Sensation and Afferent Tracts Flashcards
What are somatic senses?
Temp, touch, pressure, vibration, proprioception, pain
What are the 4 basic receptor types, based on MOA?
Mechanoreceptors (compression/stretching of receptor)
Thermoreceptors (hot/cold receptors)
Electromagnetic (light in retina)
Chemoreceptors (O2/CO2/H+, osmolarity, taste, smell etc.)
What are other receptor types are there, based on the purpose they serve?
Nociceptors - pain (mechano/thermo/chemoreceptors)
Proprioceptors (mechanoreceptors - joint position)
What is modality of receptors?
- Show high sensitivity to one type of stimulus, may respond to others
- activity in afferent nerve is always interpreted as sensation associated with receptor, regardless of cause
- LAW OF SPECIFIC NERVE ENERGY
What is the receptive field and how can it be tested?
Area monitored by single receptor
- highly sensitive - small fields (fingertips, tongue, lips <1mm)
- general body surface has larger fields (7cm)
tested by two point discrimination
What is receptor potential?
Type of graded potential (as opposed to all-or-nothing)
Transmembrane potential difference produced by activation of a sensory receptor
Often produced by sensory transduction
Generally a depolarizing event resulting from inward current flow.
E.g. in a Pacinian corpuscle
- bare neurone tip surrounded by concentric tissue layers
- local pressure - deformation of tissue
- transferred to unmyelinated fibre tip
- deformation of fibre tip creates ion channels, allowing Na+ entry
- if Na+ conc. reaches threshold, AP generated
What is sensory coding?
When receptor potential above threshold, AP generated
- as receptor stimulated, more APs propagated
Therefore, stimulus strength coded for in:
- size of receptor potential (graded)
- sensory nerve freq. of AP
Compare tonic and phasic responses.
Tonic
- adapts v slowly, receptor potential + AP maintained
- constant sensation
e. g. some proprioceptors
Phasic
- adapts rapidly, receptor potential + APs diminish
- transient info
e. g. Pacinian Corpuscle (pressure + vibration)
What are the different groups of mechanoreceptors and what do they have in common?
In common: stretch-sensitive membrane channels, respond to distortion of membrane
- Tactile receptors of skin (fine or crude touch, range in complexity)
- Deep tissue receptors (same as skin)
- Proprioceptors (muscle spindles, GTO, joint receptors)
- baroreceptors (carotid sinus, aortic arch, RA)
- inner ear receptors (cochlear + vestibular receptors)
Describe the tactile receptors of the skin.
Free nerve endings - tonic, small fields, sole cornea receptors
Root hair plexus - rapidly adapting
Merkel’s discs - v sensitive, tonic, grouped in Iggo receptor domes, v small fields, fine touch
Meissner’s corpuscles - fine touch, low freq. vibration, fast-adapting, in capsule in dermis, many in sensitive areas
Pacinian corpuscles - deep pressure, high freq. vibration, rapidly-adapting, phasic, lamellae, wide distribution
Ruffini corpuscles - skin pressure + distortion, tonic
Describe muscle spindles as proprioceptors
- Sense muscle length, trigger muscles stretch reflexes
- receptors located in central region with sensory afferents
- contractile regions either end with gamma motor fibres
- muscle stretch causes intrafusal stretch, sends info to spine
- synapses with alpha motor neurone to trigger muscle contraction + inhibit muscles opposing contraction
- contraction of extrafusal fibres (via a-mn) accompanied by contraction of intrafusal fibres in spindle (via gamma-mn) to maintain sensitivity
Describe golgi tendon organs as proprioceptors
- located in tendons
- in series with muscle
- sense muscle tension
- initiate inhibitory reflex to stop contraction for protection)
Describe joint receptors
Free nerve endings in joint capsules
- detect joint pressure, movement + tension
Describe baroreceptors
- monitor pressure changes in organ walls
- free nerve endings in elastic tissue of some distensible organs, inc. BVs, heart, portions of resp, digestive, urinary tracts
- produce different affects, according to tissue
- rapidly adapting
- monitor blood pressure in walls of major arteries, inc. carotid sinuses, aortic bodies + heart
Describe thermoreceptors
- free nerve endings in dermis
- sensitive to hot OR cold temp. ranges
- phasic
- also located in hypothalamus (co-ordinator of responses to temp., spine, liver, skeletal muscle)
- transduced by specific protein channels (TRP channels)
Describe chemoreceptors
Carotid + aortic bodies
- monitor blood pH, O2, CO2
- medulla surface monitors CSF pH + CO2
- elicit resp, CV, behavioural responses
Hypothalamic receptors
- monitor glucose, AAs, osmolarity
- hunger, thirst
GI tract stimulated in diff parts by food content
e.g. stomach reacts to presence of proteins - gastric secretions
Describe nociceptors
- bare nerve endings
- 3 populations sensitive for: mechanical damage, dissolved chemicals, temp extremes
- relatively dense distribution
- some multimodal
- many response to chemicals released by damaged cells ( K+, ATP, inflammatory mediators - 5-HT, bradykinin)
- PGs sensitize nociceptors
- carried in 2 axon types: type A(fast), type C (slow)
What is the organisation of somatosensory system pathways?
1st order neurones (primary afferent)
- nerves with receptor endings
- cell bodies in DRG
- enter spinal cord via dorsal root
Synapse with 2nd order neurones
- travel to brain via two primary ascending tracts
Connect to cerebral cortex
- 3rd order neurones
What are the 4 types of primary afferent neurones, based on conduction velocity + diameter?
Aalpha - fastest
- muscle spindle, GTP, touch + pressure
Abeta
- touch, pressure + vibration
Adelta
- touch + pressure, pain + temp
C - slowest (unmyelinated)
- pain + temp
Describe the primary afferent route to spine
Sensory receptors -> primary afferent enters spine via dorsal root of spinal nerve -> dorsal root ganglion containing cell bodies of sensory axons -> enters dorsal horn of grey matter (where it synapses with 2nd order neurons)
where do fast and slow fibres synapse in the spine
Spinal lamina
- slower fibres synapse in more dorsal lamina
- faster fibres more ventrally
What is a dermatome?
Area of skin sensation associated with a particular spinal level of afferent entry
Sensory pathways move which way in the spinal cord?
Ascend
Describe the location and properties of the spinothalamic tract?
- anterolateral
- pain + temp receptors, tickle + itch
- 2nd order neurons cross midline (synapse in grey matter)
- ascends contralateral to side of entry (in white matter)
- poor spatial discrimination, crude sensation
- conduction velocity slower (synapse more dorsally in spinal cord)
- smaller myelinated + unmyelinated fibres
- synapse in thalamus
- 3rd order neurones travel to sensory cortex
