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What is the purpose of the PNS?
Designed for rapid communication between the CNS and the remainder of the body
What structures comprise the PNS?
12 pair cranial nerves and 31 pair of spinal nerves
Are PNS nerves motor or sensory?
- Spinal nerves and most Cranial nerves are “mixed nerves” ( they carry axons of both sensory and motor neurons)
- Cranial nerves I, II, and VIII are sensory only
Cranial nerves in detail. (number, name, motor/sensory, function)
- I – Olfactory – Sensory – Smell
- II – Optic – Sensory – Vision
- III – Oculomotor – Mixed – Serves muscles of the eye
- IV – Trochlear – Mixed – Serves muscles of the eye
- V – Trigeminal – Mixed – Sensory from face to mouth, motor to muscles of mastication
- VI – Abducens – Mixed – Serves muscles of the eye
- VII – Facial – Mixed – Serves muscles of facial expression, taste
- VIII – Vestibulocochlear – Sensory – Equilibrium and hearing
- IX – Glossopharyngeal – Mixed – Serves pharynx for swallowing, tongue, taste
- X – Vagus – Mixed – Sensory from visceral organs, parasympathetic motor regulation of visceral organs
- XI – Accessory – Mixed - Serves muscles that move head, neck, and shoulders
- XII – Hypoglossal – Mixed – motor control of the tongue
Spinal nerves listed by area of spine
8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
How to spinal nerves join the spinal cord?
Spinal nerves join the spinal cord through spinal nerve roots
Spinal nerve roots detailed
- Dorsal root – carry afferent (sensory) axons into spinal cord. Dorsal root ganglion contains afferent cell bodies (pseudounioplar) outside the spinal cord
- Ventral root – carry efferent (motor) axons out of spinal cord. No ventral root ganglion, cell bodies reside in the spinal cord (multipolar)
What types of neurons do sensory neurons synapse with in the spinal cord?
They can synapse with an interneuron OR directly to motor neuron. Most of the time it will be an interneuron
What types of neurons stimulate motor neurons?
Motor neurons can be stimulated by an interneuron OR sensory neuron (depending on what the sensory neuron synapses with)
Upper motor neurons in detail
Upper motor neurons are interneurons that originate in the precentral gyrus and provide input to somatic motor neurons (lower motor neurons) in the corticospinal pathway
Lower motor neurons in detail. Alternate name?
Lower motor neurons (somatic motor neurons) are located in the spinal cord and synapse with skeletal muscle
Sensory neurons are inhibitory/excitatory?
Sensory neurons are always excitatory
Define reflex (detailed)
A simple, stereotyped response to a stimulus. It occurs automatically (without input from the brain) and is not a learned response. A reflex will occur even if the subject is asleep or unconscious
Explain the reflex arc
The reflex arc is the actual circuitry for a reflex. Action potentials “arc” into the spinal cord in a sensory neuron and out of the spinal cord in the same muscle’s motor neuron
Describe the knee-jerk reflex (detailed)
- Monosynaptic response
- A reflex hammer on the patellar ligament causes the quadriceps femoris muscle to stretch.
- This stimulates the muscle spindle (stretch receptor), which results in APs being conducted along the sensory neuron and into the spinal cord.
- In the spinal cord, the sensory neuron synapses directly with the somatic motor neuron.
- When the somatic neuron receives the excitatory signal it stimulates the same skeletal muscle
Describe reciprocal innervation
- Simultaneous activation of both excitatory (agonist) and inhibitory (antagonist) motor responses in antagonistic muscles
- Prevents both muscles from contracting simultaneously
How does reciprocal innervation work?
- Excitation from sensory neuron (directly or through interneuron) to neurons of agonist muscle
- Inhibition from inhibitory interneurons to the motor neurons of the antagonist muscle
Describe the withdrawal reflex
Activation of flexors and inhibition of extensors on ipsilateral side allows for quick withdrawal
Describe the crossed extensor reflex in relation to the withdrawal reflex
Activation of extensors and inhibition of flexors on contralateral side of withdrawal reflex
Give an example of when the withdrawal reflex and cross extensor reflex would occur
Stepping on a tack
What is a central pattern generator?
