Neural Basis of Motor Control

Chapter 4

Neurological Perspective

“A basic understanding of the physiology underlying the control of voluntary movement establishes a more comprehensive appreciation and awareness of capabilities and limitations of the people with whom a practitioner works.” (Magill, pg 65)

Nervous System

Nerves (Neurons) inside or outside the: – Central Nervous System (Nerves inside)

•  Brain and •  Spinal cord

– Peripheral Nervous System (Nerves outside CNS) •  Efferent nerves (motor) •  Afferent nerves (sensory)

Neural Activity

The focus of this chapter will be how the nervous system controls voluntary, coordinated movement.

Neuron: Basic Unit of CNS

•  Neuron are similar to other cells

It has a cell membrane, a nucleus that contains genes, contains cytoplasm, mitochrondria, and organelles. It carries out basic cellar processes such as protein synthesis and energy production

Neuron: Basic Unit of CNS

•  Neuron is a nerve

Axon takes information away from the cell body (soma) - the neuron has only one

Dendrites bring information to the cell body (soma) -neuron may have none

to as many as 1000 dendrites

Differences

Axons •  Take information away

from the soma •  Generally only 1 axon per

cell •  No ribosomes •  Can have myelin •  Branches further from the

soma

Dendrites •  Bring information to the

soma •  Rough surfaces (dendritic

spines) •  Many dendrites per cell •  Has ribosomes •  No myelin insulation •  Branches are near the

soma

Neuron Types

e.g. Retinal cells; olfactory cells

2 axons one extending toward the spinal cord and other to skin or muscle

e.g. Spinal motor neuron (serves many functions

Neuron Classification

Neurons are classified by direction that they send information:

1.  Sensory (afferent) neurons sends information from sensory receptors (e.g. skin, eyes, ears)

2.  Motor (efferent) neurons sends information AWAY from the CNS to muscles or organs.

3.  Inter-neurons: send information between sensory and motor neuron; most are located in CNS.

Sensory Neurons

•  They are unipolar, that is, they have no dentrites and only one axon.

•  Most sensory neuron are in the peripheral nervous system

Motor Neuron

•  Two types

– Alpha motor neurons (predominately in spinal cord and referred to as the motor horn cells)

– Gamma motor neurons which supply a portion of skeletal muscle called intrafusal fibers.

Interneurons

•  Originate and terminate in the brain or spinal cord

•  They function as connections between axons descending from the brain and they synapse on motor neurons and axons from sensory nerves and spinal nerves ascending to the brain.

Structures of Transmission

Communication

Neurons communicate with each other through an electrochemical process.

Neurons contain some very specialized structures (i.e., synapses) and chemicals (i.e, neurotransmitters) that enable one cell to communicate with another.

Electrochemical Charges

•  Chemicals cause an electrical signal. •  Almost every chemical in our body is “electrically

charged.” •  When they have an electrical charge, they are

call “ion.” •  The important ions in the nervous system is:

– Sodium & potassium (1 positive charge *) – Calcium (2 postive charges **) – Chloride (negative charge-)

Neural Transmission •  Each axon is enclosed in cellular (myelin) sheath of lipid

material that insulates the axon. •  The sheaths wrapped together in many layers is called

myelinated fibers. If it is only wrapped in one layer it is called unmyelinated fibers.

•  Large myelintated fibers (1-2 mm) contain gaps called nodes of Ranvier.

•  The myelinated fibers transmit neural messages up to 400 feet per second by jumping from one node to the next. Unmyelinated fibers transmit messages up to 3 feet per second.

What turns the neural system on!

•  The on or off position is determined by the distribution of charged ions (sometime called particles).

•  Ions (+ or – charged) surround the inside and outside of each cell.

•  When there is an unbalanced # of + or – ions on each side it creates a membrane potential. –  Resting membrane potential (polarization)

•  Unexcited cell (not sending a signal) –  Action membrane potential (depolarization)

•  Excited cell (sending a signal) •  Sodium (NA+) rushes which turns the cell on! •  Transmission occurs as an “all or none” situation

Resting Membrane Potential

•  Inside of neuron is negative to that of the outside.

•  Ions on the inside and outside are not completely balanced so some can flow in and out of the cell.

Resting Membrane Potential At rest, potassium ions (K+) can cross through the membrane easily. Chloride ions (Cl-)and sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane. The resting membrane potential of a neuron is about -70 mV (mV=millivolt) - this means that the inside of the neuron is 70 mV less than the outside.

There are more sodium ions outside the neuron and more potassium ions inside the neuron.

