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Lecture 6

HMB320H1 Lecture 6: HMB320 Lectures (6-8)


Department
Human Biology
Course Code
HMB320H1
Professor
Cindi Morshead
Lecture
6

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HMB320 Lecture 6 Basal Ganglia
Direct connections between cortex’s upper motor neurons lower motor neurons
Basal Ganglia does NOT have direct connections to lower motor neurons
Basal Ganglia has other functions i.e. cognitive, mood
Basal Ganglia cortex lower motor neurons
Balance between STOP and GO pathway creates control movements
--> excitatory pathway
--| inhibitory pathway
--? modulatory pathway
Str striatum = caudate + putamen
GPi globus pallidus interna | SNr substantia nigra pars reticularis (GPi + SNr both have similar functions)
GPe globus pallidus externa
SNc substantia nigra pars compacta
LMN lower motor neurons
STn subthalamic nucleus
Basal Ganglia Baseline Conditions, the thalamus is strongly inhibited = chronic inhibition
Motor cortex --> LMN (direct pathway)
Motor cortex --> stratum
Striatum --| GPi
GPi --| thalamus (thalamocortical fibres to cortex)
Thalamus --> motor cortex
Basal Ganglia GO Pathway = disinhibition of thalamus
Motor cortex --> striatum --| GPi/SNr --| thalamus --> motor cortex --> LMN (disinhibition of thalamus)
Basal Ganglia MODULATORY Pathway, from SNc that enhances GO pathway
SNc (DAergic neurons) --> striatum --| GPi/SNr --| thalamus --> motor cortex --> LMN (disinhibition of thalamus, enhances GO pathway)
GPe --| STn --> GPi/SNr (STn causes more inhibition of thalamus, GPe stops STn from inhibiting)
Basal Ganglia STOP Pathway
Motor cortex --> striatum --| GPi/SNr --| thalamus --> motor cortex --> LMN (disinhibits of thalamus here same as GO pathway)
striatum --| GPe --| STn --> GPi --| thalamus --> motor cortex --> LMN (by disinhibition of GPe, this is STOP pathway)
Basal Ganglia MODULATORY Pathway, from SNc that inhibits STOP pathway
SNc (DAergic neurons) --| striatum
Striatum Neurons
Striatum has two subpopulations of neurons, both do effectively the same thing to modify movement, both respond to SNc
1) in GO pathway, has D1 receptors, activated by DA from SNc
2) in STOP pathway, has D2 receptors, inhibited by DA from SNc
SNc (substantia nigra compacta)
Projects to all areas of striatum, both of its actions enhance action of motor cortex
Uses DA (dopamine)
Enhances GO pathway of basal ganglia (D1 receptors)
Inhibits STOP pathway of basal ganglia (D2 receptors)
Basal Ganglia Damage
Location of damage effects whether too much or too little movement results, imbalance of STOP/GO
Hyperkinetic disorder Huntington’s if damage to striatum cells in STOP pathway | ballismus if damage to subthalamic nucleus (STn)
Hypokinetic disorder Parkinson’s if damage to the substantia nigra (SN)
Parkinson’s SN neurons degenerate (less GO more STOP) hypokinetic, resting tremor (no tremor when moving)
No gross motor movement, shuffle feet, hard to start movement, onset 40-70 y/o
Akinesia = loss of voluntary motor movement
Bradykinesia = slowness of movement
Pill-rolling tremor, rigidity, postural instability, masked face
Huntington’s stratum cells involved in STOP degenerate/atrophy, hyperkinetic
Continuous involuntary movements (chorea = dance), onset 30-50, progressive
Autosomal dominant genetic CAG repeats in gene 100% penetrance
Ballismus bilateral in Huntington’s
Dementia
(Hemi)ballismus hyperkinetic, flailing movements, one side of body
Basal Ganglia Functions
Motor control sensory+motor cortices through the putamen (striatum)
Hyper and hypokinetic, eye movement, speech
Cognitive motivation, personality, through caudate nucleus (of striatum)
Dementia
Adaptive behaviour mood, fear, through nucleus accumbens (limbic cortex)
Mood disorders
Extrapyramidal disorders to separate from pyramidal (corticospinal) lesions
PARKINSON’S
HUNTINGTON’S

