S3) Somatic Sensation and Ascending Pathways Flashcards
What are the two types of sensation?
- General sensation
- Special sensation
What are the two types of general sensation?
- Somatic sensation (conscious)
- Visceral sensation (unconscious)
What are sensory modalities?
Sensory modalities are different forms of sensory experience e.g. pain, temperature which exists due to various types of receptors
Besides pain and temperature, identify five other sensory modalities
- Pressure (crude touch)
- Vibration
- Distension
- Proprioception (kinesthetic sense)
- Fine touch
Where are primary sensory neurones found?
The cell body of primary sensory neurones lies in the dorsal root ganglion and the axon transmits information to the dorsal horn of the spinal cord where the neurone synapses
Describe the relationship between the cell body and axon for primary sensory neurones
For primary sensory neurones, the axon runs ipsilaterally to the cell body
The modality of the stimulus is dependent on the type of receptor activated.
What are the different types of receptors?
- Nociceptor
- Mechanoreceptors
- Thermoreceptors
Different types of receptors adapt in different ways.
What is a tonic receptor?
Tonic receptors are slow adapting receptors, respond to the stimulus as long as it persists, and produce a continuous high frequency of action potentials e.g. all nociceptors
Different types of receptors adapt in different ways.
What is a phasic receptor?
- Phasic receptors are rapidly adapting receptors and respond quickly to stimuli but stop responding upon continual stimulation
- Action potential frequency decreases during prolonged stimulation and the receptor remains sensitive to a change/removal of stimulus
What is a nociceptor?
Nociceptors are tonic receptors, which respond to noxious stimuli (stimuli that would cause tissue injury if they were to persist) and result in the sensation of pain
What is a mechanoreceptor?
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion by the means of pressure, touch, vibration or stretch
What is a thermoreceptor?
Thermoreceptors are tonic receptors that respond to warmth and cold
What is a dermatome?
A dermatome is an area of skin supplied by a single spinal nerve
Where are second order sensory neurones found?
The cell body of second order sensory neurones lies in the dorsal horn of the spinal cord
Where are third order sensory neurones found?
The cell body of third order sensory neurones is located in the thalamus and the axon extends into the somatosensory cortex
What is the sensory homunculus?
The somatosensory cortex corresponds to the sensory homunculus, which is a map of brain areas dedicated to sensory processing for different anatomical divisions of the body
Describe the somatotropic organisation of the spinal cord tracts
- In the dorsal column pathway, the lower body maps to the medial portion of the tract
- In the spinothalamic tract, the lower body maps to the lateral portion of the tract
Different ascending tracts are found in specific locations in the spinal cord, and each of the ascending pathways carry information about specific sensory modalities.
Identify two ascending tracts of interest and their location
- Dorsal column-medial lemniscus (DCML)
- Spinothalamic tract
Information for which sensory modalities is relayed by the DCML?
The dorsal column-medial lemniscus relays information relating to conscious proprioception of the limbs, fine touch and vibration
Describe the significance of the following in the dorsal column pathway:
- Gracile/cuneate nucleus
- Medial leminiscus
- The cell bodies lie in the gracile/cuneate nucleus
- The fibres decussate to the contralateral side at the great sensory decussation in the medulla oblongata, and then ascend via the medial leminiscus and terminate at the thalamus
Describe and illustrate the organisation of the homunculus in the dorsal column pathway
In the sensory and motor homunculus, the lower limb is represented medially and the upper limb and face are represented laterally
Information for which sensory modalities is relayed by the spinothalamic tracts?
The spinothalamic tract relays information relating to pain, temperature, and pressure
What is the ventral white commissure and what is its significance in the spinothalamic pathway?
The axons of the tract cells decussate to the other side of the spinal cord via the anterior/ventral white commissure and to the anterolateral corner of the spinal cord
Illustrate the ascension of axons in the spinothalamic pathway
Describe and illustrate the organisation of the homunculus in the spinothalamic pathway
In the sensory and motor homunculus, the lower limb is represented laterally and the upper limb and face are represented medially
In Brown-Sequard syndrome, consider a complete cord hemisection causing destruction of a single cord segment resulting from trauma or ischaemia and identify the five structures that will be completely destroyed unilaterally
- The dorsal horn
- The ventral horn
- All other cord grey matter
- All white matter pathways
- Dorsal and ventral roots
How does Brown-Sequard syndrome present?
- 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 lesion
- Contralateral loss of spinothalamic modalities below the lesion
Describe the respective functions of A fibres and C fibres in analgesia
- A fibres carry impulses from mechanoreceptors in the skin
- C fibres carry pain
Explain how the activation of mechanoreceptors alleviates pain
Activation of mechanoreceptors alleviates pain (i.e. rubbing a painful area helps) by exciting inhibitory enkephalinergic interneurones in the cord
