Sensory pathways Flashcards
Somatosensory function
Ability to interpret bodily sensations eg: mechanical, thermal, proprioceptive, nociceptive (encoding of noxious stimuli, but sensation is not necessarily implied to be painful)
Somatosensory system components
- sensory receptors in skin, tissues and joints
- nerve cells and tracts in body and spinal cord
- brain centres that process and modulate sensory information
Major somatosensory modalities
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Sensory nerves
-individual axons of the sensory nerves have modified terminals with either free nerve endings or enclosed nerve endings
Sensory neuron classification (A-beta fibres)
- largest and fastest
- diameter (μm) and myelin coating increases speed of conduction (m/s)
- encode innocuous mechanical stimuli
Sensory neuron classification (A-delta fibres)
- smaller but myelin coating still present
- encode noxious mechanical and thermal stimuli
Sensory neuron clasification (C fibres)
- smallest diameter with no myelin covering so slowest speed of conduction
- encode noxious mechanical, thermal and chemical stimuli
Sensory neuron classification (A-alpha fibres)
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Receptor (sensory)
Transducers that convert energy from the environment into neuronal action potentials
Thermoreceptors
- sensitive to temperature because of specific membrane mechanisms
- thermoreception transduced by A delta (cold) fibres and C fibres (heat)
- encoded by free nerve endings
The 5 different types of mechanoreceptors
DIFFER IN STRUCTURE, LOCATION, SIZE OF RECEPTIVE FIELD AND THE INFORMATION THEY ENCODE
- Hair follicle receptors
- Meissner’s corpuscle encodes fine discriminative touch and low frequency vibration
- Merkel cells encode light touch and superficial pressure
- Pacinian corpuscle encodes deep pressure, high frequency vibration and tickling
- Ruffni endings encode continuous pressure or touch and stretch
Stimulus threshold
Point of intensity at which the person can just detect the presence of a stimulus 50% of the time
Stimulus intensity
- encoded by frequency of neuron firing (how quickly neurons fire)
- increased stimulus strength and duration increases the release of neurotransmitters-results in greater intensity
Adaptation
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Tonic receptors
- ‘slow adapting’
- detect continuous stimulus strength
- fire more quickly at the point of stimulation (burst) but then firing slows down and the receptors continue to transmit impulses to the brain as long as the stimulus is present
- keeps the brain constantly informed of the status of the body
Phasic receptors
- detect a change in stimulus strength
- transmit an impulse with burst at the start and burst at the end of the stimulus eg: only firing when a change is taking place
Receptive field
Region on the skin which causes activation of a single sensory neuron when activated
Small receptive fields
-allows for the detection of fine detail over a small area (precise perception)
Large receptive fields
-allows the cell to detect changes over a wide area (less precise perception)
Two point discrimination
Minimum distance at which two points are perceived as separate
-related to the size of the receptive field
Somatosensory dermatomes
-each spinal nerve ( consists of a dorsal and ventral root and passes through a notch in the vertebrae) has a specific dermatome (area of innervation) on the skin
-provides mapping system
Eg: C5 at clavicle level, C6 innervates thumb and back of index finger, T4 at nipple level, T10 at umbilicus level
Dorsal horn neurons
Two main types:
- projection neurons (those with axons that project to the brain)
- interneurons (those with axons that remain in the spinal cord)
Cell body arrangement
- cell bodies are in the dorsal root ganglia for the body
- cell bodies are in the trigeminal ganglia for the face
Lateral inhibition
- a receptive field can overlap with another receptive field->results in difficulty distinguishing between 2 stimulus locations
- lateral inhibition prevents overlap of receptive fields, facilitating pinpoint accuracy in localisation of the stimulus
- lateral inhibition is mediated by inhibitory interneurons within the dorsal horn of the spinal cord
Primary somatosensory cortex location
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Parietal association cortex location
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Dorsal column system (ascending pathway)
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The spinothalamic (anterolateral) pathway (ascending pathway)
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Key differences between dorsal column and spinothalamic tracts
Spinothalamic tract:
- pain, temperature, coarse touch stimulus
- decussation in spinal cord before ascending to the medulla oblongata
Dorsal column pathway:
- light touch, vibration and 2 point discrimination stimulus
- decussation in medulla oblongata
Anterior spinal cord lesions
- blocked anterior spinal artery causes ischaemic damage to the anterior part of the spinal cord->damages the spinothalamic tract causing pain and temperature loss below the level of the lesion
- light touch, vibration and 2 point discrimination are retained due to intact dorsal columns
Pain
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
Nociceptors
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Spinal cord nociceptive processing
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The Gate Control Theory
-inhibition of primary afferent inputs before they are transmitted to the brain through ascending pathways
Descending control pathways
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Descending inhibition
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Descending facilitation
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Nociceptive pain
Noxious stimulation of a nociceptor (somatic or viscera)
Neuropathic pain
Lesion or disease of the somatosensory system
Mixed pain
Combination of nociceptive pain and neuropathic pain
Peripheral sensitisation
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Central sensitisation
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Hyperalgesia
Increased pain from a stimulus that normally provokes pain
- Primary hyperalgesia=
- Secondary hyperalgesia=
Allodynia
Pain due to a stimulus that does not normally provoke pain
The pain matrix
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