2.2 Peripheral Nervous System and Motor Units
4 min read•july 30, 2024
The plays a crucial role in motor control, connecting the brain and spinal cord to muscles and sensory receptors. Motor units, consisting of motor neurons and muscle fibers, form the basic functional units of movement, allowing for precise control of force and movement.
Sensory receptors like and provide vital feedback about muscle length, tension, and joint position. This proprioceptive information is essential for coordinating movements, maintaining balance, and adapting to changing environments during motor tasks.
Motor units and their components
Composition and location of motor units
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- A consists of a single and all of the muscle fibers it innervates
- The cell body of the alpha is located in the ventral horn of the spinal cord or brainstem motor nuclei
- The axon of the alpha motor neuron exits the spinal cord via ventral roots and travels through peripheral nerves to the muscle
- The axon branches to innervate multiple muscle fibers within a muscle (e.g., biceps brachii)
Neuromuscular junction and innervation patterns
- The is the synapse between the axon terminal of the motor neuron and the muscle fiber
- The number of muscle fibers innervated by a single motor neuron varies depending on the precision and force requirements of the muscle
- Muscles requiring fine control (e.g., extraocular muscles) have a low innervation ratio, with each motor neuron innervating a small number of muscle fibers
- Muscles involved in gross movements (e.g., gastrocnemius) have a high innervation ratio, with each motor neuron innervating a large number of muscle fibers
Types of motor units
Slow-twitch motor units (Type I)
- have small cell bodies, thin axons, and innervate a small number of oxidative, fatigue-resistant muscle fibers
- They are involved in maintaining posture and producing low-force, sustained contractions (e.g., maintaining an upright posture)
- These motor units are resistant to fatigue due to their high mitochondrial content and oxidative capacity
- They have a low activation threshold and are the first to be recruited during voluntary contractions
Fast-twitch motor units (Type IIa and IIb)
- Fast-twitch fatigue-resistant motor units () have medium-sized cell bodies, axons, and innervate a moderate number of glycolytic, fatigue-resistant muscle fibers
- They are involved in producing moderate-force contractions with some resistance to fatigue (e.g., walking or jogging)
- Fast-twitch fatigable motor units () have large cell bodies, thick axons, and innervate a large number of glycolytic, fatigable muscle fibers
- They are involved in producing high-force, rapid contractions but fatigue quickly (e.g., sprinting or weightlifting)
- The size principle states that motor units are recruited in order of increasing size, with smaller motor units being recruited first for low-force contractions and larger motor units being recruited as force requirements increase
Sensory receptors in motor control
Muscle spindles and Golgi tendon organs
- Muscle spindles are sensory receptors embedded within skeletal muscles that detect changes in muscle length and velocity
- They provide information about the position and movement of limbs (e.g., detecting the degree of elbow flexion)
- Golgi tendon organs are sensory receptors located at the junction between muscles and tendons
- They detect changes in muscle tension and provide information about the force generated by the muscle (e.g., sensing the force applied during a bicep curl)
Joint and cutaneous receptors
- Joint receptors, such as Ruffini endings and Pacinian corpuscles, are located in joint capsules and ligaments
- They provide information about joint position, movement, and pressure (e.g., sensing the position of the knee joint during walking)
- Cutaneous receptors in the skin, such as Meissner's corpuscles and Merkel's discs, provide information about touch, pressure, and vibration
- This information can be important for fine motor control and manipulating objects (e.g., sensing the texture and shape of a pen while writing)
Proprioception in motor control
Definition and importance of proprioception
- refers to the sense of body position, movement, and force, which is derived from sensory information provided by muscle spindles, Golgi tendon organs, joint receptors, and cutaneous receptors
- Proprioceptive information is essential for maintaining posture, coordinating movement, and making adjustments to motor commands based on the current state of the body
- The cerebellum plays a crucial role in processing proprioceptive information and integrating it with motor commands to ensure smooth, accurate, and coordinated movements (e.g., maintaining balance while walking on uneven terrain)
Consequences of proprioceptive deficits and training
- Proprioceptive deficits, which can occur due to injury or disease, can lead to impairments in balance, coordination, and the ability to perform skilled movements (e.g., difficulty walking or grasping objects following a stroke)
- Proprioceptive training, which involves exercises that challenge balance and body awareness, can be used to improve motor control and prevent injuries in athletes and rehabilitation settings (e.g., balance training on unstable surfaces for ankle sprain prevention)
- Examples of proprioceptive training include single-leg balance exercises, wobble board training, and joint position sense drills
Key Terms to Review (25)
Afferent feedback: Afferent feedback refers to the sensory information that is sent from the body's periphery to the central nervous system. This feedback is crucial for monitoring performance and making necessary adjustments during motor activities. It helps in refining movements and enhancing skill acquisition by providing real-time data about how well a task is being performed.
