Movement I

Movement Part I Spinal Control of Movement

Overview Alpha motor neurons, which innervate the skeletal muscle fibers, are the final common pathway for behavior. They are wired into a complex set of reflex loops in the spinal cord. These reflex loops are supplemented by locomotor programs in the spinal cord which provide the basic rhythmic aspects activities such as walking.

Types of Muscle Smooth muscle digestive system & arterioles innervated by adrenergic autonomic nervous system Cardiac muscle (striated) heart muscle modulated by autonomic nervous system Skeletal muscle (striated) body and eye movement breathing controlled by lower motor neurons in spinal cord

Skeletal muscles are the effectors of movement .

Categories of Muscles Categories based on direction of motion Categories based on body location

Types by Body Location Axial muscles move trunk Proximal muscles move shoulder, elbow, pelvis, knee Distal muscles move hands, feet, digits

Muscles Are the Effectors of Movement All animal movement is based on contraction of muscles working against some type of skeleton The action of a muscle is always to contract Muscles extend only passively To move body parts in opposite directions, muscles are attached in antagonistic pairs Example: Bicep contracts  arm flexes Bicep relaxes; triceps contracts  arm extends

Types by Direction of Motion Flexors reduce angle of joints Extensors increase angle of joints Synergists all flexor muscles working together on one joint all extensors working together on one joint muscles that work in parallel Antagonists flexors and extensors for one joint muscles that work in opposition

Structure of Skeletal Muscle Formed from a hierarchy of smaller & smaller parallel units Each muscle consists of a bundle of long fibers, the length of the muscle Each fiber is a single cell with many nuclei Each fiber is a bundle of smaller myofibrils Myofibrils are formed from 2 types of myofilaments: Thick & thin Myofilaments are formed from 2 key proteins: Actin & myosin

Myofilaments Thin filaments Two strands of actin and one of regulatory protein Thick filaments Staggered arrays of myosin molecules

Sarcomeres Skeletal muscle is striated: The regular arrangement of myofilaments creates a repeating pattern of light & dark bands Each repeating unit = sarcomere The basic contractile unit of muscle

Z-Lines The borders of the sarcomere = Z-lines These are lined up in adjacent myofibrils Thin filaments are attached to the Z-lines and project toward the center of the sarcomere Thick filaments are centered in the sarcomere

Banding At rest, thick & thin filaments don’t overlap completely The area at the edge of the sarcomere where there are only thin filaments = I band The broad region of thick filaments = A-band H - zone is in the center of the A-band and contains only thick filaments The arrangement of thick & thin filaments is the key to muscle contraction

Filaments & Contraction When a muscle contracts, the length of each sarcomere is reduced The distance from one Z-line to the next gets shorter The A-bands don’t change, but the I-bands shorten The H-zone disappears

The Sliding Filament Model Neither group of filaments changes length when a muscle contracts Rather, the filaments slide past each other, so the overlap increases If the overlap increases, the area of only thin filaments (I-band) and the area of only thick filaments (H-zone) decreases

The sliding filament model of muscle contraction.

Actin & Myosin Thick & thin filaments are formed from actin & myosin The myosin “head” is the site of bioenergetic reactions that power muscle contraction

Interaction of Actin & Myosin Myosin head binds ATP and hydrolyzes it to ADP The energy released is transferred to myosin The myosin changes shape The energized myosin binds a specific site on the actin molecule, forming a cross-bridge This releases energy, relaxing the myosin head

Actin & Myosin (continued) The myosin changes shape and bends inward on itself This exerts tension on the thin filaments to which it is bound Which pulls the thin filaments toward the center of the sarcomere When a new ATP molecule binds the myosin head, the bond between myosin & actin is broken The cycle repeats

The Repeating Cycle Each of the ~ 350 myosin heads of a thick filament forms and reforms 5 cross-bridges/sec Producing muscle contractions

Energy Muscle cells store only enough ATP for a few muscle contractions They store glycogen between myofibrils Most energy for muscles is stored in phosphagens In vertebrates = creatine phosphate

Motor Neurons & Movement A muscle contracts only when stimulated by a motor neuron An action potential in a motor neuron connected to muscle causes it to contract Ca ++ ions and regulatory proteins control muscle contractions

Regulatory Proteins When a muscle is at rest, myosin binding sites on actin are blocked by regulatory proteins, tropomyosins The position of tropomyosin on the thin filaments is controled by troponin complex . Another set of regulatory proteins For a muscle to contract, the myosin binding site on actin must be exposed

