Learning Outcomes - Week 5 - Somatosensory System Flashcards
Be able to explain the difference between the afferent (sensory) and efferent (motor) divisions of the nervous system.
The afferent division of the nervous system is involved in the relay of information from peripheral tissues to the central nervous system. Consequently this division of the nervous system is responsible for detecting stimuli that act on these tissues (from both the internal and external environment) and relaying information about the location, intensity and quality of these stimuli to higher centres.
Because these stimuli usually give rise to a sensation (i.e. the conscious perception of the stimulus) this division of the nervous system is also known as the sensory division.
The afferent (or sensory) division can be subdivided into two major components:
1.The special senses are those arise from dedicated sensory organs such as the eye, ear and nose.
- The somatosensory system
NOTE: “The afferent or sensory division transmits impulses from peripheral organs to the CNS. The efferent or motor division transmits impulses from the CNS out to the peripheral organs to cause an effect or action.”
Have an understanding of what the somatosensory system is and how it is different from the special senses
The somatosensory system is concerned with sensations that arise from tissues other than specialised sense organs. These include the skin, viscera, muscles and joints and can be conveniently subdivided into three major components:
- Cutaneous Sensation - sensations that arise from the skin.
- Interoreception - sensations that arise from the viscera, muscles and joints that are NOT related to movement.
- Proprioception - sensations that arise from skeletal muscle and joints that ARE related to movement.
Know the basic structural components of the somatosensory system and where these are located in both the peripheral and central nervous system
(Label and complete diagram)
The basic organisation of the somatosensory system is summarised in the diagram below. Peripheral tissues are effectively connected to the cerebral cortex by a three neurone (disynaptic pathway). Activation of the primary sensory neurone (sometimes confusingly referred to as a receptor) by a stimulus produces action potentials that travel through this pathway and eventually reach the cerebral cortex where the stimulus is consciously perceived
Be able to explain what is meant by the terms transduction, frequency encoding, receptive field, innervation density and adaptation and be able to illustrate each of these with a physiological example
Check all individual cards for this and assess your overall knowledge with this card
Understand the difference between the seven sensations that arise from stimulation of the skin and the types of stimuli that elicit them
Humans are able to discriminate at least three major classes of sensation from stimulation of their skin:
A. Low Intensity Mechanical Stimuli
The low intensity (i.e. non-painful) mechanical stimuli are responsible for three distinct sensations:
pressure - the degree of skin indentation.
touch - the rate at which a skin indentation is applied.
vibration - the frequency of a vibratory stimulus.
These sensations are referred to collectively as mechanoreception.
B. Low Intensity Thermal Stimuli
The two sensations that can be produced by the small changes in skin temperature that occur as part of daily life are known as cold and warm. These are low intensity in the sense that they do not produce damage to the skin and are non-painful. Collectively these sensations are known as thermoreception.
C. High Intensity (Painful) Stimuli
Stimuli with sufficiently high intensity to produce damage to the skin produce the sensation of pain. It is well established that there are two distinct pain sensations and these can be clearly demonstrated by a mechanical injury such as that associated by a paper cut or pin prick. Very shortly after a mechanical injury we experience a pain that has a ‘sharp’ quality. A few seconds later we experience a second pain sensation that is usually described as being ‘burning’. We can also elicit the burning pain by high temperatures and by the application of pain-producing chemicals (such as acid). So there are two distinct pain sensations (span sharp and burning) that differ in their quality. Pain sensations are known collectively as nociception.
Understand that each cutaneous sensation is the result of…?
Activation of a specific class of primary sensory neurone and that information about these stimuli reaches the cerebral cortex through a dedicated projection pathway
What are the three for low threshold mechanoreceptors and what are the three mechanoreceptor pathways?
Low threshold mechanoreceptors
- Pressure
- Touch
- Vibration
Mechanoreception projection pathways:
Neurones involved:
- Primary Sensory Neurones
- Second-order Neurones
- Third-order Neurones
Know what Von Frey hairs are and how you can use these to measure the relative innervation density for different skin regions
Definition. Von Frey hairs (named after the German physiologist Max von Frey, 1852–1932) have been originally produced from animal and human hairs of different diameters. Nowadays they are nylon monofilaments of different diameters, each of them mounted at right angles to the end of a plastic handle.
