Session 3 Flashcards
How can sensation be classified?
• Sensation can be split into the following classifications:
o General sensation, referring to the body wall and viscera (including parietal layer of serous membranes and mucosa of pharynx, nasal cavity and anus)
General sensation is subdivided into somatic sensation (conscious), and visceral sensation (usually unconscious)
o Special sensation, referring the special senses of vision, hearing, balance, taste and smell
Describe somatic sensation
o There are a variety of modalities of somatic sensation. A modality can be thought of as a ‘unit’ of sensation, relying on a distinct receptor type
Spinothalamic system / anterolateral system:
- Temperature (thermoreceptors)
- Pain (nociceptors)
- Pressure/crude touch (mechanoreceptors)
Dorsal column-medial lemniscus system:
- Vibration (mechanoreceptors)
- Proprioception, or joint position sense, or kinaesthetic sense (detected by a variety of receptors such as muscle spindles and Golgi tendon organs)
- Fine touch (mechanoreceptors)
- Two point discrimination (mechanoreceptors)
• Encoding of sensory information
o First order Primary sensory neurones (aka dorsal root ganglion neurones or primary afferents or first order sensory neurones or psueudunipolar neurones) receive information from receptors and are responsible for the initial encoding of sensory information
Each individual primary neurone receives input from a single receptor type but multiple receptors.
Primary sensory neurones have their cell body in the dorsal root ganglion, and collect information from a single dermatome along their peripheral axon
Primary sensory neurones project into the spinal cord along their central axon
o The strength of receptor activation is converted from an analogue signal (related to ion flux during the generator potential) to a digital signal (which is the frequency of action potentials in the primary sensory neurone)
Strong receptor activation causes high frequency of action potentials in the primary sensory neurone
Weak receptor activation causes a low frequency of action potentials in the primary sensory neurone
o Rapidly adapting receptors (e.g. cutaneous mechanoreceptors) respond best to changes in strength of stimulation. However, their frequency of firing diminishes rapidly after the initial stimulus (i.e. they rapidly adapt). Adaptation of these receptors explains why you are not aware of your clothes on your skin
o Slowly adapting receptors (e.g. nociceptors) change their frequency of firing very little after the initial stimulus. This explains why pain can be so persistent, and you never really get ‘used to’ having pain
What are receptive fields?
o A single primary sensory neurone supplies a given area of skin (it’s receptive field)
o If an area of skin is supplied by sensory neurones with relatively large receptive fields, this area will have low sensory acuity (it would have poor two-point discrimination where two points would need to be far apart to be distinguished). The skin of the back has relatively low acuity
o If an area of skin is supplied by sensory neurones with relatively small receptive fields, this area will have high sensory acuity (it would have great two-point discrimination where two points could be very close together to be distinguished). The skin of the fingertip has relatively high acuity
Acuity is inversely proportional to size of receptive field and dirctly proprtional to the number of sensory neurones
o The overlap of receptive fields of primary sensory neurones from adjacent dermatomes is one of the reasons why dermatomes can have ‘fuzzy’ boundaries so when examining dermatomes, we should always use the autonomous region which is the area often most central and has the least amount of overlap.
The somatosensory system – a chain of three neurones
o First order sensory neurones:
Have their cell bodies in a sensory ganglion (most commonly dorsal root ganglion)
Communicates with a receptor
Their central axon projects into the spinal cord ipsilateral to the cell body
Project onto second order neurones
(Can also be involved in the monosynaptic reflex arc and project onto motor neurones)
o Second order sensory neurones:
Have their cell bodies in the spinal cord dorsal horn or medulla
Decussate
Project onto third order neurones
o Third order neurones:
Have their cell bodies in the thalamus
Project to the primary sensory cortex (postcentral gyrus) via the internal capsule
What is somatotopy / topographical representation?
o The principle relating to the idea that for every point on the surface of the body, an equivalent point can be identified along the sensory pathway
o With some exceptions, adjacent body regions map to adjacent regions of the sensory system (e.g. in the sensory cortex, the hand is represented adjacent to the wrist)
o This way of organising the pathways minimises the amount of ‘wiring’ required to transmit sensory information
o The motor system has a similar organisation, but running in reverse
o Information becomes reorganised as we move upwards through the neuraxis, such that at the level of spinal nerves and spinal cord we have a dermatomal organisation, but at levels of the thalamus and above we have a ‘homuncular’ pattern
o At the level of the sensory homunculus, all modalities converge (i.e. the head area of the sensory cortex deals with pain, temperature, vibration etc. all at the same time)
Ignore blue line
image 3
What is the dorsal column-medial lemniscus system (DCML)?
