The Nervous System and Exercise Science

Introduction to the Nervous System

This lecture provides an overview of the nervous system, focusing on its structure, function, and relevance to sport and exercise science. The nervous system is fundamental to understanding how the brain communicates with muscles and how this connection can be enhanced.

Nervous System Structure and Function

The nervous system is divided into two main parts:

  • Central Nervous System (CNS):
    • Brain
    • Spinal Cord
  • Peripheral Nervous System (PNS):
    • Sensory Nervous System: Detects external stimuli (e.g., touch).
    • Motor Nervous System: Controls movement.
      • Somatic Nervous System: Controls voluntary movement.
      • Autonomic Nervous System: Controls involuntary functions (e.g., heart rate, breathing).
        • Sympathetic Nervous System: Fight or flight.
        • Parasympathetic Nervous System: Rest and digest.

The Brain: Command Center

The brain, particularly the cerebrum, is the command center of the nervous system. The cerebrum is divided into four main lobes:

  • Frontal Lobe:
    • Motor Cortex: Controls voluntary movement.
    • Prefrontal Cortex: Associated with personality and memory.
    • Premotor Cortex: Involved in planning movement.
    • Broca's Area: Controls speech (typically on the left hemisphere for right-handed individuals).
  • Temporal Lobe: Associated with hearing, sound localization, and processing of written language.
  • Parietal Lobe: Processes touch, pressure, texture, and object recognition.
  • Occipital Lobe: Processes visual information.

The cerebrum is split into two hemispheres. Gray matter, the outer layer, is responsible for thinking and processing. White matter, composed of axons with myelin sheaths, facilitates signal transmission. The myelin sheath is a fatty material that insulates axons and speeds up signal transmission.

Diencephalon and Thalamus

Deep within the cerebrum lies the diencephalon, largely made up of white matter. It facilitates communication between the left and right hemispheres. An example of this communication is seen in cross-education training, where training one side of the body can lead to strength gains in the untrained side.

Early adaptations to resistance exercise are primarily driven by neural adaptations rather than muscle growth.

  • Thalamus: Relays messages throughout the body.
  • Hypothalamus: Controls homeostasis, heart rate, blood pressure, and sleep patterns; particularly important in exercise physiology for temperature regulation.
    • During exercise, ATP is used for energy. 60% of ATP is lost as heat. The hypothalamus detects & regulates the increase in temperature.
  • Pituitary Gland: Regulates hormone production, including growth hormone and cortisol.
  • Pineal Gland: Produces melatonin.

Cerebellum

The cerebellum, or small brain, is responsible for motor tone, posture, balance, and coordination. It works in conjunction with the motor cortex to control movement.

Brain Stem

The brain stem includes the midbrain, pons, and medulla oblongata.

  • Medulla Oblongata: Influences proprioception, heart rate, blood pressure, swallowing, breathing, vomiting, coughing, and sneezing.
  • Pons: Important for cerebrospinal fluid production, which nourishes, insulates, and protects the brain.
  • Midbrain: Contains the substantia nigra, which produces dopamine, a neurotransmitter associated with pleasure and addiction.

Motor Pathway

The motor pathway involves the motor cortex, descending tracts in the spinal cord, and motor units.

  • Supraspinal area: anything above the spinal cord
  • Descending Tract: The motor system carries messages away from the brain.
  • Ascending Tract: Sensory messages that travel to the brain.
  • Motor Unit: A motor neuron and the muscle fibers it innervates.

When a movement is initiated, a message travels from the motor cortex down the brain stem and spinal cord, exiting at the appropriate level to reach the muscles required for the movement.

Action Potentials

Messages are transmitted via action potentials. A typical action potential involves:

  1. Resting membrane potential: -70mV
  2. Depolarization: Sodium ions (Na+) rush into the cell through voltage-gated sodium channels, raising the charge.
  3. Repolarization: Sodium channels close, and potassium ions (K+) leave the cell, lowering the charge.
  4. Refractory period: A period where another action potential is unlikely to occur.
  5. Ions are then restored through active pumps.

The faster action potentials can be sent, the stronger the muscle contraction. Tremors occur because the nerves do not have a perfect or consistent firing rate.

Multiple sclerosis is a condition in which the myelin sheath degrades, impairing signal transmission and causing weakness and trembling.

Action potentials fire at a rate of 20-50 times per second (Hertz).

Motor Cortex Representation

The motor cortex dedicates different proportions to different body parts. Areas requiring fine motor control, such as the hands and face, have a larger representation. The body map reflecting the motor cortex is called a homunculus--it represents the proportions of the motor cortex dedicated to each region of the body.

Spinal Cord

The spinal cord transmits messages between the brain and the body. Vertebrae are split into different regions: cervical (7), thoracic (12), lumbar (5), sacral (5), coccygeal (4).

Nerves extend from the spinal cord at different points, controlling different parts of the body. Spinal cord injuries can affect movement depending on the location and the severity of the injury.

Synapses and Neurotransmitters

Synapses are junctions between two neurons. The synaptic cleft is the gap between the pre-synaptic and post-synaptic cells.

Neurotransmitters transmit information across the synapse. Key neurotransmitters include:

  • Acetylcholine
  • Adrenaline

Action potential travels down the axon terminal and releases neurotransmitters that binds to receptors on the post-synaptic neuron.

Motor Units and Myelin Sheath

A motor unit consists of a motor neuron and the muscle fibers it controls. The myelin sheath insulates the axon, with gaps called nodes of Ranvier. These nodes help to propagate the signal down the axon.

Sensory Nervous System

The sensory nervous system detects stimuli and sends messages to the brain. Different types of afferent neurons are associated with:

  • Proprioception (Type 1)
  • Touch and Pressure (Type 2)
  • Movement (Type 3)
  • Temperature (Type 4)
  • Pain
  • Fatigue

Reflexes

Reflexes are rapid, involuntary responses to stimuli. The message loops within the spinal cord.

  • Mechanoreflex: Increases heart rate & ventilation due to mechanoreceptors.
  • Metaboreflex: Detects changes in chemicals and stimulates changes in respiration.

Autonomic Nervous System

The autonomic nervous system controls involuntary functions. Includes:

  • Sympathetic Nervous System: Fight or Flight
  • Parasympathetic Nervous System: Rest and Digest

Assessing the Nervous System

The nervous system can be assessed using:

  • Magnetic Resonance Imaging (MRI): Forms images based on tissue absorbance.
  • Electroencephalography (EEG): Maps electrical activity in the brain.
  • Transcranial Magnetic Stimulation (TMS): Stimulates the brain with a localized magnet to measure the speed of impulse transmission. By Faraday's law stimulation through magnetic field.

Applied Examples

  • Functional MRI scans of professional footballers show more efficient brain activity during movements compared to amateur footballers.
  • TMS can be used to study and improve the neuromuscular system through exercise and resistance exercise training.