THE SENSES Flashcards
Head houses the “sense” organs
- Sight (eye, optic nerve)
- Smell (nose, olfactory nerves)
- Taste (tongue, CNVII/IX/X)
- Touch (post-central gyrus)
- Hearing (auditory apparatus, CNVIII)
- Balance (vestibular apparatus, CNVIII)
EXPLAIN THE RETINA
Optic disc (blind spot) is where retinal ganglion cells are leaving the eye, it is devoid of photoreceptor cells.
Macula lutea (Latin = yellow spot) is lateral to optic disc.
Tiny depression in the centre of the macula lutea is the fovea. The fovea is ~ 1.5 mm in diameter.
RETINA STRUCTURE
- Photoreceptor cells (rods & cones) are embedded in PE.
- Outer nuclear layer contains cell bodies of rods & cones.
- Inner nuclear layer contains cell bodies of bipolar cells.
- Bipolar cells integrate information from several rods & cones.
- Bipolar cells synapse on ganglion cells in the inner plexiform layer.
- Axons of ganglion cells comprise the optic nerve.
RODS
INNER SEGMENT: CONTAINS THE CELL NUCLEUS AND MITOCHONDRIA
OUTER SEGMENT: HOUSES THE DISCS THAT CONTAIN LIGHT-SENSITIVE RHODOPSIN
Sensitive to light, not frequencies.
- Rods activated with 1/100th amount of light needed to activate a cone.
- Good for seeing in dim lighting but does not allow colour distinction.
- Many rods converge onto one bipolar cell which reduces visual acuity
CONES
INNER SEGMENT: CONTAINS THE CELL NUCLEUS
OUTER SEGMENT: HOUSES THE DISCS THAT CONTAIN LIGHT-SENSITIVE IODOPSIN
- Relatively insentive to light but responsive to frequencies to allow colour (RGB) distinction.
- Less convergence of cones onto bipolar cells which increases visual acuity.
- Fovea = 1 cone: 1 bipolar cell.
Receptor Density
- Cones have highest density in the fovea.
- Cones have minimal density starting 5 degrees from fovea heading peripherally.
- Rods have highest density 20 degrees from the fovea.
- Rods absent in the fovea.
Visual Pathway (3-neuron chain)
- PRIMARY NEURON
= BIPOLAR CELL
Originates: Inner nuclear layer of retina.
Terminates: Inner plexiform layer of retina. - SECONDARY NEURON
= RETINAL GANGLION CELL (OPTIC NERVE)
Originates: Ganglion cell layer of retina.
Decussates: Nasal fibres: yes; Temporal fibres: no.
Terminates :Lateral geniculate nucleus of thalamus. - TERTIARY NEURON
= OPTIC RADIATION
Originates: Lateral geniculate nucleus of thalamus.
Terminates: Primary visual (striate) cortex in occipital lobe
cortical processing of vision = 5 steps
- The CONTRALATERAL VISUAL FEILD PROJECTS onto the Temporl or lateral portion of the RETINA.
IPSILATERAL visual feild projects onto the nasal or medial portion of the retina - OPTIC NERVE carries all info from the IPSILATERAL EYE
- OPTIC TRACT contains fibres from the contralateral visual field
- OPTIC RADIATIONS are the projections from the LATERAL GENICULATE BODY to the PRIMARY VISUAL CORTEX
Pupillary Light Reflex
Controls the pupil diameter.
Afferent limb: optic nerve (CN II) bilaterally projects to visceral oculomotor (Edinger-Westphal) nuclei.
Efferent limb: oculomotor nerve (CN III); (pre-ganglionic parasympathetic neurons which arise in the visceral oculomotor nucleus and project to the ciliary ganglion. Post-ganglionic parasympathetic neurons travel in the short ciliary nerves to synapse onto the sphincter pupillae muscles).
Pupillodilator Reflex
Situations that arouse the 4 “F”s (fighting, feeding, fleeing, fornication) stimulate the sympathetic nervous system.
Pre-ganglionic sympathetic neurons arise in the lateral horn of spinal cord grey matter between T1-L2; those heading to the head are found at the T1 level.
Pre-ganglionic sympathetic neurons synapse in the superior cervical ganglion at the most superior point of the sympathetic trunk in the cervical region (hence the name!).
Post-ganglionic sympathetic neurons are carried by the long ciliary nerves into the eye and synapse onto the dilator pupillae muscle which dilates the pupil.
ACCOMODATION OF THE EYE
- Afferent limb: optic nerve & visual pathway.
- Efferent limb: parasympathetic fibres in CN III visceral nucleus ciliary ganglion ciliary muscles & constrictor pupillae muscles.
Corneal Blink Reflex
Afferent limb: ophthalmic nerve (CN V1)
Efferent limb: facial nerve (CN VII)
CN I (olfactory nerves)
- Bipolar neurons.
- Traverse cribriform plate (ethmoid bone).
- Synapse onto secondary olfactory neurons in olfactory bulb.
- Olfactory tract carries secondary neurons (bypassing the thalamus) to the olfactory cortex in the medial temporal lobe and basal fontal lobe.
- Odorants bind to specific receptors on the primary bipolar olfactory neurons.
- Supporting cells help maintain a microenvironment conducive to olfactory function and survival.
- Basal cells are stem cells for olfactory neurons and supporting cells.
- Olfactory neurons replaced every 2-3 months.
- Olfactory glands (eponym: Bowman gland) are large serous glands producing a constant flow of fluid surrounding the olfactory cilia and facilitating the access of new odoriferous substances.
Olfactory Processing
Four types of cells in olfactory bulb:
1. Mitral
2. Tufted
3. Periglomerular
4. Granule
- Olfactory receptor neurons synapse with mitral and tufted cells.
- Olfactory receptor neurons detecting same odorant are scattered through olfactory epithelium but converge onto same glomerulus increasing sensitivity of olfaction.
- Periglomerular cells mediate contact between glomeruli.
Granule cells mediate contact between same cell types from different glomeruli.
OLFACTION SECONDARY NEURONS
Secondary neurons (axons of mitral & tufted cells) travel in olfactory tract and synapse in the olfactory cortices.
The primary olfactory cortex consists of cortices of the uncus and entorhinal area, limen insulae (collectively known as the “piriform area”) and part of the amygdala.
