Neuromotor System Flashcards
Skeletal Muscle Functions:
Primary Motor Cortex: Located in the precentral gyrus, directly responsible for voluntary movement. It is organized somatotopically, meaning body regions are mapped in specific areas, with fine movements (e.g., hands, face) occupying larger areas.
Additional Functions:
- Heat Production: Skeletal muscle generates heat through shivering thermogenesis, particularly in axial muscle groups, helping maintain body temperature.
- Metabolic Regulation: Involved in glucose and fat metabolism, with myokines influencing whole-body energy balance.
- Joint Stability: Muscle tension around joints stabilizes them, preventing dislocations.
- Protection: Shields internal organs by absorbing shock and providing a physical barrier
Excitatory Endplate Potential (EPP) Generation:
Process: Acetylcholine (ACh) released at the neuromuscular junction binds to ACh receptors on the muscle fiber, causing Na⁺ influx. This depolarizes the endplate, creating an EPP, which, if strong enough, triggers a muscle action potential
Clustering of Acetylcholine Receptors (AChRs):
Mechanism: AChRs are densely clustered under the motor nerve terminal due to interaction with proteins like MuSK, which helps maintain efficient signal transmission at the neuromuscular junction
Neuromuscular Synapse Structure and Function:
Built-in redundancy of AChRs ensures that the EPP reliably reaches the threshold to trigger an action potential, preventing transmission failure even with fluctuating ACh release.
Motor Neuron Disease (MND) & Myasthenia Gravis (MG):
MND: Progressive degeneration of motor neurons, causing muscle weakness and atrophy.
MG: Autoimmune disorder where antibodies block AChRs, impairing neuromuscular transmission and causing muscle fatigue
Treatments for Myasthenia Gravis:
First-Line Drug: Pyridostigmine, an acetylcholinesterase inhibitor, increases ACh availability by preventing its breakdown, improving muscle strength.
Other Treatments: Immunotherapy (e.g., corticosteroids) and complement inhibitors (e.g., eculizumab) are used for different types of MG.
Duchenne Muscular Dystrophy (DMD):
Cause: Genetic lack of dystrophin, leading to progressive muscle degeneration.
Treatment: Corticosteroids (like prednisone) help reduce inflammation, though long-term use has side effects. Other treatments include gene therapy for dystrophin replacement.
Rate Modulation:
Definition: Increasing the frequency of action potentials increases tension in muscle fibers by summing twitches into a stronger contraction
Motor Unit Recruitment & Henneman’s Size Principle:
Mechanism: Smaller motor units are recruited first for low-force tasks, with larger units recruited as needed for higher force, ensuring smooth increases in muscle tension
What is a motor unit?
A single motor neuron and all the muscle fibers it controls. Motor units vary in size depending on the type of movement (e.g., fine control in fingers requires smaller units, while postural muscles have larger units).
Fiber Type Grouping and Muscle Strength:
Explanation: Loss of motor neurons can lead to neighboring neurons innervating different fiber types, causing a mix of slow- and fast-twitch fibers, which can reduce overall muscle strength
Motor Neuron Activity & Muscle Fiber Type:
Relationship: Motor neuron activity patterns determine whether a muscle fiber becomes slow- or fast-twitch, adapting to the demands placed on it
Compound Muscle Action Potentials (CMAPs):
Latency Period: Reflects the time between electrical stimulation and the onset of muscle response. Important in diagnostics to measure neuromuscular function and motor unit recruitment.
CMAP and Latency: The latency period is the time between nerve stimulation and the start of a compound muscle action potential (CMAP), showing how quickly the action potential travels from the motor neuron to muscle fibers.
Motor Unit Recruitment and Tension Latency: Motor units are recruited progressively for increased force. Tension latency is longer than CMAP latency because it includes additional delays from muscle contraction processes like calcium release and cross-bridge cycling.
Latency Differences: Electrical activity latency is brief, reflecting fast impulse transmission, while tension latency is longer due to the mechanical steps needed for force generation in muscle.
Exercise and Muscle Adaptations:
Benefits of Intense (Anaerobic) Exercise:
- Increased Muscle Strength and Power: High-intensity exercises like weightlifting and sprinting help build muscle mass and improve power, beneficial for overall strength and metabolism.
