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Sensory System: Somatosensory

Terms in this set (33)

are primary sensory neurons excitatory or inhibitory?
excitatory (because they are sending a signal!)
what does it mean by axons having distinct receptive fields?
- Each nerve axon innervates a specific receptive field within its dermatome
- Receptive field sizes vary, determining precision of localisation
how is receptive field size determined?
by '2 point discrimination'
what is by '2 point discrimination'?
- Precision of sensory localisation varies greatly across the body
- Related to area of cerebral cortex devoted to each region
define dermatomes
an area of the skin that is innervated by afferent axon fibres
what does adequate action potential depend on?
the nerve ending (eg. the channels expressed by that nerve ending
what are tonic receptors?
slow/ non-adapting receptors
what are phasic receptors?
fast adapting receptors
Why are tonic receptors important?
important when maintaining info about a stimulus variable eg. amount of pain
what do tonic receptors detect?
strength of stimulus
what why are phasic receptors important?
constantly changing stimulus required. Useful when important to signal a change in stimulus also to stop paying attention to no longer important stimuli eg. tactile receptors
what do phasic receptors detect?
how fast stimulus changes
what are the 3 types of cutaneous sensory receptors?
1. Mechanoreceptors: touch, pressure, vibration
2. Thermoreceptors: hot, cold (temp.)
3. Nociceptors: noxious stimulation (pain)
describe cutaneous mechanoreceptors
• Tactile (touch) receptors at the end of Aβ fibre
• nerve ending associated with specialised sensory apparatus
• apparatus comprises a specialised cell
• structure determines function
• function indicates location
what is the function of cutaneous mechanoreceptors?
Information about surface texture, pressure and vibration
what are the 4 major types of mechanoreceptors?
• Merkel's receptor (disk) (more superficial layers of skin)
• Meissner's corpuscle (more superficial layers of skin)
• Ruffini's corpuscle (ending) (deep layers of skin)
• Pacinian corpuscle (deep layers of skin)
outline the structure and function of hair follicle receptors
- Light touch, but activation is dermis
- Rapidly adapting
- Constantly changing stimulus required
- Nerve fibre wrapping around hair
- Hair deflection detected
what are the features of thermoreceptors?
• bare nerve endings
• slowly adapting sensory receptor
• two types (in general), adequate stimulus is either warmth or cooling
• poor indicators of absolute temperature
• but very sensitive to changes in temperature
• sense of temperature comes from the comparison of the
signals from warm and cold receptors
outline the function of thermoreceptor channels (the Transient Receptor Potential family)
• non-specific cation channels
• nerve ending sensitivity dependant on which transducer channels are expressed (TRPV3/4 detects warmth, TRPM8 detects cold)
how many different transducer channels are there in total?
6
what happens when cold receptors are excited by high temps?
can cause paradoxal cold perception
what are the features of cutaneous nociceptors?
• bare nerve endings
• non-adapting sensory receptor; high threshold
• adequate stimulus must be capable of damaging tissue
what are the 2 types of nociceptor?
1. high threshold mechanoreceptors (Aδfibre): well localised 'pricking' pain
2. polymodal nociceptor: sensitive to mechanical stimulus, damaging heat (>46oC) and noxious chemicals (C fibre)(poorly localised burning pain)
what is involved in Prioprioception?
muscle spindle and Golgi tendon organ
define prioprioception
detects the mechanical status of the musculo-skeletal system
what is the purpose of prioprioception?
• proprioceptors provide information about: joint position; muscle length; muscle movement; acceleration; tension/ force
• Simple reflexes
what is the function of the muscle spindle?
detection of length and acceleration (the more stretched the muscle fibres are, the more firing occurs)
what is the function of the Golgi tendon organ?
detection of muscle tension (sensitive to tension generated by contraction)
what are the 2 central pathways?
1. Lemniscal Pathway (Dorsal Columns) (NON NOXIOUS INFO)
2. Spinothalamic Pathway (Anterolateral Tracts) (NOXIOUS INFO)
what is the Lemniscal Pathway (Dorsal Columns)?
- Large sensory A β fibres (AB fibres are thick so info is transmitted rapidly)
- Touch, vibration, two-point discrimination, proprioception
what is the Spinothalamic Pathway (Anterolateral Tracts)?
- Small sensory Aδ and C fibres
- Pain and temperature (some touch)
SAQ: Briefly describe the mechanism of adaptation of sensory responses
1. slow adapting - important to maintain info about a stimulus eg. pain:
- Merkel's receptor - detects initial indentation of skin/ light touch/ for detection of form and texture/ in epidermis around mouth/ low density in hairy skin
- Ruffini's corpuscle - responds to lateral movement or stretching of skin/ detects deeper touch and stretch/ made of collagen fibres
- thermoreceptors - bare nerve endings/ poor indicators of absolute temperature/ sensitive to change in temp.

2. fast adapting - require stimulus that is constantly changing, to detect change in stimulus, or to stop paying attention to stimuli that are no longer important:
- Meissner's corpuscle - detects light touch and vibration/ found at papillary dermis
- Pacinian corpuscle - detects deep poke or touch/ gives off vibrations/ nerve ending fully encapsulated in 'onion' structure/ onion distorted when pressure is applied/ found deep in dermis
- hair follicle receptor - nerve ending wrapped around hair/ sensitive to light touch
SAQ: describe the somatosensory system's way of perceiving temperature
- thermoreceptor channels are part of the Transient Receptor Potential family
- non-specific cation channels
- sensitivity depends on which transducer channel is expressed: TRPV3/4 = warm, TRPM8 = cold
- there are 6 types of transducer channel
- when cold receptors are excited by high temp you get paradoxal cold perception
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