3. Movement Spinal Control
Sliding-filament model of muscle contraction
- Binding of Ca2+ to troponin causes myosin to bind to actin. - Myosin "heads" pivot, cause filaments to slide. - Repetition of process "walks" myosin heads along filament. - Sarcomeres can only contract (myofibrils shorten), not stretch.
Structure of a skeletal muscle
- Each skeletal muscle is enclosed in connective tissue sheath that forms tendons at the ends of the muscle. - Each muscle consists of hundreds of muscle fibers, each of which is innervated by a single axon from the CNS. -Somatic/voluntary nervous system: muscles and parts of the nervous system that control them (voluntary control!)
Proprioception
- The brain receives and interprets information from multiple inputs: - Vestibular Organs in the inner ear send information about rotation, acceleration, and position. - Eyes send visual information - Stretch receptors in skin, muscles, and joints send information about the position of body parts.
The neuromuscular junction: excitation-contraction coupling
-Alpha motor neurons release ACh. ACh activates nicotinic receptors, produces EPSP in the muscle fiber due to Na+ influx / K+ efflux. - EPSPs evoke muscle action potential. Action potential (excitation) triggers Ca2+ release inside, which leads to contraction of the fiber.
How does Ca2+ trigger contraction: sliding filament model
-Sarcomere: smallest functional unit of a skeletal muscle; defined by two Z lines. • Z lines: division of myofibril into segments by disks • Thin filaments: actin • Thick filaments: myosin Actin and myosin filaments generate the striation of the skeletal muscle!
Structure of a muscle fiber
1) Action potential sweeps down the sarcolemma: excitable cell membrane that encloses muscle fibers. 2) T tubules (T for transverse): provide access of action potential to sarcoplasmic reticulum (SR): extensive intracellular Ca2+ storage sac. 3) Ca2+ release -myofibrils/sarcomeres contract. Ca2+ reuptake, fiber relaxes.
The neuromuscular junction: excitation-contraction coupling
Alpha motor neurons release ACh. ACh activates nicotinic receptors, produces EPSP in the muscle fiber due to Na+ influx / K+ efflux.
How does Ca2+ trigger contraction: sliding filament model
Ca+ binding to troponin allows myosin heads to bind to actin—myosin heads then pivot, causing filaments to slide.
Some complex motor patterns can be generated without participation of the brain
Considerable amount of circuitry in the spinal cord for coordinated control of movement. Descending commands from the brain access, execute, and modify these motor programs.
Control of contraction by alpha motor neurons
From muscle twitch to sustained contraction: A) Single action potential in alpha motor neuron causes muscle fiber to twitch.Summation B) Number and frequency of incoming action potentials increase: summation of twitches causes sustained contraction. Complete sustained contraction: tetanus!
The brain guides our behavior
Input: sensory neurons Output: motor neurons
Distribution of motor neurons in the spinal cord
Motor neurons are neither distributed evenly throughout the body nor within the spinal cord. • Cervical enlargement: motor neurons that innervate arm muscles. • Lumbar enlargement: motor neurons that innervate leg muscles.
Movement: Spinal control
Motor system: all muscles and the neurons that control them. - Role: generation of coordinated movements - Two main parts of motor control (motor programs): • Spinal cord : control of coordinated muscle contraction • Brain : control of motor programs in spinal cord
Alpha and gamma motor neurons and the muscle fibers they innervate
Muscle spindle contains modified skeletal muscle fibers: • Alpha motor neurons innervate extrafusal fibers (skeletal standard muscle fibers, outside of spindle)
Alpha and gamma motor neurons and the muscle fibers they innervate
Muscle spindle contains modified skeletal muscle fibers: • Alpha motor neurons innervate extrafusal fibers (skeletal standard muscle fibers, outside of spindle) • Gamma neurons innervate intrafusal fibers (specialized proprioceptors) inside the muscle spindle
Behavior of a muscle after repeated stimulation
Regulation of contraction: discharge rate of motor neurons recruitment of additional motor units. - Tetanus: sustained muscle contraction evoked when the alpha motor nerve emits action potentials at a very high rate. -Motor unit has been maximally stimulated by its motor neuron and remains that way for some time.
The somatic sensory system
Sensation (such as pressure, pain, or warmth) that can occur anywhere in the body, in contrast to one localized at a sense organ (such as sight, balance, or taste). • Touch • Thermoreception • Nociception (pain) • Proprioception (sense of the relative position of neighboring parts of the body)
What regulates alpha motor neurons?
Three major sources of input into alpha motor neurons: a) Dorsal root ganglion cells that innervate the muscle spindle (feedback about muscle length) b) Upper motor neurons in motor cortex and brain stem (initiation/control of voluntary movement) c) Interneurons in the spinal cord (largest input, excitatory or inhibitory)
Reminder
afferent and efferent neurons in the spinal cord
Reminder: ionotropic nicotinic acetylcholine receptor
• 5 subunits, activation requires binding of 2 ACh molecules to the alpha subunits. • Cation channel: both Na+ (in) and K+ (out) flow through • Found at the NMJ. • Blocked by curare (receptor antagonist), an alkaloid poison. Causes skeletal muscle weakness and eventually death by asphyxiation. • Responds to nicotine (agonist).