The Role of Ca ++ When Ca ++ binds to troponin alters the tropomyosin-troponin complex, exposing the mysosin binding sites on actin. When Ca ++ is present, filaments can slide and muscles contract When Ca ++ levels decrease, contraction stops

The Sarcoplasmic Reticulum Ca ++ in the cytosol of a muscle cell is regulated by the sarcoplasmic reticulum (specialized type of ER) Surrounds myofibrils; sequesters and releases calcium The membrane of the sarcoplasmic reticulum (SR) actively transposrt Ca++ from the cytosol to the interior of the SR An interior storehouse for Ca++

Motor Neurons Spinal organization Lower motor neurons Alpha motor neurons Motor units Motor neuron pools

Spinal Organization Lower Motor Neurons Motor neuron fibers exit the spinal cord in the ventral root of each spinal segment cell bodies in ventral horn Cell bodies have a somatotopic arrangement There are bulges in the ventral horn because of the large number of motor neurons for the arms and for the legs

Alpha Motor Neurons Neuron directly responsible for synapsing on muscle fibers and causing movement final common pathway for behavior Sources of direct input Sensory input from muscle spindles Input from spinal interneurons Descending input from upper motor neurons (e.g. motor cortex) Controlling the force of muscle contraction

The Neuromuscular Junction Action potential in a motor neuron connected to a muscle causes contraction The synaptic terminal of the motor neuron releases acetylcholine at the neuromuscular junction, depolarizing the muscle cell Sarcolemma external, electrically excitable membrane of a muscle fiber

Excitation  Contraction How the action potential in a motor neuron causes muscle contraction Nicotinic ACh receptors (transmitter-gated ion channel) open Na + channels  EPSP Muscle fiber generates action potential which sweeps down the sarcolemma

Transverse Tubules Transverse (T) tubules = infoldings of sarcolemma (membrane) Conduct the action potential inward Depolarization of T-tubules activates a voltage sensitive protein that plugs Ca ++ channels in SR Where the T-tubules touch SR, the action potential changes the permeability of the SR, causing release of Ca ++ Calcium is released and floods myofibrils Ca ++ binds to troponin, allowing the muscle to contract

Relaxation Contraction stops when the SR pumps Ca++ out of the cytosol and troponin-tropomyosin complex blocks myosin binding sites as Ca++ concentration decreases Calcium ions are sequestered by SR via an ATP-driven Ca ++ pump Myosin binding sites on actin are covered by troponin

Graded Contractions Muscle contractions are graded some are strong, some are weak We can voluntarily alter the strength of a contraction At a cellular level, the response is all or none Any stimulation that depolarizes the plasma membrane of a single muscle fiber triggers a contraction Like in a neuron So how are contractions graded?

Creating Graded Responses Nervous system can vary the frequency of action potentials in motor neurons Action potential summation  gradation Rate coding each action potential produces a muscle twitch fire faster and produce stronger contraction If the rate of stimulation is fast enough, individual twitches become one smooth contraction = tetanus Not the same as the disease!

Temporal summation of muscle contraction: muscle tension resulting from 1, 2, or a series of action potentials.

The Motor Unit One alpha motor neuron and all the muscle fibers it innervates Each muscle fiber is innervated by only one motor neuron Each motor neuron may synapse with many muscles cells Motor units range in size from 1:3 (fine control) to 1:1000 (leg muscles)

Structure of a vertebrate motor unit.

The Role of Motor Units When a motor neuron fires, all of the muscle fibers it controls contract as a group Graded contraction then depends on how many motor units are activated and whether they are small or large motor units Motor units are recruited in the order of increasing size i.e. small units are always recruited first

Motor Neuron Pool All of the motor neurons that innervate a single muscle All the muscle fibers enclosed in a single sheath with a single tendon e.g. biceps brachii, gastrocnemius

Recruitment Muscle tension can be increased by activating more of the motor neurons controlling a muscle = recruitment The brain recruits motor neurons based on the task Recruiting synergists activate more motor units that work to move in same direction, produce more force

Duration An action potential triggers a muscle to contract The duration is controlled by how long the Ca ++ concentration in the cytosol is elevated Muscle fibers are specialized for fast or slow contraction The type of motor neuron determines the type of muscle fiber

