Neuro Flashcards
What are the functions and characteristics of the cornea?
Transmission of light
- Refraction
- Must be transparent
- Must have a smooth spherical surface
- Dehydrate endothelium (no repair)
- the innermost layer of the cornea, maintain corneal clarity by pumping water out (since water molecules alter the regular spacing between collagen fibres & cause opacity)
- Surface epithelium (capable of repair)
- outer layer of the eye, many layers that slough off and are constantly regenerated
What are the functions and characteristics of the sclera?
Forms the white capsule around the eye, except at its anterior surface where it is specialised into the clear cornea
- Offers protection; formed of a tough outer layer of collagen - Serves as an insertion point for the external muscles of the eye
- Continuation of dura mater and cornea
What are the functions and characteristics of the iris?
Specialised section of choroid
- Contains & controls the size of the pupil - which lets light in
- Sphincter muscles (circular fibres) make the pupil smaller [parasympathetic]
- Dilator muscles (radial fibres) make the pupil larger [sympathetic]
- Gives eyes their colour
What are the functions and characteristics of the ciliary body?
Glandular epithelium
- produces; aqueous humour & nutrients for cornea & lens
- Aqueous humour: mainly water & electrolytes, located in the anterior chamber important in maintaining intraocular pressure (15mmHg)
- Made of smooth muscle, which controls accommodation; the adjustment of the lens in the eye so that clear images of objects at different distances are formed on the retina
- Receives innervation from the parasympathetic system
What are the functions and characteristics of the choroid?
Important for the nutrition of the outer retina (photoreceptors)
- Acts as a heat sink
- Darkly pigmented so that it can absorb stray photons
What are the functions and characteristics of the retina?
Layered structure
- Produces vitreous humour: which acts as a collagen scaffold, helps maintain intraocular pressure and is important in the transmission of light
- Light passes through the pupil from the visual field to project an image onto the retina. An object that attention is focused on, projects an image that is centred near the posterior pole of the eye along the visual axis, this point is known as the FOVEA CENTRALIS and the surrounding 1cm is known as the MACULA LUTEA at these points the retina is specially modified for maximal visual acuity (resolving power) - Medial to the macula is a region where retinal axons accumulate to leave the eye this is the optic disc (where the optic nerve forms) - photoreceptors are absent in this region so its called the blind spot
- Retinal pigement epithelium (RPE)
- Contains photoreceptors (rods & cones) so it is able to convert light into electric impulses (PHOTOELECTRIC TRANSDUCER) which are transmitted to ganglion cells which go on to make optic fibres and eventually the optic nerve
- Rods: important to vision in dim lighting - very sensitive to light, also important for peripheral vision
- Cones: important for colour vision
What are the three layers of tear film and where do they come from?
Anterior Lipids (oils): secreted by meibomium glands, provides a hydrophobic barrier to prevent the aqueous layer evaporating
Middle aqueous (water,electrolytes & proteins): secreted by lacrimal glands, regulates transport through the cornea and prevent infection
Posterior mucous:- secreted by goblet cells, provides a hydrophilic layer that allows for the even distribution of the tear film
Layers through which a photon must travel through the eye?
- Tear film (transmission)
- Cornea (transmission & refraction (contributes to 2/3rds of refraction)
- Aqueous humour (transmission)
- Lens (transmission & refraction)
- Vitreous humour (transmission)
- Ganglion cell (transmission)
- Amacrine cell (transmission)
- Bipolar cell (transmission)
- Horizontal cell (transmission)
- Cone (transduction)
- Rods (transduction)
- Pigmented epithelium (absorption of excess photons)
What are the branches of the internal carotid artery in relation to the eye?
Opthalmic artery Central retinal artery - which passes into the optic nerve Ciliary arteries Lacrimal artery Ethmoid Eyelid artery
What are the branches of the external carotid artery in relation to the eye?
Facial artery - supplies medial lid & orbit
What is the effect of damage to the left optic nerve?
no vision through the left eye
What is the effect of damage to the optic chiasm?
