Touch, Dermatomes and Referred Pain Flashcards
What are Somatic Sensations?
Sensation from the skin/bones/tendons/joints
- Initiated by a variety of sensory (somatic) receptors
- Receptors are distributed throughout the body
- Respond to multiple kinds of stimuli
Somatic Sensations:
- Touch
- Pressure
- Temperature
- Pain
- Awareness of the position of the body parts and their movements (Proprioception)
How does the sensation of touch come about?
Exteroceptors in the skin called Mechanoreceptors (afferent nerve endings) reside in the dermis
- Receptors are sensitive to physical distortion
- Present throughout the body and monitor contact with the skin
What’s the touch pathway?
Called the Dorsal Column-medial lemniscal pathway
- Peripheral afferent axons join a posterior root that enters the spinal cord and ascend to the brain stem in the posterior column of the spinal cord
- Roots up to T7 ascend the spinal cord in the ipsilateral GRACILE FASCICULUS
- T6+ and cervical roots collect in the ipsilateral CUNEATE FASCICULUS
- Gracile and Cuneate Fasciculi are collectively called the posterior funiculus or posterior column
- Ascends the spinal cord in the Posterior Funiculus up to the Medulla without synapsing - In the Medulla, Gracile synapse in the Gracile Nucleus & Cuneate synapses in Cuneate Nucleus
- The axons of these nuclei (2nd afferents) pass anteriorly and decussate to form the medial lemniscus, contralateral to their cells of origin.
- Above the level of the nuclei each half is represented within the medial lemniscal pathway - The 2nd medial lemniscal afferent ascend the brain stem in the Medial Lemniscus to the Diencephalon
- Terminate in the Ventral Posterolateral (VPL) Nucleus of the Thalamus
- Cutaneous information terminates in core of VPL
- Proprioceptive info terminates in surrounding shell of VPL - Axons of the VPL 3rd afferent neurons travel in the posterior limb of the internal capsule and terminate in the Postcentral Gyrus and Posterior paracentral lobule of the parietal lobe
- The postcentral gyrus and posterior paracentral lobule are called the PRIMARY SOMATOSENSORY CORTEX
- Endings are grouped according to the location of the receptor
What are the different types of Afferent Sensory nerve endings?
- Free nerve endings (pain & temp)
- Pacinian Corpuscle (mechanical distortion)
- Meissner’s corpuscle (Fine touch)
- Muscle spindles (responds to muscle stretch)
Outline the Trigeminal Touch Pathway
- Peripheral processes are located in the trigeminal nerves and for mechanoreceptors in the skin which send their central axons to the brain stem - SYNAPSE IN THE MAIN SENSORY TRIGEMINAL NUCLEUS (2nd order afferent)
- Trigeminal 2nd degree afferent axons order immediately on leaving the trigeminal nucleus and JOIN the CONTRALATERAL VENTRAL TRIGEMINAL LEMINSCUS
- 2nd order main sensory trigeminal afferent in the ventral trigeminal leminscus ascend to the DIENCEPHALON and TERMINATE IN the VENTRAL POSTEROMEDIAL (VPM) NUCLEUS of the THALAMUS
- The axons of the 3rd order main sensory trigeminal afferents (VPM neurons) travel in the posterior limb of the internal capsule and end in the postcentral gyrus of the parietal lobe
- The postcentral gyrus is part of the primary cortical receiving area of the somatosensory system
- These action potentials initiate the release of neurotransmitter from the 3° afferent axon terminals onto cortical neurons and initiate the higher-order processing of the stimulus information generated by the Merkel cell (mechanoreceptor). The point-to-point connections within the pathway provide the basis for a somatotopic map that is used to locate the area of contact with the stimulus
How long does it take sensory symptoms from a migraine attack/Vascular lesion/ Sensory epilepsy/ Spinal cord lesion take to manifest?
Migraine attack = 20-30 mins to spread one half of the body
Vascular lesion - Instantaneous
Sensory epilepsy - Seconds
Spinal cord lesion - hours or days to spread to limbs and trunk
What’s the pattern of sensory disturbance associated with Peripheral nerve lesions?
Glove & Stocking manifestation
- sensory loss and simple paraesthesia (pins & needles)
- Hands and feet
What’s the pattern of sensory disturbance associated with Nerve root lesions?
Associated with dermatomal pattern of sensory loss
Common symptom of pain
What’s the pattern of sensory disturbance associated with Spinal cord lesions?
Brown Sequard Syndrome = unilateral lesion in the spinal cord = sensory loss on the same side
Syringomyelia = central cord lesion = sensory loss is dissociated = e.g. both arms and chest
What’s the pattern of sensory disturbance associated with Brain Stem lesions?
Dorsal column sensory system cross the midline in the upper medulla - lesion causes loss affecting contralateral side of the body (right side face + left side body)
What’s the pattern of sensory disturbance associated with Hemisphere lesions?
Loss of sensation over the whole contralateral half of the body (one side)
What are the sensory & motor deficits in peripheral nerve lesions of the Ulnar nerve?
Sensory:
- Loss of sensation in little & 3rd finger
Motor:
- Small muscles of the hand except abductor pollicis brevis
- Ulnar flexors of little and ring finger (auriculaire + annulaire) and wrist
What are the sensory & motor deficits in peripheral nerve lesions of the Medial nerve?
Sensory:
- Loss of sensation in thumb, index, middle and half ring finger
Motor:
- Loss of Abductor Pollicis brevis
What are the sensory & motor deficits in peripheral nerve lesions of the Radial nerve?
Sensory:
- Loss of sensation below thumb and index finger on posterior surface of hand
Motor:
- Loss of finger extensors
- Thumb extensors and abductors
- Wrist dorsiflexors
- Brachioradialis
What is a dermatome?
Area of the skin supplied by a particular spinal/cranial nerve
- Created a dermatone map which is useful in clinical neurology to identify the location of pathology related to sensory deficit
- Useful to note there is a marked overlap between dermatomes of adjacent nerves