreceptive

Nociceptive

Mechanoreceptive

stimulated by mechanical displacement.

Tactile: touch, pressure, vibration, tickle, itch

position or Proprioceptive: static position, rate of change

Thermoreceptive

detect heat and cold

Nocicipetive

detect pain and are activated by any factor that damages tissue

Free nerve endings

detect touch and pressure

found everywhere in the skin and other tissues

Meissner corpuscles

rapidly adapting (within a fraction of a second) receptors that detect movement of light objects over skin

Found on nonhairy skin (glabrous skin) (fingertips, lips, eyelids) -located in the dermal papillae of hairless skin

Merkels discs

respond rapidly at first and then slowly adapt, detect the "steady state"

found on glabrous and nonglabrous skin (hairy and nonhairy)

stratum basale and then into dermis

Hair end organ

adapts rapidly and detects movement over the body

Ruffini's end organ

slowly adapting and respond to continual deformation of skin - tugging

Pacinian corpuscle

very rapidly adapting and is stimulated only by rapid

movement & deep pressure

detects deep pressure and deep vibration

Tactile Receptors

Free nerve endings

Meissner's corpuscles

Merkel's discs

Hair end organ

Ruffini's end organ

Pacinian corpuscle

Tactile Sense Transmission - fast

Meissner's corpuscles, hair receptors, Pacinian

corpuscles and Ruffini's end organs transmit signals

in type Ab nerve fibers FAST

30-70 m/s

Tactile Sense Transmission - slower

Free nerve endings transmit signals in type Ad nerve

fibers at 5-30 m/sec, some by type C unmyelinated

fibers at

Tactile Sense Transmission - The more critical the information

the faster the rate of transmission

Almost all sensory information enters the spinal

cord through

dorsal roots of spinal nerves

Pathways for the Transmission of Sensory Information

2 pathways for sensory information:

1. dorsal column - medial leminscal system

2. anterolateral system

Dorsal Column System

◻ Contains large myelinated nerve fibers for FAST TRANSMISSION

◻ High degree of spatial orientation maintained

throughout the tract

◻ Transmits information rapidly

Dorsal Column System - examples

1. touch

2. vibration

3. position

4. fine pressure

Anterolateral System

• Smaller myelinated & unmyelinated fibers for SLOW TRANMISSION

• Low degree of spatial orientation

• Transmits a broad spectrum of modalities

Anterolateral System - examples

1. pain

2. temp

3. crude touch

4. tickle and itch

5. sexual sensation

Somatic Sensory Cortex location

postcentral gyrus

Somatic Sensory Cortex

receives sensory input that gives rise to our sensations of heat, touch, and cold and to our senses of balance and body movement

Somatic Sensory Cortex organization

• Highly organized distinct spatial orientation

• Each side of the cortex receives information from opposite side of the body

Unequal representation of the body - Somatic Sensory Cortex

lips have greatest area of representation followed by the face and the thumb - trunk and lower body have the least area

Cellular Organization of the Cortex

- Six separate layers of neurons

layer I is near the surface of cortex

layer VI is deep within the cortex

cortex incoming signals enter what layer

layer IV (4)

Layers I and II receive

diffuse input from lower brain centers

Layer II and III neurons send axons to

closely related areas of cortex (similar function)

layer V and VI send axons to

distant parts

VI to thalamus

V to brainstem and cord

Further cellular organization of Cortex

- Within the layers the neurons are also arranged in

columns

- Each column serves a specific sensory modality (ex. strength, pressure, touch)

- Different columns interspersed among each other (may have 2 strength, then 2 pressure, than 3 touch, then 1 pressure, etc.)

Cellular Organization of the Cortex - Layer I

near the surface of the cortex

Cellular Organization of the Cortex - layer VI

deep in the cortex

Cellular Organization of the Cortex - IV

incoming signals enter

Cellular Organization of the Cortex - Layer I & II

receive input from lower brain centers

Cellular Organization of the Cortex - layer II & III

closely regulated cortex nearby areas

send axons to communicate to areas receiving similar signals

Cellular Organization of the Cortex - layer V and VI

more distant parts

V - brainstem and spinal cord

VI - thalamus

Function of the Somatic Sensory Cortex: Destruction of Somatic area I results in

1. lose discrete localization ability (touch)

2. lose ability to judge degree of pressure

3. lose ability to determine weight of object

4. asterognosia

5. inability to judge texture

asterognosia

inability to determine shape of an object by touch

Graphesthesia

ability to "read" a number by having it traced on the skin

Somatic Association Areas - location

Located behind the somatic sensory cortex - parietal area of cortex

Somatic Association Areas - receive input from where?

Association areas receive input from somatic sensory

cortex, ventrobasal nuclei of the thalamus, visual and

auditory cortex

Somatic Association Areas - function

decipher sensory meaning

Somatic Association Areas - loss fo these areas

results in loss of ability to recognize complex objects - smell, feel, etc

also even loss of self (self awareness)

pain occurs when

tissue is being damaged

Pain function

protection

two types of pain

fast pain and slow pain

fast pain

felt within 0.1 sec of stimulation (type A fiber)

described as sharp

easily localized by patients

slow pain

can take up to 1 sec or more

- throbbing or aching pain

- general areas of pain, less localized

All pain receptors are

free nerve endings

Pain Receptors can be stimulated by

- can be stimulated by: mechanical stretch, thermal (hot/cold) and chemical (bradykinin, serotonin, histamine, prostygandins, substance P)

Bradykinin

chemical that causes the most pain and most responsible chemical for causing pain