sensation and perception discussion

Class announcement

• DB#2 due next Wed, 1/26, 11:59pm

• Quiz/Exam #1 next MONDAY, 1/31 - on the content from DBs 1 and 2 (neurons, SDT, psychophysics, touch, etc)

– Review questions on canvas and check out the DBs

Skin

• Skin is the largest organ in the body, both by weight and surface area.

– In adults, skin accounts for about 16% of your total body weight and a 6 foot person has a surface area of ~ 22 ft2

Skin • The skin serves many purposes:

– Has esthetic and beauty qualities

– Stabilizes body temperature

– Regulation of blood flow

– Serves as a barrier to the environment

• Holds in bodily fluids

• Protects us from water loss, friction, wounds, infection

– Wards off harmful sun rays

• Uses specialized pigment cells to protect us from ultraviolet rays of the sun

• Produces vitamin D in the epidermal layer, when it is exposed to the sun's rays

• Has many receptors…

Sensory receptors

• Receptors convert stimulus energy into nervous signals → signal transduction

• There are different receptors for different kinds of stimuli

Let’s talk about what it means to have different receptors…

Law of specific nerve energies

• 1830s, Johannes Muller – We are not aware of objects directly.

– Rather, we are aware of the neural signals transmitted through our nerves.

– Each nerve fiber carries a specific type of information.

“touch”

Law of specific nerve energies

• What is important is which nerve is stimulated and how they were. – “labeled lines”

Sensory receptive fields (RFs)

• Levine & Shefner (1991) define a receptive field as an "area in which stimulation leads to response of a particular sensory neuron" (p. 671).

• Each nerve fibers responds to a particular region.

• This particular region is called the neuron’s receptive field.

Sensory Receptor Properties

• Stimulus specificity – Receptors respond to different qualities (i.e. pressure, vibration,

temperature, etc)

• Receptive field size and location – Receptors are activated when stimulation is applied to a particular area of

the body

• Rate of adaptation – The pattern of activity of a neuron might indicate the strength of the

stimulus. Does it give a burst of activity (fast-acting) or a steady stream (slow-acting)?

Skin and your senses of touch

• Discrimitive touch – perception of pressure, vibration, texture

• Meissner’s corpuscles, Pacinian corpulses, Merkel’s disks, Ruffini endings

• Pain and temperature

• Free nerve endings

• Proprioception

• Registration of tension and stress in muscles and joints

Let’s talk about your mechanoreceptors

Mechanoreceptors

• Your skin contains 4 different types of mechanoreceptors

– About 17,000 total receptors

• These are the transducers of touch stimuli – pressure on or indentation of the skin

• Each receptor has a distinct shape and location which gives a hint to its function

Receptive field properties and the “Four channel” model of mechanoreception

• Each mechanoreceptor has a nerve fiber and a particular type of ending

• Spatial properties

– Punctate

– Diffuse

• Temporal properties

– Slowly adapting

– Rapidly adapting

Spatial and temporal receptive field properties

ulnar nerve

median nerve

“touch”

“touch”

slowly adapting

Fast adapting

punctate

diffuse

Which type of RF would be better at responding to small stimuli?

Which type of RF would respond better to continued stimulation?

Receptive field properties and the “Four channel” model of mechanoreception

• Spatial properties

– Punctate → small RF with defined borders

– Diffuse → large, ill-defined RF

• Temporal properties

– Slowly adapting → respond more or less constantly as long as stimulus present

– Fast adapting → respond vigorously at first, then stops; also responds when stimulus removed

Receptive field properties and the “Four channel” model of mechanoreception

• Spatial properties

– Punctate (small)

– Diffuse (large)

• Temporal properties

– Slowly adapting

– Fast adapting

Slowly- Adapting

(SA)

Fast- Adapting

(FA)

largesmall

SA-I (Merkel)

SA-II (Ruffini)

FA-I (Meissner)

FA-II (Pacinian)

Different channels “record” different types of touch information.

Quiz

Mechanoreceptors and FA Fibers

• Meissner corpuscles (FA I) – Located near skin surface

– When stimulated, “flutter” or “wobble”

– Respond best to low frequencies • Coffee cup slipping

• Something rubs against the skin

• Pacinian corpuscles (FA II) – Largest, least plentiful, most deeply

located

– When stimulated, diffuse “buzzing”

– Respond best to high frequencies • Any minute indentation: mosquito

• Air against the skin

“touch”

Fast adapting → responds best to changes in stimulation

Mechanoreceptors and SA Fibers

• Merkel’s Disks (SA-I) – Located near skin surface

– When stimulated, feel “pressure”

– Responds best to steady pressure of a small object

• Texture and pattern perception: Braille

• Ruffini’s endings (SA-II) – Located deep within

– When stimulated, feel nothing (need to stimulate more than 1 fiber)

– Responds to steady pressure on the skin and to stretching of the skin

• Fingers and hand moving and grasping

“touch”

slowly adapting → responds best to sustained stimulation

Mechanoreceptors

Slowly- adapting

Fast- adapting

Large (diffuse)Small (punctate)

Meissner corpuscle

Merkel disks

Ruffini endings

Pacinian corpuscle

Other types of receptors

• Thermoreceptors – temperature – warmth fibers

– cold fibers

• Nocioreceptors – pain fibers – A-delta fibers – fast conduction, respond best to strong pressure or

heat

– C fibers – slower conducting, respond to various types of intense stimulation (pressure, temperature, chemical)

Questions?

Wilder Penfield 1891 – 1976 "Brain surgery is a terrible profession. If I did not feel it will become different in my lifetime, I should hate it." (1921)

Primary somatosensory cortex (S-1)

- Receives input from the thalamus

- Input is contralateral

- Multiple “body maps”

Stimulating “auras” / “engrams”…

Somatosensory Cortex

The sensory homunculus

London Natural History Museum

Sensory and Motor homunculi London Natural History Museum

Somatosensory Cortex receptive fields

• Contralateral

• In general, areas with larger representation have smaller receptive fields

– Lips versus arm

• Receptive field locations in each body map strip have rough correspondence with one another

– Skin of right index finger versus joints of right index finger

• Receptive field properties of cortical neurons differ from that of peripheral nerve fibers

– Orientation and movement specific

Two-point thresholds

• Receptive field size can be measured using the two-point threshold

• Diffuse? Punctate?