Spinal Control of Movement

Chapter 13: Spinal Control of Movement

Overview of Motor Programs

  • Motor System:

    • Definition: The collective group of muscles and neurons that control muscular movements in the body.

    • Role:

    • Generation of coordinated movements.

    • Two Connected Motor Systems:

    • Spinal Cord:

      • Responsible for controlling motor programs that generate coordinated muscle contractions, primarily through reflexive actions. This will be explored in-detail in this chapter.

    • Brain:

      • Regulates motor programs in the spinal cord, a topic which will be addressed in the following chapter.

The Somatic Motor System

  • Types of Muscles:

    • Smooth Muscles:

    • Location: Found in the digestive tract and arteries.

    • Striated Muscles:

    • Types include:

      • Cardiac Muscles: Present in the heart.

      • Skeletal Muscles: Comprise the bulk of body muscle mass.

    • Functionality: Muscles exist in opposing pairs; they work to either flex or extend joints, with the caveat that a muscle can only contract or relax, not push. An example includes:

      • Flexor (e.g., Biceps)

      • Extensor (e.g., Triceps)

    • Innervation: Each muscle fiber is innervated by one axon.

Lower Motor Neurons (LMNs)

  • Definition: Often considered the 'final common pathway' for motor control.

  • Location: Found in the ventral horn of the spinal cord.

  • Function: Sends axons through the ventral root to activate muscles directly.

  • Distinction: Lower motor neurons are distinctly different from upper motor neurons located in the primary motor cortex (to be discussed in the next chapter).

Types of Lower Motor Neurons

  1. Alpha Motor Neurons:

    • Function: Directly trigger muscle contraction.

    • Motor Unit: Defined as all muscle fibers innervated by one alpha motor neuron.

    • Motor Neuron Pool: Refers to all alpha motor neurons innervating a single muscle (e.g., biceps), consisting of multiple motor units.

  2. Gamma Motor Neurons:

    • Function: Involved in proprioception, helping muscles adjust their sensitivity.

Muscle Contraction Mechanism

  • Ca2+ Liberation:

    • Essential for muscle contraction. The following steps occur:

    1. An action potential is generated in the alpha motor neuron.

    2. Acetylcholine (ACh) is released onto the neuromuscular junction, binding to nicotinic ACh receptors (nAchRs).

    3. Sodium (Na+) channels in the sarcolemma open, leading to depolarization (EPSP).

    4. This depolarization triggers calcium (Ca2+) release from the sarcoplasmic reticulum, which facilitates the interaction between actin and myosin filaments, resulting in muscle contraction. The contraction leads to the shortening of muscle fibers, producing force and enabling movement. Furthermore, this force generation is influenced by the frequency of action potentials, as higher frequencies result in increased calcium release and stronger contractions. Additionally, the timing of these action potentials is crucial for smooth and coordinated movements, highlighting the importance of precise neuronal control in modulating muscle activity and ensuring that the body's responses are appropriately timed.

    5. The depolarization in T-tubules triggers the release of Ca2+ from the sarcoplasmic reticulum into the muscle fiber.

  • Excitation-Contraction Coupling: This process converts the electrical stimulus into mechanical contraction.

Sliding Filament Model of Muscle Contraction

  • Within each myofibril, alternating thin (actin protein) and thick (myosin protein) filaments are organized into functional units called sarcomeres.

  • Mechanism:

    • Full Contraction: Ca2+ released from the sarcoplasmic reticulum binds to proteins on the actin filament, which leads to the following sequence:

    • Myosin heads pivot and walk along actin filaments as long as Ca2+ and ATP are present.

    • When ATP binds, myosin heads disengage from actin, allowing the cycle to continue.

    • Muscle relaxation involves the reuptake of Ca2+ into the sarcoplasmic reticulum.

Graded Control of Muscle Contraction

  • Muscle Twitch: A single action potential in an alpha motor neuron leads to a muscle twitch.

  • Summation of Twitches: An increased frequency of incoming action potentials leads to sustained contractions.

  • Smoothest Contraction: Achieved at the highest firing frequency.

Types of Motor Units and Contractile Properties

  • Motor Unit Definition: Comprised of all muscle fibers innervated by one alpha motor neuron.

  • White Muscle Fibers:

    • Characterized by few mitochondria.

    • Contract rapidly and come in two types:

    1. Fast Fatigable (FF) Fibers: Powerful but fatigue quickly.

    2. Fatigue-Resistant (FR) Fibers: More resistant to fatigue but less powerful.

    • Functions in flight or fight responses and found in arm muscles.

  • Red Muscle Fibers:

    • Contain many mitochondria, contract slowly, and are fatigue-resistant.

    • Suitable for sustained contraction and found in antigravity muscles of the legs.

  • Distribution: All three muscle fiber types can coexist within a single muscle, but each motor unit contains fibers of only one type.

Organization of Lower Motor Neurons in the Spinal Cord

  • Segmental Organization:

    • Cervical Enlargement: Motor neurons here innervate arm muscles.

    • Lumbar Enlargement: Motor neurons in this region innervate leg muscles.

Functional and Anatomical Organization of Lower Motor Neurons

  • Functional Organization: Motor neurons that control flexors are located dorsal to those controlling extensors.

  • Anatomical Organization: Motor neurons controlling axial muscles are situated medially relative to those controlling distal muscles.

