HP LEC 9: Cutaneous Sensations/ Eyes and Vision

Cutaneous Receptors

1. Pain, cold, and heat receptors are naked dendrites of sensory neurons

• Free nerve endings

2. Touch and pressure receptors have special structures around their dendrites

• Merkel’s disks

• Meissner’s corpuscles

• Pacinian corpuscles

• Ruffini corpuscles

Cultaneous Receptors: Cold Receptors-  (location, what is it stimulated by, what can it also respond to, temp range)

Transient Receptor Potential (TRP) Channel

a) there are MANY more receptors that respond to cold than hot

b) located close to the epidermis

c) stimulated by cold and inhibited by warm

d) some cold receptors also respond to menthol

e) the temperature range of response is 8 to 28  degrees C

Cultaneous Receptors: Warm Receptors- (location, stimulated)

Transient Receptor Potential (TRP) Channel

a) located deeper in the dermis

b) excited by warming & inhibiting by cooling

c) different from receptors that detect painful heat

Cultaneous Receptors: Hot Receptors- (location, , activated)

Transient Receptor Potential (TRP) Channel

a) the pain experienced by a hot stimulus is sensed by a special nociceptor called a capsaicin receptor

b) also a receptor for the chemical found in chili peppers (capsaicin)

c) activated at 43 degrees C or higher

Noiceptors- Pain Receptors (different types, activation)

a) Nociceptors can be myelinated or unmyelinated

• sudden, sharp pain is transmitted by myelinated neurons

• dull, persistent pain is transmitted by UNmyelinated neurons 

b) Nociceptors may be activated by chemicals released by damaged tissues, such as ATP, or by pH change or mechanical stimulus

Itch Sensation

• acute itch: stimulated by histamine release from mast cells and basophils

• chronic itch: stimulated by other chemicals and does not respond to antihistamines

• receptors stimulate Unmyelinated sensory axons to the spinal cord

Neural Pathway for Somesthetic sensations- Tract: Dorsal Column- Medial Leminscus (origin)

Dorsal Column- Medial Leminscus (origin)

• peripheral afferent neurons; ascends on ipsilateral side of spinal cord but crosses over in medulla

Dorsal Column- Medial Leminscus (Termination)

Dorsal Column- Medial Leminscus (Termination)

• Nucleus gracilis and nucleus cuneatus of medulla; eventually thalamus, then cerebral cortex

Dorsal Column- Medial Leminscus (Function)

Dorsal Column- Medial Leminscus (Function)

• conducts sensory impulses from skin, muscles, tendons, and joints, which are interpreted as sensations of cutaneous touch and pressure, and body position

Neural Pathway for Somesthetic sensations- Tract: Anterolateral Spinothalamic (origin)

Anterolateral Spinothalamic (origin)

• posterior horn on ONE side of cord but crosses to opposite side

Anterolateral Spinothalamic (Termination)

Anterolateral Spinothalamic (Termination)

• thalamus, then cerebral cortex

Anterolateral Spinothalamic (Function)

Anterolateral Spinothalamic (Function)

• conducts pain and temperature impulses that are interpreted within cerebral cortex

Receptive Fields and Sensory Acuity

Receptive Field: the area of skin that when stimulated, changes the firing rate of a neuron

a) size of a receptive field depends on the density of receptors in that region of skin

b) there are few receptors in the back and legs, so the RF are large

c) there are many receptors in the fingertips, so the RF are small

d) the more receptors, the smaller the field, the larger the area of the somatosensory cortex

e) a small receptive field = GREATER tactile acuity = sharpness of the sensation

Two-point touch threshold ( RF is measured by by what, ex)

a) receptive fields can be measured by seeing at what distance a person can perceive TWO SEPERATE points of touch

b) measures tactile acuity

c) important in spacing the raised dots in braille symbols

Two-point touch threshold table

Intro to Vision (where does it come from?)

