Sensory Systems Flashcards
What is the process of a specialised afferent nerve ending being propagated toward the CNS?
- In sensory receptors that are specialised afferent neuron endings, stimulus opens stimulus-sensitive channels, permitting net Na+ entry that produces receptor potential
- Local current flow between depolarised receptor ending and adjacent region opens voltage-gated Na+ channels
- Na+ entry initiates action potential in afferent fibre that self-propagates to CNS
Provide some information about hair receptors
- afferent neuron endings spiral around the base of hair follicles
- stimuli: mechanical displacement of the hair leads to receptor potential
- fast adapting (phasic) receptor
- sustained stimulation does not produce further action potentials
- respond best to moving objects and signal the direction and speed of the moving object
- discriminative touch
Discuss features of the merkel cell-neurite complex
- found in hairy and not hairy skin in basal layer of the epidermis
- comprised of two receptor types, Markel cell and Merkel disk
- specialised receptor cell
- enlarged afferent neuron endings from Merkel disk
- respond to fine tactile stimuli and slow vibration
- force applied to skin leads to distortion of the cell receptor potential = neurotransmitter release
- Merkel cell leads to action potential in Merkel disk
- each afferent neuron innervated by up to 90 Merkel cells in a discrete patch of skin
- small receptive fields in fingers, larger in palm of hand and legs
- slowly adapting
- responds best to steady pressure from small objects
What is the Meissner complex?
- encapsulated receptor found within the dermal papillae in non-hairy skin
- most sensitive to flutter and movement
- stack of epithelial cells with afferent fibre endings interspersed between the cells
- force applies to skin causes epithelia cells to slide past each other = distortion of membranes of axon terminals = receptor potential
- single afferent neuron forms many Meissner corpuscles
- rapidly adapting response
- small receptive field
- respond best to rubbing against skin, movement across a surface
What is the Ruffini corpuscle?
- encapsulated mechanoreceptor
- sensitive to skin stretch, sustained pressure
- found deep in the skin, in joint ligaments and capsules
- touch and proprioceptor
- contains strands of collagenous fibres, continuous with connective tissue
- afferent fibre branches within the capsule, endings are intertwined with collagenous fibres
- tugging or stretching in the dermis detected
- slowly adapting receptor
- monitor slippage of object (modulation of grip)
What is the Pacinian corpuscle?
- encapsulated (onion shaped with layers of epithelial cells surrounding afferent neuron endings)
- sensitive to vibratory pressure, deep touch
- in subcutaneous tissue beneath the dermis, connective tissues of bone, body wall and body cavity = cutaneous and proprioceptive function
- epithelial cells contain fluid that is displaced when force is applied, movement of fluid dissipates force
- mechanical deformation = opening of pressure sensitive channel = receptor potential
- fast adapting
- function (grasping, releasing objects, surface texture discrimination)
List some features of free nerve endings, including where in the body they are located as well as what they are receptors for.
- found throughout the body (skin, muscles, tendons, cornea, viscera, etc.)
- receptors for crude touch, pain and temperature
- morphologically very similar but different functions, each with its own ascending pathway
- adaption depends on specific functional type, but mostly slow adapting
- small unmyelinated fibres
Contrast fast and slow pain
Fast:
- occurs on stimulation of mechanical and thermal nociceptors
- carried by small, myelinated A-delta fibres
- produces sharp, prickling sensation
- easily localised
- occurs first
Slow:
- occurs on stimulation of polymodal nociceptors
- carried by small, unmyelinated C fibres
- produces dull, aching, burning sensation
- poorly localised
- occurs second, persists for longer time, more unpleasant
What are the two best known pain neurotransmitters>
- Substance P (activates ascending pathways that transmit nociceptive signals to higher levels for further processing)
- Glutamate (major excitatory neurotransmitter)
How does the analgesic system of the CNS work?
- CNS suppresses transmission in pain pathways as they enter spinal cord
- depends on presence of opiate receptors (endogenous opiates - endorphins, enkephalins, dynorphin)
Describe ‘equilibrium’
- sense of body orientation and motion
- body orientation with respect to gravity
- discriminate self-generated movements from external forces
- activates reflex pathways - compensatory body movements
- also activates pathways that project to the cortex
What is the vestibular apparatus comprised of?
- semicircular canals (rotational equilibrium)
- otolith organs (gravitational equilibrium)
Describe the semicircular canals
- fluid filled canals (endolymph(
- detect rotational or angular acceleration or deceleration
- canals are perpendicular to each other (map 3D space)
- receptors (hair cells) are located in the cristae ampullaris within the ampulla of each cell
- hairs are embedded in gelatinous material called the cupula
What structures are involved in transduction?
