Somatosensation, Reflexes, Locomotion and Posture Flashcards
Which mechanoreceptors are deep in the skin (dermis and subcutaneous layer)?
- Ruffini organs
- Pacinian corpuscles
Which mechanoreceptors are located in the epidermis?
- Meissner corpuscles
- Merkel complexes
Which mechanoreceptors or slowly adapting?
Merkel complexes respond to indenation
Ruffini organs have a sustained response to skin movement
Which mechanoreceptors are rapidly adapting?
- Meissner corpuscles have a transient response to skin movement
- Pacinian corpuscles respond transiently to vibration
Where are Merkel complexes located?
tips of epidermal ridges
Where are ruffini organs located?
Upper dermis
Where are meissner corpuscles located?
epidermis
Where are pacinian corpuscles located?
dermis and hypodermis
What do Meissner corpuscles encode?
rate of force
what do Merkel complexes encode?
grip force
What do ruffini corpuscles encode?
hand posture
Where is somatosensation in upper limbs is topographically located?
More lateral in the dorsal columns
Where is somatosensation of lower limbs topographically located?
more medial in dorsal columns
What is the main tactile mechanoreceptive pathway (dorsal column-medial lemniscus)?
- nerve fibre travels to dorsal column white matter
- travels to medulla where it synapses with secondary neuron
- secondary neuron in medulla projects to gracile and cuneate nuclei
- fibres decussate in caudal medulla
- fibres project up to medial lemniscus in brainstem
- info about lower limbs is now more lateral and info about upper limbs is now more medial
- secondary neurons synapse with tertiary neurons of VPN in the thalamus
- thalamic nucleus projects to primary somatosensory cortex
Which dorsal column nucleus is associated with lower limb somatosensation?
gracile nucleus
Which dorsal column nucleus is associated with upper limb somatosensation?
cuneate nucleus
Where are somatosensory areas located?
Postcentral gyrus - posterior to central sulcus
Where are motor areas located?
Precentral gyrus
What is somatosensory plasticity?
The functions of cortical areas that are no longer used can be adopted by other territories
Where does spinal cord end?
Level of T12
In spinal cord, what is the location of white matter and grey matter?
White matter is outside and grey matter is inside
in brain, white matter is inside and grey matter is outside
What are the main motor neurons that innervate muscle?
alpha
What is the topographical location of muscle innervation in the spinal cord?
- distal muscles are innervated by more lateral motor neurons
- proximal muscles are innervated by more medial motor neurons
What is a motor unit?
1 motor neuron innervates a collection of muscle fibres
What stimulus does the muscle spindle detect?
muscle stretch
runs parallel to muscle
What are Group I and II afferent axons?
conducting fibres which wrap around muscle fibres and transduce stretch into nerve impulses
What stimulus does the golgi tendon organ respond to?
responds to force
runs in series to muscle
What is the monosynaptic reflex?
-one central synapse
-occurs in spinal cord at a segmental level
-afferent arc: AP travels to dorsal root in spinal cord and makes an excitatory connection with MNs that innervate that same muscle
APs excite interneurons which send inhibitory signals to MN of flexor muscle (antagonist muscle)
What happens when there is a load applied to biceps?
Increased weight disturbs the joint causing biceps to stretch
a signal will be sent to counteract the stretch and so biceps will contract
inhibitory interneuron is excited which tells triceps to relax
What happens when golgi tendon organ detects excess force applied by a muscle?
Golgi tendon organ feedsback via inhibitory interneuron to decrease the amount of contraction and via excitatory interneuron to antagonist muscle to increase its contraction
this is not a monosynaptic reflex
What stimulus do Pacinian corpuscles respond to?
Vibration causes transmembrane Na+ channels to open
When Na+ ions are sufficient to reach threshold, nerve fibre generates APs
What is the cross-extensor reflex?
muscle is activated via nociceptors, not through muscle sense
stepping on sharp object will result in withdrawal such that flexors are activated and extensors are inhibited while extension is increased in the other limb and flexors are inhibited to maintain stability
What is the difference between short propriospinal and long propriospinal tracts?
Short propriospinal tend to be in the lateral part of spinal cord and control distal muscle. Distal muscles tend to be involved in complex, dextrous movement.
Long propriospinal tend to be in medial part of spinal cord and control proximal muscles. Midline muscles tend to be involved in postural control
What is an UMN?
Any neuron that an affect excitability of a LMN, mostly via interneurons
Do not leave the brain
What are LMNs?
Neurons which innervate muscle
What are consequences of stroke and spinal cord injury on monosynaptic reflexes?
Stroke - exaggerated reflexes on contralateral side
Spinal cord lesion - exaggerated reflexes below the level of injury
What are signs and symptoms of UMN lesion?
UMN synapse with interneurons which tend to be inhibitory
UMN lesions associated with loss of inhibition - LMN more excitable
-Weakness
-Spasticity: increased tone, hyperactive reflexes, clonus
-Positive Babinski sign
-Loss of fine voluntary movements
What are signs and symptoms of LMN lesion?
- weakness or paralysis
- decreased superficial reflexes
- hypoactive deep reflexes
- decreased tone
- fasciculations and fibrillations
- muscle atrophy
At what level do fibres of lateral corticospinal tract decussate?
Caudal medulla
At what level do fibres of nociceptive pain pathway decussate?
Within spinal cord - spinal cord neuron decussates and and travels through anterolateral pathway
How does TRPV1 channel act as a nociceptive transducer?
Channel is activated by H+, heat and capsaicin
Stimuli cause the channel to become more permeable to Ca++ & Na+
Influx of positive charge into nerve terminal causes nerve to depolarise
Which nociceptive fibre if unmyelinated?
C fibre
What are signs of decerebrate rigidity?
Upper and lower limbs extended as a result of overactivity of extensor muscles relative to flexor muscles
What are signs of decorticate rigidity?
Upper limbs flexed while lower limbs extended
If there is weakness of inferior facial muscles, what neurons are affected?
UMN
If there is weakness to superior and inferior facial muscles, what neurons are affects?
LMN
What are possible consequences of loss of cortical motor control to ventromedial pathways?
Brainstem may be able to compensate for postural maintenance but motor control also involves anticipation/planning and so brainstem may not be ale to compensate for anticipatory postural maintenance
What are the topographic locations of movement in the primary motor cortex?
More medially, lower limbs
More laterally, upper limbs and face
What structures form basal ganglia?
In forebrain: caudate nucleus, putamen, globus pallidus, subthalamic nucleus
In brainstem: substantia nigra
What side do symptoms appear if there is a cerebellar lesion?
Cerebellar pathway double crosses:
cerebellum feeds to contralateral PMC
contralateral PMC feeds to contralateral muscle
symptoms appear ipsilateral
