Skilled Performance and Motor Learning

Introduction to Motor Control

This unit introduces the fundamental processes by which information is transmitted in the nervous system, forming the basis for understanding motor control and skilled performance. The focus is on the structure and function of neurons, the organization of the nervous system, and the mechanisms underlying neural communication.

Organization of the Nervous System

Main Divisions

• Central Nervous System (CNS): Composed of the brain and spinal cord. Responsible for integrating sensory information and coordinating bodily functions.

• Peripheral Nervous System (PNS): Consists of peripheral nerves and ganglia. Connects the CNS to limbs and organs.

• Functional Interconnection: Although anatomically distinct, the CNS and PNS are functionally interconnected, allowing for coordinated responses to stimuli.

Major Regions of the CNS

1. Spinal Cord: Transmits neural signals between the brain and the rest of the body.

2. Brainstem: Includes the medulla, pons, and midbrain; controls basic life functions.

3. Cerebellum: Coordinates voluntary movements and balance.

4. Thalamus: Part of the diencephalon; acts as a relay station for sensory information.

5. Cerebral Hemispheres (Forebrain): Responsible for higher cognitive functions.

Directional Terms in Neuroanatomy

• Dorsal vs Ventral: Back vs belly side

• Superior vs Inferior: Above vs below

• Anterior vs Posterior: Front vs back

• Rostral vs Caudal: Toward the nose vs toward the tail

• Medial vs Lateral: Toward the midline vs away from the midline

• Distal vs Proximal: Farther from vs closer to the point of attachment

• Ipsilateral vs Contralateral: Same side vs opposite side

• Planes: Horizontal, coronal, and sagittal planes are used to describe anatomical sections.

Types of Neurons

• Sensory Neurons: Transmit sensory information from receptors to the CNS.

• Motor Neurons: Convey commands from the CNS to muscles and glands.

• Interneurons: Connect neurons within the CNS and integrate information.

Membrane Potential and Action Potentials

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the neuronal membrane when the neuron is not actively transmitting a signal. It is typically about -70 mV, with the inside of the cell being more negative than the outside.

• Ion Distribution:

  • Inside: High concentration of negatively charged molecules (A-) and potassium ions (K+), some sodium (Na+) and chloride (Cl-).

  • Outside: High concentration of Na+ and Cl-, some K+.

• Cause: The imbalance of ions across the membrane creates the potential difference.

Action Potential

An action potential (AP) is a rapid, transient change in membrane potential that propagates along the axon of a neuron.

• Threshold: If the membrane voltage reaches a certain threshold, voltage-gated sodium channels open, causing depolarization.

• Depolarization: Influx of Na+ ions makes the inside of the cell more positive (up to +30 mV).

• All-or-None Principle: Action potentials are generated fully or not at all, and travel down the axon to the axon terminals.

Equation:

Propagation of Action Potentials

• Myelin Sheath: Fatty covering that insulates axons and increases the speed of impulse conduction.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated, allowing for saltatory conduction (jumping from node to node).

Synaptic Transmission

Synapse Structure and Function

A synapse is the junction between two neurons where information is transmitted from one cell to another.

• Presynaptic Neuron: Releases neurotransmitters from vesicles into the synaptic cleft.

• Postsynaptic Neuron: Has receptors that bind neurotransmitters, leading to the opening of ion channels and changes in membrane potential.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, bringing it closer to threshold for firing an action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, making it less likely to fire an action potential.

• Summation: EPSPs and IPSPs can summate spatially (from multiple neurons) or temporally (from rapid, repeated input from one neuron) to influence whether the postsynaptic neuron reaches threshold.

Neuronal Integration

• Convergence: Multiple presynaptic neurons synapse onto a single postsynaptic neuron, integrating information.

• Divergence: One presynaptic neuron synapses onto multiple postsynaptic neurons, distributing information.

Termination of Synaptic Potentials

• Reuptake: Neurotransmitters are taken back into the presynaptic terminal.

• Enzymatic Degradation: Neurotransmitters are broken down by enzymes in the synaptic cleft.

• Drug Effects: Some drugs can alter synaptic transmission by affecting neurotransmitter release, reuptake, or receptor activity.

Examples of Drug Effects

• Ethanol (Alcohol): Facilitates GABAergic inhibition by keeping inhibitory channels open longer, resulting in larger or longer IPSPs.

• Cocaine: Blocks reuptake of dopamine, increasing its presence in the synapse and enhancing post-synaptic transmission, leading to euphoria and increased energy.

Summary of Information Transmission Steps

1. Deformation of receptor membrane

2. Generation of the action potential

3. Propagation of action potential

4. Depolarization of presynaptic membrane

5. Release of neurotransmitters

6. Stimulation of receptors on postsynaptic membrane

7. Opening of ion channels

8. Generation of synaptic (local) potential

9. Generation of action potential (if threshold is reached)

10. Propagation of action potential (repeats as necessary)

11. Depolarization of presynaptic membrane (cycle continues)

12. Release of neurotransmitters (cycle continues)

Table: Comparison of EPSP and IPSP

Additional info: Understanding these processes is essential for comprehending how the nervous system controls movement and how drugs or diseases can affect motor performance.