Principles of sensory physiology Flashcards
Special senses
Carried by cranial nerves
- Olfaction i
- Vision ii
- Taste vii and ix
- Hearing and balance viii
General/somatic senses
Detected from all parts of body, transmitted to CNS by:
- trigeminal v
- All spinal nerves except C1
Sensory receptors are transducers
Convert one form of energy to another
Detect various stimuli + convert to APs
Photoreceptors
Detect light. rods and cones of retina
Thermoreceptors
Detect changes in temp, central (hypothalamus) and peripheral (skin)
Nociceptors
pain
Mechanoceptors
Mechnical stimuli, divided into:
- Exteroceptors: stimuli from outside body, ex touch
- Proprioceptors: info about body position, ex muscle spindles
Sensory receptor
- depolarized to threshold to generate AP (opens voltage-gated Na+ channels)
- Generator potential: depolarization caused by opening/closing of ion channels in response to sensory stimulus.
- In rods and cones, GP is hyperpolarization
- If GP big enough to reach threshold, APs produced, propagate to CNS
- In myelinated sensory axons: AP initiated at 1st node of ranvier
GP of somatosensory mechanoreceptors
- Direct effect of stretch on stretch-sensitive channels
- Allow both Na+ and K+ to pass
- Net depolarization due to greater driving force for Na+
GP of nociceptors, photoreceptors, chemoreceptors
G-protein coupled mechanism, influence ion channels indirectly
How is stimulus intensity coded?
- Frequency coding
- Population coding
Frequency coding
Greater stimulus intensity, greater freq of APs in ind axons
- Not a linear function
Population coding
Greater stimulus intensity, more ind receptors recruited
Receptor adaption
Slowly adapting and rapidly adapting.
- Adaptation in mechanoreceptors is due to accessory structures surrounding axon terminal
- These structures modify physical stimulus
Slowly adapting
AKA tonic
- Monitor static, unchanging stimuli
- Maintained muscle length
- Maintained pressure
insert pic
Rapidly adapting
AKA phasic
- Detect onset of stimulus
- Change in time, eg vibration
- Change in space
insert pic
Tactile receptors
- Fast adapting: Meissner’s corpuscules (change in space), Pacinian corpuscules (vibration), endings surrounding hair follicles
- Involved in discriminative touch
- Meissner’s corpuscules abundant in fingertips
- Slow adapting: Non-changing features of tactile stimuli (eg maintained pressure)
Proprioception
- Muscle spindles:
- Primary endings: rate of change of muscle length
- Secondary endings: absolute muscle length
- Golgi tendon organs: tension receptors, lets us know if there’s too much tension in tendons, reflexively causes relaxation
- Joint receptors: Detects joint angles
- Ruffini endings, pacinian corpuscules
- In joint capsules and ligaments
- Skin receptors: deformed by changes in joint angle
Pain and temp
Receptors are free nerve endings
- No capsule or specialization
Conduction velocity classification of peripheral nerve fibers
Group A: fastest, large diameter, myelinated
- Sub-divided: Aalpha, Abeta, Adelta, Agamma
Group B: smaller, still myelinated
Group C: slowest, smallest, unmyelinated
- Usually used for motor neurons
alpha motor neurons
voluntary movements
gamma motor neurons
coordination
Diameter classification of peripheral nerve fibers
I - thickest, fastest
II
III
IV - thinnest, unmyelinated, slowest
- Used for sensory axons
Anatomy of spinal cord
- X shaped central portion of grey matter
- Many cell bodies, and dendrites/synapses)
- Outer portion: white matter (axons)
- Descending motor tracts from brain
- Ascending sensory tracts to brain
insert pic
- Spinal nerves divide into dorsal root and ventral root
White matter of spinal cord can be divided into…
Dorsal, lateral, ventral white columns
Dorsal root
Carries sensory info into spinal cord
- Cell bodies in dorsal root ganglia (periphery)