Loss of vision of the temporal visual fields - this is called hemianopia (since half/hemi of vision has been lost)
What is the effect of damage to the left optic tract?
Loss of vision of the temporal field of the left eye & the loss of the nasal field of the right eye - another type of hemianopia
What is the effect of damage Meyer’s loop carrying information from the inferior retina and thus the SUPERIOR VISUAL FIELD?
resulting in loss of vision in the superior nasal field of the left eye and the superior temporal field of the right eye
What is the effect of Damage to the left Baum’s loop carrying information from the superior retina and thus the INFERIOR VISUAL FIELD?
resulting in loss of vision in the inferior temporal field of the right eye and the inferior nasal field of the left eye
What are the 7 extraocular muscles, what are their movements, and what are their innervations?
Levator Palpabrae Superiosis- Oculomotor nerve, raises the eyelid
Superior Rectus- Oculomotor nerve, lifts the eyeball superiorly and posteriorly
Lateral rectus- Abducens Nerve, Abducts the eyeball
Inferior rectus- Oculomotor nerve, pulls the eyeball inferiorly and posteriorly
Medial Rectus- Oculomotor nerve, adducts the eyeball
Superior Oblique- Trochlear nerve, External rotation is known as extortion (AWAY from MIDLINE)
Inferior Oblique- Oculomotor nerve, Internal rotation is known as intortion (TOWARDS MIDLINE)
Define Anterograde?
Transport from neuronal cell bodies to axon terminals
Define retrograde?
Transport from axonal terminals to neuronal cell bodies
Neurones do not project from A —> B instead its from A—> B —> C —> D etc.
Define a Class A experiment?
DIAGNOSIS: - Some behavioural, physiological or pharmacological variable is manipulated and the consequent effects on brain activity/structure are measured i.e. DIAGNOSIS
Define a Class B experiment?
TREATMENT: - Some aspect of the brain structure (lesion) or activity (stimulation/inhibition) is manipulated and the consequent effects on behaviour/physiology/ endocrinology is measured i.e. TREATMENT
Define Gastrulation?
single layer blastula developing into tri-laminar disc (gastrula)
The embryo develops into a tri-laminar disc made up of:
- Ectoderm
- Mesoderm
- Endoderm
Define neuralation?
the process of formation of the embryonic nervous system
Describe the formation of the neural tube?
1 The ECTODERM thickens in the mid-line to form the neural plate in the 3rd week of embryonic development
2 - The ectoderm then undergoes differential mitosis to cause the formation of a mid-line groove known as the neural groove
3 - This groove deepens and eventually detaches from the overlying ectoderm to form the neural tube
4 - Lateral to the neural plate lie presumptive neural crest cells which run dorso-laterally along the neural groove
5 - Neural crest cells develop to form many cell types:
• Sensory (dorsal root) ganglia of the spinal cord and cranial nerves V,VII, IX & X
• Schwann cells
• Pigment cells
• Adrenal medulla
• Bony skull
• Meninges
Dermis
• [Quite a lot of the head & neck are made up of neural crest cells]
6 - During embryonic development the rostral (i.e. superior) portion of the neural tube, which develops into the brain (CNS), grows faster than the caudal (i.e.inferior) portion, which develops into the spinal cord (the central cavity within the neural tube becomes the central canal of the spinal cord and the ventricles of the brain)
7 - By the 5th week of embryonic development, three primary brain vesicles can be identified: 1. Prosencephalon (forebrain) 2. Mesencephalon (midbrain) 3. Rhombencephalon (hindbrain)
8 - By the 7th week, further differentiation occurs resulting in the formation of secondary brain vesicles: 1. Prosencephalon —> Telencephalon & Diencephalon 2. Mesencephalon —> Mesencephalon 3. Rhombencephalon —> Metencephalon & Myelencephalon
9 - The secondary brain vesicles give rise to derivatives in the mature brain: • Telencephalon —> Cerebral hemisphere & Lateral ventricles • Diencephalon —> Thalamus, Hypothalamus & Third ventricle • Mesencephalon —> Midbrain (colliculi) & Aqueduct • Metencephalon —> Cerebellum, Pons & Upper part of fourth ventricle • Myelencephalon —> Medulla oblongata & Lower part of fourth ventricle
10 - As the brain develops, its central cavity also undergoes considerable changes in size and shape, forming a system of chambers called ventricles with contain cerebrospinal fluid
Describe abnormalities in the spinal cord development?
