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
Describe the dorsal column
- proprioceptors/touch/vibration info
- ascend by ipsilateral pathway (no crossing over in spinal cord)
- no synapse in spinal cord
- synapse with 2nd order neuron in dorsal column nuclei
- cross midline in medulla, travels to thalamus (synapse with 3rd order)
- larger, myelinated fibres
- faster conduction
- 3rd order neuron to sensory cortex
Describe the spinocerebellar tract
- originates in spinal cord
- 2nd order neurones ascend, some ipsilaterally, some contralaterally
- terminates in cerebellum
- conveys info to cerebellum about limb and joint position (proprioception) from muscle spindles and GTO
- some cutaneous afferent info
Describe the somatosensory cortex
Primary sensory cortex in post-central gyrus
- receives info from sensory receptors
- allows conscious awareness of sensations
What other sensory cortex areas are there?
Occipital lobe - visual cortex
Frontal lobe - gustatory cortex
Temporal - auditory + olfactory
Somatic sensory association areas
- monitor activity in primary sensory cortex
- integrates somatic senses with memories, emotions, state of arousal by pathways to other brain areas
Special senses have own association areas
What kind of symptoms will peripheral lesions cause compared to brain/spine lesions?
Peripheral - localized
Brain/spine - more widespread
What are primary sensory axons affected by? What does this lead to?
Trauma
Nerve root damage
Neuropathy - diabetes, alcohol, MS, chemotherapy
Leads to:
- numbness
- pins + needles
- all modalities usually affected
What is the most common type of ascending tract damage? Where is sensory loss seen?
bilateral spinal damage
- sensory loss in all modalities below level of lesion
What happens to sensation in unilateral spinal damage?
joint position - sense lost on same side of lesion (hemiparaplegia - weakness/paralysis)
temp + pain senses lost on opposite side (hemianesthesia)
Brown-Sequard syndrome
How can somatosensory cortex be damaged?
usually caused by stroke
- damaging cortex or tracts from thalamus to cortex (internal capsule)
- causes sensory loss from opposite side of body
- motor deficits from nearby affected areas common