1. Vision comes from light energy transduced (converted) into nerve impulses

2. Only a limited part of the electromagnetic spectrum can excite photoreceptors in the eye

Structure of the Eye (5)

1. Sclera

2. Pupil

3. Lens

4. Retina

5. Fovea-Macula

sclera

• tough outer layer of the eyeball (white part of eye)

• cornea: the slight buldge in the sclera at the front of the eye

• Cornea directs light rays into the eye and helps focus them on the retina

Pupil

• the opening in the colored part of the eye (which is called the iris)

• allows light to pass to the lens

• black

Lens

• normally clear

• located behind the iris

• small muscles attached to the lens can change its shape- which allows the eye to focus on near or far objects

Retina

• thin nerve tissue

• that lines the back of the eye

• detects light entering the eye and converts it into electrical impulses

Fovea-Macula

• is the part of the retina that provides the sharp, detailed central vision that allows you to focus on what is directly in the line of sight

• the rest of the retina provides side (peripheral) vision, which allows you to see shape but not fine details

General Pathway of Light through the Eye

1. Light passes through the cornea and into the anterior chamber of the eye

2. Next, it passes through the pupil, which can change shape (due to the pigmented iris muscle) to allow more or less light in

3. Then it passes through the lens, which can change shape to focus the image

4. Then it passes through the posterior chamber and vitreous body

5. Finally, it hits the retina, where photorecptors are found

Structures of the Eye: Pupil & Iris

1. The iris sphincter muscle (pupillary sphincter, pupillary dilator)- can increase or decrease the diameter of the pupil

• Constriction: contraction of pupillary sphincter via parasympathetic stimulation

• Dilation: contraction of pupillary dilator via sympathetic stimulation

2. The iris also has a pigmented epithelium for eye color

Intro to the Retina (thickeness, what does it consist of)

1. The retina is approx 0.55 mm thick and lines the back of the eye. Retina consist of:

• single cell pigmented epithelium

• rods & cones: photoreceptor neurons

• layer of other neurons

Intro to Retina (photosensors, ganglion cells)

2) the photosensors(rods & cones) lie outermost in the retina against the pigmented epithelium

3) The ganglion cells (the output neurons of the retina) lie innermost in the retina “closest” to the lens and the front of the eye

4) the optic nerve contains the ganglion cell axons running to the brain

Layers of Retina: Rods & Cones 

Rods & Cones

• Photoreceptors (rods & cones) are in the inner layer- toward the vitreous body, back of the eye

• they synapse on a middle layer of bipolar cells, which synapse on the outer layer of ganglion cells

• there are also horizontal cells and amacrine cells within the layers

Rods

• 100 M per retina

• Vision in shades of gray

• High sensitivity

• Low Acuity

• Night vision

• Much convergence in retina

• more numerous peripherally

Cones

• 3 M per retina

• Color Vision

• Low sensitivity

• high acuity

• day vision

• little convergence in retina

• concentrated in fovea

Electrical Activity of the Rods - “On pathway”

1. contain the pigment rhodopsin- rhodopsin is EXTREMLY sensitive to light- when rhodopsin is exposed to light, it immediately photobleaches

Effects of Light on the Retina- “On pathway”

When Light Hits Photoreceptors

a) dissociation of rhodopsin activates a G protein/2nd messenger system, which CLOSES Na+ channels

• G-proteins are called transducins

• activation of the enzyme phosphodiesterase converts cGMP to GMP

• this closes Na+ channels

b) photoreceptors are hyperpolarized, and inhibition on bipolar cells is lifted

c) bipolar cells activate ganglion cells that transmit action potentials to the brain

Glutamate Receptors (3 major classes)

1. AMPA receptors (iGluR- ligand)

2. NMDA receptors (iGluR- ligand and voltage)

3. Metabotrophic glutamate receptors

• metabotrophic receptors act through a second messenger system to create slow, sustained effects on their targets

Neural Pathway for Vision (2 systems)

1. Geniculostriate System

2. Tectal System

Geniculostraite System (what synapses on what, what kind of info does it carry?)

• axons from ganglion cells synapse on the lateral geniculate nucleus of the thalamus by way of the optic chiasma

• information from the lateral portion of the retinas does not cross sides, but info from the medial portion does

• neurons from the thalamus synapse on the visual cortex of the occipital lobe

• carries info of “what” is seen

Tectal System (what synapses on what, what kind of info does it carrry?)

• 20 to 30% of the ganglion cell axons synapse on the superior colliculus of the midbrain, which helps with eye and body movements

• carries info about “where” the object is