The neural tube usually closes at the end of the 4th week
Failure of the tube to close in the spinal cord results in - spina bifida
Failure of the tube to close in the cephalic region (brain) results in - anencephalus
The reasons for failure to close could be due to faulty induction or environmental teratogens (any agent that can disturb the development of the embryo) acting on neuroepithelial cells
What are the development milestones in neural development?
By 3 weeks there is eye formation 10 weeks = cerebral expansion & commissures 3 months = basic structures established 5 months = myelination has begun 7 months = lobes cerebrum has formed 9 months = gyri and sulci formed
What are the critical periods of neural development?
Abnormalities to the CNS are dependent on time of infection
6th week - eye malformations occur e.g cataracts
9th week - deafness can occur e.g. malformation of the organ of corti
5th to 10th week - cardiac malformation occur
In general CNS disorders generally occur in the 2nd trimester
The risk of disorders falls after 16 weeks due to the fact that most of the structures of the CNS have developed by this time
Describe the development of sensation?
There is innervation of dermal skin from 28 weeks
The dorsal root ganglion connects to the spinal cord from 8 weeks but this is nonnoxious (i.e. no pain is detected)
C-fibre connection (noxious (painful) stimuli) from 19+ weeks
Organised thalamus from 8+ weeks
Retinal inputs arrive at 14-16 weeks
Myelination occurs from 25 weeks (speed of conduction increases with myelination)
Connections from the thalamus to the cortex occur from 24 weeks
Describe the features of the blood brain barrier?
Endothelial tight junctions
Astrocyte end feet
Pericytes
Continuous basement membrane, lacks fenestrations (windows)
Requires specific transported for glucose, essential ions etc.
Certain parts of the brain lack the blood-brain barrier, these are called CIRCUMVENTRULAR ORGANS e.g. posterior pituitary - they need to be in contact with the blood for a sensory role to monitor
Describe CSF and its transport?
Circulates through the subarachnoid space (around the brain and spinal cord) and within ventricles - offers protection by cushioning brain from gentle movements)
There are four ventricles: 1. Lateral (paired) 2. III 3. IV • Ventricles & subarachnoid spaces connect via cisterns
Entries CSF is around 120mls
Its a clear, colourless liquid which contains; protein, urea, glucose & salts
Produced by ependymal cells in the choroid plexuses of the lateral ventricles (mainly)
Choroid plexus: - Formed from modified ependymal cells - They from around a network of capillaries, large surface area
Absorbed via arachnoid granulations (VILLI) e.g. in the superior sagittal sinus
What is hydrocephalus?
Abnormal accumulation of CSF in ventricular system. Often due to a blocked cerebral aqueduct - Accumulation of fluid leads to a build up of pressure which can damage brain tissue since the skull in hard in adults - In children with soft skull the pressure will cause the soul to bulge and look abnormal as well as damaging the brain
What is gastrulation?
single layer blastula developing into tri-laminar disc (gastrula)
The embryo develops into a tri-laminar disc made up of:
- Ectoderm
- Mesoderm
- Endoderm
What is neurultion?
the process of formation of the embryonic nervous system
Describe the formation of the neural tube?
