Download CNS consists of brain and spinal cord PNS consists of nerves CNS

CNS consists of brain and spinal cord
PNS consists of nerves
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As with sensory input, motor output is organized in central nervous system
Peripheral Nervous system divides efferent signals somatotopically
Signals to skeletal muscles (including those transmitting voluntary
movements) originate and are carried in SOMATIC NERVOUS SYSTEM
Signals to other effector cells and organs - viscera, smooth and cardiac
muscle-containing structures, glands – originate and are carried in
AUTONOMIC NERVOUS SYSTEM
Adult brain regions
1.
Cerebral hemispheres
2.
Diencephalon
3.
Brain stem (midbrain, pons, and medulla)
4.
Cerebellum
areas of gray matter (cell bodies) in brain organized in the cerebral hemispheres and
cerebellum
Outer gray matter called cortex
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Main aspects of gyral patterns similar from person to person
Central sulcus identifiable
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Precentral gyrus – frontal continuation goes towards the front.
Note left right asymmetry
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Five lobes
Frontal
Parietal
Temporal
Occipital
Insula
Cortical areas functionally distinct. Motor areas in frontal lobe – most posterior
aspect – near primary sensory cortex.
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Obvious difference between motor and sensory cortex across central sulcus
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(Korbinian) Brodmann’s areas based on location and cytoarchitecture.
Intuition that different cell arrangements had different functions turns out to have
been correct
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The basal ganglia are subcortical nuclei
Caudate nucleus
Putamen
Globus pallidus
Caudate nucleus + putamen = striatum
Associated with subthalamic nuclei (diencephalon) and substantia nigra (midbrain)
Caudate and putamen not completely separate – actually single nucleus with fibers
from cortex passing through
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Internal capsule is bunched up corona radiata
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Corona radiate are axons to and from cortex to lower areas
Association fibers are axons between areas of the same cortical hemisphere
Commisural fibers are axons between areas of the two different hemispheres
Internal capsules are axon bundles between cortex, basal ganglia, thalamus,
brainstem and spinal cord
Functions thought to be
Influence muscle movements
Role in cognition and emotion
Regulate intensity of slow or stereotyped movements
Filter out incorrect/inappropriate responses
Inhibit antagonistic/unnecessary movements
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Direct circuitry is cortex to putamen to globus pallidus (interna) to thalamus to cortex
– inhibition of inhibition of excitation leads to enhanced thalamic drive of cortical
activity (excitatory)
Indirect circuitry is cortex to putamen to globus pallidus (externa) to subthalamic
nucleus to globus pallidus (interna) to thalamus to cortex - inhibition of inhibition of
excitation of inhibition of excitation leads to reduced thalamic stimulation of cortex
(inhibitory).
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Corona radiate (internal capsule) include axons PROJECTING to other parts of nervous
system – corticopontine, corticobulbar, corticospinal tracts
Three paired structures
Thalamus
Hypothalamus
Epithalamus
Encloses third ventricle
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Thalamus (singular) main sensory path to cortex (third order neurons)
Major component of motor circuitry
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[Repeat of slide 17]
Direct circuitry is cortex to putamen to globus pallidus (interna) to thalamus to cortex
– inhibition of inhibition of excitation leads to enhanced thalamic drive of cortical
activity (excitatory)
Indirect circuitry is cortex to putamen to globus pallidus (externa) to subthalamic
nucleus to globus pallidus (interna) to thalamus to cortex - inhibition of inhibition of
excitation of inhibition of excitation leads to reduced thalamic stimulation of cortex
(inhibitory).
