Orbital Region

General Sensation and Vision

Generalized Sensation Physiology

• Sensation –state of awareness of external and internal

conditions of the body

• Four conditions for sensation:– 1. Adequate stimulus– 2. Adequate receptor (transducer)– 3. Conduction– 4. Translation (interpretation)

Brain’s Assumption in Translating

Assumption: Source of light is from above

Reversing X pattern; notice shading.

How many triangles?

Assumption: Straight lines should connect.

Which dark line is larger?

Assumption: perspective

Characteristics of Sensation

• Modality –ability to interpret nerve impulses differently

• Projection –referral of sensation to point of origin

• Adaptation –decrease in sensitivity of receptors to continued stimulation

– Phasic (fast)– Tonic (slow)– Some receptors never adapt (pain, cold, etc.)

• Afterimage –persistence of sensation after stimulus ceases

Classification of Receptors• Type of Sensory Information Relayed

– Exteroreceptors, Enteroreceptors, Proprioceptors

• Type of Stimulus Transduced– Mechanoreceptors, Thermoreceptors, Chemoreceptors,

and Nociceptors

• Complexity of Receptor– Simple structure (usually single cell) –most general senses

– Complex structure (many cells) –special senses

Receptor Physiology• Stimulation of a receptor leads to the generation of a

receptor (generator) potential in its membrane.

• These are usually excitatory, and are similar to the EPSPs found in neurons

• If the receptor potential reaches the threshold potential for the sensory neuron; it fires an AP into the CNS

• Strength of the stimulus is therefore encoded by the number of AP generated

• Sensations may be sharpened through Lateral Inhibition

Lateral Inhibition

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General Senses

• Cutaneous –skin receptors

• Proprioception –sense of body position

• Nociception –pain perception (chemoreceptors that perceive locally secreted warning hormones (prostaglandins))

Distribution of Cutaneous Receptors

Sensitivity of Skin due to Receptive Fields

Cutaneous Receptors

Proprioceptors

Muscle sensors

Muscle spindles (Intrafusal fibers:-senses degree of length of muscle fibers and the rate of change in length

Golgi Tendon organs –sense tension within tendon

Relationship of Stretch to AP

Referred Pain

Illustrates projection.

General Pathway of Perception

Orbital Region

Accessory Structures of the Eye

Eyelid

Lacrimal Apparatus

Outer Eye

Extrinsic Ocular Muscles

Arrangement of Posterior Orbit

Optic Nerve

Occulomotor -to sup. rectus, med. rectus,inf. rectus, inf. oblique, andlevator palpebrae superiorisAbducens

-to lateral rectus

Trochlear-to sup. oblique

Action and Innervation of Ocular Muscles

Action and Innervation of Ocular Muscles

Which cranial nerve and muscle is injured?

Orbital Blood Supply

Internal Carotid

Middle Meningeal

Ophthalmic

Facial v.

to Cavernous sinus

Ophthalmic

Eye Anatomy

(Canal of Schlemm)

Aqueous Humor Flow

Glaucoma results frominadequate drainage ofAqueous Humor leading to increase pressure in the eye.

Iris controls amount of light entering the eye.

Distant Vision

Near Vision (Accomodation)

Near and Far-sightedness

Astigmatism and Presbyopia

• Astigmatism –results from imperfections in the resolving structures in the eye (lens and/or cornea)

• Presbyopia is the loss of near vision with age; resulting from a decrease in elasticity of the lens.

Increase lens opacity with age.

Cataract

Retina Cellular Arrangement

Special Areas of Retina

Blind Spot

Optic N.

Optic Disc(blind spot)

Photoreceptors: Cones and Rods

Three populations of cones

Rod’s and Cone’s Photopigments

Retinal Bleaching

Light Signal Transduction

Visual Processing