Sensory Systems Flashcards
What are the two receptor types for General sensory afferents (GSA)?
Exterorecptors
Proprioceptors
What are exterorecptors for?
Pain and temperature (A delta, C fibres)
Discriminative touch (A beta fibres)
What are proprioceptors for?
Joints - Ib fibres
Tendons - Golgi tendon organs Ib fibres
Muscles - muscle spindles Ia and ll fibres
Describe the conscious state of sensation
Exteroceptive - from external world (body surface - temperature, pressure, vision, hearing
Proprioceptive - from within the body (muscles, tendons, joints, position sense)
Describe the unconscious state of sensation
Enteroceptive - from the viscera (homeostasis)
Proprioceptive - from with the body (muscles, tendons, joints, position sense) for coordination and refinement of movement
Describe the primary motor cortex
Receives input from the ventral Post nucleus of the thalamus
Arranged somatopically
Cortex devoted to each body part is correlated to the density of sensory input received from that body part
Describe how hair follicles impact the somatosensory system
Embedded in the skin
Innervated by nerve endings
Hair is bent resulting in mechanical deformation
This activates the nerve ending
How do pacinian corpuscles affect the somatosensory system?
Located in the dermis of both hairy and non hairy skin
Found in the hands, feet, nipples, mammary glands etc
Concentric lamallae of flattened cells
Spaces between lamallae are filled with fluid
They are low threshold, rapidly adopting and sensitive to high vibration
What cells detect touch, pressure and vibrations
Hair follicles
Pacinian corpuscles
Meissners corpuscles
Merkels discs
Ruffinis corpuscles
How are meissners corpuscles part of the somatosensory system?
They are stacks of horizontally flattened epithelial cell in a connective tissue sheath
Found beneath the epidermis of and in ridges of glaborus skin (no hair)
Low threshold, rapidly adapting sensitive to touch and vibration
How are merkels discs part of the somatosensory system?
Located in the epidermis of glaborus skin
Found at the lips, extremities, and external genetilia
Consist of a nerve terminal and a flattened non neuronal epithelial cell
Spaces between lamellae are filled with fluid
Low threshold, slowly adapting and sensitive to pressure
What are ruffinis corpuscles?
Found in the epidermis of both hairy and non hairy skin
They are encapsulated bundles of collagen fibrils connected to fibrils of dermis
Low threshold, slowly adapting and sensitive to stretching skin
What do joint capsules receptors do?
Generate conscious awareness of kinesthesia and provide sensory info to the cerebral cortex
Where are joint capsules receptors found?
Receptor In ligaments and joint capsules consist of free nerve endings and encapsulated receptors which are low threshold mechanoreceptors
What is the adaption state of joint capsules receptors?
Some are slowly adapting to provide info on the ability of an individual to judge the position of a joint without seeing it and without moving
Others are rapidly adapting and provide info about the ability of an individual to perceive the movement of a joint and judge the direction and velocity of its movement
Describe the dorsal column / medial lemniscal system
The dorsal Post funiculus (dorsal column) contains two long ascending tracts
Axons mediating tactile sensations and proprioception ascend the spinal cord in the ipsilateral dorsal column
Axons from the sacral region are located medially
Axons added at higher levels of the cord will occupy more lateral positions
What are the two fascicles of the dorsal column from T6 up?
Gracile - lower body
Cuneate - upper body
Describe the fasiciculus gracilis of the dorsal column
Located medially
Fibres from
Sacral
Lumbar
Lower thoracic segments (T6-12)
1st, 2nd, 3rd order neurons
Describe the 1st order neurons of the gracile fascicle
Dorsal root ganglia peripheral processes Innervate the pacinian and meissners corpuscles of the skin and proprioceptors
Ascend ipsilaterally in the spinal cord
Describe 2nd order neurons of the gracile fascicle
Found in the ipsilateral nucleus gracilis in the medulla
Travel as the internal accurate fibres
Decussate at medial lemniscus
Ascend as the medial lemniscus
Describe the 3rd order neurons of the gracile fascicle
Synapse with the 2nd order neurons in the contralateral ventral posterolateral nucleus of the thalamus
They terminate in the medial aspect of the sensorimotor cortex
Describe the fasciculus cuneatus
Joins the spinal cord at level T6
Fibres from the upper body
Enters the dorsal funicuclus medial to the dorsal horn and ascends the dorsal horn in a lateral position
Describe the 1st order neurons of the cuneatus fascicle
Dorsal root ganglia peripheral processes Innervate the pacinian and meissners corpuscles of the skin and proprioceptors
Ascend ipsilaterally in the spinal cord
Describe the 2nd order neurons of the cuneatus fascicle
Are in the ipsilateral nucleus cuneatus in the medulla
Travel as the internal accurate fibres
Decussate at medial lemniscus
Ascend as the medial lemniscus
Describe the 3rd order neurons of the cuneatus fascicle
Synapse with 2nd order neurons in the contralateral ventral posterolateral nucleus of the thalamus
Terminate in the lateral aspect of the sensorimotor cortex
Describe the trigeminal touch pathway
Axons from the sensory receptors synapse with 2nd order neurons in the ipsilateral trigeminal nucleus
Decussate and project to the ventral post nucleus
Sensory info relayed to the somatosensory cortex
What happens due to a lesion of the dorsal column?