Complex circuitry that allows reflexive behaviors without communication to the brain
Give an example of a central pattern generator
Mike the headless chicken being able to walk without forebrain
What is a plexus?
Axons from different levels of the spinal cord come together to form very large nerves (such as the sciatic nerve)
How do somatic motor neurons connect to skeletal muscle?
SMNs synapse with skeletal muscle fibers at neuromuscular junctions (1 NMJ per 1 fiber)
Describe how a SMN makes a skeletal muscle contract (detailed w/ neurotransmitters, etc).
- APs cause SMN to release ACh at NMJ.
- Motor end plates have Nicoctinic AChR, and are stimulated by a single AP (no threshold needed)
- The EPSPs generated by the Nicotinic AChR result in contraction of the muscle
What is a motor unit?
- A motor unit is comprised of a single SMN and all the muscle fibers it innervates.
- SMNs divide into numerous collateral axon branches
- Each SMN synapses with many muscle fibers (1 axon branch per fiber, many axon branches per SMN)
How does a motor unit stimulate multiple skeletal muscle fibers?
When a SMN fires an AP it releases ACh onto ALL muscle fibers that it innervates, so muscle fibers contract as a single unit
Difference between large and small motor units?
- Motor units can be small (~4) or large (>1,000)
- Small motor units are designed for fine motor control (such as extrinsic eye muscles)
- Large motor units are designed for powerful contraction (such as the quadriceps muscle)
Difference between somatic motor neurons and autonomic motor neurons in exciting effectors?
- Somatic motor neurons innervate skeletal muscle exclusively via NMJ that targets specific fibers
- Autonomic motor neurons innervate smooth muscle, cardiac muscle, and glands via “spraying” mechanism that does not target specific fibers
What structure controls the autonomic motor neurons? Voluntary/involuntary?
- The hypothalamus controls the autonomic system, but is not conventionally named as part of the autonomic two-neuron pathway.
- Involuntary (hence the hypothalamus control)
Sympathetic division characteristics
- "Fight or flight” response
- Increased heart rate, blood pressure, respiration rate, perspiration
- Decreased digestive activitiy
Parasympathetic division characteristics
- "Rest and digest” response
- Decreased heart rate, blood pressure, respiration rate
- Increased digestive activity
Do sympathetic/parasympathetic systems turn off?
No, sympathetic/parasympathetic systems work simultaneously in a push/pull mechanism
Describe the general autonomic efferent pathway
- Two neuron pathway.
- Preganglionic neuron – cell body in CNS, synapses with postganglionic neurons in the autonomic ganglion (located in the PNS). Always releases ACh to postganglionic neuron.
- Postganglion neuron – cell body in autonomic ganglion (always
- contain Nicotinic AChR – excitatory response), synapses with effector organ
How do autonomic efferent pathways differ from somatic efferent pathways?
- Autonomic pathways have two motor neurons, while somatic pathways have just one.
- Autonomic pathways originate in the hypothalamus, somatic pathways the precentral gyrus
- Autonomic pathways are involuntary, somatic pathways are voluntary
- Autonomic pathways can use NE as a neurotransmitter, somatic pathways only use ACh
- Autonomic pathways use “spray mechanism,” somatic pathways directly innervate a single muscle fiber
Relationship between adrenal medulla and autonomic system?
