Flashcards in Sensory pathways; touch and proprioception Deck (34):
Which receptors detect 'touch and proprioception', temperature and pain?
Touch and proprioception'; Mechanoreceptors
What are mechanoreceptors?
They are modified terminals of the peripheral aeon's of the primary sensory neurones. They transduce a mechincal stimulus into an electrical.
What pathway transmits info for touch and propriception and what are the stimuli?
Dorsal columns/medial lemniscus
Touch; light touch, pressure, vibration
Proprioception; mucche tension, muscle length, joint position
What are the different types of mechanoreceptor endings?
Complex; peritrichial ending, merkel ending in epidermis, ruffini ending, Pacinian corpuscle, Meissner's corpuscle
Use of Pacinian corpuscles
Formed of concentric rings of modified epithelial cells which make them more sensitive to vibrations
Use of Ruffini endings
Sensitive to stretch
Use of Meissner's corpuscles
Sensitive to light touch
What determines the function of the mechanoreceptors?
Degree of specialisation
Physiological properties e.g. activation threshold
What is the difference between slow and fast adapting fibres?
Slow adapting: fire as long as the stimulus is present but frequency may decline over time
Fast adapting: fire at high frequency when the stimulus is first applied (fires at start, end or both)
Describe the properties of mechanoreceptors
Low activation threshold
Large and fast conducting
What is an early sign of neurodegeneration in terms of vibrations?
Elevation of vibratory threshold
What are the main receptors for proprioception?
Muscle spindles; detects changes in muscle length
Golgi tendon organs; detects changes in muscle tension
What type of axons do receptors for proprioception have?
Innervated by Type 1 axons
What type of axons do mechanorecptors for touch have?
Type II/Aβ axons
Does the amplitude or frequency matter in intensity of the stimulus?
Frequency of AP generated
[+ number of adjacent receptors stimulated i.e. recruitment]
Amplitude does NOT change.
What is meant by receptive field?
The number of receptors innervated by one sensory nerve/ area from which a stimulus elicits a neuronal response.
What would you expect the receptive field to be like in a sensitive part of the body?
Small receptive fields, High density innervation = better acuity
What is lateral inhibition?
Activation of one neural unit inhibits the activation of others.
Mediated by inhibitory interneurones, takes place in the gracile and cuneate nucleus.
Allows for better sensory perception.
What is two point discrimination and two-point threshold?
The ability to distinguish that two stimuli are distinct from each other.
Minimum distance between two stimuli required in order to perceive they are two distinct stimuli.
What does two point discrimination depend on and is related to?
Depends on: peripheral mechanoreceptors, posterior spinal column, cortical function
Relates to: density of innervation, area of receptive field, sensory homunculus
What is meant by neural adaptation and what fibres do phasic and tonic receptors have?
It is a form of desensitisation, when a stimulus is maintained at a constant strength for a period of time, firing rate decreases.
Phasic receptors; rapidly adapting
Tonic receptors; slow adapting
Describe the pathway of sensory information from the body
Information comes in and reaches the dorsal columns.
Info from leg down: medial gracile fasciculus (sacral and lumbar)
Info from waist up: lateral cuneate fasciculus (thoracic and cervical)
Information travels up and in the medulla it synapses with either the cuneate or gracile nuclei.
They then send their axons across the midline/decussation to form a new pathway; the median lemniscus -> thalamus -> vental-posterolateral nucleus -> tertiary neurones -> internal capsule -> primary somatosensory cortex
What are the functions of the different nerve fibres; Aa, Ab, Ag, Ad, B, C fibres?
Aa; proprioception, somatic motor
Ab; light touch, pressure
Ag; motor to muscle spindle
Ad; pain, cold, touch
B; Postganglionic autonomics
C; pain, temperature, mechanoception
Which thalamic neurones have an important role in nociceptive modulation?
Ventrobasal and nucleus reticularis
What is the function of cortical neurones?
Perception and interpretation of pain
Describe the pathway of sensory information from the head
Main sensory nerve is trigeminal nerve (CNV)
Trigeminal nerve enters at the pons and synapses with the second order trigeminal cranial nucleus.
The axon of this crosses the midline and joins the medial end of the medial lemniscus.
They relay through the ventral posteromedial nucleus and the axon of the third neurone relays to the primary somatosensory cortex.
Where does the ascending spinothalamic pathway decussate?
Decussation in the spinal cord level of which it enters via interneurones
What is descending modulation?
Descending pathways from the brainstem can have excitatory or inhibitory influence on spinal nociceptive transmission.
Mediated by spinal 5HT, GABA, Glycine, NA
Loss of physiological inhibition may result in pathological hypersensitivity
Where is the location of the primary and secondary somatosensory cortices?
S1: postcentral gyrus
S2: receives intracortical projections from S1, located inferoposteriorly to i, bounded by the lateral fissure
What is the function of the posterior parietal cortex?
Interprets spatial relationships
What do the cortical neurones in the primary somatosensory cortex analyse?
Abstract properties e.g. movement
Stimulus properties e.g. pressure, vibration
What is the clinical implication of damage to the dorsal columns?
Anaesthesia (loss of sensation)
Paraesthesia (change in sensation)
What are some causes of peripheral neuropathy?
Some hereditary diseases e.g. Charcot Marie-tooth disease