Afferent pathways: Afferent pathways are neural pathways that carry sensory information from sensory receptors to the central nervous system (CNS). These pathways play a crucial role in how the body processes sensory information and translates it into appropriate motor responses, connecting sensory input to motor output.
Alpha motor neuron: An alpha motor neuron is a type of lower motor neuron located in the spinal cord that innervates skeletal muscle fibers, directly controlling voluntary muscle movements. These neurons play a crucial role in transmitting signals from the central nervous system to the muscles, facilitating the execution of movements through neuromuscular junctions. Alpha motor neurons are essential for the regulation of muscle tone and coordination of motor activities.
Autonomic Nervous System: The autonomic nervous system (ANS) is a component of the peripheral nervous system that regulates involuntary bodily functions, including heart rate, digestion, and respiratory rate. It operates automatically without conscious control, allowing the body to maintain homeostasis and respond to stress or relaxation through its two main divisions: the sympathetic and parasympathetic systems.
Contractile properties: Contractile properties refer to the characteristics of muscle fibers that determine their ability to contract and generate force. These properties include aspects like the speed of contraction, the amount of force generated, and the duration of the contraction. Understanding these properties is crucial for analyzing how motor units function within the peripheral nervous system and how they contribute to movement and muscle control.
Efferent Pathways: Efferent pathways refer to the neural routes that carry signals away from the central nervous system (CNS) to various target organs, muscles, or glands. These pathways play a crucial role in executing motor commands and coordinating movements based on sensory input and learned behaviors. They are essential for translating decisions made by the brain into physical actions, ensuring an effective response to stimuli.
Fast-twitch motor units: Fast-twitch motor units are specialized groups of muscle fibers that contract quickly and powerfully but fatigue faster than slow-twitch fibers. These motor units are essential for high-intensity activities like sprinting or weightlifting, as they generate rapid bursts of force necessary for explosive movements.
Force production: Force production refers to the ability of muscles to generate tension and exert force, which is essential for movement and physical activity. This process is heavily influenced by the coordination of motor units, which are made up of motor neurons and the muscle fibers they innervate, along with the peripheral nervous system's role in controlling muscle contractions.
Golgi tendon organs: Golgi tendon organs are proprioceptive sensory receptors located at the junction of muscles and tendons that detect changes in muscle tension. They play a crucial role in providing feedback about force and load on muscles, helping to regulate muscle contractions and prevent injury during movement.
Motor neuron: A motor neuron is a type of nerve cell responsible for transmitting signals from the central nervous system to the muscles, facilitating movement. These neurons play a crucial role in motor control by sending electrical impulses that initiate muscle contractions, allowing for coordinated and voluntary movements. They are essential components of motor units, which consist of a motor neuron and the muscle fibers it innervates.
Motor unit: A motor unit is the functional unit of muscle contraction, consisting of a motor neuron and all the muscle fibers it innervates. It plays a crucial role in controlling muscle force and coordinating movement, with the number of fibers in a motor unit varying depending on the precision required for specific tasks.
Motor unit recruitment: Motor unit recruitment refers to the process by which the nervous system activates additional motor units to increase muscle force production during physical activities. This mechanism is crucial for ensuring that muscles can generate the necessary force to perform tasks ranging from fine movements to powerful actions. As more motor units are recruited, the strength and precision of muscle contractions improve, allowing for a wide range of physical performance.