The adrenal medulla is innervated by sympathetic pre-ganglionic fibers. When they fire, the secretory cells in the medulla release adrenaline into the blood stream. Adrenaline activates the same receptors as norepinephrine from the postganglionic sympathetic axon terminals
Efferent pathway of sympathetic division in detail
- Preganglionic cell bodies in spinal cord T1-L2 levels
- Preganglionic axons enter sympathetic chain ganglia (also T1-L2 levels) just lateral to spinal cord through the spinal rami communicantes
- Synapse with postganglionic motor neurons in sympathetic chain ganglia
- Postganglionic neurons send axons to effector organs
- Postganglionic neurons use NE (norepinephrine) for neurotransmitter (adrenergic synapse)
- Adrenergic receptors also activated by adrenaline released from adrenal medulla
Efferent pathway of parasympathetic division in detail
- Preganglionic cell bodies in brainstem and sacral cord
- Preganglionic axons synapse with postganglionic neurons at terminal ganglia (adjacent to or within the target organ)
- Postganglionic neurons send axons to effector organs
- Postganglionic neurons use ACh for neurotransmitter (cholinergic synapse) to Muscarinic AChR
Sympathetic vs. Parasympathetic Pre/Postganglionic lengths
- Sympathetic – short preganglionic, long postganglionic
- Parasympathetic – long preganglionic, short postganglionic
Describe dual innervation
Many organs receive both sympathetic and parasympathetic input
Give examples of the varied responses gained by dual innervation
- Iris/pupil – sympathetic dilates, parasympathetic constricts
- Heart – sympathetic increases heart rate, parasympathetic decreases heart rate
- Digestion – sympathetic decreases stimulation, parasympathetic increases stimulation
- Respiration – sympathetic opens bronchioles, parasympathetic constricts bronchioles
What does the ANS (autonomic nervous system) consist of?
Sympathetic and parasympathetic pathways and their CNS control centers
What type of motor neurons are carried to organs by spinal nerves?
Both SMNs and autonomic motor neurons bound for blood vessels, sweat glands, and arrector pili
Cholinergic v. Adrenergic?
- Cholinergic synapses use ACh
- Adrenergic synapses use norepinephrine
What comprises the sensory systems?
Sensory receptors and their pathways
What are some basic characteristics of sensory systems?
- Monitor internal/external environment
- detect stimuli (changes in the environment)
- transduction (conversion of stimulus to electrical signal (receptor potential))
- critical for homeostasis (input for many negative feedback loops)
Types of sensory receptors (functional) by modality + information?
- Chemoreceptors – changes in chemical concentration (smell, taste, CO2 levels, glucose levels, etc)
- Thermoreceptors – temperature changes (separate receptors for heat and cold)
- Mechanoreceptors – mechanical energy (touch, pressure, vibration)
- Photoreceptors – light (rods and cones of the retina)
- Nociceptors – tissue damage (pain)
What makes nociceptors different from other sensory receptors?
They are categorized by generation of a specific sensation (pain), unlike the rest which are categorized by the stimulus they receive
What is the law of specific nerve energies?
A specific sensation is felt for each receptor that is stimulated even though APs are identical, due to specific pathways leading to specific brain regions
Define sensory adaptation. What are the two types of possible receptors (detailed)?
- Response of sensors to constant stimulation
- Tonic receptors exhibit little adaptation. The firing rate of APs remain constant for as long as stimulus is applied (vision)
- Phasic receptors exhibit sensory adaptation. The firing rate of APs decrease with constant stimulus (smell, temperature of water)
Receptor membrane potential information (detailed)
- Sensors are most sensitive to one stimulus modality.
- Stimulation of sensor results in receptor membrane potential (excitatory) (generated via transduction)
- If receptor potential causes threshold to be met then an AP is generated
- Increased stimulus strength to sensor results in more frequent (but identical) APs
What is a receptive field?
Area/space/location of stimulus that leads to the response of a sensory neuron (touch, pain, vision, etc)
What does a two-point discrimination test measure?
The density/size of receptive field (not sensitivity)
Receptive field re: localization and convergence
- Localization of stimulus is a measure of the size of receptive field (more accurate localization = smaller field)
- Convergence of signals is directly related to size of receptive field (more convergence = larger receptive field, poorer localization, wider two point discrimination)
Why does convergence affect receptive field?
- Convergence is the merging of multiple receptors into a single axon.
- When a large amount of signals are merged the brain receives only one stimulus, and has no way to determine which of the many original receptors it came from
Conduction vs. perception?