1 The ECTODERM thickens in the mid-line to form the neural plate in the 3rd week of embryonic development
2 - The ectoderm then undergoes differential mitosis to cause the formation of a mid-line groove known as the neural groove
3 - This groove deepens and eventually detaches from the overlying ectoderm to form the neural tube
4 - Lateral to the neural plate lie presumptive neural crest cells which run dorso-laterally along the neural groove
5 - Neural crest cells develop to form many cell types:
• Sensory (dorsal root) ganglia of the spinal cord and cranial nerves V,VII, IX & X
• Schwann cells
• Pigment cells
• Adrenal medulla
• Bony skull
• Meninges
Dermis
• [Quite a lot of the head & neck are made up of neural crest cells]
6 - During embryonic development the rostral (i.e. superior) portion of the neural tube, which develops into the brain (CNS), grows faster than the caudal (i.e.inferior) portion, which develops into the spinal cord (the central cavity within the neural tube becomes the central canal of the spinal cord and the ventricles of the brain)
7 - By the 5th week of embryonic development, three primary brain vesicles can be identified: 1. Prosencephalon (forebrain) 2. Mesencephalon (midbrain) 3. Rhombencephalon (hindbrain)
8 - By the 7th week, further differentiation occurs resulting in the formation of secondary brain vesicles: 1. Prosencephalon —> Telencephalon & Diencephalon 2. Mesencephalon —> Mesencephalon 3. Rhombencephalon —> Metencephalon & Myelencephalon
9 - The secondary brain vesicles give rise to derivatives in the mature brain: • Telencephalon —> Cerebral hemisphere & Lateral ventricles • Diencephalon —> Thalamus, Hypothalamus & Third ventricle • Mesencephalon —> Midbrain (colliculi) & Aqueduct • Metencephalon —> Cerebellum, Pons & Upper part of fourth ventricle • Myelencephalon —> Medulla oblongata & Lower part of fourth ventricle
10 - As the brain develops, its central cavity also undergoes considerable changes in size and shape, forming a system of chambers called ventricles with contain cerebrospinal fluid
Describe abnormalities in the spinal cord?
The neural tube usually closes at the end of the 4th week
Failure of the tube to close in the spinal cord results in - spina bifida
Failure of the tube to close in the cephalic region (brain) results in - anencephalus
The reasons for failure to close could be due to faulty induction or environmental teratogens (any agent that can disturb the development of the embryo) acting on neuroepithelial cells
What are the developmental milestones?
By 3 weeks there is eye formation 10 weeks = cerebral expansion & commissures 3 months = basic structures established 5 months = myelination has begun 7 months = lobes cerebrum has formed 9 months = gyri and sulci formed
What are the critical period of neural development?
Abnormalities to the CNS are dependent on time of infection
6th week - eye malformations occur e.g cataracts
9th week - deafness can occur e.g. malformation of the organ of corti
5th to 10th week - cardiac malformation occur
In general CNS disorders generally occur in the 2nd trimester
The risk of disorders falls after 16 weeks due to the fact that most of the structures of the CNS have developed by this time
Describe the development of sensation?
There is innervation of dermal skin from 28 weeks
The dorsal root ganglion connects to the spinal cord from 8 weeks but this is nonnoxious (i.e. no pain is detected)
C-fibre connection (noxious (painful) stimuli) from 19+ weeks
Organised thalamus from 8+ weeks
Retinal inputs arrive at 14-16 weeks
Myelination occurs from 25 weeks (speed of conduction increases with myelination)
Connections from the thalamus to the cortex occur from 24 weeks
What is a fasciculi?
Nerve axons run up and down the spinal cord in bundles called fasiculi
The axons in a fasiculus are from neurones that come from or are going to similar locations
Where does the dorsal/medial leminiscal column decussate?
In the medulla
What sensation is carried by the dorsal/medial leminiscal column?
Proprioception, vibration, and descriminative touch
Describe the pathway of the dorsal/medial leminiscal column?
Fasciculus cuneatus - LATERAL and carries information from the UPPER body to the cuneate tubercle in the medulla
Fasciculus gracilis - MEDIAL and carries information from the LOWER body to the gracile tubercle in the medulla
Ascends to the medulla and then DECUSSATES to become the medial lemniscus then ascends to the thalamus then to the cortex
transmits to the pre-central gyrus - primary somatosensory cortex
• There are 4 sensory nerve endings that may sense fine touch (in the skin) (beginning the pathway):
1. Meissner’s corpuscle
2. Pacinian corpuscle
3. Ruffini endings
4. Merkel endings
• There are three neurones in this pathway:
- in dorsal root ganglion
- in the cuneate & gracile nuclei
- in the ventral posterolateral nucleus of the thalamus
What are the ascending tracts?