Muscle activation: Muscle activation refers to the process of initiating muscle contractions through neural signals, which ultimately leads to movement and force generation. This process is essential for any physical activity, as it involves the recruitment of motor units, which are made up of motor neurons and the muscle fibers they control. Understanding muscle activation is key to comprehending how the peripheral nervous system communicates with muscles to produce coordinated movements.
Muscle fiber types: Muscle fiber types refer to the different classifications of muscle fibers based on their physiological and biochemical properties. These types include slow-twitch (Type I) fibers, which are more resistant to fatigue and are suited for endurance activities, and fast-twitch (Type II) fibers, which generate more force and are better for short bursts of power. The characteristics of these muscle fiber types influence how muscles respond to training and physical demands.
Muscle spindles: Muscle spindles are specialized sensory receptors located within the belly of muscles that play a critical role in proprioception, which is the body's ability to sense its position and movement. They contain intrafusal muscle fibers and are sensitive to changes in muscle length and the rate of that change, providing important feedback to the central nervous system about muscle stretch and tension. This information helps regulate muscle tone, reflexes, and overall movement coordination.
Neuromuscular Junction: The neuromuscular junction is a specialized synapse between a motor neuron and a muscle fiber, allowing for the transmission of signals that initiate muscle contraction. This crucial connection facilitates communication between the nervous system and the muscular system, making it essential for voluntary movement and muscle control.
Nicholas Bernstein: Nicholas Bernstein was a pioneering Russian physiologist and biochemist known for his significant contributions to understanding motor control and coordination. He emphasized the importance of the relationship between the nervous system and movement, particularly through his exploration of motor units and how they work in coordination during complex actions like gait. His work laid the groundwork for later research into the dynamics of movement and the neural control of locomotion.
Peripheral Nervous System: The peripheral nervous system (PNS) is the part of the nervous system that connects the central nervous system (CNS) to the limbs and organs. It plays a crucial role in transmitting sensory and motor information between the brain and the body, facilitating coordination and movement.
Proprioception: Proprioception is the body's ability to sense its position, movement, and equilibrium through sensory receptors located in muscles, tendons, and joints. This internal feedback system is crucial for coordinating movements and maintaining balance, allowing individuals to perform motor tasks effectively and adapt to changing environments.
Richard Schmidt: Richard Schmidt is a prominent figure in the field of motor learning and control, known for his significant contributions to understanding how humans acquire and refine motor skills. His work emphasizes the importance of feedback, practice variability, and the theoretical frameworks that explain how motor skills are learned and executed.
Slow-twitch motor units: Slow-twitch motor units are specialized muscle fibers that are designed for endurance activities, primarily using aerobic metabolism to generate energy. These motor units are characterized by a high resistance to fatigue, allowing them to sustain prolonged contractions, making them essential for activities like long-distance running or cycling. They are innervated by smaller motor neurons and have a slower contraction speed compared to fast-twitch motor units.
Somatic Nervous System: The somatic nervous system is a component of the peripheral nervous system responsible for transmitting sensory and motor information to and from the central nervous system. It controls voluntary movements by activating skeletal muscles, allowing for conscious control over activities like walking or picking up objects, and is essential in the functioning of motor units that facilitate these movements.
Type I: Type I refers to a category of muscle fibers, also known as slow-twitch fibers, that are characterized by their ability to sustain prolonged activity due to their high oxidative capacity. These fibers are rich in mitochondria and myoglobin, which enables them to use oxygen efficiently for energy production, making them ideal for endurance activities like distance running or cycling. Type I fibers are crucial for maintaining posture and performing activities requiring stamina.
Type IIA: Type IIA muscle fibers, also known as fast oxidative glycolytic fibers, are a type of muscle fiber characterized by their ability to generate force quickly and sustain activity for longer periods compared to Type I fibers. These fibers are essential for activities that require both speed and endurance, making them vital in sports and physical tasks that involve moderate to high intensity over extended durations.
Type IIB: Type IIB fibers are a classification of muscle fibers that are fast-twitch and primarily anaerobic, allowing for rapid and powerful contractions. These fibers are particularly important for activities requiring quick bursts of energy, such as sprinting or weightlifting, and are characterized by a higher diameter, greater force production, and fatigue more quickly than other fiber types.