Information Transmission in the Nervous System: Foundations of Motor Control

Skilled Performance and Motor Learning

Introduction to Motor Control

This unit introduces the basic principles of how information is transmitted within the nervous system, focusing on the processes that underlie motor control. Understanding these mechanisms is essential for comprehending how skilled movements are learned and executed.

Organization of the Nervous System

Central and Peripheral Components

The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). These components are anatomically separated but functionally interconnected.

• CNS: Consists of the brain and spinal cord.

• PNS: Includes peripheral nerves and ganglia.

Major Regions of the CNS

• Spinal Cord

• Brainstem (Medulla, Pons, Midbrain)

• Cerebellum

• Thalamus (part of diencephalon)

• Cerebral Hemispheres (Forebrain)

Directional Terms and Planes

Specific terms are used to describe locations and directions in the nervous system:

• Dorsal vs Ventral

• Superior vs Inferior

• Anterior vs Posterior

• Rostral vs Caudal

• Medial vs Lateral

• Distal vs Proximal

• Ipsilateral vs Contralateral

Three anatomical planes: Horizontal, Coronal, and Sagittal.

Types of Neurons

Classification of Neurons

Neurons are the fundamental units of the nervous system and are classified into three main types:

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

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

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

Function of the Nervous System

Information Flow

The nervous system coordinates motor behaviors by integrating sensory, motor, and motivational systems. Information flows from sensory receptors through neurons to the CNS, where it is processed and results in motor output.

Information Transmission in Neurons

Membrane Potential

The membrane potential is the difference in electrical charge across the cell membrane, resulting from the distribution of ions (charged atoms) inside and outside the neuron.

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

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

This imbalance creates a resting membrane potential, typically around -70 mV, with the inside of the neuron more negative than the outside.

Action Potential

An action potential (AP) is a rapid change in membrane potential that occurs when a neuron is stimulated. If the membrane voltage reaches a threshold, sodium channels open, allowing Na+ ions to enter and depolarize the cell.

• Membrane potential shifts from -70 mV to approximately +30 mV.

• Action potentials are all-or-none events.

• Generated at the initial segment of the axon and travel to the axon terminals.

• Dependent on voltage-gated ion channels.

Key Equations:

• Resting membrane potential: $V_{rest} \approx -70\ \text{mV}$

• Threshold for action potential: $V_{threshold} \approx -55\ \text{mV}$

Propagation of Action Potential

Action potentials propagate along the axon. In myelinated axons, the myelin sheath increases conduction speed by allowing the action potential to jump between nodes of Ranvier (saltatory conduction).

Synaptic Transmission

Synapse Structure and Function

A synapse is the junction between two neurons where information is transmitted via an electro-chemical mechanism.

• Presynaptic neuron releases neurotransmitters into the synaptic cleft.

• Neurotransmitters bind to receptors on the postsynaptic neuron, opening ion channels.

Neurotransmitter Release

Neurotransmitters are released from vesicles in the presynaptic terminal and bind to postsynaptic receptors, leading to changes in the postsynaptic membrane potential.

Postsynaptic Potentials

Postsynaptic potentials can be either excitatory or inhibitory:

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the membrane, bringing it closer to threshold.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the membrane, moving it farther from threshold.

Summation of Signals

Neurons integrate multiple inputs through summation:

• Spatial Summation: Inputs from many presynaptic neurons at once.

• Temporal Summation: Rapid, successive inputs from a single presynaptic neuron.

• The net effect is the sum of all EPSPs and IPSPs affecting the neuron.

Convergence and Divergence

Neural networks exhibit convergence (many neurons synapse onto one) and divergence (one neuron synapses onto many), allowing complex integration and distribution of information.

Termination of Postsynaptic Potentials

Mechanisms of Termination

Postsynaptic potentials are terminated by:

• Destruction of neurotransmitters

• Reuptake into the axonal terminal

Drugs can affect these processes by altering neurotransmitter uptake or receptor stimulation.

Effects of Drugs on Synaptic Transmission

Drug Actions

Drugs can modify behavior by influencing synaptic transmission:

• Interfere with ion channel opening/closing

• Block or facilitate postsynaptic effects

Examples

• Ethanol (Alcohol): Facilitates postsynaptic GABA stimulation, keeping inhibitory channels open longer and increasing IPSPs.

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

Summary of Information Transmission Steps

Step	Description
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
10	Propagation of action potential
11	Depolarization of presynaptic membrane
12	Release of neurotransmitters

Example Application

When learning a new motor skill, such as playing a musical instrument, repeated activation and transmission of signals through these neural pathways lead to changes in synaptic strength and efficiency, underlying the process of motor learning.

Additional info: This content is foundational for understanding biological psychology, sensation and perception, and learning, as outlined in standard psychology curricula.