- Conduction relays information through a specific sensory pathway to a CNS region
- Perception is the conscious awareness of a stimulus or evaluation of a stimulus that occurs when the information reaches the appropriate CNS region
List the classifications of sensory input
Somatosensory input, special senses input, and visceral sensory input
Somatosensory input sensors/pathway
- Sensors located over wide areas of body
- Information conducted to spinal cord then somatosensory cortex of brain
Special senses input sensors/pathway
- Changes detected only by specialized sense organs in the head (cranial nerves)
- Information conducted to specific brain areas
Visceral senses input sensors/pathway
- Sensors in internal organs
- Information conducted to hypothalamus, limbic system, RF, etc (not conscious)
Somatosensory receptors with information
- Proprioceptors – sensory receptors in muscles, tendons, and joints. Detect stretch in muscles, limb movements, position of body parts, etc. (golgi tendon organs (tension) and spindle fibers (stretch)
- Cutaneous receptors – sensory receptors in the skin. Detect somatesthetic senses (touch, pressure, temperature, pain, etc.) that use the spinothalamic pathway/fine touch and proprioception pathway
What are the special senses?
Taste, smell, equilibrium, hearing, and vision
Describe the structure of a taste bud
- Barrel-shaped cluster of both taste cells (functional) and supporting cells (space fillers) found in tongue
- Taste cells have microvilli (taste hairs) that protrude into mouth through taste pore.
- Taste cells synapse with sensory neurons at base
Describe depolarization/creation of stimulus by taste bud
- Cells depolarize when taste hairs are stimulated, release neurotransmitter to associatied sensory neurons.
- The information is relayed (via CN VII and IX) to the thalamus and then to the inferior postcentral gyrus for perception
List the different taste sensations and the molecules that trigger them
- Salty (high [Na+])
- Sour (high [H+])
- Sweet (various organic molecules)
- Bitter (toxins)
- Umami (glutamate)
Describe how the different molecules of taste actually cause depolarization
- Salty – depolarization is driven by a direct flow of Na+ through an ion channel and into the cell
- Sour – depolarization is driven by a direct flow of H+ through an ion channel and into the cell OR H+ blocking K+ outflow from a cell
- Sweet/bitter/umami – binding of molecules to receptor proteins activates second messenger pathways that lead to depolarization
How do taste buds with different taste cells create a singular taste impulse?
Each sensory neuron can only be stimulated by a single type of taste cell. If all five taste cells simultaneously trigger in one taste bud only one will stimulate the sensory neuron for that taste bud
Describe olfactory receptor structure
- Olfactory receptors are bipolar neurons.
- Their dendrite extends into the nasal epithelium, and ends in a ciliated knob (with the cilia exposed to the inner nose)
- Their axon travels through the cribiform plate of the ethmoid bone to the olfactory bulb (CN I)
Describe depolarization/creation of stimulus by an olfactory receptor
- Odorants bind to receptor proteins on the cilia, which elicits a G-protein response (leading to depolarization)
- APs in the olfactory receptor axons travel to the olfactory bulb via CN I
- In the olfactory bulb they synapse with interneurons (this area is galled a glomerulus)
- The information is the relayed to the olfactory cortex ( temporal lobe) and the limbic system, bypassing the thalamus
How many different genes code for olfactory receptor proteins?
How many different smells can humans distinguish
How do olfactory receptors create various smell sensations?
Various combinations of odorants binding to receptors create unique smell sensations
What is the bony labyrinth?
A complex hollow area within the temporal bone which is filled with perilymph and houses the membranous labyrinth
What structures make up the bony labyrinth and what are their general functions?
- Vestibule (Utricle and Saccule) – equilibrium
- Cochlea – hearing
- Semicircular canals – equilibrium
What is the membranous labyrinth?
The membranous labyrinth is like a water balloon filled with endolymph that resides inside the perilymph of the bony labyrinth
What ion is highly concentrated in the endolymph? Why is this relevant?
K+ (creates gradient which enables depolarization of stereocilia)
What is equilibrium (sense) and what is useful for?
Position/motion of the head, useful for balance/coordination of body movement
What structures comprise the vestibular apparatus?
The semicircular canals and the vestibule
What are hair cells? What is their basic structure?
- Sensory cells in the vestibular apparatus.
- They have small stereocilia (mechanoreceptors) on their apical surface
Describe depolarization/creation of stimulus in the vestibular apparatus
- Bending stereocilia open K+ mechanically-gated ion channels which leads to depolarization of membrane.
- Depolarization causes hair cell to release neurotransmitter to sensory nerve endings at the base of cell.
