Special Senses Lecture 2 - Vision

How are sensory signals modified before they reach higher levels of CNS

Lateral inhibition, pathways descending from higher centers of the brain, synapses on axon terminals of primary afferent neurons (presynaptic inhibition), indirectly by interneurons

Cortex role in signal modification

Inhibition to sensory fibers and projection neurons (turns down the volume of these neurons)

What happens when you remove inhibitory effect of cortex

Amplification of sensory input

Setup of neurons from skin to brain

Sensory endings --> afferent neuron --> projection neuron -->higher brain sensors

What does the afferent neuron synapse to

Travels to spinal cord and synapses with projection neuron

Where does the projection neuron synapse to

Brain centers, such as the cortex

How to inhibitory neurons work

Release inhibitory neurotransmitters onto projection neurons or sensory afferent neurons, reducing glutamate release. Turns down the response of projection neuron

What is the use of inhibition of pain signals

Based on the situation we are in, we will feel more or less stimulation from pain
- Stepping on our foot at school (hurts) vs at a track meet (not as much) vs in war (not at all)

What happens if we don't have pain tolerence

We have no signal to tell us to remove ourself from painful stimuli

What are the 2 ways information ascends to the cortex

Anterolateral system (spinothalamic system) and the Dorsal column system

Anterolateral system (spinothalamic system)

Pathway that carries pain and hot/cold information up to the somatosensory cortex

Dorsal column system

Pathway that carries information on fine touch mechanoreceptors to the somatosensory tract

Anterolateral system pathway

1. Painful stimulus from right side activates free neuron endings
2. Action potentials generated into mixed peripheral nerve
3. Synapse between sensory receptor neuron and second neuron on the dorsal horn of the right side of the spinal cord
4. Second neuron crosses spinal cord and travels up the left side of the spinal cord
5. Synapse in thalamus with a cortically projecting neuron
6. Cortically projecting neuron takes information to somatosensory cortex

Dorsal system pathway

1. Painful stimulus from right side activates free neuron endings
2. Action potentials generated into mixed peripheral nerve
3. Sensory neuron travels through the dorsal root of spinal cord, and travels up the right side of the spinal cord
4. Sensory neuron synapses with secondary neuron in brainstem
5. Secondary neuron crosses over to left side of brain stem and synapses with cortically projecting neuron
6. Cortically projecting neuron takes information to somatosensory cortex

Similarities of anterolateral and dorsal system pathway

Both pathways end up in the brain on the opposite side the stimulus

Difference between the anterolateral and dorsal system pathway

Anterolateral - secondary neuron crosses over in spinal cord

Dorsal - Secondary neuron Crosses over in brainstem

Somatosensory cortex location

Behind the motor cortex and central sulcus

Function of somatosensory cortex neurons

Activate motor cortex neurons, which control movement

Role of motor cortex neurons

Travel down spinal cord via descending systems to motor neurons, activating them based on how we want to move

Somatosensory cortex regions

Each region of the body maps to a region of the somatosensory cortex

How is each body part's amount of room taken up in the somatosensory cortex determined?

Based on how densely innervated the body part is with sensory receptors

High density innervated areas of the body

Fingers, face, lips

Low density innervated areas of the body

Trunk, neck, hips

Photoreceptor depolarization/hyperpolarization

Photoreceptors are depolarized at rest, and hyperpolarized when activated (opposite of most systems)

Components of eyes

Optical component and neural component

Optical component of the eye

Focuses visual image on receptor cells - the front part of the eye

Neural component of the eye

Back part of the eye - transforms visual image into a pattern of graded and action potentials

What light do humans see

In the visible range

When we look at an object, what do we see

Light reflected off the object, and hitting the photoreceptors of our eyes

Sclera

White of the eye - membrane surrounding eyeball

Extarocular muscle

Muscle that is responsible for eye movements, attached to the sclera

Cornea

Clear tissue at front of the eye that refracts light waves, causing them to converge on photoreceptors

Pupil

Hole that allows light to pass through to photoreceptors

Iris

Colored part of the eye that regulates the size of the pupil - controls amount of light that enters eyeball

How is the iris innervated

By the autonomic nervous system
- Sympathetic - causes pupil constriction
- Parasympathetic - causes pupil dilation

Lens

Transparent structure that works with cornea to focus the image on the retina. Can change shape based on where viewed object is

