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

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

Movement I

by Edward Tsien

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Movement I Presentation Transcript