EHS_385_nervous_s25_cfs

EHS 385: The Nervous System

• The Nervous System: A highly complex physiological system that conveys information and regulates homeostasis.

Organization of the Nervous System

• Central Nervous System (CNS)

  • Comprises the brain and spinal cord.

• Peripheral Nervous System (PNS)

  • Links the CNS to the body.

  • Afferent Nerves: Carry sensory information from the body to the brain.

  • Efferent Nerves: Transmit motor commands from the brain to the body.

Peripheral Nervous System

• Afferent Nerves:

  • Carry information towards the CNS from various body areas, including blood vessels, internal organs, special senses, skin, muscles, and tendons.

• Five primary types of receptors:

  • Mechanoreceptors: Respond to mechanical stimuli.

  • Thermoreceptors: Respond to changes in temperature.

  • Nociceptors: Respond to damaging/possibly damaging stimuli.

  • Photoreceptors: Respond to light.

  • Chemoreceptors: Respond to chemical stimuli.

Efferent Nerves

• Autonomic Nervous System:

  • Sympathetic: Activates the "fight or flight" response.

  • Parasympathetic: Responsible for the body's housekeeping functions.

• Somatic Nervous System:

  • Involves motor neurons that act on skeletal muscles.

Neurons: An Overview

• Neurons: The functional units of the nervous system.

• Parts of a Neuron:

  • Cell Body: Central operational hub of the neuron.

  • Dendrites: Receptive area that conducts electrical impulses toward the cell body.

  • Axon: Carries action potentials away from the cell body to other neurons or effector organs.

  • Myelin Sheath: Insulating layer that increases conductivity of the neuron.

  • Nodes of Ranvier: Gaps in the myelin sheath that facilitate fast signal transmission.

Electrical Activity in Neurons

• Resting Membrane Potential:

  • The inside of a neuron is negatively charged (-70 mV), while the outside is positively charged, resulting in polarization.

• Ion Movement:

  • Drive changes in electrical charge through permeability of the cell membrane to certain ions (e.g., Na+, K+).

Mechanisms of Ion Movement

• Ions move in/out of the neuron through voltage-gated channels.

  • High concentrations of Na+ outside and K+ inside the cell dictate charge dynamics.

• Resting Condition:

  • K+ channels are mostly open, allowing positive charge to leak out, maintaining the negative charge inside.

Action Potential Generation

• Process of Action Potential:

  • Initiated when a sufficient stimulus reaches the neuron membrane, opening Na+ gates, causing depolarization.

  • When the threshold is reached, more Na+ gates open, leading to further depolarization and generation of a nerve impulse.

• Phases of Action Potential:

  • Depolarization: The cell becomes more positive as Na+ rushes in.

  • Repolarization: K+ exits the cell to restore the negative interior charge.

Role of the Sodium-Potassium Pump

• Na/K Pump: Maintains ion concentrations and resting potential, allowing for continued neuron excitability and action potential generation.

Synaptic Transmission

• How Neurons Communicate:

  • Nerve impulses reaching the end of an axon trigger neurotransmitter release into the synapse.

  • Neurotransmitters bind to receptors on the postsynaptic neuron, causing depolarization and potentially generating an action potential in that neuron.

Neurotransmitter Functions

• Excitatory Neurotransmitters:

  • Promote depolarization through mechanisms like temporal and spatial summation.

• Inhibitory Neurotransmitters:

  • Cause hyperpolarization, making it harder for a neuron to depolarize and generate an action potential.

Sensors and Reflexes

• Proprioceptors: Sensors that inform the CNS about body position.

  • Joint Receptors: Monitor the position and movement of joints.

  • Muscle Spindles: Located within muscles, they detect changes in muscle length and provide information about stretch.

  • Golgi Tendon Organs: Found at the junction between muscles and tendons, they sense tension and prevent excessive force.

Joint Receptors

• Free Nerve Endings: Most abundant type of joint proprioceptors; sensitive to touch and pressure; initially strongly stimulated, then adapt.

• Golgi-type Receptors: Found in ligaments and around joints; functionally similar to free nerve endings.

• Pacinian Corpuscles: Located in tissues around joints; detect rate of joint rotation.

Muscle Proprioceptors

• Mechanoreceptors: Sensitive to mechanical changes such as length and pressure.

• The nervous system must receive continuous feedback from muscles, providing information on both tension developed by the muscle and muscle length.

• Muscle Spindles: Indicate the relative length of the muscle.

• Golgi Tendon Organs: Monitor the tension developed by the muscle.

