Sensory pathways: somatosensory system Flashcards
What can we use to salvage limbs
Free tissue transfer
What is important to remember about noicieptors
they encode noxious stimuli- not just pain
Summarise what is meant by somatosensory function
ability to interpret bodily sensations i.e. Mechanical, thermal, proprioceptive, nociceptive (noxious, not necessarily painful) - system consists of sensory receptors in skin/tissues/joints, nerve cells/tracts, and brain centres that process and modulate sensory information
These include all the conscious senses other than vision, hearing, balance, sense and smell.
Gives you a lot of information- can identify where your body is touched, amount of pressure, whether it’s sharp or a threat- without looking at it
List the main sensory modalities
mechanical - touch (light mechanical stimuli)
thermal- detection of temperature
proprioceptive- detection of the mechanical displacement of muscles and joints- positional sense
nociceptive - noxious or potentially damaging stimuli
What does the somatosensory system consist of
sensory receptors in skin, tissues & joints
nerve cells & tracts in body & spinal cord
brain centres that process & modulate sensory information
What re each of the senses referred to as
Modality, and modality refers to the type of information encoded
What is important to remember about sensory neurones
‘’Individual axons of sensory nerves have modified terminals’’
Describe some of the modifications of sensory neurones
Free nerve endings:
thermoreceptors &
nociceptors
Enclosed nerve endings:
mechanoreceptors
- What receptors are responsible for the sensory modalities of touch and proprioception?
Mechanoreceptors
How do sensory neurones vary in their properties
size and conduction velocity
What are the classifications of sensory neurones based on
Anatomical = based on axon diameter (labelled using LETTERS)
Physiological = based on conduction velocity (labelled using ROMAN NUMERALS)
As axon diameter and conduction velocity are related, there is a lot of overlap in the classifications
Describe A- alpha fibres (group 1)
Large diameter (13-20 micrometres)
Fast conduction velocity ( 80-120m/sec)
Proprioceptors of skeletal muscle, also somatic motor
Myelinated
Describe A-beta fibres (group 2)
Medium diameter (6-12 micrometres)
Medium conduction velocity (35-75 m/secs)
mechanoreceptors of skin- innocuous mechanical stimulation
Myelinated
Describe A-delta fibres (group 3)
Small diameter ( 1-5 micrometers)
Relatively slow conduction velocity (5-30 m/secs)
Myelinated
noxious mechanical (pain) and thermal stimulation
Describe C fibres (group 4)
Small diameter (0.2-1.5 micrometres) Slow conduction velocity (0.5-2 m/secs) Unmyelinated noxious mechanical, thermal and chemical stimulation itches too
Define the term ‘receptor’
“sensory receptors are transducers that convert energy from the environment into neuronal action potentials
Describe the activation of receptors
The receptor membrane depolarizes in response to its modality stimulus, causing a generator potential. If sufficient, this causes the axon to depolarize to its threshold level and produce an action potential. Because the axon recovers after its refractory period, a long-lasting generator potential will cause the axon to fire a train of impulses whose frequency will be proportional to the magnitude of the generator potential. Essentially, all stimuli are encoded as analogue signals and the sensory systems function as analogue-digital ‘translators’.
What is the ultimate role of thermoreceptors
To allow us to detect small changes in temperature both in our environment and inside our bodies
What is important to remember about thermoception
Thermoreceptors are not evenly distributed throughout the body
One area may be sensitive to cold- but an adjacent area sensitive to heat
Temperature detection is most sensitive on the face and chest.
What does the sensitivity of the thermoreceptor depend on
Depends on the ion channel type Transient receptor potential (TRP) ion channels 4 heat activated: TRPV1-4 (also capsaicin), 3 -camphor 2 cold activated TRPM8 ( mint) TRPA1 (horseradish, garlic, cinnamon)
At what temperature if TRPV1 activated by
43 degrees Celsius- described as painful in psychosis clinical experiments
What can also activate cold ion channels
Wasabi- 15 degrees Celsius
What is important to remember about these agents that can bind to thermoreceptors and describe their potential in clinical use
They bind to ion channels for a long period of time
Why the spice sensation lasts for a while
Decreasing the ability for other stimuli- to have a painful response- so can be used as a topical analgesic
Describe the thresholds for temperature pain
<32 degrees- painful cold
32- neutral
37-43- warmth
>43 -painful hot
Summarise the thermoreceptors
Aδ- (cold) and C-fibres (heat)
Free nerve endings
How many mechanoreceptors are there and how do they differ
5
Structure, size of receptive fields, tactile information they transmit and location
What are the different types of mechanoreceptors
Meissner’s Corpuscle: fine touch discrimination and low frequency vibration
Merkel cells: light touch and superficial pressure
Pacinian Corpuscle: deep pressure and high frequency vibration (putting hand on speaker) and tickling
Ruffini endings: continuous pressure/touch and stretch- monofilament can squash the tissue underneath the finger and activate these receptors
Hair follicles- Light touch (hairy skin)
What type of axons are the mechanoreceptors
A-beta
What type of axons are the noicieptors
Sharp pain (Pin prick)- A-delta Thermal (cold)-A-delta
Dull pain, ache- C
Thermal (heat)- C
What is meant by the 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)’’
When a subliminal stimuli is applied- use monofilament- apply more pressure each time
Summarise what is meant by stimulus threshold
Stimulus threshold – described as the weakest stimulus detectable;
o Also known as – adequate stimulus required to elicit a specific response or reflex.
o Also known as – the minimum stimulus that is detected >50% of the time.
o Varies in location to anatomical location and with individual (personal) differences.
