Neurology 7 - Sensory Pathways Flashcards
What is the somatosensory cortexes function?
- The ability to interpret body sensations
- Mechanical, thermal, proprioceptive and nociceptive (not necessarily painful)
What does the function of mechanoreceptors depend on?
- Degree of specialisation (from free nerve endings to elaborate accessory structures)
- Location (in layers of skin, around hair shaft, in muscles and tendons)
- Physiological properties (activation threshold determines sensitivity (low threshold)
What is nociception?
- Provides information about unpleasant or harmful stimuli
- When this information is processed by the brain it is percieved as pain
- It can be measured
How is visceral pain transmitted?
- Carried peripherally by autonomic nerves, and centrally in the spinothalamic and dorsal columns pathways
- Localisation is mainly referred to the body wall at the same spinal level
How is somatic pain and termperature information transmitted?
Carried to the brain by the spinothalamic tract
List the characteristics of receptors involved in nociception
- Polymodal (mechanical, thermal or chemical stimulus)
- Free nerve endings
- High threshold
- Slow adapting (continue to fire impulses as long as the stimulus is present)
What is the function of taking information via the spinothalamic tract to the somatosensory I and II cortex?
Analysis of localisation and intensity of the noxious stimulus
What is the function of taking information via the spinothalamic tract to the forebrain?
Perception of pain
What is the function of taking information via the spinothalamic tract to the midbrain?
Inhibit pain
Where do the axons of the spinothalamic tract send collateral branches to?
- Brainstem (reticular formation)
- Thalamus (intralaminar nuclei)
- Hypothalamus and some cortex
- This triggers an increase in awareness and registers the unpleasantness of the stimulus
Describe central inhibition
- Descending pathway in the brainstem is triggered which inhibits the nociceptive pathway in the dorsal horn
- Pathway uses endogenous opiods and other transmitters
Describe the consequences of nociceptive dysfunction
- Reduced pain but disposed to increased injury
- Some changes may exacerbate pain (eg. windup in dorsal horn, thalamic syndrome and phantom pain)
List and define the major somatosensory modalities
- Touch (detection of light mechanical stimuli)
- Thermosensation (temperature)
- Nociception (noxious or potentially damaging stimuli)
- Proprioception (mechanical displacement of muscles and joints)
Where are free nerve endings used?
Thermoreceptors and nociceptors
Where are enclosed nerve endings used?
Mechanoreceptors
Describe the classification of sensory neurons.
- A beta are largest and fastest (mechanoreceptors of skin, touch and proprioception)
- A delta are middle size and middle speed, they convey pain and temperature
- C fibres are small and unmyelinated and therefore slow (temperature, pain and itch)
Define the term receptor
Transducers that convert energy from the environment into electrical stimulus
What type of fibres are used by thermoreceptors?
- A delta and C-fibres
- Free nerve endings
- TRP (transient receptor potential) ion channels - heat activated TRPV 1-4, cold activated TRPM8 and TRPA1
List the types of mechanoreceptors and their use
- Meissners corpuscle (fine discriminative touch, low frequency vibration - phasic)
- Merkel cells (light touch and superficial pressure - tonic)
- Pacinian corpuscle (deep pressure, high frequency vibration and tickling - phasic)
- Ruffini endings (continuous pressure or touch and stretch - tonic)
Define stimulus threshold
A threshold is the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)
Define stimulus intensity
Increase strength and duration of stimulus results in increased neurotransmitter release and greater frequency of action potential
What is the function of tonic receptors?
- Detect continuous stimulus strength
- Transmit impulses to the brain as long as the stimulus is present
- Keeps the brain informed of the status of the body (eg. Merkel cells, which slowly adapt)
What is the function of phasic receptors?
- Detect changes in stimulus strength
- Transmit impulses at the start and end of the stimulus - adapt quickly, fade in the middle
- Eg. Pacinian corpuscle, sudden pressure excites the receptor and transmits a signal again when the pressure is released.
Compare the receptive fields of the arm, fingers and back
- Arm has small receptive fields to allow detection of fine detail
- Fingers have densely packed mechanoreceptors with small receptive fields
- Large receptive fields allow the cell to detect changes over a wider area, less precise