Anatomy 2 Flashcards
Describe the corneal refelex (blink)
The corneal reflex, also known as the blink reflex, is an involuntary blinking of the eyelids elicited by stimulation of the cornea (such as by touching or by a foreign body):
- Sensory (afferent limb - AP’s are conducted from cornea via CN V1 branches, then to trigeminal ganglion & then along CN V to the pons
- Motor (efferent) limb - AP’s are conducted via CN VII to the eyelid part of orbicularis oculi (blink)
What is the vestibulo-ocular reflex?
The vestibulo-ocular reflex (VOR) is a reflex acting to stabilize gaze during head movement, with eye movement due to activation of the vestibular system:
- This is where the person turns their eyes in the opposite direction to a head movement.
- The purpose of this is to allow gaze to be stabalised on an object even during head movement
- CNS connections between CN VIII & CNs III, IV & VI are involved in the reflex
What is the oculocardiac reflex?
- This is a reflex bradycardia (decreased in HR) in response to tension (traction in surgery) on extraocular muscles or pressure on eye
- CNS connections between CN V1 & CN X involved
- Tx = atropine
What is the effect of sympathetic innveration on the eye ?
- Eyes open wider, to get more light in and to focus on far away objects
- Also potentially involved in emotional lacrimation (to express emotions)
What is the effects of parasympathetic innveration on the eye ?
- Get less light into eyes (to protect the retina from bright light or when asleep)
- Focus on near objects
- Reflex lacrimation (to wash away the stimulant foreign body & clean the cornea)
How does sympathetic innervation result in the eyes opening wider ?
It innervates the levator palpebrae superioris which elevated the eyelid
Describe the route of postsynaptic fibres to the orbital structures e.g. levator palpebrae superioris
- Superior cervical sympathetic ganglion
- Internal carotid nerve
- Internal carotid plexus
- Axons carried on the ophthalmic artery and on its branches to the orbital structures
What muscle causes constriction of the pupil ?
Sphincter pupillae fibres which encircle the pupil
What is the innervation of the sphincter pupillae ?
Parasymathetics from CN III
What are the normal physiological causes of a constricted pupil?
In bright light and “rest & digest”
What are the causes of non-physiological constriction of the pupil, a pin-point pupil and a fixed dilated pupil?
- A non-physiologically constricted pupil is a miotic pupil e.g. a component of Horner’s syndrome
- A fixed “pin point” pupil is often a serious pathological sign e.g. opiate drugs
- A “fixed-dilated” (“blown”) pupil is often a serious pathological sign e.g. of CN III pathology (possibly caused by raised ICP)
What causes dilatation of the pupil and how are they arranged ?
- Dilator pupillae fibres which are radially arranged
- They originate around the external circumference of iris – fixed
- They nsert around the internal circumference of iris - mobile
What is the dilator pupillae innervated by ?
Sympathetics
What is a physiological & a non-physiological cause of dilated pupil?
- Physiological - sick patient
- Non-physiological (known as a mydriatic pupil) - mydriatic drugs induce dilation of the pupil
Describe the pupillary light reflex
- Special sensory (afferent) limb of the reflex is the ipsilateral CN II
- CNS connections occur in the midbrain
- Motor (efferent) limb of the reflex is bilateral via CNs III
- Direct light reflex occurs in the stimulated eye
- A consensual light reflex occurs in the non-stimulated eye
What connects the circumference of the lens of the eye to the cilliary body ?
The suspensory ligament of the lens
Describe the lens accomodation reflex in response to ‘far & near vision’ ?
- The ciliary muscle relaxes in “far vision”, no parasympathetics involved. The suspensory ligament tightens & lens flattens to focus on an object in the distance
- The ciliary muscle contracts in “near vision”under influence from parasympathetic innervation. The suspensory ligament relaxes & lens becomes spherical to focus on near objects
Note - it is CN III parasympathetics