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Controls autonomic nervous system (e.g., blood pressure, rate and force of heartbeat,
digestive tract motility, pupil size)
Physical responses to emotions (limbic system)
Perception of pleasure, fear, and rage
Biological rhythms and drives - Suprachiasmatic nucleus (biological clock)
Regulates body temperature – sweating/shivering
Regulates hunger and satiety in response to nutrient blood levels or hormones
Regulates water balance and thirst
Regulates sleep-wake cycles Suprachiasmatic nucleus (biological clock)
Controls endocrine system
Controls secretions of anterior pituitary gland
Produces posterior pituitary hormones
Controls autonomic nervous system (e.g., blood pressure, rate and force of heartbeat,
digestive tract motility, pupil size)
Corpora quadrigemina— dorsal protrusions
Superior colliculi—visual reflex centers
Inferior colliculi—auditory relay centers
Substantia nigra—functionally linked to basal nuclei
Red nucleus—relay nuclei for some descending motor pathways; part of reticular
formation
11% of brain mass
Dorsal to pons and medulla
Input from cortex, brain stem and sensory receptors
Allows smooth, coordinated movements
Each hemisphere has three lobes
Anterior, posterior, and flocculonodular
Arbor vitae—treelike pattern of cerebellar white matter
Cerebellar hemispheres connected by vermis
Folia—transversely oriented gyri
All fibers in cerebellum are ipsilateral
Three paired fiber tracts connect cerebellum to brain stem
Superior cerebellar peduncles connect cerebellum to midbrain
Middle cerebellar peduncles connect pons to cerebellum
Inferior cerebellar peduncles connect medulla to cerebellum
Cerebellum receives impulses from cerebral cortex of intent to initiate voluntary
muscle contraction
Signals from proprioceptors and visual and equilibrium pathways continuously
"inform" cerebellum of body's position and momentum
Cerebellar cortex calculates the best way to smoothly coordinate muscle contraction
"Blueprint" of coordinated movement sent to cerebral motor cortex and brain stem
nuclei
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Cerebellum receives impulses from cerebral cortex of intent to initiate voluntary
muscle contraction
Signals from proprioceptors and visual and equilibrium pathways continuously
"inform" cerebellum of body's position and momentum
Cerebellar cortex calculates the best way to smoothly coordinate muscle contraction
"Blueprint" of coordinated movement sent to cerebral motor cortex and brain stem
nuclei
Role in thinking, language, and emotion
May compare actual with expected output and adjust accordingly
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Motor pathways (somatic and autonomic) with nuclei embedded in white matter
Controls automatic behaviors necessary for survival
Contains fiber tracts connecting higher and lower neural centers
Nuclei associated with 10 of the 12 pairs of cranial nerves
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Deliver efferent impulses from brain to spinal cord
Two groups
Direct pathways—pyramidal tracts
Indirect pathways—all others
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Motor pathways involve two neurons:
Upper motor neurons
Pyramidal cells in primary motor cortex – yellow in picture
Lower motor neurons
Ventral horn motor neurons – blue in picture
Innervate skeletal muscles
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Impulses from pyramidal neurons in precentral gyri pass through pyramidal
(corticospinal)l tracts
Descend without synapsing
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Axons synapse with interneurons or ventral horn motor neurons
Direct pathway regulates fast and fine (skilled) movements
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Complex and multisynaptic
Includes brain stem motor nuclei, and all motor pathways except pyramidal pathways
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These pathways regulate
Axial muscles maintaining balance and posture
Muscles controlling coarse limb movements
Head, neck, and eye movements that follow objects in visual field
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Reticulospinal and vestibulospinal tracts—maintain balance
Rubrospinal tracts—control flexor muscles
Superior colliculi and tectospinal tracts mediate head movements in response to
visual stimuli
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Cerebellum and basal nuclei are ultimate planners and coordinators of complex
motor activities
Complex motor behavior depends on complex patterns of control
Segmental level
Projection level
Precommand level
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Precommand Level
Neurons in cerebellum and basal nuclei
Regulate motor activity - Precisely start or stop movements - Coordinate
movements with posture
Block unwanted movements - Monitor muscle tone
Perform unconscious planning and discharge in advance of willed movements
Cerebellum - acts on motor pathways through projection areas of brain stem
- acts on motor cortex via thalamus to fine-tune motor activity
Basal nuclei - inhibit various motor centers under resting conditions
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Projection Level consists of
Upper motor neurons that initiate direct (pyramidal) system to produce
voluntary skeletal muscle movements
Brain stem motor areas that oversee indirect (extrapyramidal) system to
control reflex and CPG-controlled motor actions
Projection motor pathways send information to lower motor neurons, and keep
higher command levels informed of what is happening
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Segmental level
Lowest level of motor hierarchy
Reflexes and automatic movements
Central pattern generators (CPGs): segmental circuits that activate networks of
ventral horn neurons to stimulate specific groups of muscles
Controls locomotion and specific, oft-repeated motor activity
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Components of a reflex arc (neural path)
1.
Receptor—site of stimulus action
2.