o System responsible for carrying impulses concerning light touch, vibration, two point discrimination and proprioception
o Axons of first order neurones ascend ipsilaterally through the dorsal columns of the spinal cord (dorsal fanniculus)
o Concerning first order neurones of the DCML system:
Those from the lower body (T7 and below) ascend through the gracile fasciculus to the gracile nucleus in the medulla)
Those from the upper half of the body (T6 and above) ascend through the cuneate fasciculus to the cuneate nucleus in the medulla
o Concerning second order neurones of the DCML system:
Neurones in the gracile nucleus project to the contralateral thalamus in the medial lemniscus
Neurones in the cuneate nucleus project to the contralateral thalamus in the medial lemniscus
o Concerning third order neurones of the DCML system:
Thalamic neurones receiving information ultimately from the lower half of the body (via gracile nucleus) project to the medial part of the primary sensory cortex
Thalamic neurones receiving information ultimately from the upper half of the body (via cuneate nucleus) project to the lateral part of the primary sensory cortex
o Topographical organisation of the dorsal columns
Axons from the lower parts of the body run most medially
Axons from progressively superior body segments are added laterally to the dorsal columns
The spinothalamic pathway or anterolateral system or spinothalamic tract (STT)
o Responsible for carrying impulses concerning pain, temperature and crude touch
o Axons of first order neurones project to the ipsilateral dorsal cord, but the spinothalamic tract supplies the contralateral half of the body
o Concerning first order neurones of the STT:
They project onto second order neurones in the ipsilateral spinal cord dorsal horn in the segment at which they enter the cord through the dorsal root (generally)
o Concerning second order neurones of the STT:
Their cell bodies are in the dorsal horn
Their axons decussate in the ventral white commissure of the cord and then go on to form the spinothalamic tract
The spinothalamic tract projects to the thalamus
o Concerning third order neurones of the STT:
Thalamic neurones receiving information ultimately from more inferior parts of the body project to the medial part of the primary sensory cortex
Thalamic neurones receiving information ultimately from more superior parts of the body project to the lateral part of the primary sensory cortex
o Topographical organisation of the spinothalamic tract
Axons from the lower parts of the body run most laterally/superficially
Axons from progressively superior body segments are added medially/deeper onto the spinothalamic tract
This is the opposite of the situation for the dorsal columns, and is due to the decussation of the STT second order neurones at the level of entry of the first order neurones
What is Lissauers tract?
In the spinothalamic system, primary sensory neurones can ascend a couple segments to synapse with second order neurones via lissauers tract. This means that the spinothalamic sensory loss may be lower than the dorsal column medial lemniscus sensory loss.
What is Brown-Sequard syndrome? (limited to sensory features)
o If we consider a complete cord hemisection causing destruction of one lateral half of a single cord segment resulting from trauma or ischaemia, the following structures will be completely destroyed unilaterally:
The dorsal horn
The ventral horn
All other cord grey matter
All white matter pathways
Dorsal and ventral roots
o This will lead to the following signs (making reference to the side of the lesion): Ipsilateral complete segmental anaesthesia affecting a single dermatome (due to destruction of dorsal root and dorsal horn)
Ipsilateral loss of dorsal column modalities below the destroyed segment
Contralateral loss of spinothalamic modalities at and below the destroyed segment (although level can be up to a couple of segments lower due to ascent of some primary afferents in Lissauer’s tract)
Modulation of pain
• A fibres carry impulses from mechanoreceptors in the skin
• C fibres carry pain
Activation of mechanoreceptors alleviates pain (i.e. rubbing a painful area helps) by exciting inhibitory enkephalinergic interneurones in the cord which inhibit the second order neurone preventing pain transmission to the brain. The synapse between first and second order neurones in the dosal horn are the target region of opiates.
o Second order neurones of the spinothalamic system dealing with pain receive nociceptive primary afferents as well as inhibitory interneurones which contain the endorphin encephalin
o These encephalinergic interneurones can be activated by incoming impulses from mechanoreceptors (hence explaining why rubbing a sore area relieves the pain)
o Additionally, these encephalinergic interneurones can also be activated by descending inputs from higher centres such as the periaqueductal grey matter or the nucleus raphe magnus