- Dorsal horn has cell bodies of interneurons upon which sensory neurons terminate
Ventral root
Motor neuron axons
- Cell bodies in ventral horn (grey matter of sp cord)
- Ventral horn larger in cervical (upper limb) and lumbar (lower limb) regions
Intermediolateral horn
Contains cell bodies of preganglionic autonomic efferent neurons
- T1-L2, S2-S4
Reflexes
Automatic, unchangeable response to stimulus
- Activity in series of neurons (reflex arc)
1. Sensory receptor in muscle
2. Sensory axon, goes to spinal cord
3. Integration centre in CNS
4. Motor neuron and axon
5. Effector organ (Muscle contraction, gland)
Muscle spindle
Detects stretch (change in length) of muscle (proprioception)
- w/i skeletal muscle
- contains several intrafusal muscle fibers
- Two types:
1. Nuclear bag fibers (one per spindle)
2. Nuclear chain fibers (several per spindle)
1. Single group Ia fiber enters
2. Forms spiral endings around all intrafusal fibers (primary endings, rapidly adapting, change in length)
3. Several group II fibers enter
4. Innervate only nucl chain fibers (secondary endings, slow adapting, absolute length
5. Motor innervation via gamma-Mn
- Causes contraction of intrafusal fibers, stretching receptive portion
- Incr discharge in Ia and II fibers
Muscle spindles when relaxed
Sensitive to stretch
Contracted muscle w no spindle coactivation
Not sensitive to stretch
Contracted muscle w normal spindle coactivation
sensitive to stretch
Spindle coactivation
When skeletal muscles are shortened, gamma-MNs activate to prevent slack
Monosynaptic stretch reflex
- Primary endings (Ia) activated by rapid muscle stretch
- Central branch of Ia excites dendrites of motor neurons supplying same muscle
- Simultaneously recruits inhibitory neuron
- Motor neurons of antagonist flexor muscles inhibited (hyperpolarized) (reciprocal inhibition)
- Contraction
Polysynaptic flexor-withdrawal reflex
- Activation of pain stimulates nociceptors
- Travels through group III or IV axons (slow)
- Excites interneuron in spinal cord
- Interneuron excites ipsilateral flexor motor neurons
- Reciprocal inhibition: extensor MNs on same side are inhibited
- Interneuron also excites contralateral extensor MNs
- Reciprocal inhibition: flexor MNs on other side inhibited
- these same MNs normally controlled by higher centres
Muscle tone
Also a spinal reflex.
Background contraction of muscle in absence of movement
- Cont excitation of gamma-MNs
- From reticular formation of brainstem
- Spindle constantly stretched
- Tonic activity in group II fibers
- Reflex origin initiated centrally
Vestibular apparatus
Info enters CNS from VA via viii cranial nerve (vestibulocochlear)
- Info: posture, movement of body and eyes
- Consists of: 3 semicircular canals, 2 chambers (utricle and saccule)
-
Cochlea
Detection of sound
- Associated w VA
- 2.5 turn conical helix
- 3 fluid-filled cavities: scala vestibuli, tympani, and media
- Bases of hair cells attached to basilar membrane (vibrates) (w/i inner membrane - surrounded by endolymph)
- Project into tectorial membrane
Tectorial membrane
Stiff
Scala vestibuli and tympani
Connected at apex (oval and round window)
- contain perilymph
Scala media
AKA cochlear duct
- Inner membrane, filled w endolymph
VA and cochlea
- In temporal bone
- Bony labyrinth lined w membrane
- b/w membr and bone: perilymph (ionic comp similar to plasma, 150mM Na, 7mM K)
- In membrane: endolymph w hairs projecting (16mM Na, 150mM K). (+) compared to surr tissue
Semicircular canals
horizontal, anterior, posterior
- Each canal opens into utricle
- Near canal-utricle junction is wider area (ampulla)
- Hair cells on ridge at base of ampulla (crista)
- In any ampulla, hair cells always oriented in same direction (depolarize/hyperpolarize at the same time)
- hair cells embedded in gelatinous cupula
Insert pic of cupula and orientation of semicircular canals
Hair cells
Transducers
