Lecture 20- sensation and afferent tracts Flashcards
Modality and receptors
modality- high sensitivity to one type of stimulus, but may respond to others
activity in afferent nerve always interpreted as sensation associated with its receptor, regardless of cause
example- photo stimulation of retina: but pressure/ electrical stimulation of rods and cones give visual sensation
What is the receptive field?
Area monitored by single receptor- highly sensitive areas have small receptive fields, e.g fingertips, tongue, lips
larger on general body surfaces (torso, legs, arms)- tested by two point discrimination
What is the receptor potential, e,g in pacinian corpuscle
Receptor consists of bare neuron tip surrounded by concentric tissue layers- local pressure causes deformation of tissue and transferred to unmyelinated fibre tip
deformation of tip allows local depolarising na+ entry- RP graded with stimulus strength
What is sensory coding?
When RP above threshold membrane potential, AP generated- as receptor stimulated more APs propogated
SO- SS coded for in
- size of receptor potential (graded)
- sensory nerve frequency of action potentials
Adaptation- phasic and tonic responses
Phasic- adapts rapidly, receptor potential and resultant AP diminsih- transient information e.g pacinian corpuscle
Tonic- adapts v. slowly, receptor potential/ AP maintained- constant sensation e.g some proprioceptors
What are the 3 groups of mechanoreceptors?
Tactile- fine/crude touch receptors, range in complexity from free nerve endings to sensory complexes
Proprioceptors- muscle spindles, golgi tendon organs, joint receptors
Baroreceptors- CV system (carotid sinus, aortic arch, right atrium)
also deep tissue and inner ear receptors ( cochlear/ vestibular)
Tactile receptors- different types
Free nerve endings- tonic discharge, small fields- sole cornea receptors
Root hair plexus- sensory dendrites surrounding hair follicle, rapid adaptation
Merkel’s discs- v. sensitive tonic touch receptors- dendrites assc. w/large epithelial cells, small fields
Meissner’s corpuscles- fine touch, low freq vibrations- fast adapting, finger tips, eyelids, lips etc
Pacinian corpuscles- deep pressure, high freq. vibration- rapidly adapting, fingers and viscera
Ruffini corpuscles- skin pressure and distortion- tonic discharge
Proprioceptors- role of muscle spindles
Sense muscle length, trigger muscle stretch reflex
Receptors in central region with sensory afferents, contractile regions either end with gamma motor fibres
Stretch causes intrafusal stretch, info to spine, synapse w A MN to trigger muscle contraction to oppose stretch- also inhibits muscles opposing contraction
Contraction of extrafusal fibres ( A- MN), accm. by contraction of intrafusal fibres of spindle to maintain sensitivity
Proprioceptors- role of golgi tendon organs and joint receptors
GTO- tendon, in series with muscle- sense muscle tension and initiate inhibitory reflex
JR- free nerve endings in joint capsules- detect joint pressure, movement and tension
Role of baroreceptors
Pressure changes in organ walls- free nerve endings in elastic tissue of some distensible organs, inc. BV, heart, portions of resp/digestive/ urinary tracts
Produce different effects depending on tissue: rapidly adapting
Blood pressure in major arteries- carotid sinus, aortic bodies, heart
Temperature receptors
Free nerve endings located in dermis-sensitive to hot/cold temperature ranges
phasic receptors, also located in HT, spine, liver and skel. muscle
TRPV1- >43 degrees and capsaicin (chilli)
TRPV2 > 52 degrees
TRPA1 < 18 degrees and menthol
Chemoreceptors
Carotid and aortic bodies- monitor blood pH, CO2 and O2: MO surface monitors CSF pH and CO2
elicit respiratory, CV and behavioural responses
Hypothalamic receptors- monitor glucose, AAs and osmolarity
GI tract stimulated in diff. parts by food content- stomach reacts to presence of proteins especially: triggers gastric secretions
Nociceptors- role and populations of free nerve endings
Mechanical damage, dissolved chemicals and temperature extremes
-relatively dense distribution, some multimodal, many respond to chemicals released by damaged cells e.g k+, ATP, inflamm. mediators (5-HT, bradykinin)
discharge does not adapt, prostaglandins sensitise nociceptors (inhibitors e.g aspirin are analgesic)
What two axon types are nociceptor signals passed in
Type A- fast pain (e.g pin prick, burning- mechanical or temperature stimuli)
Type C- slow pain (tissue destruction, chemical stimuli)
Somatosensory system pathways
1st order neurones (primary afferent)
- nerves with receptor endings, cell bodies in DRG
- enter spinal cord via dorsal roots
Synapse with 2nd order neurones- travel to brain via 2 primary ascending tracts
Connect to cerebral cortex- 3rd order neurones
What are the 4 types of primary afferent neurones
Aa- largest and fastest, muscle spindle, GTO, touch and pressure
AB- touch, pressure and vibration
Ad- touch and pressure, pain and temperature
C- smallest and slowest, pain and temperature
Primary afferent route to spine
sensory endings > primary afferent enters spine via spinal nerve > DRG (posterior) containing somata of sensory axons/ Ventral (ant) root, containing motor and autonomic axons > white matter (Myelinated axons)/ gray matter (neurones, synapses etc)
Spinothalamic tract
pain/temp receptors, tickle and itch cross to contralateral side via synapse
2nd order neurons cross midline of spine: ascends contralateral side to entry in ST tract
poor spatial discrimination, crude sensation
slow conduction, smaller myelinated/ unmyelinated fibres, synapse in thalamus
3rd order neurones travel to sensory cortex
Dorsal column
proprioceptor/touch/vibration info- ascends by ipsilateral dorsal column
fine senses, good spatial discrimination: no synapse in spine
synapse w/ 2nd order neurons in dorsal column nuclei- cross midline in medulla to thalamus
larger unmyelinated fibres, faster conduction
Spinocerebellar tract
originates in spinal cord- 2nd order neurones ascend, some ipsilaterally/some contralaterally
terminates in cerebellum- conveys info to cerebellum about limb/ joint position (proprioception)
some cutaneous afferents
Primary sensory cortex and different lobes
PSC in post-central gyrus: receives info from sensory receptors and allows conscious awareness of sensations
Occipital- visual cortex
Frontal- gustatory cortex
Temporal- auditory + olfactory cortex
Somatic sensory association areas monitor activity in PCS, integrate somatic senses with memories, emotions, state of arousal by pathways to other brain areas
special senses have own assc. areas
Somatosensory defects
Peripheral lesions- localised symptoms
Brain/spine lesions- more widespread symptoms
Primary sensory axons affected by trauma, nerve root damage, neuropathy (diabetes, alcohol, MS, chemo)
> numbness, pins and needles, all modalities usually affected
Ascending tract damage
Bilateral spinal damage (most common) - sensory loss in all modalities below level of lesion
Unilateral spinal damage (rare)- joint position sense lost on same (ipsilateral) side of lesion, temp and pai senses lost on opposite (contralateral) side
Brown- sequard syndrome
Somatosensory damage with brain
Stroke, damaging cortex or tracts from thalamus to cortex (internal capsule)
sensory loss from opposite side of body
motor deficits from nearby affected areas common