Action Potential An action potential

occurs when a neuron send information down an axon, away from the soma. Neuroscientists use the words such as “spike” or “impulse” for the action potential.

There is an explosion! Action potential is an

explosion of electrical activities that is created by a depolarization current. This means that some stimulus cause resting potential to move toward 0mV. When it reaches about -55 mV a neuron will fire an action potential.

All or none principle at -55 mV.

Threshold potential then is -55 mv

Action Potential (Depolarizing current) Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. This causes the action potential to go back toward -70 mV (a repolarization). Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV.

A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron

How does it transmit from one Neuron to another?

Neural chains are created which is called synaptic transmission

Without a synapse there is no communication between neurons and target site such as muscles*

There is not an all or none transmission at the junction (transmission may blocked, reduced, amplified or changed)

Consists of pre-synaptic neuron (axon button), synaptic cleft, and post-synaptic neuron (receiving axon)

The neurotransmitter is manufactured by the neuron and stored in vesicles at the axon terminal.

When the action potential reaches the axon terminal, it causes the vesicles to release the neurotransmitter molecules into the synaptic cleft.

The neurotransmitter diffuses across the cleft and binds to receptors on the post-synaptic cell.

The neurotransmitter molecules are released from the receptors and diffuse back into the synaptic cleft.

Synaptic Transmission Presynaptic neuron releases a chemical transmitter.

Transmitter influences the communication

Transmitter can be excitatory or inhibitory.

Most common transmitter is acteyclohline.

Central Nervous System

•  Functions as the command center •  Comprises the the brain and spinal cord •  Integrates and organizes sensory and

motor information to control movement

CNS Components most involved in Motor Control

•  Cerebrum

•  Diencephalon

•  Cerebellum

•  Brainstem

Cerebrum •  Divided into two the right and left cerebral hemispheres •  Each hemisphere is covering with gray matter of 2-5 mm thick, folded tissue

of nerve cell bodies called the cerebral cortex •  The gray matter contains neurons that send signals from the cortex to other

parts of the CNS (pyramidal cells) or non pyramidal cells. •  Each hemisphere of the cortex consist of four lobes

Parietal Frontal Occipital Temporal

Four Lobes

•  Frontal lobe controls voluntary movement. •  Parietal lobe is a key player for controlling

perception of sensory information. •  Occipital lobe is important for visual

perception. •  Temporal lobe plays important roles in

memory, abstract thought, and judgment.

Sensory-Specific Areas (Figure 4.4 below)

Proximity of primary sensory and motor cortex areas and their association areas allows interaction between perceptual and higher-order cognitive functions.

Cortex Areas Related to Controlling Movement

•  Primary motor cortex –  Critical for movement initiation and coordination for fine motor skills.

•  Premotor area –  Controls the organization of movements before they are initiated and

rhythmic coordination during movement that requires sequential movements

•  Supplementary motor area (SMA) –  Controls sequential movements and preparation and organization of movement.

•  Parietal Lobe –  Control of visual and auditory selective attention, visually tracking a target and grasphing. –  Integration of movement preparation and execution processes by interacting with the

premotor cortex, primary motor cortex, and SMA

Diencephalon

•  Diencephalon (contains the thalamus & Hypothalamus) lies between cerebrum and the brainstem

–  Hypothalamus lies under the Thalamus controls the endocrine and the body homeostasis including temperature, hunger, thirst, and regulation of carbohydrate energy use.

–  Thalamus is a relay station for sensory and motor information that transmits pulses from one cerebral hemisphere to another and interconnects the other areas of the brain, plays important roll in controlling attention, mood, & perception of pain.

Thalamus

Hypothalamus

Cerebellum

1.  Attaches to the brain stem & Located behind the cerebral hemispheres

2.  Regulates the accuracy & smoothness of movement.

3.  Controls eye-hand movements

4.  Movement timing

5.  Posture Control

6.  It detects errors and corrects errors in movement*

cerebellum

Pons 1.  Pons considered to be part of the brainstem.

2.  Top of the brain stem

3.  Bridge between cerebral cortex and cerebellum

4.  Connects the two hemispheres of the cerebellum

5.  Acts as a relay for the auditory system and movement

6.  Involved in controlling chewing, swallowing, salivating, and respiration

7.  Plays a role in balance Pons

Medulla or Medulla Oblongata 1.  Considered to be part of the brainstem

2.  Serves as regulatory agent for various internal physiologic processes such as heart beat, respiration, and gastrointestinal functions.

