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Questions and Answers
What is the primary source of input to Betz cells in the motor cortex?
How is the direction of movement determined in the M1?
What happens to M1 microstimulation effects after cutting the nerve supplying whisker muscles?
What are the consequences of cerebellar lesions?
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What is the key function of the cerebellum in motor control?
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What is the corticopontocerebellar projection in relation to the cerebellum?
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What characterizes the movement coding in M1?
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What is meant by ataxia in the context of cerebellar function?
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What is the primary function of muscle spindles?
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Which type of nerve fibers are associated with muscle spindles?
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What information do Golgi tendon organs primarily provide?
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What occurs when a muscle is stretched in relation to muscle spindles?
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Which statement is true regarding Golgi tendon organs?
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What happens to muscle spindle activity during muscle contraction?
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What is the primary purpose of reflexes mediated by muscle spindles?
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What is the primary role of an agonist muscle during movement?
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Which tissue connects collagen strands in Golgi tendon organs to muscle fibers?
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Which reflex is characterized as monosynaptic and helps maintain muscle tone?
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What defines the flexor reflex?
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Which of the following best describes the function of synergist muscles?
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What is a key characteristic of the Golgi tendon reflex?
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Which reflex is characterized by an involuntary response that occurs when a muscle is stretched?
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What is the primary role of the ventral horns in the spinal cord?
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Which structure in the brain is primarily involved in the planning and execution of movement?
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Which of the following spinal cord components is responsible for carrying sensory input to the spinal cord?
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Which spinal cord structure is described as a bridge of gray matter that connects masses of gray matter?
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What is the primary function of the Golgi tendon organ reflex?
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Which white matter column in the spinal cord is located at the back and generally carries sensory information?
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Which neuronal groups are found in the gray matter of the spinal cord for processing visceral functions?
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What is the highest level of control in the hierarchy of motor control?
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Which type of pathways is responsible for delivering impulses from the brain to the spinal cord?
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What type of neurons are involved in the motor pathways to innervate skeletal muscles?
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Which tract is responsible for controlling fast and skilled movements directly?
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Which of the following pathways helps maintain balance by varying the tone of postural muscles?
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What is the role of the rubrospinal tract?
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Which area of the cerebral cortex is referred to as the primary motor cortex?
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What type of movements do indirect pathways primarily regulate?
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Which areas are involved in decision-making and planning actions in the motor cortex?
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Which tract is involved in mediating head movements in response to visual stimuli?
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What type of muscle control do axial muscles provide?
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Which area of the brain contributes sensory information to the motor control hierarchy?
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Which signaling pathway transforms actions into specific performance instructions?
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Study Notes
Motor Cortex: Initiating Movement
- Betz cells, located in layer 5 of the primary motor cortex (M1), are pyramidal cells that play a critical role in movement initiation.
- Two primary sources of input to Betz cells:
- Cortical areas
- Thalamus
- Many neurons in M1 encode force and direction of movement, although their tuning is broad.
- A single neuron's activity reflects a "vote" toward a specific direction; the collective activity of many neurons determines the actual direction of movement.
- Population vectors represent the collective activity of neurons, providing a comprehensive representation of movement direction.
Malleable Motor Maps
- Microstimulation of the M1 cortex typically triggers whisker movement.
- However, if the nerve supplying whisker muscles is severed, microstimulation will instead lead to forelimb or eye movements, highlighting the plasticity of motor maps.
Cerebellum: Fine-Tuning Movement
- Cerebellum is crucial for the coordinated execution of voluntary movements.
- Plays a significant role in motor learning, creating and refining motor programs to ensure smooth movement.
The Cerebellar Loop
- The corticopontocerebellar projection is a massive pathway between the motor cortex and the cerebellum, twenty times larger than the pyramidal tract.
- Axons from layer V pyramidal cells in the sensorimotor cortex form the majority of this projection.
Spinal Cord Anatomy
- Resembles a butterfly or the letter "H" in cross-section.
- Three areas of gray matter on each side of the cord, mirror images:
- Dorsal horns: interneurons receiving somatic and visceral sensory input.
- Ventral horns: some interneurons and somatic motor neurons.
- Lateral horns (thoracic and superior lumbar regions only): sympathetic neurons.
- Gray commissure connects the two halves of the gray matter, enclosing the central canal.
- Ventral roots: carry motor neuron axons exiting the spinal cord.
- Dorsal roots: carry sensory input to the spinal cord.
- Dorsal root (spinal) ganglia: contain cell bodies of sensory neurons.
- Spinal nerves: formed by the fusion of dorsal and ventral roots.
The Stretch Reflex
- Known as the knee-jerk or myotatic reflex.
- Usually monosynaptic.
- The brain sets a muscle's length, and the stretch reflex ensures it stays at that length, contributing to muscle tone and posture.
The Flexor Reflex
- Activated by painful stimuli, for example, stepping on a nail.
- Also known as the flexion, withdrawal, or "ouch" reflex.
- Always polysynaptic.
The Golgi Tendon Reflex
- Also known as the autogenic inhibition or inverse myotatic reflex.
- Always polysynaptic.
- Protects muscles from excessive tension.
Brain Control of Movement
- Hierarchical control of movement:
- Highest level: strategy
- Middle level: tactics
- Lowest level: execution
- The sensorimotor system utilizes sensory information at all levels of motor control.
Descending Motor Pathways
- Deliver efferent impulses from the brain to the spinal cord.
- Two main groups:
- Direct pathways: pyramidal tracts
- Indirect pathways: all other pathways
- Two types of motor neurons involved:
- Upper motor neurons: pyramidal cells in the primary motor cortex.
- Lower motor neurons: ventral horn motor neurons, innervate skeletal muscles.
Corticospinal Pathway (Direct)
- Impulses from pyramidal neurons in the precentral gyri travel through the pyramidal (lateral and ventral corticospinal) tracts.
- They descend directly to the spinal cord, synapsing at the end of the tract.
- In the spinal cord, axons synapse with either interneurons (lateral tract) or ventral horn motor neurons (ventral tract).
- Regulates fast and fine movements.
Indirect Pathways
- Also known as multineuronal pathways, complex and multisynaptic.
- Includes brain stem motor nuclei and all motor pathways besides the pyramidal pathways.
- Regulate:
- Axial muscles, maintaining balance and posture.
- Muscles controlling coarse limb movements.
- Head, neck, and eye movements tracking objects.
- Major indirect pathways:
- Reticulospinal and vestibulospinal tracts: regulate balance by adjusting postural muscle tone.
- Rubrospinal tracts: control flexor muscles.
- Tectospinal tracts: originate in the superior colliculi, mediating head movements in response to visual stimuli.
Planning of Movement by the Cerebral Cortex
- Motor cortex: areas 4 and 6 of the frontal lobe.
- Area 4: primary motor cortex (M1).
- Area 6: "higher" motor area, referred to by Penfield.
- Lateral region: premotor area (PMA).
- Medial region: supplementary motor area (SMA).
- Motor maps in PMA and SMA have similar functions but innervate different muscle groups.
Posterior Parietal and Prefrontal Cortex Contributions to Motor Control
- These areas represent the highest levels of motor control.
- They contribute to:
- Decisions about actions and their consequences.
- Area 5: receives input from areas 3, 1, and 2.
- Area 7: receives input from higher-order visual cortical areas, such as MT.
Higher-Level Motor Control
- Anterior frontal lobes: abstract thought, decision-making, and anticipating consequences of actions.
- Area 6: converts actions into signals specifying how they will be performed.
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