- Improved Metabolic Rate: Anaerobic exercise boosts metabolic rate even after workout completion, promoting calorie burn and fat loss.
- Enhanced Insulin Sensitivity: Intense exercise improves insulin sensitivity, aiding in blood sugar regulation and reducing diabetes risk.
- Improved Bone Density: Weight-bearing exercises strengthen bones, reducing the risk of osteoporosis.
- Cardiovascular Health Boost: High-intensity interval training (HIIT) has been shown to improve cardiovascular efficiency and reduce blood pressure.
Benefits of Endurance (Aerobic) Exercise:
- Enhanced Cardiovascular Health: Endurance exercises like running, cycling, and swimming improve heart and lung function, lowering risks of heart disease and hypertension.
- Improved Stamina and Endurance: Regular aerobic exercise enhances the body’s ability to sustain prolonged activity, improving aerobic capacity and endurance.
- Boosted Mental Health: Aerobic exercise releases endorphins, reducing stress, anxiety, and depression while enhancing mood and cognitive function.
- Better Weight Management: Sustained aerobic exercise helps maintain a healthy weight by burning calories and enhancing fat metabolism.
- Reduced Chronic Disease Risk: Endurance exercise lowers the risk of conditions like type 2 diabetes, certain cancers, and stroke.
Three Levels of Input Control to Motor Neurons:
Higher Brain Centers: The motor cortex and brainstem initiate and adjust voluntary and involuntary movements.
Spinal Interneurons: Central pattern generators and reflex arcs in the spinal cord manage rhythmic movements and reflex responses.
Sensory Feedback: Muscle spindles and Golgi tendon organs provide real-time feedback on muscle length and tension, adjusting α-motor neuron activity to maintain coordination and prevent injury.
Sensory receptors that monitor muscle and movements = Proprioception
Proprioception involves sensory receptors that monitor muscle activity and body movement, providing feedback on body position and movement. Key proprioceptive receptors include:
Muscle Spindles: Located within muscles, they detect changes in muscle length and rate of stretch, helping to adjust muscle tone and coordinate movements.
Golgi Tendon Organs (GTOs): Found at the muscle-tendon junction, they monitor muscle tension and prevent excessive force by inhibiting muscle contraction if tension is too high.
Joint Receptors: Situated in joint capsules and ligaments, they sense joint angle, speed, and direction of movement, contributing to spatial awareness.
What is the major proprioceptor for sensing and controlling movement ?
The muscle spindle is the major proprioceptor responsible for sensing and controlling movement. Located within skeletal muscles, muscle spindles detect changes in muscle length and the rate of stretch, providing critical feedback to the nervous system.
Through this feedback, muscle spindles:
- Help regulate muscle tone, adjusting contraction to maintain stability.
- Enable reflexive adjustments, such as the stretch reflex, to prevent overstretching and ensure coordinated movement.
- Contribute to precise, real-time control of movements by continuously relaying information about muscle dynamics.
How are motor neurons organized within the spinal cord
Motor Pools: Motor neurons controlling a specific muscle are grouped into clusters called motor pools, which are located in the ventral (anterior) horn of the spinal cord. Each motor pool sends signals to a particular muscle, allowing for organized and specific motor control.
Somatotopic Arrangement: Neurons are positioned according to the muscles they control; proximal muscles are managed by medially located neurons, while distal muscles are managed by lateral neurons.
Segmental Organization: Each spinal segment controls muscles at its respective body level, with cervical segments for upper limbs, thoracic for the trunk, lumbar for lower limbs, and sacral for lower trunk and legs.
Interneuronal Connections: Interneurons connect motor and sensory neurons, enabling reflexes, rhythmic movements, and coordinated muscle actions.
How do groups of motor neurons (motor pools) work together to mediate movement ? The role of interneurons –
as seen in complex reflexes. Key components of neuromotor circuits.
Motor Pool Coordination: Motor pools work together to control specific muscles, adjusting contraction strength and timing for precise movements.