Force thresholds of the 4 types of mechanoreceptive units in the glabrous skin area of the. human hand as measured with von Frey hairs
The device can be used to stimulate either the skin or dura mater and consists of a solenoid-driven plunger to which are fixed interchangeable von Frey hairs.
Be able to demonstrate the two-point discrimination test and be able to explain how this test can be used to assess innervations density
The two-point discrimination test is used to assess if the patient is able to identify two close points on a small area of skin, and how fine the ability to discriminate this are. It is a measure of tactile agnosia, or the inability to recognize these two points despite intact cutaneous sensation and proprioception.
Be able to explain the structural and functional aspects of the mechanoreception projection pathway.
Draw the pathway.
Where does it cross midline?
In the section above we have seen that there are three major classes of low threshold mechanoreceptors that encode information about different aspects of low intensity mechanical stimulation of the skin. All three classes of mechanoreceptor have large diameter myelinated axons. Consequently these are able to conduction action potential very rapidly (30-70 m.sec-1). But how does the information they carry reach the cerebral cortex and thereby elicit a sensation?
The pathways for all three types of neurones are essentially the same so we will consider them together.
Be able to describe the physiological properties of the two classes of primary sensory neurone responsible for thermoreception and explain the type of information they encode
Cold and warm sensations are enabled by the presence of two distinct classes of temperature-sensitive primary sensory neurones known as thermoreceptors.
A. Thermoreceptors
(i) Cold Receptors.
This class of primary sensory neurone is spontaneously active at normal environmental skin temperatures and shows a fairly linear increase in action potential frequency as the skin temperature decreases (see opposite).
These neurones typically have small diameter myelinated axons and consequently conduct action potentials in the range of 12 - 30 m.sec-1.
The functional properties of these neurones clearly indicates that they are able to reliably encode a decline in skin temperature (i.e. skin cooling).
Unlike low threshold mechanoreceptors the peripheral endings of these neurones have no specialised receptor endings. Instead the axons simply terminate blindly amongst the cells of the skin. These types of peripheral endings are known as free nerve endings to reflect their simplicity.
(ii) Warm Receptors
The second class of thermoreceptors present in human skin are also spontaneously active but their action potential frequency increases as skin temperature increases (see opposite).
Warm receptors have small diameter unmyelinated axons so conduction action potentials fairly slowly (0.5 - 2.5 m.sec-1). Like cold receptors these neurones have free nerve endings in the skin.
Clearly these neurones are ideally suited to encode information about increases in skin temperature in the normal (i.e non-painful) range.
Be able to explain the structural and functional aspects of the thermoreception projection pathway.
Draw the pathway
B. Thermoreception Projection Pathway
As we have seen above, there are two classes of primary sensory neurones that encode information about low threshold changes in skin temperature. In this section we will see how this information reaches the cerebral cortex.
(i) Primary Sensory Neurones
The small diameter axons of thermoreceptors project through peripheral nerves and enter into the spinal cord through the dorsal roots. These axons enter into the grey matter of the dorsal (posterior) horn of the spinal cord where they form axodendritic synapses with the second-order neurones.
(ii) Second-order Neurones
The second-order neurones of the thermoreceptive pathway have their cell bodies with the dorsal (posterior) horn of the grey matter of the spinal cord. Their axons project down and across the midline underneath the central canal and enter into the white matter on the contralateral (opposite) side of the spinal cord. Because of its anatomical position midway between the lateral and ventral white matter this region is known anatomically as the ventrolateral funiculus. Physiologically these neurones form part of a projection pathway known as the spinothalamic tract because the axon terminals of these second-order neurones project out of the spinal cord through the brainstem and terminate with the ventrobasal complex of the thalamus where they synapse with the third-order neurones.
(iii) Third-order Neurones
The cell bodies of the third-order neurones of the thermoreceptive pathway are located in the ventrobasal complex of the thalamus. These third-order neurones have axons that project up into parietal lobe of the cerebral cortex and terminate within the postcentral gyrus (i.e. somatosensory cortex) where the conscious perception of either skin cooling or warming occurs.
Note that just like mechanoreception, the thermoreceptive pathway crosses the midline. Consequently thermal stimuli reach consciousness in the parietal lobe on the opposite side of the body from where the stimulus occurs. The major difference is that in the thermoreceptive pathway the pathway crosses the midline at the level of the spinal cord.