- In cochlea, utricle, saccule, semicircular canals
- Each cell has 60-100 cilia projecting from apical pole
- Stereocilia in rows of ascending height, tallest being next to Kinocilium
- At base of cell: vesicles containing glutamate
- Attached to e/o at apical tips by gap junctions
- Only apical tips and cilia exposed to high K endolymph
Hair cell depolarized
Stereocilia bent toward kinocilium
- Release glutamate
- Activate viii
Hair cell hyperpolarized
Stereocilia bent away from kinocilium
- Less glutamate
- Less activation of viii
Hair cell activation
- Cilia bent toward kinocilium
- Stretch-sensitive ion channels on cilia open
- K+ from endolymph enters -> depolarization
- Voltage sensitive Ca2+ channels open at base
- Glutamate release
- AP stimulated in post-synaptic axon
- Cilia away from kinocilium is opposite effect
Rotation of head (horizontal canals)
- Rotation to the right
- Endolymph has inertia, lags behind: both cupulae pushed to left
- Constant velocity: fluid catches up, and cupula goes back to starting position
- Stopping: fluid has momentum, continues pushing to right (gives opposite neuronal firing)
- Direction of head movement opposite to direction of fluid movement
Main role of semicircular canals
Control position of eyes during head movement
As head moves to one side, eyes move to opposite (fixation).
Vestibulo-ocular reflex:
- ii (optic nerve - visual feedback)
- iii (oculomotor - t/w midline)
- vi (abducens - away midline)
- viii (vestibulocochlear)
Utricle and saccule
Contain macula: equivalent to crista of canals
- contains hair cells which project into otolith membrane (jelly)
- Otolith membr contains crystals of calcium carbonate (makes it have density 5x heavier than surroundings)
- Responds to gravity and acceleration
- Hair cells not all oriented in same direction
Utricle
- macula forms floor (horizontal)
- Some kinocilium anterior edge, posterior, medial, lateral
- Linear movement and static tilt: Info on front-back, left-right movements; maintained tilt
Saccule
- Macula forms wall (vertical)
- Some kinocilium anterior, posterior, superior, inferior
- Linear movement and gravity: Info on front-back and up-down
Utricle and saccule main role
To cause reflex adjustments in:
- head via neck + trunk muscles
- body position via limb muscles
VA’s central connections
- Input from VA enters brainstem
- Synapses on 4 vestibular nuclei (VN) at pontomedullary junction
Superior VN
Semicircular canals -> medial longitudinal fasciculus (coordinates eyes)
Medial VN
Utricle + saccule -> neck and trunk via medial vestibulospinal tract
Lateral VN
Utricle + saccule -> limbs via lateral vestibulospinal tract
Inferior VN
All components -> cerebellum (coordinates all motor activities)
Outer ear
Pinna + auditory canal
- Collects sound/oscillations in air pressure
- Directs to eardrum
Middle ear
Sound transmitted + amplified w assistance of ossicles (little bones)
- Malleus, incus, stapes -> attaches to oval window
Oval window
access point to inner ear and fluid-filled cochlea
Sound transduction
- Sound collected in outer ear, directed to eardrum
- Eardrum vibration transfers to ossicles - amplify x20
- Oval window vibrations transmitted thru fluid compartments (scala)
- Basilar membrane vibrates, tectorial membrane immovable
- Hairs deflected, transduction initiated
Frequency coding of sound
Basilar membr at base of cochlea more easily displaced by high frequency
At apex, low frequency
- viii axons arising from base carry info abt high freq sound
- viii axons further along carry info abt low freq
Central connections of viii nerve
- Info goes to cochlear nuclei in brainstem
- Then to medial geniculate nucleus in thalamus
- Then to auditory cortex in temporal lobe
- Each temporal lobe receives input from both ears
- If auditory pathway w/i CNS damaged on one side, both sides can still be heard
