Ascending tracts, sensory receptors Flashcards
What are sensory receptors?
- specialized cells which convert stimuli into AP via transduction which is then interpreted by CNS
What are the somatic senses?
- temp
- touch
- pressure
- vibration
- proprioception
- pain
What are the special senses?
- vision
- hearing
- smell
- taste
- equilibrium
What is the mechanism of activation of mechanoreceptors?
- receptor stretched or compressed
- (stretch-sensitive)
What is the mechanism of activation of thermoreceptors?
- hot and cold receptors
What is the mechanism of activation of electromagnetic receptors?
- light in retina (only in eyes)
What is the mechanism of activation of chemoreceptors?
respond to chemical stimulus:
- O2
- CO2
- H+
- osmolarity (hypothalamus)
- taste
- smell
How can receptors be classified?
1) mech of action (mechano- thermo- chemo- electro-)
2) purpose (nociceptors or proprioceptors)
What are nociceptors?
- give rise to pain sensation
- usually mechano, thermo and chemo
What are proprioceptors?
- give info about joint position
- usually mechano
What does a receptors modality refer to?
- show high sensitivity to one type of stimulus
- can still respond to others
How does the level of sensitivity and the size of receptive field relate?
- more sensitive the area the smaller the receptive field
- e.g. finger tips small receptive field
What is the structure of a pacinian corpuscle?
- bare neurone ending
- surrounded by layers of tissue
How do pacinian corpuscles transmit a signal?
- membrane surrounding receptor must be distended
- this causes pressure from distension to pull apart the ion channels
- allowing Na+ ions to enter down conc gradient
- if enough sodium can result in threshold depolarisation where AP transmitted down axon
What does the strength of stimulus effect about the signal?
- size of receptor potential
- sensory nerve frequency of AP
Describe a phasic receptor response?
- adapts rapidly
- receptor potential and resultant AP diminish
- will deliver transient information
- example pacinian corpuscle
Describe a tonic receptor response?
- adapts slowly
- receptor potential and resultant AP maintained
- give constant sensation information
- e.g. some proprioceptors
Give example of mechanoreceptors?
- tactile receptors of skin and deep tissue
- proprioceptors
- baroreceptors (CV system)
- inner ear receptors
What are the different types of tactile mechanoreceptors found in the skin?
- free nerve ending receptors (bare dendrites), very sensitive
- root hair plexus (sensory dendrites) surround hair follicles
- Merkels receptors (surface of skin)
- Meissners corpuscle (encapsulated nerve ending) very sensitive found in fingertips
- Pacinian corpuscle
- Ruffini corpuscle
What is the function of muscle spindles?
stretch receptors which sense muscle length and trigger muscle stretch reflexes
What is the structure of muscle spindle?
Intrafusal
- central region of fibre
- contains receptors with sensory afferents
- at either end have contractile regions with gamma motoneurones
Extrafusal
- alpha motorneurone supply
- (oustide muscle spindle)
How do muscle spindles work?
- muscle stretch causes intrafusal stretch
- which sends this information to spine
- spine synapses with alpha motor neurones to trigger muscle contraction which opposes the stretch
- opposing muscle is also inhibited
- intrafusal fibres contracted along with extrafusal to avoid distorsion of intrafusal fibres maintaining sensitivity
What is the function of the Golgi tendon organs?
- sense muscle tension
- initiate muscles inhibitory reflex
- located in tendons
- help protect muscle from excessive tension
What do proprioceptor joint receptors detect?
joint
- pressure
- movement
- tension
Where are the baroreceptors located?
- glossopharyngeal nerve
- carotid sinus (bifurcation of carotid arteries)
- aortic arch
- heart muscle
- pulmonary artery
Where would you find temperature receptors?
- dermis
- hypothalamus
- spine
- liver
- skeletal muscle
(all send info to hypothalamus which can respond)
What type of receptors are temperature receptors (phasic or tonic?
Phasic
What channels open in response to temperature changes?
TRPV1 > 43 degrees (also responds to capsaicin found in chillies)
TRPV2 > 52 degrees
TRPA1 < 18 degrees (also responds to menthol)
What are examples of chemoreceptors?
sense changes in chemicals
- carotid body sense O2, CO2 and PH changes
- medulla oblongata sensitive to H2 and CO2 ions (function in respiration)
- hypothalamic receptors help signal hunger and thirst, sense glucose and osmolarity
- GI tract receptors, trigger gastric secretions in response to proteins
What stimuli can trigger nociceptors?
- mechanical damage
- dissolved chemicals
- temperature extremes
- (often response due to chemicals released by damaged cells)
What chemical mediator can sensitize nociceptors?
- prostaglandins
- reduce sensitivity so reduce pain
What are the types of primary afferent neurones?
- A(alpha) (Fastest)
- A(beta)
- A(delta)
- C (Slowest)
What are examples of receptors which use C fibres?
- pain and temp
What are examples of receptors which use A(alpha) fibres?
muscle spindle
Golgi tendon organ
touch and pressure
What factors effect the speed of conduction in a nerve fibre?
Faster
- larger diameter
- myelinated
What is the difference between white and grey matter?
axons myelinated when white, unmyelinated when grey
What is the pathway through which a signal passes from sensory receptors into the spinal cord?
- From sensory receptor signal carried via primary afferent nerve
- enters spinal cord via dorsal root ganglion
Where will faster and slower fibres synapse within the spinal lamina?
- slower fibres synapse more dorsally
- faster synapse more ventrally
Give examples of sensory pathways ascending from the spinal cord to the cortex?
- spinothalamic tract
- dorsal column
- spinocerebeller tract
What receptors use the spinothalamic tract?
- temp
- pain
- touch
Describe the spinothalamic pathway?
- receptor signal passes into SC via dorsal root ganglion and synapses in dorsal region in unilateral region
- 2nd order neurone crosses over to contralateral side and ascends on that contralateral side to the thalamus
- 3rd order neurone carries signal from thalamus to sensory cortex
Describe speed of conduction and sensitivity of signal in spinothalamic tract?
- slower
- poor spatial discrimination
- crude sensation
What receptors use the dorsal column tract?
- proprioceptor
- touch
- vibration
Describe the speed and sensitivity of signals in dorsal column?
- good spatial discrimination
- fine sensation
- faster
Describe the dorsal column pathway?
- enter SC via primary neurone
- primary neurone ascends immediately via dorsal column and synapses at dorsal column nuclei
- 2nd order neurone crosses within brainstem to thalamus and goes to sensory cortex
What receptors use the spinocerebellar tract?
proprioceptors
Describe the spinocerebellar pathway?
- 1st order neurone carries proprioceptive information from spinal cord to cerebellum via different routes
- some can cross at spinal cord and ascend contralaterally
- all terminate at cerebellum
Where will sensory signals from the CS synapse in the brain?
Sensory cortex in posterior central sulcus
What are resulting symptoms of damage to the ascending tracts?
Bilateral spinal damage (common)
- sensory loss in all modalities below lesion level
Unilateral spinal damage (rare)
- ipsilateral loss of joint sensation
- contralateral loss of temp & pain sensation
- Brown-Sequard syndrome
What is Brown-Sequard syndrome?
- damage to one side of spinal cord resulting in paralysis and loss of proprioception on ipsilateral (same) side
- and loss of pain and temperature on contralateral side