Interneurons in Reflexes: Interneurons modulate motor pools in complex reflexes, coordinating muscle groups for quick, balanced responses like the withdrawal reflex.
Sensory Feedback: Sensory receptors provide real-time muscle status, allowing for immediate adjustments in motor output.
Neuromotor Circuit Components: Central pattern generators and descending brain signals integrate with motor pools for rhythmic and voluntary movement control. And sensory receptors provide feedback on muscle status and position.
What is reciprocal inhibition?
Reciprocal inhibition is a neural process where the contraction of one muscle is accompanied by the relaxation of its opposing muscle. This coordination is crucial for smooth and efficient movement.
When a motor neuron activates a muscle (agonist), inhibitory signals are sent via interneurons to the motor neurons of the opposing muscle (antagonist), causing it to relax.
unctions of Muscle Spindles, Golgi Tendon Organs, and Joint Receptors:
Muscle Spindles: Detect changes in muscle length and the rate of stretch, helping to regulate muscle tone and triggering stretch reflexes for posture and balance.
Golgi Tendon Organs (GTOs): Located at muscle-tendon junctions, they monitor muscle tension, inhibiting motor neurons when tension is too high to prevent muscle damage.
Joint Receptors: Found in joint capsules, they provide information on joint angle, speed, and movement direction, contributing to proprioception and aiding in coordinated movement.
Role of Gamma Motor Neurons:
Gamma Motor Neurons maintain sensitivity of muscle spindles. They adjust the length of muscle spindles in response to muscle contraction, keeping spindles active even when the muscle is shortened, thus allowing continuous monitoring of muscle length and ensuring accurate reflex responses.
Reciprocal Inhibition in the Knee-Jerk Reflex:
In the knee-jerk reflex, tapping the patellar tendon stretches the quadriceps muscle, activating muscle spindles and triggering a reflex contraction of the quadriceps. Simultaneously, reciprocal inhibition ensures the hamstring (antagonist muscle) relaxes by inhibiting its motor neurons through interneuron pathways, allowing smooth knee extension without resistance.
Enhancing a Stretch Reflex:
A stretch reflex can be enhanced by increasing muscle spindle sensitivity via gamma motor neuron activation or by an external increase in muscle stretch. Additionally, voluntary pre-contraction or mental focus on the target muscle can also enhance the response, often seen in “Jendrassik maneuver,” which amplifies reflexes by engaging upper limb muscles.
Complex Reflexes and Interneurons:
Complex reflexes, such as the crossed extensor withdrawal reflex, involve interneurons and motor neuron groups. For example, if you step on something sharp, the withdrawal reflex causes the stimulated leg to flex, while interneurons cross to the opposite side of the spinal cord to activate extensors in the other leg, providing balance and support as you withdraw the injured leg.
Postural Adjustment Mechanisms
Feedback (Reflex) Mechanism: This mechanism provides immediate corrections in response to unexpected disturbances in posture. It relies on sensory feedback from proprioceptors (e.g., muscle spindles and Golgi tendon organs), visual inputs, and the vestibular system to adjust body posture and prevent falls. Reflexive actions are generated through spinal circuits and brainstem pathways to activate muscles quickly, stabilizing the body. Provides quick corrective responses to balance disturbances, relying on sensory data from proprioceptive, vestibular, and visual systems.
Feedforward (Anticipatory) Mechanism: Anticipatory adjustments are planned responses that activate before a voluntary movement. These are based on previous experiences and rely on input from brainstem nuclei and sensory receptors to pre-emptively stabilize posture. For example, before reaching out to grab an object, anticipatory adjustments shift body weight to maintain balance, ensuring smooth and stable movement. Anticipates postural changes based on prior experience, helping stabilize the body during planned movements.
Anticipatory Mechanisms: These include inputs from proprioceptors, vestibular inputs, and brainstem nuclei like the vestibular nuclei, which play a role in stabilizing posture by coordinating muscle activity to anticipate shifts in balance.
Major Brainstem Nuclei
Vestibular Nuclei: Integrate signals from the vestibular apparatus to maintain balance and coordinate eye movements with head and body movements.
Reticular Formation: Modulates muscle tone, cardiovascular control, and motor control by influencing the activity of motor neurons.