Loss of tactile sense (vibration/ deep touch) and kinaesthetic sense (position / movement) below the level of the lesion
Why is a patient with a dorsal column lesion clumsy and with no sense of touch?
They have lost conscious proprioception of their position in space leading them to be unable to identify the position of their limbs with their eyes closed
What is asteregnosis?
When a dorsal column lesion affects the cervical region of the spine the patient cannot identify the shape, size or texture of an object in their hand ipsilateral to the lesion
What is Tabes dorsalis?
It represents the late stage consequences of a syphilitic infection
(Tertiary syphilis / neurosyphilis)
What happens due tabes dorsalis?
The large diameter central processes of the dorsal root ganglion neurons degenerate (especially in the lower thoracic and lumbosacral segments)
Gracile fascicle degenerates and their is a loss of vibration sensation, two point discrimination and conscious proprioception
What is ataxia?
The deprivation of sensory feedback signals that detect the position of the lower limbs at any given time
(Neurosphyilis)
What tract is part of the direct anterolateral system?
The spinothalamic tract
What does the lateral spinothalamic tract transmit?
Pain and temperature
What does the anterior spinothalamic tract transmit?
Non discriminative touch
What are the two pathways of the spinothalamic tract?
The direct pathway
The indirect pathway
Describe the direct pathway of the spinothalamic tract
Comprises the neospinothalamic tract
For pain, temperature and simple tactile sensations
Also known as the lateral sensory discriminative pathway
Describe the indirect pathway of the spinothalamic tract
Comprises the paleospinothalamic, spinoreticular and spinomesencephalic tracts
Involved in the autonomic, endocrine, motor and arousal components of pain, temperature and crude touch
What are the steps of the indirect pathway of the spinothalamic tract?
Involved in activation of pain inhibiting mechanisms
Ascend the cord bilaterally (poor somatotopic organisation
Connects with the reticular formation, hypothalamus and limbic system
Mediates affective motivational pathways
What are the steps of the direct pathway of the spinothalamic tract?
Axons from ruffinis corpuscles, nociceptors, and thermoreceptors synapse with 1st order neurons in the dorsal root ganglion
Central axons enter spinal cord, branches travel up the tract of lissauer
Synapse with 2nd order neurons in proper sensory nucleus
Axons cross and enter contralateral side
Ascend in the lateral funiculus
Synapse with 3rd order neurons in VPN
Project to sensorimotor cortex
How are the fibres arranged when the ascend in the direct pathway?
Somatotpically
Fibres from lower body ascend dorsolaterally
Fibres from upper extremities and neck ascend ventromedially
What is Brown-Sequard syndrome?
Damage to the neosphinothalamic tract and dorsal columns
What are the symptoms of Brown-Sequard syndrome?
Hemisection of the spinal cord leads to the patient experiencing a loss of discriminative touch, vibration and proprioception on the ipsilateral side to the lesion due to disruption of the uncrossed dorsal columns
Pain and temperature sensation are lost contralateral to lesion due to disruption of the neosphinothalamic tract
Transaction of the corticospinal fibres will lead to hemiparesis on the ipsilateral side to the lesion
What is the paleosphinothalamic tract?
Located in the dorsal horn and intermediate grey matter
Sense non discriminative touch, pain and temperature
Describe the process of the paleosphinothalamic tract
Axons ascend bilaterally in the ventrolateral quadrant of the spinal cord
Make several synapses in the reticular formation for the brainstem
Finally project to the midline and intralaminar thalamic nuclei
These nuclei then project to the cerebral cortex especially the limbic regions such as the cingulate gyrus
Describe the spinoreticular tract
Sense pain
Neurons located in the dorsal horn and intermediate grey matter
1st order neurons from receptor to dorsal horn
2nd order neurons synapse in medullary and pontine reticular formations
Describe the projections of the spinreticular tract
Projections from the spinal cord to the brain are crossed and uncrossed
The ascending fibres are believed to transmit sensory information to the reticular formation
(Influences levels of consciousness)
Activated cerebral cortext through secondary and tertiary projections via the midline and intralaminar thalamic nuclei
The thalamocortical projections are highly diffused and influence wide areas of the cerebral cortex
Describe the spinomesencephalic tract
Located in the dorsal horn and intermediate grey matter
Axons ascend to the midbrain
Terminate in the periaqueductal gray
Activate PAG neurons
Send descending fibres to the spinal cord to inhibit pain sensation
Thought to transmit sensory info to the amygdala via parabrachial nuclei