- Excitation of axons sends an AP along CN VIII to medulla oblongata which relays impulses to the cerebellum and to the thalamus which relay impulses to the cerebral cortex
Where are the hair cells of the vestibule located?
Which structures of the vestibule detect which stimuli?
- Utricle – linear movement of head (horizontal)
- Saccule – direction of gravity (vertical)
Where are the otolith organs found?
What is the otolithic membrane?
A gelatinous membrane that contains otoliths (CaCO3 crystals) and holds hairs cells in the maculae
What effects do linear movement and titling of the head have on the otolithic membrane?
Causes otolithic membrane to sag which bends the stereocilia, causing depolarization of the hair cells
Where are the hair cells of the semicircular canals located?
In the ampulla
What is the cupula?
Gelatinous membrane that holds the hair cells of the ampulla
Difference between hair cells of the vestibule and semicircular canals?
Vestibule hair cells are held in otolithic membrane (in maculae)Semicircular canal hair cells are held in cupula (in ampulla)
What happens in semicircular canals when you rotate your head (not spin around, just rotate)?
Cupula and stereocilia bend in the opposite direction of the rotations (like a car making a hard turn) resulting in a depolarization
What is vestibular nystagmus?
Involuntary rapid movement of the eye in a given direction
What causes nystagmus?
- When spinning the eyes receive some information to keep you focused while your head moves.
- When spinning stops abruptly your eyes continue to jerk in that direction for a short period of time.
What causes “dizziness” from rotation?
- When the head initially rotates the ampula (hair cells) move in the opposite direction (like a car quickly accelerating), but the endolymph (due to inertia) stays still.
- After time the endolymph begins to spin (due to friction from ampulla), but if an abrupt stop occurs the endolymph will continue to spin, when the ampulla stops.
- This will cause hair cells to be turned the opposite direction (as the liquid pores over them) causing a sensation of rotation in the opposite direction
Difference between stereocilia moving in different directions?
One direction is depolarization (excitation) and the other direction is hyperpolarization (inhibition) (two distinct responses)
What are sound waves actually?
Variations in air pressure
What is pitch? What aspect of a sound wave affects pitch? What is it measured in?
- High/low tones, affected by the frequency of sound waves.
- Hertz (Hz)
What is the relationship between frequency and pitch?
As frequency increases, so does pitch (direct)
What range of frequencies can humans detect?
Between 20 and 20,000 Hz, any more or less will not be heard by the human ear
What is loudness? What aspect of a sound wave affects loudness? What is it measured in?
- Volume, affected by the amplitude of sound waves.
- Decibels (dB)
Describe the functions of each structure in the outer ear with relation to hearing
- Pinna (auricle) – collects and channels soundwaves from directly in front of you
- External auditory meatues – entrance into the temporal bone
- Tympanic membrane – vibrates when struck by sound waves
Describe the functions of each structure in the middle ear.
- Eustachian tube – connects middle ear to pharynx to equilibrate pressure with the atmosphere. (typically collapsed, but opens during yawning, etc)
- Auditory ossicles (malleus, incus, and stapes) – transmit and amplify vibrations from the tympanic membrane to the oval window
Describe the functions of each structure in the inner ear with relation to hearing
Cochlea – receives vibrations from oval window, vibrations cause pressure waves which lead to depolarization in the Organ of Corti (a part of the cochlea)
Describe the structure of the cochlea
- The structures of the cochlea are continuous with the bony and membranous labyrinth of the rest of the inner ear.
- Bony cochlea – filled with perilymph (forms scala vestibuli and scala tympani)
- Cochlear duct – part of the membranous labyrinth (filled with endolymph) (scala media)
- Organ of Corti – basilar membrane of the cochlear duct (sensory receptor for hearing)
Describe the structure of the organ of corti in depth
- Forms the floor of the cochlear duct.
- Composed of both hair cells and supporting cells.
- The hair cells are located on the basilar membrane, are flexible, and vibrate when they receive sound stimulation.
- The stereocilia of the hair cells is embedded in the tectorial membrane which causes the stereocilia to stay completely still (unaffected by sound stimulation)
What is the tectorial membrane?
the gel-like substance that holds the stereocilia of hair cells in the organ of corti
Describe hearing transduction (from the sound wave to the brain)
- Sound waves are turned into vibrations of the tympanic membrane.