Dorsal/medial leminiscal column, spinothalamic tract, spinocerebellar tract, spinoreticular tract
Where does the spinothalamic tract decussate?
Across the white anterior commissure in the spinal cord
What sensation is carried by the spinothalamic tract?
LATERAL: pain & temperature
MEDIAL: crude touch
Describe the pathway of the spinothalamic tract?
Enters the spinal cord at lassauer’s fasciculus
Ascends 1-2 segments ipsilaterally then synapses with the cell bodies of the substantia gelatinosa (dorsal horn) - first order neurone
The secondary afferent decussates immediately across the white anterior commissure - second order neurone
The two tracts (lateral and medial) ascend and eventually join at the medulla to form the spinal lemniscus and ends at the thalamus - second order neurone
• Tract travels from the dorsal horn to the CONTRALATERAL thalamus
• The tract terminates at the thalamus
- From the thalamus the tertiary afferent goes from the thalamus to the somatosensory cortex - third order neurone
• Three neurones pathway:
- in the dorsal root ganglion
- in the dorsal horn of the spinal cord
- in the thalamus
What sensation is carried by the spinocerebellar tract?
Ventral spinocerebellar tract: carries information on proprioception to the ventral CONTRA & IPSILATERAL cerebellum
Dorsal spinocerebellar tract: carries information on proprioception to the dorsal IPSILATERAL cerebellum
What sensation is carried by the spinoreticular tract?
Carries deep/chronic pain
What is an overview of the descending tracts?
Originate from the cerebral cortex and brainstem (UPPER MOTOR NEURONES)
Their role concerns the control of movement, muscle tone, spinal reflexes, spinal autonomic functions & transmission of sensory information to higher centres
Divided into pyramidal & extrapyramidal
There are no synapses within the descending pathways. At the termination of the descending tracts, the neurones synapse with a lower motor neurone. Thus, all the neurones within the descending motor system are classed asupper motor neurones. Their cell bodies are found in the cerebral cortex or the brain stem, with their axons remaining within the CNS.
What are the divisions of the descending tracts and explain them?
Pyramidal-• 2 neurone pathway originating in the cerebral cortex of cranial nerve nucleus (for facial innervation)
DECUSSATE in the medulla and descend CONTRALATERALLY
• Neurones innervating our axial muscles (muscles of the trunk and head) mostly do not decussate
• Synapse with the cell bodies of the ventral horn of the spinal grey matter
Extra-pyramidal- • Originate in the brainstem and carry motor fibres to the spinal cord
• Responsible for involuntary autonomic control of all musculature
• Decussating tracts - contralateral innervation:
Which descending tracts are pyramidal?
Corticospinal tract and the Corticobulbar tract
Which descending tracts are extra-pyramidal?
Tectospinal tract, Rubrospinal tract, Reticulospinal tract, vestibulospinal tract
Where does the corticospinal tract decussate?
- Lateral (75%): pyramidal (medulla) decussation - limb muscles
- Medial (25%): decussates as it leaves via the anterior white commissure (a bundle of nerve fibres that cross the mid-line of the spinal cord) - axial muscles (head & trunk)
What innervation does the corticospinal tract provide?
Transmits control of voluntary muscles (motor)
What is the pathway of the corticospinal tract?
• Begins in the pre-central gyrus - primary motor cortex
receivea range of inputs:
Primary motor cortex
Premotor cortex
Supplementary motor area
• Neurones ultimately innervate the axial and limb muscles - they leave the cortex by descending through the internal capsule and into the brainstem
• Upper motor neurones (UMN) originate in the motor cortex - a UMN lesion can occur anywhere from the cortex all the way down to the ventral horn
• Neurones (cell bodies) located in the ventral horns project to limb and axial muscles - these are the lower motor neurones
• Two neurones involved:
- Upper motor neurones:
- Limb efferents decussate at medullary pyramids
- Axial efferents decussates at appropriate levels
Theanterior corticospinal tractremains ipsilateral, descending into the spinal cord. They then decussate and terminate in the ventral horn of thecervicalandupperthoracicsegmental levels.