Inputs to Alpha Motor Neurons

  1. Upper Motor Neurons:

    • Origin: Brain (to be discussed next chapter).

  2. Proprioceptive Inputs:

    • From muscle spindles and enter through the afferent dorsal horn.

  3. Spinal Interneurons:

    • Provide both excitatory and inhibitory inputs to alpha motor neurons.

Proprioceptive Control of Motor Units

  • Sensory Feedback: Proprioceptive input from muscle spindles (stretch detectors within muscle fibers).

  • Signal transmission:

    • Carried by large and myelinated Aα sensory axons (Group Ia sensory axons).

Myotatic (Stretch) Reflex

  • Definition: A reflex mechanism involving muscle contraction when a muscle is stretched.

    • The process occurs in a monosynaptic feedback loop:

    • When a muscle is pulled, the spindle stretches, resulting in muscle contraction (e.g., knee jerk reflex).

  • Discharge Rate Dependency: The rate of the Aα Ia sensory axon discharge relies on the length of the stretched muscle.

Knee-Jerk Reflex (Example of Myotatic Reflex)

  • Mechanism:

    • Tapping the tendon of the quadriceps muscle induces a small stretch in the quadriceps muscle.

    • The muscle spindle also stretches, leading to the discharge of the Aα Ia sensory axon, which then triggers the alpha motor neuron to contract the quadriceps muscle reflexively.

  • The entire reaction occurs rapidly and is characterized as a monosynaptic reflex.

Maintenance of the Myotatic (Stretch) Reflex

  • Gamma Motor Neuron Function:

    • Innervates intrafusal fibers within the muscle spindle to adjust the set point of the myotatic feedback loop.

    • Maintains tension on intrafusal fibers, ensuring continued proprioceptive feedback even during muscle contraction.

  • Stages of the Reflex:

    • a & b: Initiation of reflex (complete knee jerk)

    • c: Maintenance of reflex aimed at preserving balance.

Other Proprioceptive Control: Golgi Tendon Organ (GTO)

  • Definition: Functions as a sensitive strain gauge, monitoring muscle tension (the force applied).

  • Location: Situated at the junction of muscle and tendon.

  • Interaction with Muscle Spindles: Together with muscle spindles, GTOs help in proprioception where the muscle spindles detect muscle length and GTOs detect muscle tension.

  • Mechanism: GTO activation happens when muscle fibers contract, leading to tension increase that activates mechanoreceptors.

Golgi Tendon Organ Circuitry

  • Feedback Loop Functionality:

    • Maintains muscle tension within an optimal range using feedback circuits.

    • Activation of the Ib (Aα) sensory axon by the GTO can also activate inhibitory interneurons that inhibit alpha motor neurons, thereby reducing force of contraction and allowing for finer motor control, especially while handling delicate or fragile objects.

Proprioception from the Joints

  • Proprioceptive axons are also located in joint tissues:

    • Respond to movements in joints, capturing information on:

    • Angle, Direction, and Velocity of movement.

  • This information synergizes with other proprioceptive data (muscle spindles and GTOs) to resolve body position in a three-dimensional space.

Reminder: The Somatic Motor System

  • Key Features of Skeletal Muscle:

    • Located in striated form, comprising the bulk of body muscle mass.

    • Operate in pairs to manage joint flexion and extension, functioning solely through contraction and relaxation.

Reciprocal Inhibition in the Same Joint

  • Definition: The contraction of one muscle pair is accompanied by the relaxation of its antagonist muscle.

  • Example: In the arm,

    • When biceps contract (monosynaptic reflex), the triceps (the antagonist) must simultaneously relax (polysynaptic reflex). Inhibitory interneurons play a critical role in this process.

Circuitry of the Flexor Withdrawal Reflex

  • Functionality of Reflex Arc: Allows for limb withdrawal from aversive stimuli (e.g., stepping on a tack).

    • Involves activation of Aδ (pain) axons that activate excitatory interneurons across adjacent spinal cord segments to trigger alpha motor neurons innervating flexor muscles, resulting in limb withdrawal.

    • Note: Extensor muscles on the same side would need relaxation for effective withdrawal.

Circuitry of the Flexor Withdrawal Reflex and Crossed-Extensor Reflex

  • Flexor Withdrawal Reflex:

    • Facilitates limb withdrawal to avoid aversive stimuli. However, the crossed-extensor reflex helps maintain balance: it activates appropriate extensor muscles and inhibits flexors on the opposite leg side, involving both inhibitory and excitatory interneurons, thus preventing the individual from falling.

Spinal Motor Programs for Walking

  • Central Pattern Generator:

    • A network of spinal interneurons that generates rhythmic activity, required for locomotion without the constant engagement of the brain.

    • The mechanism involves NMDA Receptors that play a critical role in depolarization leading to rhythmic activation as follows:

    1. Glutamate binds to NMDAR, leading to cell membrane depolarization.

    2. Sodium (Na+) and Calcium (Ca2+) ions flow into the cell via NMDAR.

    3. The Mg2+ block is removed, allowing continued activation pathways until necessary repolarization occurs through K+ efflux.

  • The cyclic process enables smooth walking patterns.

Hypothetical Circuit for Rhythmic Alternating Activity

  • Walking involves excitation and inhibition through interneurons in the spinal cord, generating rhythmic activity via central pattern generator action, allowing for the coordinated movement of flexors and extensors, leading to locomotion without conscious thought.