- The middle ear bones transmit and amplify these vibrations to the oval window.
- The oval window creates waves of perilymph in the cochlea.
- Waves of perilymph cause the basilar membrane and hair cells to vibrate
- The vibration causes the hair cells to move against the tectorial membrane (which remains still), bending the stereocilia
- This causes the hair cells to depolarize, and a neurotransmitter is released to the nerve endings of CN VIII.
- The signal is relayed through the brainstem (including the inferior colliculus) to the thalamus.
- The thalamus relays the signal to the primary auditory cortex (temporal lobe) and then the auditory association cortex
How do the hair cells of the cochlea depolarize?
Fluid pressure waves in perilymph cause basilar membrane to vibrate, which causes hair cells to vibrate while stereocilia stays still, bending the stereocilia
How is depolarization of hair cells in the cochlea affected by louder sounds?
Bigger vibration, which results in more frequent APs
Describe the tonotopic organization of the cochlea
- Low pitches and high pitches cause vibrations in different parts of the basilar membrane.
- High – closer to the oval window
- Low – closer to the apex/ farther from the oval window
What is vision?
The perception of light by the brain
What type of receptors are used for vision? What stimulates them?
Photoreceptors located in the eye are stimulated by photons of light
- Thousands of photopigments are located on a given photoreceptor.
- Photopigments undergo a chemical change in response to light. This response induces changes in the photoreceptor, leading to the generation of receptor potential (transduction)
Describe the vision pathway (starting with retina)
Signals sent from retina -> CN II -> optic chiasm -> optic tract -> thalamus -> primary visual cortex (occipital lobe) -> visual association cortex
Describe the anatomy of the eye (with layers and structures - no functions).
- Outer Layer – Sclera, Cornea
- Middle Layer – Iris, pupil, ciliary body, choroid layer
- Inner Layer – Retina
- Internal Structures – lens, vitreous chamber
Sclera structure and function.
- White of the eye, tough CT
- Protects inner structures, maintains eye shape
Cornea structure and function
- Continuous with sclera, transparent (not CT)
- Lets light pass into the eye
- Fixed, convex (curves outward) lens that bends light inward
- Covers the anterior chamber
Anterior chamber vs Posterior chamber vs Vitrious chamber (location and fluid).
- Anterior chamber – space between cornea and iris (aqueous humor)
- Posterior chamber – space between iris and ciliary body/lens (aqueous humor)
- Vitrious chamber – space behind ciliary body/lens (vitrious humor)
Iris structure and function
- Thin ring of pigment and smooth muscle in front of lens
- Alters pupil size to regulate the amount of light that passes into the eye
Pupil structure and function
- Opening at the center of the iris
- Allows light into the eye
Which iris muscles control the pupil in which ways? Which autonomic system?
- Radial (dilator) muscles – open the pupil to allow more light (sympathetic)
- Circular (sphincter) muscles – close the pupil in response to bright light or close objects (parasympathetic)
Ciliary body structure and function
- Ring-shaped smooth muscle encircling the lens, attached to lens by suspensory ligaments
- Secretes aqueous humor, adjusts the shape of the lens to focus light (via suspensory ligaments)
Choroid layer structure and function
- Vascular and melanin-pigmented layer between the sclera (outer) and retina (inner)
- Supplies nutrients to photoreceptor cells, pigment absorbs stray light in eye
Lens structure and function.
- Elastic and flexible mass in the eye. Cells made mostly of protein (crystallin)
- Used to focus light onto the retina
Vitreous humor characteristics and function
Viscous, transparent fluid in the vitreous chamberAllows light rays to pass freely. If humor is cloudy then light rays are bent/interfered with)
What is the accommodation reflex?
- The ability for our eyes to focus on close objects.
- Achieved by contracting the ciliary body in varying degrees, pupil constriction (for close objects), and bilateral contraction of the medial rectus muscles (for close objects)
Explain how the accommodation reflex works for far objects.
- Light from object enters eye from narrow range of angles.