What is carried by the corticobulbar tract?
• Innervation of the skeletal muscles of the head and neck via the cranial nerves
Describe the corticobulbar tract?
It has no decussation
• Bilateral innervation
• EXCEPT for cranial nerves; facial (VII), hypoglossal (XII) & glossopharyngeal (IX) which have contralateral innervation
The corticobulbar tracts arise from the lateral aspect of theprimary motor cortex. They receive the same inputs as the corticospinal tracts. The fibres converge and pass through the internal capsule to thebrainstem.
The neurones terminate on the motor nuclei of thecranial nerves.Here, they synapse with lower motor neurones, which carry the motor signals to the muscles of thefaceandneck.
Clinically, it is important to understand the organisation of the corticobulbar fibres. Many of these fibres innervate the motor neuronesbilaterally. For example, fibres from the left primary motor cortex act as upper motor neurones for the right and left trochlear nerves. There are a few exceptions to this rule:
Upper motor neurones for thefacial nerve(CN VII) have a contralateral innervation.
Upper motor neurons for thehypoglossal(CN XII) nerve only providecontralateralinnervation.
What is the function of the tectospinal tract?
The tectospinal tract coordinates movements of the head in relation tovision stimuli.
What is the pathway of the tectospinal tract and where does it decussate?
Arises from the tectum; inferior (auditory) & superior (visual) colliculus
This pathway begins at thesuperior colliculusof the midbrain. The superior colliculus is a structure that receives input from theoptic nerves.The neurones then quickly decussate, and enter the spinal cord. They terminate at the cervical levels of the spinal cord.
What is the function of the rubrospinal tract?
- Assists in motor functions
* Fine hand movements
What is the pathway of the rubrospinal tract and where does it decussate?
The rubrospinal tract originates from thered nucleus, a midbrain structure. As the fibres emerge, they decussate (cross over to the other side of the CNS), and descend into the spinal cord. Thus, they have acontralateralinnervation.
What is the function of the vestibulospinal tract?
- Muscle tone
* Balance & posture by innervating the antigravity muscles
What is the pathway of the vestibulospinal tract and where does it decussate?
no decussation
• From the vestibular nuclei
There are two vestibulospinal pathways; medial and lateral. They arise from thevestibular nuclei, which receive input from the organs of balance. The tracts convey this balance information to the spinal cord, where it remainsipsilateral.
Fibres in this pathway controlbalanceandpostureby innervating the ‘anti-gravity’ muscles (flexors of the arm, and extensors of the leg), via lower motor neurones.
Lateral:
Fibres descend ipsilaterally though the anterior funiculus of the same side of the spinal cord, synapsing on the extensor antigravity motor neurons
Medial:
Descends bilaterally in the medial longitudinal fasciculus
Synapses with the excitatory and inhibitory neurons of the cervical spine
What is the role of the reticulospinal tract?
Responsible for spinal reflexes
The two recticulospinal tracts have differing functions:
Themedial reticulospinal tractarises from thepons. It facilitates voluntary movements, and increases muscle tone.
Thelateral reticulospinal tractarises from themedulla. It inhibits voluntary movements, and reduces muscle tone
What is the pathway of the reticulospinal tract and where does it decussate?
- Medial pathway: Mediated by the pons which controls the extensors acting to increase muscle tone thereby facilitating voluntary movement
- Lateral pathway: Mediated by the medulla which controls the flexors acting to decrease muscle tone thereby inhibiting voluntary movement
No decussation
Explain Brown-Sequard Syndrome and what would be experienced?
