Sensory & Motor Neurons Flashcards
Describe Aa sensory fibres
Largest diameter, myelinated, proprioceptors of skeletal muscle
How do sensory neurons encode stimulus intensity?
Through amplitude of receptor potential within their dendrites (EPSP) and transformation into a digital pulse code
Describe Ab sensory fibres
Large diameter, myelinated, mechanoreceptors of skin
Describe A& sensory fibres
Smaller diameter, lightly myelinated, receptors for pain and temperature
Describe C sensory fibres
Smallest diameter, unmyelinated, slowest transmission speed, receptors for temperature, pain and itch (e.g. burning sensation)
What are the types of sensory receptors?
Thermoreceptors, chemoreceptors, mechanoreceptors and photo (electromagnetic) receptors.
Nociceptors fall under all 4 categories
What are the outputs of chemoreceptors?
Chemical outputs e.g. taste, smell, CO2, O2 etc.
What is the output of photoreceptors?
Electromagnetic radiation (light)
What are the four types of temperature recognised by thermoreceptors?
Cold, cool, warm and hot
What are thermoreceptors driven by?
Transient receptor potential (TRP) channels
Cold: TRP(A)1 (cold/frigid) and TRP(M)8 (cold/cool)
Warm: TRP(V)1/TRP(V)2 (hot/burning) and TRP(V)3 (warm)
What are the three hypotheses regarding how mechanoreceptors work?
A: Direct physical stretching open channels
B: Second messenger system initiated by force detection/chemical stimuli
C: Force conveyed through structural proteins
What do mechanoreceptors respond to?
Physical distortion - touch, deformation and proprioception
What are the four types of mechanoreceptors and what do they detect?
1) Meissner corpuscle: Initial contact (motion/stroking/slippage)
2) Merkel’s disks: Pressure/texture
3) Pacinian corpuscle: Vibration
4) Ruffini endings: Skin stretch
Which mechanoreceptors adapt
A) rapidly
B) slowly
A) Meissner corpuscle and Pacinian corpuscle
B) Merkel’s disks and Ruffini endings
What are the two classes of nociceptors?
Class 1: Innervated by A& fibres, respond to sharp/prickly pain
Class 2: Innervated by C fibres, respond to dull/burning pain, not localised, poorly tolerated
What is sensory receptor adaptation?
When a continuous stimulus is applied, receptor response decreases over time until extinction
What are tonic receptors and what is their function?
Slowly adapting receptors that transmit impulses for long periods of time (hours/days) while a stimulus is present.
Keep the brain informed about the status of the body and indicate the duration of a stimulus
What are phasic receptors and what is their function?
Rapidly adapting receptors that only respond to change, indicating if there is a change in stimulus strength or a new stimulus.
Important for balance and movement
What is the dorsal column-medial lemniscal responsible for? Describe its pathway.
Touch and proprioception.
1) Signal enters through the dorsal root and synapses with interneurons in the dorsal horn
2) Signal travels in the ipsilateral dorsal column to the medulla, where it synapses with second order neurons and decussates
3) Signal travels in the medial lemniscal pathway to the thalamus and synapses with ventrobasal complex
4) Signal travels to somatosensory cortex via internal capsule
What is the anterolateral system responsible for? Describe its pathway.
Pain, itch and thermal sensation.
1) Signal enters through the dorsal root and synapses with interneurons in the dorsal horn
2) Signal decussates to the contralateral side and enters the anterolateral pathway
3) Signal travels to the thalamus and synapses with ventrobasal complex
4) Signal travels to somatosensory cortex via internal capsule
Describe the anatomy of the sensory cortex.
Located in the precentral gurus, highly organised, receives information from the opposite sides of the body and gives an unequal representation of the body
What is the medial brain stem pathway responsible for?
Posture
What is the lateral brain stem pathway responsible for?
Self-movement of distal limbs
What are the primary motor pathways?
Medial/lateral brain stem pathways, corticobulbar pathway, ventral/lateral corticospinal pathway, rubrospinal pathway
What is the corticobulbar pathway responsible for?
Facial movements
What is the corticospinal pathway responsible for?
Spinal movements
What do muscle spindles respond to?
Muscle tone and length and the rate of change in muscle tone and length
What fibres are muscle spindles composed of?
Intrafusal muscle fibres
What are the two types of intrafusal fibres?
Nuclear bag and nuclear chain
What are the two types of sensory receptor endings and what do they respond to?
Primary (Type 1A): Static tone and sudden dynamic changes
Secondary (Type 2): Static tone
How is a muscle spindle activated to increased muscle tone?
Weight added to muscle, type 1A neurons increase firing, alpha motor neurons are activated, muscle contracts, intrafusal fibres lose tension, gamma motor neurons are activated, intrafusal fibres become tense again
What do Golgi tendon organs respond to and what is their function?
Stretching of the tendon
- inhibition of anterior motor neurons
- deforms terminals of sensory receptors (opens calcium channels to allow for action potential)
Protect against excessive tension of the muscle and tendon
What fibres do alpha neurons innervate and what do they control?
Extrafusal fibres
Muscle tone and contraction
What fibres do gamma motor neurons innervate and what do they control?
Intrafusal fibres
Intrafusal tone and allow sensitivity to changes in length
What is the function of lateral inhibition and through what cells does it occur?
Used to strengthen/sharpen signals, occurring through Renshaw cells
What is an example of a monosynaptic reflex?
Stretch reflex
What is an example of a polysynaptic reflex
Pain reflex
What pathways are stimulated in a pain reflex?
Motor neurons (withdrawal reflex and reciprocal inhibition)
Reverberating circuits
Anterolateral pathway