3.  Site where sensory (ascending) and motor neural (descending) pathways cross over the body midline and merge on their way to the cerebellum and cerebral cortex.

medulla

Important Subcortical Component

•  Basal ganglia plays critical role in planning and initiating movement, control of antagonist muscles during movement, and the control of force.

•  People with Parkinson’s disease and cerebral palsy affect the basal ganglia’s functions

•  People with basal ganglia limitations experience:

–  Bradykinesia (slow movements) –  Akinesia (reduced amount of movements) –  Tremor –  Muscular rigidity

Basal ganglia

Reticular Formation

Located just above pons and contains the reticular formation which is considered to be part of the brainstem)

Plays a major role in arousal, consciousness, states of sleep, and relaxation

Primary role is as an integrator of sensory and motor neural impulses, that is, inhibits or increases neural impluses which in turn influences skeletal muscle activity.

Reticular

Limbic system

Limbic system controls behaviors including emotions, motivation, and learning which provides impetus for goal directed movement in environmental contexts.

Pleasure center of the brain.

Consists of the frontal and temporal lobes of cerebral cortex, thalamus, hypothalamus, and interconnections nerves of CNS

Spinal Cord Two major portions of the spinal cord are the gray and white matter.

Gray matter is butterfly shape, central part of spinal cord, containing 2 pairs of horns.

Dorsal horn involve cells are involved in transmitting sensory information.

Ventral horns contain Alpha Motor Neuron whose axons terminate on skeletal muscles.

Spinal cord contains interneurons call Renshaw cells. Nerve fibers descend from the brain terminate on interneurons which influence Alpha Motor Neuron activity.

Transportation of sensory information to the brain

•  Sensory neural pathway (ascending track) –  Passes through the spinal cord to brain stem to

thalamus to the sensory areas of cerebral cortex and to the cerebellum

–  There are different specific ascending tracks: •  Vision has it’s own track to the cerebral cortex •  Audition has it own track to the cerebral cortex •  Sensory information has it own tracks to the cerebral cortex.

–  Ascending tracks cross at the brain stem from one side of the body to another which means information from one side of the body is received in the opposite side of the brain.

Next Slide

Transportation of Movement information from the Brain to the Muscles

•  Transport of movement information (descending track) that will execute the movements: –  Via two distinct motor neural pathways that function

together •  Pyramidal (corticospinal tract)

–  Transmits neural information that arises from the cerebral cortex with axons projecting into the spinal cord that cross over to the opposite side of the body.

–  Primarily associated with fine motor skills (mostly discrete in nature or what neural scientists call – fractionated movements).

•  Extrapyramidal (brainstem pathways) –  Transmits neural information that arises in the brainstem with

axons descending into the spinal cord with many of fibers not crossing over to the opposite side of the body

–  Chiefly found in the reticular formation of the pons and medulla. –  Primarily associated with postural control and muscle control of

flexion and extension of hands and fingers.

Motor Unit

The end of transmission of motor neural information is the motor unit.

Commonly defined as the Alpha motor neuron and muscle fibers it innervates (motor unit)

Connection between an Alpha motor neuron and skeletal muscle occurs at the neuromuscular junction located at the middle of the muscle.

This synapse allows nerve impulses to be transmitted so he muscle contracts and movement occurs.

Alpha Motor Neuron

Performing a Voluntary Motor Skills

Neural system is hierarchical organized.

1.  One needs to have cognitive intent to move.

2.  Various structures work both hierarchically (top-down or down-up) and in parallel (same time).

3.  Different neural activity occurs when we perform different skills.

Producing a Motor Skill

Important part of performing a motor skill comes from knowing “what to do” and “how to do” the motor skill

– What to do is called declarative knowledge

– How to do the skill is called procedural knowledge

Motor Control of Goal Oriented, Voluntary Movement or Tasks

What to do (declarative knowledge) is a brain function of planned movement.

-Involves the limbic & association cortex systems

- In short, limbic and association cortex function cooperatively to guide goal-directed voluntary

movements.

Motor Control of Goal Oriented, Voluntary Movement or Tasks

How to do it (procedural knowledge) is another brain function associated with planned movement.

- The projection system (basal ganglia, cerebellum and the motor cortex) provides detailed to motor and sensory information of how to do it that matches with what to do (limbic or association cortex systems) in the situation in which the movements or skill is to be performed.

Controlling Voluntary Coordinated Movement

•  Involves the limbic, association cortex, and projection systems. For example: – basal ganglia (magnitude of movement) – cerebellum (detection & correction of errors) – pre and motor cortex (command center) –  thalamus (relay station) – hypothalamus (body regulation) –  frontal lobe of cerbral cortex (interprets)

THE END