Red Nucleus: Involved in motor coordination and fine-tuning movements, especially important in animals, although it has a reduced role in human motor control due to corticospinal tract dominance.
Vestibular Apparatus and Postural Control
The Vestibular Apparatus in the inner ear provides continuous information about head orientation and movement relative to gravity. By detecting linear and angular accelerations, it helps adjust motor neuron output to extensor muscles that stabilize posture. This feedback is crucial for maintaining balance and coordinating head, eye, and body movements.
Modification of Postural Reflexes by Anticipatory Mechanisms
Reflexive postural responses, such as the stretch reflex, are modified by anticipatory signals that prepare the body for expected postural shifts. This anticipatory input helps refine reflexive responses, creating smoother and more controlled reactions to disturbances and enhancing movement stability.
Cerebellum Structure and Divisions
Vestibulocerebellum: Manages balance and eye movements by coordinating closely with the vestibular system, particularly for stabilizing gaze and maintaining upright posture. Interacts with the vestibular system for balance and eye movement coordination.
Spinocerebellum: Controls muscle tone, posture, and limb position, receiving proprioceptive inputs from the spinal cord to adjust movements during physical activities. Regulates posture and muscle tone by integrating proprioceptive input.
Cerebrocerebellum: Involved in planning, timing, and learning of complex voluntary movements, coordinating closely with the motor cortex. Works with the cerebral cortex to plan and time complex, voluntary movements.
Cerebellum as a Comparator
The cerebellum functions as a comparator, comparing the intended movement plan (from motor cortex input) with sensory feedback of the actual movement. When discrepancies are detected, it issues corrective signals to adjust muscle activity, ensuring that movements are accurate, fluid, and coordinated.
Motor Cortex Functions and Organization
Primary Motor Cortex (M1): Located in the precentral gyrus, M1 is responsible for initiating and directing voluntary movements. It is organized somatotopically, meaning different regions of M1 correspond to specific body parts, with larger areas dedicated to fine control (e.g., hands and face).
Pre-Motor Areas: The supplementary motor area (SMA) and premotor cortex assist with planning complex, coordinated movements, especially those requiring bilateral coordination or sensory input. They work closely with M1 to execute movement plans and integrate sensory feedback to adjust motor plans.
Corticospinal Tract and Motor Control
The Corticospinal Tract (pyramidal tract) originates in the motor cortex and descends through the internal capsule and brainstem, decussating at the medulla-spinal cord junction. This pathway allows precise voluntary control by synapsing directly onto lower motor neurons or interneurons in the spinal cord, which activate specific muscle groups.
Neurotransmitters in Motor Control
Glutamate (+): The primary excitatory neurotransmitter in the CNS, crucial for activating motor pathways.
Acetylcholine (+): Acts at the neuromuscular junction, directly stimulating muscle contraction.
Glycine (-): An inhibitory neurotransmitter within the spinal cord, balancing excitation to prevent overstimulation.
GABA (-): The main inhibitory neurotransmitter in the CNS, essential for regulating motor control and reflex activity.
Differences Between Upper and Lower Motor Neuron Lesions
Upper Motor Neuron Lesions: Damage above the spinal cord (e.g., in the cortex or corticospinal tract). Symptoms include spasticity, hyperreflexia, and a positive Babinski sign due to loss of inhibitory signals.
Lower Motor Neuron Lesions: Damage at the spinal or peripheral nerve level. Symptoms include muscle atrophy, hyporeflexia, fasciculations, and weakness, resulting from loss of direct muscle innervation
Cerebellum as a Comparator
Compares intended movement (from motor cortex) with actual performance (sensory feedback). The cerebellum sends corrective signals to adjust and refine ongoing movements, ensuring accuracy and fluidity
Effects of Cerebellar Lesions on Movement
Ataxia: Uncoordinated movements and poor balance due to disrupted cerebellar processing.
Dysmetria: Inaccurate movement range and force, causing overshooting or undershooting of targets.
Intention Tremor: Tremor that worsens with voluntary movement, resulting from an inability of the cerebellum to provide smooth movement adjustments.