Types of Motor Units Fast motor units Muscle fibers used for short, rapid, powerful contractions rapidly fatiguing, white muscle fibers burst firing patterns in motor neuron Slow motor units slowly fatiguing, red muscle fibers slow, steady firing patterns in motor neuron Can sustain long contractions Often found in muscles that maintain posture

Specialization of Slow Muscle Fibers Slow muscle fibers must sustain long contractions Have less SR Slower Ca ++ pumps Many mitochondria for a steady energy supply Contain myoglobin – Specialized oxygen storing protein Greater affinity for oxygen than hemoglobin, so it can extract oxygen from the blood

Motor Units & Activity Activity (exercise, athletic training) may change the type of motor neuron Patterns of activity may change motor unit type Levels of activity increase muscle bulk (especially isometric exercise)

Spinal Control of Motor Units How a motor neuron is controlled Sensory feedback from the muscles Muscle spindles Specialized structures within skeletal muscles Specialized muscle fibers contained in a fibrous capsule Muscle fibers are wrapped in the middle with with Ia sensory axons Spindles & their Ia axons are specialized to detect changes in muscle length (stretch)

Proprioception Proprioception = “body sense” Understanding how our body is positioned and moving in space Muscle spindles and Ia axons are proprioceptors Part of the somatic sensory system Myotactic reflex provides one path of sensory input to the spinal cord

Myotatic or Stretch Reflex When a muscle is stretched by an external force, the opposite muscle is also stretched Stretching a muscle spindle increases firing rate of the associated nerve Nerve makes excitatory synapse with a motor neuron Alpha motor neuron increases firing rate Muscle fibers contract, muscle spindle is no longer stretched, firing rate decreases, alpha motor neuron excitation is reduced, muscle contraction is reduced Serves to maintain muscle tone and compensate for the effects of gravity during movement

Intra & Extrafusal Muscle Fibers Extrafusal skeletal muscle fibers The bulk of muscle fibers Outside the muscle spindle Innervated by alpha motor neurons Intrafusal skeletal muscle fibers Modified skeletal muscle fibers found only in the muscle spindle Innervated by gamma motor neurons at ends to control length of spindle

Gamma Motor Neurons Motor neuron for the muscle spindle If not for gamma motor neurons, contraction of muscle would turn off muscle spindles During voluntary movements, alpha and gamma motor neurons are co-activated The gamma loop: gamma motor neuron  muscle fiber  afferent neuron  alpha motor neuron  opposite muscle fiber The gamma loop controls the set point of the myotatic reflex feedback control loop

Golgi Tendon Organs Another sensor of proprioception Monitors muscle tension Wired in series with whole muscles in tendons Excite inhibitory interneurons which inhibit alpha motor neurons in the motor neuron pool for that muscle Mediates reverse myotatic reflex When force being generated is too great, the alpha motor neurons are turned off Reduces force toward the limits of extension of a joint Reduces force when limb hits an immovable object Regulate fine motor movements of fragile objects such as picking up an empty egg shell

Proprioception from Joints Receptors in joint capsules Most are rapidly adapting (movement) a few are slowly adapting (stationary position) Input is combined with information from muscle spindles and Golgi tendon organs Replacement-joint patients still have ability to determine position of limbs

Spinal Interneurons Inhibitory interneurons Mediate inverse myotatic reflex Mediate coordination of synergists and antagonists by reciprocal inhibition Excitatory interneurons Mediate polysynaptic flexor reflex - withdrawal of foot when one steps on a tack Sometimes excitatory and inhibitory interneurons work together Crossed-extensor reflex which tends to keep you from falling when you step on a tack

Spinal Locomotor Programs Circuits of neurons which produce rhythmic motor activity central pattern generators Different circuits use different mechanisms Simplest pattern generators are neurons that serve as pacemakers One proven example: swimming in a lamprey Results from activation of NMDA receptors on spinal interneurons

NMDA Receptors NMDA (N-methyl-D-asparate) receptors Glutamate-gated ion channels Allow more current to flow into the cell when postsynaptic membrane is depolarized Admit Ca ++ as well as Na + into the cell

NMDA Receptors & Locomotion Glutamate activates NMDA receptors Na + and Ca ++ flow into cell as membrane depolarizes Ca ++ activates Ca ++ activated K + channels K + flows out of cell - cell hyperpolarizes Ca ++ stops flowing into cell K + channels close - ready for another cycle Central pattern generators for walking are in spinal cord modulated by higher motor neurons

Movement I

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