- Ciliary body relaxes which increases tension on suspensory ligaments which causes the lens to flatten
- A flatter (less convex) lens results in less refraction (bending of light) from the lens
Explain how the accommodation reflex works for close objects.
- Light from object enters eye from a wide range of angles
- Ciliary body contracts which decreases tension on suspensory ligaments which causes the lens to thicken
- A more convex lens results in more refraction (bending of light) from the lens
- Parasympathetic firing causing simultaneous pupil constriction and ciliary muscle constriction
Describe myopia, its causes, and its correction
- Nearsightedness – a distant object is brought into focus in front of the retina (too much refractive power to see distant objects)
- Caused by elongated eyeball or abnormally high curvature of cornea or lens
- Corrected with biconcave (curved inward) lenses
Describe the flow of aqueous humor and its function
- Secreted by ciliary body -> posterior chamber (provides nutrients to lens) -> anterior chamber -> drainage (continuous flow)
- Maintains intraocular pressure/ eye shape and provides nutrients to avascular lens
What is glaucoma?
Buildup of pressure from aqueous humor buildup in eye. (eye carries no pain receptors, so affected person may have no idea it is happening)
Describe hyperopia, its causes, and its correction.
- Farsightedness – a close object is brought into focus behind the retina (too little refractive power to see near objects)
- Caused by shortened eyeball or abnormally low curvature of cornea or lens
- Corrected with biconvex (curved outward) lenses
Describe presbyopia, its causes, and its correction
- Close objects are blurry.
- With age (~45-50 years) the proteins making up the lens lose their flexibility, and can no longer thicken for accommodation (even when the ciliary muscle contracts).
- Correction with reading glasses or bifocals (convex lenses)
Retina structure and function (basic) + major substructures (macroscopic).
- Inner layer of eye (lining of the vitreous chamber) that contains photoreceptors.
- Fovea centralis – point where light from the center of the visual field is focused (highest desnity of cone cells, sharpest visual acuity). Macular degeneration (degeneration of that area) can cause blindness)
- Optic disc – where optic nerve joins the eye. “blind spot”
Describe the layers of retinal cells (in order) and their functions
- Pigmented epithelium – supporting cells (absorb some stray light and various activities)
- Photoreceptors (rods and cones) – sensitive to light
- Bipolar cells – interneurons between photoreceptors and ganglion cells
- Ganglion cells – receive signals from bipolar cells. Axons for CN II and send signals to the thalamus and other visual centers (eg superior colliculus)
What does the term inverted retina mean?
The photoreceptors are “buried” underneath the other cells, so the electrical signal is sent in the opposite direction (up) of incoming light (down)
General rod information? Synapse structure? Function in detail?
- More numerous than cones, cannot distinguish between colors, night vision.
- Many rods converge onto several bipolar cells which synapse with one ganglion cell, causing massive summation, high sensitivity to low light levels, low visual acuity, and a large visual field per ganglion cell
General cone information? Synapse structure? Function in detail?
- Found mainly in fovea centralis, can distinguish colors, day vision.
- One or very few cone cells per ganglion cell (little to no convergence), causing a higher threshold of light than rods (less sensitivity), high visual acuity, and a small visual field per ganglion cell
Describe photoreceptor structure
- Outer segment of photoreceptors have many discs with photopigments (visual pigments).
- Rods have rhodpsin
- Cones have opsins (3 kinds: red, green, or blue) – each cone has mostly one opsin and will respond more to a certain wavelength of light (similar to taste buds)
How does rhodopsin respond to light?
Light stimulous causes a change in the shape of the molecule (retinal + opsin) which leads to a membrane potential change. This molecule is “used” and must be reformed before it can fire again.
How does opsin respond to light?
Opsin responds to various wavelengths of light (either red, green, or blue)
Describe the two different pathways from the eye (from CN II)
- VISION – optic nerve -> optic chiasm (partial crossing) -> optic tract -> thalamus -> primary visual cortex -> visual association cortices
- VISUAL REFLEXES – optic nerve -> optic chiasm (partial crossing) -> hypothalamus (suprachiasmatic nuclei) AND optic nerve -> optic chiasm (partial crossing) -> optic tract -> superior colliculus
Inferior olive structure and function
- Cluster of nerve cell bodies which serve as a relay to the cerebellum.