Sensory neuron—transmits afferent impulses to CNS
3.
Integration center—either monosynaptic or polysynaptic region within
CNS
4.
Motor neuron—conducts efferent impulses from integration center to
effector organ
5.
Effector—muscle fiber or gland cell that responds to efferent impulses
by contracting or secreting
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Inborn (intrinsic) reflex - rapid, involuntary, predictable motor response to stimulus
Example – maintain posture, control visceral activities
Can be modified by learning and conscious effort
Learned (acquired) reflexes result from practice or repetition,
Example – driving skills
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Visceral reflex arcs have same components as somatic reflex arcs
But visceral reflex arc has two neurons in motor pathway
Visceral pain afferents travel along same pathways as somatic pain fibers,
contributing to phenomenon of referred pain
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Functional classification
Somatic reflexes
Activate skeletal muscle
Autonomic (visceral) reflexes
Activate visceral effectors (smooth or cardiac muscle or glands)
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Composed of 3–10 modified skeletal muscle fibers - intrafusal muscle fibers wrapped in connective tissue capsule
Effector fibers – extrafusal muscle fibers
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Noncontractile in central regions (lack myofilaments)
Two types of afferent endings
Anulospiral endings (primary sensory endings)
Endings wrap around spindle; stimulated by rate and degree of stretch
Flower spray endings (secondary sensory endings)
Small axons at spindle ends; respond to stretch
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Contractile end regions innervated by gamma () efferent fibers - maintain spindle
sensitivity
Note: extrafusal fibers (contractile muscle fibers) innervated by alpha () efferent
fibers
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Excited in two ways
1.
External stretch of muscle and muscle spindle
2.
Internal stretch of muscle spindle
Activating  motor neurons stimulates ends to contract, thereby
stretching spindle
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Stretch causes increased rate of impulses to spinal cord
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Contracting muscle reduces tension on muscle spindle
Sensitivity lost unless muscle spindle shortened by impulses in  motor neurons
– coactivation maintains tension and sensitivity of spindle during muscle
contraction
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Contracting muscle reduces tension on muscle spindle
Sensitivity lost unless muscle spindle shortened by impulses in  motor neurons
– coactivation maintains tension and sensitivity of spindle during muscle
contraction
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Spinal somatic reflexes
Integration center in spinal cord
Effectors are skeletal muscle
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Maintains muscle tone in large postural muscles, and adjusts it reflexively
Causes muscle contraction in response to increased muscle length (stretch)
How stretch reflex works
Stretch activates muscle spindle
Sensory neurons synapse directly with  motor neurons in spinal cord
 motor neurons cause stretched muscle to contract
All stretch reflexes are monosynaptic and ipsilateral
Reciprocal inhibition also occurs—IIa fibers synapse with interneurons that inhibit 
motor neurons of antagonistic muscles
Example: In patellar reflex, stretched muscle (quadriceps) contracts and
antagonists (hamstrings) relax
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Polysynaptic reflexes
Helps prevent damage due to excessive stretch
Important for smooth onset and termination of muscle contraction
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Produces muscle relaxation (lengthening) in response to tension
Contraction or passive stretch activates tendon reflex
Afferent impulses transmitted to spinal cord
Contracting muscle relaxes; antagonist contracts (reciprocal
activation)
Information transmitted simultaneously to cerebellum and used to adjust
muscle tension
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Flexor (withdrawal) reflex
Initiated by painful stimulus
Causes automatic withdrawal of threatened body part
Ipsilateral and polysynaptic
Protective; important
Brain can override
E.g., finger stick for blood test
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Crossed extensor reflex
Occurs with flexor reflexes in weight-bearing limbs to maintain balance
Consists of ipsilateral withdrawal reflex and contralateral extensor reflex
Stimulated side withdrawn (flexed)
Contralateral side extended
e.g., step barefoot on broken glass
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Startle reflex in newborn
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Elicited by gentle cutaneous stimulation
Depend on upper motor pathways and cord-level reflex arcs
Best known:
Plantar reflex
Abdominal reflex
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Test integrity of cord from L4 – S2
Stimulus - stroke lateral aspect of sole of foot
Response - downward flexion of toes
Damage to motor cortex or corticospinal tracts  abnormal response = Babinski's
sign
Hallux dorsiflexes; other digits fan laterally
Normal in infant to ~1 year due to incomplete myelination
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