- hair cell - separate receptor cell
- hair cell consists of 1 kinocilium and 20-50 stereocilia (microvilli)
- hairs are connected by tip links
- mechanically gated ion channels open/close depending on direction of bending
Describe the process of transduction with specific mention of how it works when the head is turning.
- when head is turned, endolymph lags behind due to inertia
- endolymph (in the canal that is in the same plane as the movement) pushes on cupula and bends hair cells in the opposite direction of head movement = appropriate signal
- if head movement continues in same direction, endolymph moves at the same speed as the head movement, no force in the cupula = hair cells are no longer bent
- if head movement stops, the reverse happens
Provided a detailed explanation of the process of transduction, with mention of its afferent neurons
- depolarisation in hair cell induces increased neurotransmitter release and the consequent increase in frequency of action potentials
- semicircular canals work in pairs: depolarisation on one side, hyperpolarisation on other
- when fluid stops moving, hair cells straighten = state of no signal, which is the resting membrane potential
- no signals when head keeps turning at a constant speed or when head is motionless
- axons of afferent neurons from the vestibular nerve
What are the otolith organs?
- provide information about position of head relative to gravity and changes in rate of linear motion
1. utricle
2. saccule - sack like structures in bony capsule between semicircular canals and cochlea
- receptors: hair cells arranged in maculae
- overlying hair cells is a gelatinous sheet with many calcium carbonate crystals (otoliths)
- added weight of otoliths increases inertia
- when in upright position, hairs of utricle are orientated vertically and those in saccule horizontally
What is the utriculus, and what does it respond to?
- responds if head is tilted so that it is no longer vertical
- depending on the direction of the head tilt (up or down), hair cells depolarise or hyperpolarise
- the utriculus also responds to changes in horizontal linear motion (moving forwards, backwards or sideways)
- heavy otoliths lag behind as movement starts, which causes hair cells to bend = appropriate signal
- no signals detected when the movement constant
What is the sacculus, and what does it respond to?
- responds if head is tilted away from horizontal position
- also responds to vertically directed linear acceleration
- depending on the direction of the head, hairs cells depolarise or hyperpolarise
- no signals are detected when the movement constant
Where do projections from the vestibular apparatus go, and to what effect?
- vestibulocochlear nerve carries afferent from the vestibular apparatus to vestibular nuclei (brainstem) and then to the cerebellum
- the cerebellum then maintains balance and posture, controls eye movements and perceives motion and orientation
What is the vestibulo-ocular reflex?
- reflex eye movement (Nsytagmus)
- when vestibular system detetcs a head rotation, eyes move in opposite direction = stabilisation of images in the retina
- test for function of the vestibular system
- stabilises images on the retina during head movement, preserves image on the centre of visual field
- does do not depend on visual input, works in darkness
What are the symptoms, causes and treatment of motion sickness?
Symptoms:
- nausea, vomiting, pale skin, dizziness
Causes:
- conflict of information between sense, in particular visual and vestibular information
Treatment:
- minimise movement
- fix vision on stable object - horizon
- medication
List some properties of rods (vision)
- 120 million per retina
- more numerous in periphery
- high sensitivity
- night vision
- low acuity
- much convergence in retinal pathways
- vision in shades of grey
List some properties of cones
- 6 million per retina
- concentrated in fovea
- low sensitivity
- day vision
- high acuity
- little convergence in retinal pathways
- colour vision
What is the fovea?
- pin-sized depression in exact centre of retina
- point of most distinct vision
- has only cones
- bipolar and ganglion cells are ‘pulled aside’ - light can strike cones directly
What is the macula lutea?
- area immediately surrounding fovea
- fairly high acuity
What is the clinical significance of the pupillary light reflex?
- allows testing for integrity of sensory and motor functions of the eye
- lack of the pupillary reflex or an abnormal pupillary reflex can be caused by optic nerve damage, oculomotor nerve damage, brain stem death and depressant drugs
What are the two aspects of the neural perception of sound energy?
- identification of sounds (‘what’)
- localisation of sounds (‘where’)
Contrast the inner and outer hair cells of the ear.
Inner:
- main sensory cell
- single line
- receptor potential following bending of stereocilia
Outer:
- receive efferent input
- move to amplify the wave in the basilar membrane
- tunes and enhances response by inner hair cells