- Separates CN XII from CN IX-XI
How are spinal nerves named?
- Based on what level of the vertebre they extend from.
- Spinal cord actually ends ~L1/L2, but nerves extend downward and continue to exit the spine
What type of sensory neuron does not have a myelin sheeth?
What pathway do SMNs belong to?
What significance does the cell body have for sensory neurons?
Required to keep cell alive, but completely uninvolved for transmission of electric signal
What is sensitivity?
Strength of stimulus needed to start an action potential, inversely related to threshold
What is sensation?
Conscious awareness of stimulus. Occurs in related portion of brain NOT sensory neuron.
How are various smells created? What other sense functions similarly?
Various levels of stimulation to different receptors integrated together (similar to vision).
Where does the organ of corti reside (specific part of the cochlea)?
The organ of corti forms the base of the cochlear duct
Purpose of vestibule vs purpose of semicircular canals?
Vestibule – detects motion (horizontal/vertical)Semicircular canals- detect angle/rotation of head
Describe the carpal tunnel test and carpal
- Test: two point-discrimination test on finger tips (palm side). Normal range is 4-6mm.
- Symptoms: 6+mm discrimination, diminished grip strength
What are the three SFST tests?
Horizontal Gaze Nystagmus, Walk and Turn, and One Leg Stand
Describe the HGN test (including indicators of impairment and # needed for a positive test). Any other possibilities for a positive test?
- Alcohol impaired person has exaggerated nystagmus and difficulty smoothly tracking a moving object.
- Test: Observe eyes of suspect as suspect follows a slowly moving object (pen or flashlight)
- Three indicators (per eye): eye cannot follow moving object smoothly, jerking is distinct when eye is at maximum deviation, angle of onset of jerking within 45 degrees of center.
- Positive test: 4+ indicators between two eyes. (88%)
- Could also indicate consumption of seizure medications, phencyclidine, inhalants, barbiturates, and other depressants.
Describe the WAT test (including indicators of impairment and # needed for a positive test)
- Divided attention test, requires subject to follow instructions while performing simple physical movements.
- Test: subject is directed to take nine steps (heel-to-toe) along a straight line, turn on one foot, and return in the same manner.
- 8 indicators: suspect cannot keep balance, begins test before instructions are completed, stops while walking to regain balance, does not touch heel-to-toe, steps off the line, uses arms to balance, makes an improper turn, takes an incorrect number of steps.
- Positive test: 2+ indicators (79%
Describe the OLS test (including indicators of impairment and # needed for a positive test)
- Divided attention test, requires subject to follow instructions while performing simple physical movements
- Test: subject is instructed to stand with one foot ~6in off the ground and count aloud by the thousands. Officer times subject for 30 seconds.
- 4 indicators: swaying while balancing, using arms to balance, hopping to maintain balance, putting the foot down
- Positive test: 2+ indicators (83%)
Describe the Rinne test. What does it test for? What test should accompany a Rinne test and why
- Compares the perception of sounds transmitted by
- air vs conducted through the mastoid
- Test: Tuning fork is placed on mastoid until sound is no longer heard, then held to ear.
- tests for conductive hearing loss
- A Weber test should accompany to detect sensorineural hearing loss
What are the possible results of a Rinne test?
- Normal ear: positive (AC>BC)
- Conductive hearing loss: negative (BC>AC)
- Sensorineural hearing loss: should give positive (equal depreciation) but could give false neg
Describe the Weber test. What does it test for?
- Compares the perception of sounds transmitted by each ear. Can detect unilateral conductive hearing loss and unilateral sensorineural hearing loss.
- Test: Stem of tuning fork is placed centrally on either the top of a patients skull, forehead, or above upper lip. Patient reports which ear hears the sound louder
Weber test results: unilateral conductive vs unilateral sensorinueral and why?
- Conductive: tuning fork loudest in the affected ear.
- Conduction problem masks the ambient noise of the room, but inner ear picks up vibrations through the bone.
- Sensorineural: tuning fork loudest in the unaffected ear. Affected ear is less effective at picking up sound even when vibrating through the bone