Zonular fibers

Fibers that attach the lens to the ciliary muscles

Ciliary muscles

Muscles that contract/relax to change the shape of the lens

Retina

Light sensitive part at the back of the eye where photoreceptors are found

Types of photoreceptors

rods and cones

Rods

Receptors that are monochromatic and activated in low light

Cones

Receptors that are responsible for color vision and activated with more light

Retinal ganglion cells

Take information from the rods and cones to the brain

Optic nerve

Nerve that travels towards the thalamus and cortex

What is the optic nerve made of

axons of retinal ganglion cells

Aqueous Humor

Gelatinous fluid that fills the space between the lens and the cornea

Vitreous humor

Gelatinous fluid that fills the space behind the lens

What is refraction

When light travels to a more dense medium (air to cornea), the light waves are bent to hit the retina

How does the image change in the eye

The image is inverted, the brain flips the image around

What happens if the amount of refraction is innapropriate in the eye

The image is constructed in front or behind the retina, and the image is not focused

What parts of the eye are responsible for refraction

Cornea refracts light, lens changes shape to focus light on the retina

How does the eye work to see an image up close

Ciliary muscle contracts, causing the lens to get fatter and shorter. The lens increases the amount of refraction, allowing the image to focus on the retina

Accommodation

the process by which the eye's lens changes shape to focus near or far objects on the retina

What age do humans lose the ability to accomodate

Around 45 years of age, due to the breakdown of ciliary muscles that control the lens

Presbyopia

Loss of elasticity of the lens, leading to inability to accommodate for near vision. Refraction only comes from the cornea

Myopia (near sightedness)

Eyeball is too long, and too much refraction occurs. The image is reconstructed in front of the retina and the image is out of focus

How to fix myopia

Wear lenses with a concave shape - reduces refraction so the image can land on the retina

Hyperopia (far sightedness)

Eyeball is too short, and not enough refraction occurs. The image is reconstructed behind the retina and the image is out of focus

How to fix hyperopia

Wear lenses with convex shape - increases refraction so image can land on the retina

Astigmatisim

Oblong shape of eyeball causes blurry vision. Corrected for with glasses or complex laser surgery

Glaucoma

Aqueous humor builds up, pushes on lens. The lens pushes back on the vitreous humor, which pushes on the retina, damaging photoreceptors

How to treat glaucoma

No treatment is available.

Cataracts

Cells of the lens die, and debris builds up, causing greying of lens and inability to see

How to treat cataracts

Remove lens and put in a silicone lens. New lens cannot accommodate (no activation by ciliary muscles)

What interneurons do rods and cones connect to

Horizontal, bipolar, amacrine cells

Role of interneurons in the eye

Take information from photoreceptors and transfer the info to retinal ganglion cells

Bipolar eye cells

Interneurons which take information from the photoreceptors tot he retinal ganglion cells

What do cone synaptic terminals contact

Bipolar cells

Parts of a cone

Cell body, inner segment, outer segment

Disks

Little layers of outer segment where processing of visual information occurs

Phototransduction from cones when light is not present

1. Guanylyl cyclase converts GTP to cGMP
2. cGMP binds to GMP-gated cation channels
3. Sodium and calcium flow into the disk, causing depolarization

Phototransduction for cones when light is present

Disk contains photopigment, containing retinal (chromophore)
1. Light hits photopigment, causing retinal to change confirmation
2. Confirmation activates cyclic GMP phosphodiesterase
3. Cyclic GMP phosphodiesterase converts cGMP to GMP
4. cGMP is removed from ion channel.
5. Sodium and calcium can no longer enter the cell, and the photoreceptor becomes hyperpolarized

Photoreceptor polarizaton

Reverse from normal cells - activated photoreceptors are hyper polarized (-75mV), and inactivated cells are depolarized (-35 mV)

OFF pathway phototransduction

1. Cone cell is depolarized from cGMP binding to ion channel, allowing in sodium and calcium
2. Glutamate released from photoreceptor
3. Glutamate release activated OFF bipolar cell, causing the release of glutamate onto the OFF retinal ganglion cell.
4. Glutamate release inhibits ON bipolar cell, causing little release of glutamate from the cell to the ON retinal ganglion cell. No action potential generated in the ON ganglion cell
5. Action potential sent to the cortex

Where are action potentials generated when there is no light present

AP generated in OFF pathway, but not the ON pathway

Are action potentials generated in photoreceptor and bipolar cells

No. They have no voltage gated sodium channels. Graded potentials are generated

Are action potentials generated in retinal ganglion cells

Yes. They have voltage gated sodium channels

ON Pathway signal transfuction

1. Cone cell is hyper polarized due to lack of cGMP binding to ion channel - not a lot of sodium and potassium enter cell
2. Very little glutamate is released by cone cell, which does not activate the OFF pathway
3. Reduced glutamate release causes the ON bipolar cell to be released from inhibition (inhibition caused by glutamate)
4. ON bipolar cell is activated, and graded potentials are generated
5. Glutamate is released from the ON bipolar cell, activating the ON ganglion cell
6. Action potential is generated in retinal ganglion cell, and travels to the geniculate nucleus