Understand the thermal paradox and how the physiological responses of thermoreceptors can explain it
Data on human warm fibers have been described in several papers.27,37,38,84 They are mechano-insensitive and have small innervation territories. They are activated by moderate warming, but may also encode increasing temperature into the noxious range. Their low number and small receptive fields result in a sparse innervation for warmth. This may explain early impairment of warmth detection in peripheral neuropathy as compared to heat-pain thresholds, which may increase at a later stage of the disease.
The phenomenon of paradoxical hot sensation upon mild cooling under a differential A-fiber block has provided evidence for cold-specific C fibers.81 Also, the existence of cold-sensitive C fibers has been suggested as the explanation of a heat-pain illusion occurring on simultaneous stimulation with non-noxious warm and cold (thermal grill illusion,11,12 Thunberg effect83). Recently, recordings of C fibers responsive to mild cooling have been identified in humans.8 Their activation thresholds were about 29 °C, which is compatible with a role of this class of C fibers in the paradoxical hot sensation. Interestingly, not only did these fibers differ in their receptive properties, but their axonal characteristics also clearly distinguished them from C nociceptors. Activity-dependent hyperpolarization of axons, which leads to slower conduction velocities, was much less pronounced in C-cold fibers as compared to the nociceptors
Be able to describe the physiological properties of the two classes of nociceptor and explain the type of information they encode
Any stimulus applied to the skin that is sufficiently intense to cause damage usually results in the sensation of pain. As was alluded to earlier in this lesson, there are two types of pain sensation; burning and sharp. Thermal and chemical stimuli produce a sensation that has a burning quality. High intensity mechanical stimuli initially produce pain that has a sharp quality, but this is followed a few seconds later by burning pain. Because of the different temporal aspects of pain produced by a mechanical injury we usually describe these as fast-sharp pain and slow-burning pain.
These two different pain sensations are the result of two distinct classes of pain signalling primary sensory neurones known as nociceptors.
A. Nociceptors
(i) High Threshold Mechanoreceptors
As its name suggests, this class of nociceptor is selectively activated by high intensity mechanical stimuli such as a pin-prick, laceration or pinch . In fact any mechanical stimulus that has the potential to damage the skin will elicit action potentials in this class of neurone.
High threshold mechanoreceptors are NOT activated by either high temperatures or pain-producing chemicals 0.
Morphological analysis of this class of nociceptor has revealed that they don’t have specialised receptors (i.e. they have free nerve endings) and that they have small diameter myelinated axons. Because of this myelin sheath they conduct action potentials in the range of 12 - 30 m.sec-1.
High threshold mechanoreceptors are responsible for the fast-sharp pain associated with a mechanical injury.
(ii) Polymodal Nociceptors
This class of nociceptor is activated by high intensity thermal, chemical and mechanical stimuli (hence the name ‘polymodal’). Like high threshold mechanoreceptors they are stimulated by high intensity mechanical stimuli (pin-prick, laceration or pinch) . In addition they are activated by temperatures greater than 45 oC and a range of pain-producing chemicals (including acid, histamine, bradykinin and a whole host or irritants found in stinging plants and animals) 0.
Polymodal nociceptors have small diameter unmyelinated axons (so they have conduction velocities in the range 0.5 - 2.5 m.sec-1) and free nerve endings in the periphery.
Polymodal nociceptors are responsible for the burning pain associated with a thermal or chemical injury.
These neurones are also activated by high intensity mechanical stimuli but have conduction velocities that are much slower than high threshold mechanoreceptors. As a result, information about a mechanical injury encoded by these neurones reaches the central nervous system a lot slower than that encoded by high threshold mechanoreceptors. Consequently these neurones are responsible for the slow-burning pain associated with a mechanical injury.
Be able to explain why a high intensity mechanical stimulus such as a paper cut initially produces pain that has a sharp quality but is followed a few seconds later by pain that is described as having a burning quality
Morphological analysis of this class of nociceptor has revealed that they don’t have specialised receptors (i.e. they have free nerve endings) and that they have small diameter myelinated axons. Because of this myelin sheath they conduct action potentials in the range of 12 - 30 m.sec-1.
High threshold mechanoreceptors are responsible for the fast-sharp pain associated with a mechanical injury
This results in the pain sensation reaching your brain a lot faster than polymodal nocireceptors which are responsible for the burning pain associated with a thermal or chemical injury.