It is key to think about both ascending and descending tracts:
• Damage to a hemi-section of the spinal cord
• Ipsilateral & contralateral are in relation TO THE LESION
• Ipsilateral weakness (i.e less motor etc.) below the lesion - due to damage to the ipsilateral descending motor corticospinal tract (decussated at the medulla already)
• Ipsilateral loss of dorsal column proprioception below lesion - since the ascending tracts are damaged before they could decussate in the medulla
Contralateral loss of spinothalamic pain & temperature below the lesion since spinothalamic fibres decussate just after entering cord within the spinal cord
• Overall: - Ipsilateral loss of; proprioception, motor & fine touch - Contralateral loss of; pain, temperature & crude touch
What is the human hearing range and at what range are our ears most sensitive?
Hearing range is from 20 to 20,000 Hz
The ear is most sensitive at 1000 - 4000Hz
What are the three segments of the ear and what is a brief overview of their roles?
Outer Ear: helps you collect sound
Middle Ear: transmission of sound
Inner Ear: conversion of sound into neural impulses
Describe the process of sound transmission in the external ear?
- Sound first enters the ear through the pinna (or auricle) which is the exterior part of the ear
- It then enters the ear via the external auditory canal/meatus
- The shape of both the pinna (or auricle) & external auditory canal/meatus help to amplify and direct the sound
- The sound then makes its way through the canal, to the tympanic membrane (eardrum)
- As the air molecules push against the membrane, it causes the tympanic membrane to vibrate at the same frequency as the sound wave
- The membrane vibrates slowly to low frequency sounds and very rapidly to high frequency sounds
- The tympanic membrane marks the end of the external ear and marks the start of the middle ear
What is the middle ear, which nerve provides sensation, and what is the role of eustachian tube in the ear?
• An air-filled cavity in the temporal bone of the skull
• Sensation of the middle ear is provided by the glossopharyngeal nerve CNIX
The pressures in the external auditory canal/meatus and middle ear cavity are normally equal to atmospheric pressure
• The middle ear is exposed to atmospheric pressure via the eustachian tube (or auditory tube) which connects the middle ear to the pharynx
• The eustachian tube opens into the pharynx through a slit-like opening which is normally closed, EXCEPT when muscle movements result in the opening of the tube during swallowing, yawning or sneezing
• A difference in pressure between the middle and external ear occurs due to changes in altitude
• When the pressure outside the ear and in the external auditory meatus change, the middle ear initially remains constant due to the fact that the eustachian tube is closed
• This constant pressure can stretch the tympanic membrane resulting in pain - which can be relieved by yawning/swallowing which in turn results in the opening of the eustachian tube thereby allowing the pressure in the middle ear to equilibrate with the external atmospheric pressure
What is the process of sound transmission in the middle ear?
- The vibrations of the tympanic membrane are transmitted to the inner ear (for processing) through a moveable chain of three bones - the ossicles (the smallest bones in the body)
- Vibrations from the tympanic membrane are transmitted into the inner via firstly through the malleus then incus then stapes (MIS)
- These bones have synovial joints between them
- They act as a piston and couple the vibrations of the tympanic membrane to the OVAL WINDOW (a membrane covered opening between the middle and inner ear)
- The total force of a sound wave applied to the tympanic membrane is completely transferred to the oval window
- However, due to the fact that the oval window is much smaller than the tympanic membrane, the force per area is much greater which is required to adequately transmit the sound energy through the FLUID FILLED COCHLEA
How is the amount of energy transferred to the inner ear controlled?
The amount of energy transmitted to the inner ear can be lessened by the contraction of two small muscles in the middle ear - the tensor tympani (innervated by V3 (mandibular branch trigeminal) & stapedius (innervated by CN7 (facial))
• The tensor tympani attaches to the malleus, and contraction of the muscle dampens the bones movement - innervated by the mandibular division (V3) of the TRIGEMINAL NERVE CN5
• The stapedius attaches to the stapes and similarly controls it - innervated by the FACIAL NERVE CN7
• These muscles act reflexively to CONTINUOUS LOUND NOISE to protect the delicate receptor apparatus in the inner ear
• These muscles CANNOT protect the inner ear from SUDDEN INTERMITTENT LOUND SOUNDS
Describe the anatomy of the inner ear?