What is meant by stimulus intensity
How quickly the neurone fires
increased strength and duration of the stimulus leads to greater NT release and greater intensity - encoded by frequency of neurone firing
timulus intensity discrimination:
o Information regarding stimulus intensity is conveyed by a variation of:
Frequencies of APs generated.
Number of separate receptors activated (recruitment).
o The final relationship between stimulus intensity and ultimate sensory discrimination may be linear or logarithmic.
- What is the difference between slow adapting and fast adapting receptors?
Slow adapting receptors continue firing impulses for as long as the stimulus is present
Fast adapting receptors tend to fire at the start of the stimulus and sometimes when the stimulus switches off but they tend to fade in the middle
Describe how the adapting fibres work together
These two categories work in harmony to send different information about the same stimulus. The different signalling depends either on the linkage of the receptor to its incident energy or on a property called adaptation (i.e. a decline in receptor responsiveness even though the stimulus is still present). As a general rule, slowly adapting receptors signal the magnitude or location of a stimulus, whereas rapidly adapting receptors signal its rate of change and duration.
Essentially, what does the term ‘adaptation’ refer to
ability of a receptor to stop firing while a stimulus is in place
Summarise tonic receptors
Detect continuous stimulus strength
Continue to transmit impulses to the brain as long the stimulus is present
Keeps the brain constantly informed of the status of the body
e.g. Merkel cells
Slowly adapt allowing for superficial pressure and fine touch to be perceived.
Tonic receptors – do not adapt or adapt very slowly
Give an example of tonic receptors in use
Sitting on a chair- only notice it when you first sit on it- but tonic receptors are continuously relaying information to your brain without you realising it- like a backup on a computer checking for viruses and ensuring that everything is working properly
Summarise phasic receptors
Detect a change in stimulus strength
Transmit an impulse at the start and the end of the stimulus
e.g. when a change is taking place
The pacinian receptor
Sudden pressure excites receptor
Transmits signal again when pressure is released
Adapt quickly
Describe the passage of stimulus activation
Stimulus
Receptor potential generated
Action potentials in sensory neurone
Summarise cutaneous thermoreceptors
Thermoreceptors are slowly adapting and are poor indicators of absolute temperature. Cold receptors increase their discharge frequency in response to decreasing temperature whereas warm receptors increase their discharge frequency in response to increasing temperature
Summarise what is meant by the term receptive field
The receptive field is the region on the skin which causes activation of a single sensory neuron when activated’’
Largely a function of the branching characteristics of the afferent within the skin; smaller arborisations result in smaller receptive fields.
Compare the small and large receptive fields
Small receptive fields allow for the detection of fine detail over a small area. Precise perception
Large receptive fields allow the cell to detect changes over a wider area (less precise perception)
Describe the regional differences in receptive fields
The fingers have many densely packed mechanoreceptors with small receptive fields
There are systematic regional variations in the average size of afferent receptive fields that reflect the density of afferent fibres supplying the area.
The receptive fields in the areas with dense innervation (fingers, lips, toes) are relatively small compared to those in the forearm or back, that are innervated by a smaller number of afferent fibres.
Explain what is meant by two-point discrimination
Minimum distance at which two points are perceived as separate
Related to the size of the receptive field (region on the skin which causes activation of a single sensory neuron)
Regional differences in receptive field size and innervation density are the major factors which limit the spatial accuracy with which tactile information can be sensed. Thus, measures of two-point discrimination-the minimum inter stimulus distance required to perceive two stlmmutaneously applied stimuli as distinct- vary dramatically across the skin surface
Summarise two-point discrimination
The ability to detect that two stimuli are distinct from each other.
Two-point Threshold – the minimum distance required between 2 stimuli in order to perceive that they are two separate stimuli.
Depends on – peripheral mechanoreceptors, spinal posterior column and cortical function.
Magnitude varies – back is 65mm, fingers are 2mm.
Related to – density of innervation, area of receptive field and sensory homunculus.
Summarise somatosensory dermatomes
Each spinal nerve has a specific dermatome on the skin
Mapping system to localise sensation
C5- Clavicles
T4- nipples
T10 -umbilicus
Where are the key cell bodies of sensory neurones found
dorsal root ganglia (body) trigeminal ganglia (face)
Describe the pathway of sensory information from the face
Cutaneous mechanoreceptor information from the face is conveyed centrally by a separate set of first-order neurones that are located in the trigeminal ganglion at the level of the pons
Corresponds to the dorsal column nuclei
Decussate at the pons
Second-order neurones cross the midline and ascend to the ventral posterior medial nucleus of the thalamus by way of the trigeminal leminiscus
Neurones in the VPM, send their axons to the ipsilateral cortical areas SI and SIII