These neurones are also activated by high intensity mechanical stimuli but have conduction velocities that are much slower than high threshold mechanoreceptors. As a result, information about a mechanical injury encoded by these neurones reaches the central nervous system a lot slower than that encoded by high threshold mechanoreceptors. Consequently these neurones are responsible for the slow-burning pain associated with a mechanical injury.
Polymodal nociceptors have small diameter unmyelinated axons (so they have conduction velocities in the range 0.5 - 2.5 m.sec-1) and free nerve endings in the periphery.
This results in the pain sensation reaching your brain slower than that produced from high threshold mechanoreceptors
Be able to describe the structural and functional aspects of the nociception projection pathway
B. Nociception Projection Pathway
In the skin there are two classes of primary sensory neurones that encode information about high intensity stimuli applied to the skin. In many respects the projection pathway for nociception is the same as that already described for thermoreception, except of course in for the types of stimuli that activate neurones in this pathway.
(i) Nociceptors
The small diameter axons of high threshold mechanoreceptors and polymodal nociceptors enter into the spinal cord through the dorsal roots. These axons enter into the grey matter of the dorsal (posterior) horn of the spinal cord where they form axodendritic synapses with the second-order neurones.
(ii) Second-order Neurones
The second-order neurones of the nociceptive pathway have their cell bodies with the dorsal (posterior) horn of the grey matter of the spinal cord. Their axons project down and across the midline underneath the central canal and enter into the white matter on the contralateral ventrolateral funiculus. Together with the neurones of the thermoreceptive pathway these neurones form the spinothalamic tract. The axon terminals of these second-order neurones project out of the spinal cord and terminate with the ventrobasal complex of the thalamus where they synapse with the third-order neurones.
(iii) Third-order Neurones
The cell bodies of the third-order neurones of the nociceptive pathway are located in the ventrobasal complex of the thalamus. These third-order neurones have axons that project up into parietal lobe of the cerebral cortex and terminate within the postcentral gyrus (i.e. somatosensory cortex) where the conscious perception of either sharp or burning pain occurs.
Explain transduction and illustrate with example
Transduction is the mechanism by which stimulation of a peripheral tissue induces action potentials in the primary sensory neurone (receptor) .
Although the molecular mechanism of transduction for some stimuli has yet to be established, in most cases the stimulus appears to evoke a depolarising graded potential (known as a generator potential) in the primary sensory neurone .
If the generator potential is large enough to reach threshold then action potentials are produced in the primary sensory neurone .
The generator potential appears to involve the opening of stretch-gated ion channels for mechanical stimuli and ligand-gated ion channels when chemical stimuli are involved. More recently some temperature-gated ion channels have been identified that appear to be responsible for the generator potentials associated with changes in temperature.
Explain frequency encoding and illustrate with example
Like the rest of the nervous system, the somatosensory system uses the principle of frequency coding to communicate the size of peripheral stimuli .
So a small stimulus produces a low frequency (i.e. a small number of action potentials per second) response in the primary sensory neurone.
A higher intensity stimulus produces a higher frequency response.
These different frequency responses are relayed through the remaining two neurones in the pathway and are interpreted by the cerebral cortex as stimuli of differing intensities.
Explain receptive field and illustrate with example
If you record the membrane potential of a primary sensory neurone and then stimulate the peripheral tissue that it innervates you will eventually locate the region of that tissue that produces action potentials in that neurone.
This region is referred to as the neurone’s receptive field.
Using your cursor scan the peripheral tissue opposite and see of you can identify the receptive fields of three different neurones.
The receptive field of a neurone is quite closely related to the extent of the axon terminals of the primary sensory neurones. Interestingly however the size of receptive fields varies quite significantly in different parts of the body.
Describe adaptation and illustrate with example
The term adaptation refers to the way in which a primary sensory neurone responds to a sustained stimulus .
In response to a sustained stimulus (e.g. skin deformation) some neurones show little change in their action potential frequency until the stimulus is removed . These types of neurones exhibit very little adaptation so are said to be are said to be slowly adapting.
Other neurones stop responding after a few seconds of a sustained response and are said to be rapidly adapting .