- The inner ear is called the cochlea (the organ of hearing)
- The cochlea is a spiral-shaped (coiled around 2.5 - 2.75 times), fluid filled space in the temporal bone
- The cochlea is almost completely divided lengthwise by a membranous tube called the cochlear duct - which contains the sensory receptors of the auditory system
- The cochlea duct is filled with a fluid called endolymph - a compartment of extracellular fluid containing a high concentration of K+ and a low concentration of Na+ (this arrangement of concentrations is normally seen in intracellular fluid)
- On either side of the cochlear duct are compartments filled with perilymph, which is similar in composition to cerebrospinal fluid (CSF)
- The scala vestibuli is above the cochlear duct and begins at the oval window - remember it since vestibule is like an entrance so forms the entrance to the inner ear from the oval window
- The scale tympani is below the cochlear duct and connects to the middle ear via a second-membrane covered opening, the round window
- The scala vestibuli and tympani are continuous at the far end of the cochlear duct at the helicotrema
What is the process of sound transmission in the inner ear?
- Sounds waves from the external acoustic meatus cause the tympanic membrane to move in and out which in turn is transmitted to the ossicles which in turn transmit this movement to the oval window
- This results in the oval window moving in and out of the scala vestibuli
- This movement creates waves of pressure in the scala vestibuli
- The majority of these waves of pressure are transmitted across the cochlear duct with some being transmitted toward the helicotrema and into the scala tympani where the pressure is relieved by the movements of the membrane of the round window
- The side of the cochlear duct closest to the scala tympani is formed by the basilar membrane, upon which sits the ORGAN OF CORTI - contains the ears sensitive receptor cells, pressure difference across the duct causes the basilar membrane to vibrate
- The base of the basilar membrane is narrow & stiff and thus sensitive to high frequencies
- The apex of the basilar membrane is wider & less stiff and thus is sensitive to low frequencies
Explain the hair cells of the organ of corti?
- These cells are mechanoreceptors that have hairlike stereo-cilia protruding from one end
- Some antibiotics can damage the stereocilia of the hair cells
- There are two anatomically separate groups of hair cells; a SINGLE ROW of inner hair cells & 4-5 ROWS of outer hair cells
- The stereocilia of inner hair cells extend into the endolymph fluid and convert pressure waves caused by the movement of fluid in the cochlear duct into receptor potentials
- The stereocilia of the outer hair cells are embedded in the overlying tectorial membrane (see diagram above) and mechanically alter its movement to sharpen frequency tuning at each point along the basilar membrane
- The tectorial membrane overlies the organ of corti, as the pressure waves displace the basilar membrane, the hair cells move in relation to the stationary tectorial membrane resulting in the bending of the stereocilia
- When the stereocilia bend towards the tallest member of the bundle, fibrous connections called TIP LINKS pull open mechanically gated K+ channels, resulting in an influx of K+ from the surrounding endolymph (K+ rich) thereby depolarising the membranes
- This change in voltage triggers the opening of voltage-gated Ca2+ channels near the base of the cell, which in turn triggers neurotransmitter release (since Ca2+ causing neurotransmitter containing vesicles to migrate to the presynaptic membrane)
- Bending the hair cells in the opposite direction, slackens the tip links thereby closing the channels and allowing the cell to rapidly repolarize
- The neurotransmitter released from the hair cells is GLUTAMATE which in turn binds to and activates proteinbinding sites on the terminals of the afferent neurones
- As sound waves vibrate the basilar membrane, the stereocilia are bent back and forth, the membrane potential of the hair cells rapidly oscillates and bursts of glutamate are released onto afferent neurones
- This results in the generation of action potentials in the neurons, the axons of which join to form the COCHLEAR BRANCH of the VESTIBULOCOCHLEAR NERVE (CN VIII)
- The greater the energy (loudness) of the sound wave, the greater the frequency of action potentials generated in the afferent nerve fibres
- Due to its position on the basilar membrane, each hair cell respond to a limited range of sound frequencies, with one particular frequency stimulating it most strongly
