Special Senses

Sensory system overview

Sensory receptors receive stimuli from the extneral or internal environment, stignals carreid to brain or spinal cord, different parts of the brain process information

Somatosensory system

Concered with the conscious perception of touch, pressure, pain, temperature, position, movement, vibration which arise from muscles, joints, skin, fascia

Somatic sensation

Sensation from the skin, muscles, bones, tendons and joints initiated by somatic receptors

Stimulus motility

What we perceive after a stimulus ( a form of sensory perception)
- Ex. Light, sound, temperature

Somatosensation

Process that conveys information regarding body surface and interaction with environment

Propioception/Kinesthesia

Sense of posture and movement - position with body parts and muscle contraction in space

Individuals who lost propioception

Loss of sensory nerves (sensation of touch)

Modality

A form of sensory perception. Each modality has it's own sensory receptor

Receptor potentials

Graded potentials initiated by receptor cells for stimulus response

Photoreceptors

Respond to light

Mechanoreceptors

Respond to pressure

Thermoreceptors

Respond to temperatures

Auditory receptors

Respond to sound

5 types of somatosensory receptors

Meissner's Corpuscles, Merkel's corpuscles, Free neuron ending, Pacinian corpuscles, Ruffini corpuscle

Meissner's Corpuscles

mechanoreceptors that respond to touch and pressure (rapidly adapting)

Merkel's Corpuscles

mechanoreceptor that responds to touch and pressure (slowly adapting)

Free neuron ending

Close to skin surface (nociceptors, thermoreceptors, mechanoreceptors)

Pacinian corpuscle

Responds to vibration and deep pressure (rapidly adapting)

Ruffini corpuscle

Responds to skin stretch (slowly adapting mechanoreceptor)

How are afferents activated

Somsatosensory receptors are activated by touch or pressure (mechanoreceptors)
- Sensory receptor opens up cationic channels to move down concentration gradient. Sodium flows down concentration gradient, causing depolarization

Types of sensory receptors

Sensory receptor located directly on afferent fiber, and sensory receptors located on specialized receptors

In both types of sensory receptors, where does the signal go?

To the spinal cord (CNS)

What is the receptor potential with a weak stimulus

Receptor potential is generated, but there is not enough stimulus to bring first node a Ranvier to threshold. No AP, no NT release

What is the receptor potential with a medium stimulus

Receptor potential is greater than in weak stimulus. There is enough graded polarization to bring neuron to threshold, and starting a AP. NT is released to projection neurons that stimulate the cortex, and we are aware of being poked

What is the receptor potential with a strong stimulus

Larger graded potential is generated, more action potentials are fired, causing more release of NT. Projection neurons send more signals to the cortex, and we are aware of a hard poke in the arm

What determines stimulus intensirty

More receptor terminals are activated, causing activation of more action potentials.
- More graded depolarization of the afferent

Different types of somatic receptors in the same area

Receptors function in the same wall, but differ in the speed which they adapt to a stimulus (ex. Meissner's corpuscle rapid adaptation, Merkel's corpuscle slow adaption)

Merkel's corpuscle adaptation (slow)

Slow decay of receptor potential. Potential is activated during full time of stimulus
- Frequency of action potentials decrease over time, but they remain on during poke

Meissner's corpuscle adaptation (fast)

Receptor potential immediately generated, but quickly decays. Action potentials are only generated at the start and end of an action potential

Use of rapidly adapting mechanoreceptors

Filter out unimportant information (time felt sitting in a chair). We know when we start sitting and when it ends

Example of a slowly adapting receptor

Proprioception - need to know where our body position is at all times (ex. arms sticking out straight in front of us)

Factors that affect our ability to localize a stimulus

Receptive field size, density of innervations, overlapping receptive fields

Receptive field size

Extent of the body that feels stimulus

Densitiy of innervation

Number of sensory receptors within a certain area that receive stimulus.

Overlapping receptive fields

if the branches of two adjacent units both sense the stimulus.

Density of inneration effect on stimulus localization

More dense receptors = greater ability to localize stimulus (lips poked with forceps, feel two touches. Back poked with forceps, only feel one touch)

Receptive field size effect on stimulus localization

Smaller receptive fields allow for better localization (specific cortical neuron is activated)

Overlapping receptive fields effect on stimulus localization

Poke in the middle of receptive field triggers many action potentials, and small amount in nearby fields. If between fields, medium amount of potentials generated in each field

Lateral inhibition

Overlapping receptive fields help ID specific stimulus sites. Information from directly stimulated neurons is activated, while receptor neurons at edge of stimulus are inhibited

In which sensory systems is lateral inhibition present

Somatosensation and vision (not auditory)

How is localization possible

Lateral inhibition removes information from peripheral regions

What neurons are inhibited in lateral inhibtion

All of the neurons are inhibited, but the peripheral neurons are turned down a lot. The central neuron is only slightly

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