The gamma stretch feedback loop: function of gamma motor neurons
• Activation of alpha motoneurons (a) shortens extrafusal muscle fibers. If spindle becomes slack, it no longer signals muscle length! • Consequence: this changes the set point of the stretch feedback loop: activation of gamma motoneurons (b) contracts poles of the spindle, keeping its sensitivity. • 'Parallel' configuration of muscle and spindle! Keeps spindle active and 'on the air'
Proprioception from Golgi tendon organs
• Additional proprioceptive input—acts like very sensitive strain gauge— monitors muscle tension (force of contraction). • Golgi tendon organs in series with fibers (not in parallel like spindles!). Located at muscle junctions and innervated by Ib axons that are entwined along collagen fibrils
ALS (Lou Gehrig's disease)
• Amyotrophic Lateral Sclerosis • Degeneration of large alpha motor neurons. • Many possible causes are being discussed; one suspected cause is excitotoxicity (overstimulation with glutamate toxic, block can slow disease). • Initially muscle weakness and atrophy; over 1-5 years, all voluntary movement is lost (walking, speaking, swallowing, breathing). -Result: complete paralysis. Death usually due to respiratory failure. • No effect on sensations, intellect, cognitive function
Reminder: The human nervous system
• Central nervous system (CNS) brain and spinal cord • Peripheral nervous system (PNS) somatic/voluntary NS - afferent neurons: from periphery to CNS - efferent neurons: CNS to periphery and autonomic (internal organs) NS
Proprioception summary: Muscle spindles vs. Golgi tendon organs
• Different arrangement: Muscle spindles in parallel with muscle fibers, Golgi tendon organs in series. • Different types of sensors to spinal cord: Ia activity from spindles signals muscle length, Ib activity from Golgi tendon organs signals muscle tension.
The Stretch Reflex (monosynaptic) Example: knee-jerk reflex
• Doctor taps tendon beneath kneecap, tendon briefly stretches the quadriceps muscle. • Ia axons respond, alpha motor neurons cause quadriceps muscle to reflexively contract, causing the leg to extend. • Procedure tests the intactness of nerves and muscles in this reflex arc.
Alpha motor neurons
• Excite skeletal muscles. Directly trigger generation of force by muscles. • Motor unit: single alpha motor neuron and all the muscle fibers it innervates. Elementary component of motor control. Contraction result of combined actions of motor units. • Motor neuron pool: all the alpha motor neurons that innervate a single muscle.
Golgi tendon organ circuit: the inverse stretch reflex (polysynaptic)
• Function of Golgi tendon organs: regulate muscle tension within optimal range (no over-stretching). Normal function: allows fine motor acts, steady (not too powerful) grip, e.g. for handling fragile objects. May prevent muscle from being overloaded. • Ib axon of Golgi tendon organ excites inhibitory interneuron, inhibits alpha motor neurons on same muscle. Inhibition slows muscle contraction.
Proprioception from Golgi tendon organs
• Golgi tendon organs respond to increased tension on the muscle and transmit this information to the spinal cord via group Ib axons. • Activated muscle does not change length in this example, therefore Ia axons remain silent.
Regulation of muscle contraction through synergistic motor units
• Mechanisms to achieve control of the strength of muscle contraction: a) Varying the firing rate of motor neurons (affects ACh release). b) recruiting additional synergistic motor units (smallest neurons recruited first, are more easily excited). • Number of muscle fibers innervated by a motor neuron critical for strength of contraction. • Vertebrate muscles usually have several hundred to thousand motor units.
The Stretch Reflex (monosynaptic)
• Monosynaptic: only one synapse separates primary sensory input from motor output. • Stretch reflex: muscle pulled Ia axons respond - muscle pulls back (contract) due to activation of alpha motor neurons. • Spindle measures muscle length: discharge rate of sensory axons related to muscle length (goes up when muscle is stretched). • Involves sensory feedback from muscle - absent when Ia axons are cut, loss of muscle tone.
Proprioception from muscle spindles
• Proprioceptors inform brain about position and motion of body in space and the relative position of body parts. • Sensory feedback from muscle spindles (stretch receptors) and their associated Ia axons deep within most skeletal muscles. • Specialized skeletal muscle spindles contained in fibrous capsule.
The somatic motor system: two major types of muscles
• Smooth muscle: digestive tract (peristalsis: movement of materials through intestines), arteries, related structures. Innervated by nerve fibers from the autonomic nervous system (ANS). • Striated muscle: cardiac (heart, rhythmic contraction even in absence of innervation; ANS slows or accelerates heart rate) and skeletal (bulk of body muscle mass).
Muscle innervation by lower motor neurons
• Somatic musculature is innervated by lower motor neurons in the ventral horn of the spinal cord. (Upper motor neurons of the brain supply input to the spinal cord). • Lower motor neurons directly command muscle contraction.