Describe low threshold mechanoreceptors - pressure:
Humans are very good at detecting very subtle differences in indentations of their skin. This is enabled by a class of primary sensory neurone that responds in a sustained fashion to skin indentation such that their action potential frequency is directly proportion to the magnitude of the stimulus .
In these neurones, small indentations 0 produce a low frequency of action potentials and progressively larger indentations 0 produce a higher frequency response.
Because of their slowly adapting response to a sustained stimulus these neurones are referred to as slowly adapting mechanoreceptors. Clearly these neurones are well suited for encoding the duration and magnitude of skin indentation.
Slowly adapting mechanoreceptors have specialised receptor endings associated with each of the branches of their axons in the skin. There are two major classes of receptor endings associated with slowly adapting mechanoreceptors. These are known as Merkel’s corpuscles and Ruffini’s endings after the microscopists who first identified them.
Describe low threshold mechanoreceptors - pressure:
Humans are very good at detecting very subtle differences in indentations of their skin. This is enabled by a class of primary sensory neurone that responds in a sustained fashion to skin indentation such that their action potential frequency is directly proportion to the magnitude of the stimulus .
In these neurones, small indentations 0 produce a low frequency of action potentials and progressively larger indentations 0 produce a higher frequency response.
Because of their slowly adapting response to a sustained stimulus these neurones are referred to as slowly adapting mechanoreceptors. Clearly these neurones are well suited for encoding the duration and magnitude of skin indentation.
Slowly adapting mechanoreceptors have specialised receptor endings associated with each of the branches of their axons in the skin. There are two major classes of receptor endings associated with slowly adapting mechanoreceptors. These are known as Merkel’s corpuscles and Ruffini’s endings after the microscopists who first identified them.
Describe low threshold mechanoreceptors - touch
Psychophysical studies in humans have also revealed that we are able to detect very subtle differences in the rate of application of a skin indentation. This is because we have a population of primary sensory neurones that are selectively sensitive to the speed at which a stimulus is applied .
Whilst a relatively slowly applied indentation produces a low frequency of action potentials, this increases in a linear fashion as the rate of application increases . Note that these neurones do not continue to respond throughout the skin indentation and so are referred to as rapidly adapting mechanoreceptors.
Clearly these neurones can provide little information about the duration of a skin deformation (because action potentials cease before the stimulus is removed) but they are very well suited to encode the rate at which a mechanical stimulus is applied.
In glabrous (hairless) skin rapidly adapting mechanoreceptors have specialised receptor endings known as Meissner’s corpuscles. In hairy skin these neurones innervate hair follicles and are responsible for detecting the rate of hair movement.
Describe low threshold mechanoreceptors - vibration
Most primates are very sensitive to vibration and can discriminate between very small differences in vibration frequency in the range of 50 to 300 Hz. Vibration is in effect a cyclical skin indentation and encoding of this stimulus is enabled by a population of very rapidly adapting mechanoreceptors.
These neurones adapt so quickly that only one action potential is produced in response to each skin indentation . Clearly these neurones can provide very little information about the magnitude, duration or rate of application of a skin indentation. However because each phase of the vibration produces a single action potential, these neurones can distinguish between different vibration frequencies quite readily.
Like the other classes of mechanoreceptors, very rapidly adapting mechanoreceptors have specialised receptor endings known as Pacinian corpuscles. These corpuscles can be up to 2 mm long and 1 mm wide and consist of concentric layers of connective tissue surrounding the axonal ending.
Be able to describe the physiological properties of the three classes of primary sensory neurones responsible for mechanoreception and explain the type of information they encode:
Primary Sensory Neurones:
The large diameter myelinated axons of low threshold mechanoreceptors course through the peripheral nervous system and enter into the spinal cord through the dorsal roots. These axons take up a position in the white matter on the dorsal (posterior) aspect of the spinal cord just lateral to the midline. These structures run the length of the spinal cord and are known as the dorsal columns. The axons of these mechanoreceptors project through the spinal cord in the dorsal columns and synapse with the second-order neurones on the posterior surface of the medulla oblongata in structures known as the dorsal column nuclei.
What is the homunculus?
The end result of this convergence and organised pattern of terminations is that within somatosensory cortex there is a precise map of the body surface (see opposite) . In effect this means that if you recorded from adjacent neurones in somatosensory cortex they would have adjacent receptive fields on the skin. This map is referred to as a homunculus and is shown opposite.