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Somatosensation
5 basic sensory modalities: smell, vision, taste, hearing, touch
Have different receptors, pathways, targets
All mediated through cranial nerves if related to the face, spinal nerves in soma
Submodalities touch = pain, temp, simple/crude touch, discriminative touch, vibration sense, proprioception
Spinothalamic pathway = pain, temp, simple/crude touch
Dorsal column pathway = discriminative touch, vibration sense, proprioception
Sensory Neurons
Pseudounipolar: one from cell body that bifurcates into peripheral + central process
One process towards CNS (central process), one process towards periphery (peripheral process)
Peripheral ganglia can be dorsal root ganglia OR cranial nerve ganglia (trigeminal, vestibular ganglia…)
Dorsal root ganglia beside spinal cord, where soma of all somatosensory cells are, afferents to somatosensory cortex via dorsal roots
Myelinated afferent = nerve fiber with larger diameter, discriminative touch, proprioception, vibration
Unmyelinated afferent = nerve fiber with smaller diameter, pain, temp
Receptors
Specialized for modalities, morphologically and functionally different receptors
Found In skin, muscles, joints
Soma of all sensory nerves regardless of nerve endings are in dorsal root ganglia | afferents to CNS through dorsal roots
Cutaneous receptors in skin, discriminative touch (pressure, vibration, texture)
Free nerve endings in skin, unencapsulated simple nerve endings (pain, temp)
Muscle/joint receptors in golgi tendon organs and muscle spindles (proprioception = location of limbs, muscle tension)
Motor vs. Sensory
Motor descending pathway crosses at decussation of pyramids (80-90%), cross at spinal cord ventral funiculus (10%)
Sensory ascending pathway crosses at
Motor Pathway
2 neuron chain: upper motor neuron, lower motor neuron
Somatosensory Pathway
3 neuron chain: primary neuron, secondary neuron, tertiary neuron
Primary sensory neuron pseudounipolar, soma in dorsal root ganglia, axon in dorsal root projects towards CNS (spinal cord), peripheral process is the receptor
Secondary sensory neuron soma in dorsal gray column of spinal cord or brain stem, this neuron crosses the midline at or above spinal level, synapses in thalamic nuclei
Axon tract in spinal cord = fasciculus cuneatus or fasciculus gracilis | axon tract in brainstem becomes medial lemniscus
Tertiary sensory neuron soma in thalamus, axon projects through internal capsule, corona radiate, to somatosensory cortex
Dorsal Column Medial Lemniscus (DC-ML Pathway)
Discriminative touch (touch, pressure), vibration sense, proprioception
Conscious awareness of modalities requires information to reach cerebral cortex,
some information (proprioception) goes to cerebellum too
3 neuron chain
Dorsal root ganglia, pseudounipolar cell large myelinated senses environment dorsal root to CNS
ascending axon = fasciculus cuneatus (upper limb) synapses in medulla on secondary neuron
secondary neuron axon = medial lemniscus medial lemniscus crosses midline in brainstem at junction of medulla & cord
secondary neuron medial lemniscus synapses on VPN (ventral posterior nucleus) of thalamus
third order neuron goes through internal capsule, corona radiate synapses on sensorimotor cortex
Upper limb information carried in fasciculus cuneatus in dorsal column of spinal cord (arm info)
Lower limb information carried in fasciculus gracilis in dorsal column of spinal cord (leg info)
Dorsal column spinal cord = fasciculus cuneatus + gracilis
Fasciculus cuneatus more lateral,
larger in higher spinal cord sections because it’s carrying upper limb information
no fasciculus cuneatus in lower lumbar spinal cord sections
Fasciculus gracilis more medial, smaller because it’s only carrying lower limb information
only fasciculus gracilis in lower lumbar region (only have lower limb info)

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Lower Medulla of Brainstem
Nucleus gracilis and nucleus cuneatus (where secondary sensory neurons’ fasciculus cuneatus/gracilis synapse)
Internal arcuate fibers are the axons of the second order neurons, cross the midline, synapse onto nucleus g/c
Medulla
Medial lemniscus second order neurons’ axons = crossing internal arcuate fibers, medial and thin
Olive, pyramid
Pons
Medial lemniscus higher in brain stem, becomes more lateral and flat
Midbrain
Medial lemniscus higher in brain stem, becomes even more lateral and crescent-shaped
Thalamus
Second order neurons’ axons = medial lemniscus, synapses in the thalamus’ VPN (ventral posterior nucleus)
Soma of third order neurons in VPN, project through thalamocortical fibers of posterior limb of internal capsule
corona radiata primary sensorimotor cortex (Somatotopically organized)
Internal Capsule
Anterior limb of the internal capsule thalamocortical fibers, corticothalamic fibers
Genu of the internal capsule corticobulbar fibers
Posterior limb of the internal capsule corticospinal, thalamocortical, corticothalamic
Sublenticular limb of the internal capsule auditory radiations
Retrolenticular limb of the internal capsule optic radiations
Spinothalamic Tract (Pathway)
Pain, temperature, crude tough (itch, tickle)
Neurons from spinal cord thalamus
(DC-ML went to thalamus from medulla)
3 neuron chain
dorsal root ganglia has cell body,
small unmyelinated peripheral process with free nerve endings
central process towards CNS through dorsal roots
first order neuron can run up spinal cord rostral or caudal
in the tract of Lissauer, white matter beside dorsal horn
this makes pain more diffuse!
Still feel pain if you lesion some area of dorsal horn
synapses in same level (dorsal horn, substantia gelatinosa)
cross midline in spinal cord at same level
spinothalamic tract runs up contralateral, in ventral horn
spinothalamic tract = spinal lemniscus in brainstem
second order neuron synapses on thalamus
second order neuron bifurcates in thalamus,
synapses onto third order neurons
posterior limb of internal capsule (thalamocortical fibers)
corona radiata
synapses in sensorimotor and cingulate gyrus
(cingulate gyrus part of limbic system, emotional pain); pain hard to manage because it has lower-level processing in thalamus
Ventral lateral funiculus hugs ventral horn, where spinothalamic tract ascends, axons of contralateral second order
Motor neurons gray matter of spinal cord, movement of trunk more MEDIAL, lower/upper limbs more LATERAL
Lateral horn indicates thoracic region of spinal cord, gray matter has lateral horn
Dorsal horns soma of sensory neurons, has layers (lamina),
Lamina all gray matter in spinal cord is divided into these lamina/layers
o Substantia gelatinosa (layers 3-5) dorsal horn gray matter, where spinothalamic first order neurons synapse with second order
Also has descending projects from cortex to regulate pain, a lot of endorphin receptors
o Nucleus proprius, dosal nucleus, intermedolateral nucleus (in lateral horn), motor neurons
Spinothalamic Tract Somatotopy
Area of white matter in cord
Ventral/lateral,
Upper limb innermost
Trunk middle
Lower limb outermost
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