MovementPart 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

Actin & Myosin Interaction

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