Muscle Spindle

• Fibrous Capsules: Located within the muscle belly.

• Function: Sense a stretch or lengthening of the muscle fibers.

• Signal Transmission: When stretch is sensed, a signal is sent through the CNS (spinal cord).

• Neuronal Interaction: Sensory neurons synapse with alpha motor neurons.

• Outcome: Action potential

Muscle Spindles

• Muscle Spindles: Sensitive to changes in muscle LENGTH.

• Signal Transmission: Sends impulses to the spinal cord, synapses with motor neurons to facilitate muscle contraction.

• Purpose: Protects the body from injury caused by overstretching and maintains muscle tone.

Golgi Tendon Organ

• Encapsulated Sensory Organs: Located within muscle tendons.

• Function: Sense small changes in tension and monitor force development in muscle.

• Reflex Stimulation: Stimulates a reflex in which the muscle relaxes.

• Injury Prevention: Can prevent muscle injury when there is excessive force.

Neuromuscular System

• Activation of Muscles:

  • Arrival of the action potential at the nerve terminal causes release of acetylcholine.

  • Action potential is generated across the sarcolemma.

  • Fiber contracts.

• Control of a Muscle: Depends on the number of muscle fibers within each motor unit.

Neuromuscular System

• Activation of Muscles:

  • Arrival of the action potential at the nerve terminal causes release of acetylcholine.

  • Action potential is generated across the sarcolemma.

  • Fiber contracts.

• Control of a Muscle: Depends on the number of muscle fibers within each motor unit.

• All-or-None Principle: All muscle fibers in a motor unit contract and develop force at the same time. There is no evidence that a motor neuron stimulus causes only some of the fibers to contract.

• Strength of Contraction:

  • A stronger action potential does NOT produce a stronger contraction.

  • Strength of contraction is dependent on the types of muscle fibers and the number of fibers recruited

Neuromuscular System

• Muscle Fiber Types:

  • Type I (Slow-Twitch): Designed for endurance and are fatigue-resistant.

  • Type IIa (Fast-Twitch): Intermediate fibers that can use both aerobic and anaerobic metabolism.

  • Type IIx (Fast-Twitch): Primarily anaerobic, designed for quick bursts of power but fatigue quickly.

• Motor Units: Muscle fibers with specific characteristics that determine functional capacity.

• Force Output of Muscle: Depends on frequency of activation and changes in the number of activated fibers.

The Motor Unit

• Motor Neuron (Alpha Motor Neuron):

  • Leaves the spine and reaches all the way out to the muscle.

  • At the muscle, it branches, and each branch reaches a single fiber.

  • Can have more than one motor neuron for a muscle.

• Motor Unit: Motor neuron and all the muscle fibers it innervates.
  
  A motor unit consists of an alpha motor neuron and ALL of the muscle fibers that it innervates.

• Multiple motor units (sometimes thousands) comprise a whole muscle (e.g. biceps).

• Motor units operate on an ALL-or-NONE principle, similar to action potentials, meaning that the firing of a motor unit is either full or none at all.

How do I use my biceps?I can use my biceps to pick up a pen or the desk that the pen is sitting on by activating the appropriate motor units to control the force required for each task.

Types of Motor Units

Motor units are NOT all the same and can be categorized into three types:

• Slow (Type I): Small, with high aerobic capacity. Also referred to as ‘aerobic,’ ‘red,’ or ‘slow oxidative’.

• Fast Fatigue Resistant (FR; Type IIa): Intermediate size, sometimes called ‘mixed’ or ‘fast oxidative’.

• Fast Fatigable (FF; Type IIx, IIb): Larger size with significant anaerobic power. Also known as ‘glycolytic’ or ‘white’.

Note: The type of motor unit is dictated by the muscle and the nerve, NOT just the muscle.

The Size Principle of Motor Unit Recruitment

• Motor Units are recruited to complete a task from smallest (usually Type I) to largest (Type IIx, if necessary).

Neuromuscular System

• GXT (Graded Exercise Test):

  • Stage I: Easy work -> mostly Type S-fibers (slow twitch, Type I).

  • Stage III: Harder work -> Type FR (fast-twitch resistant, Type IIa).

  • Stage V: Heavy work -> Type FF (fast-twitch fatigable, Type IIx).

• Purpose: This recruitment system allows for the delay of fatigue.

Neuromuscular System

• Short Term Explosive Movements:

  • In movements such as an Olympic lift, Fast Fatigue Resistant (FR) and Fast Fatigable (FF) motor units are recruited first, as the movement is explosive, short, and power-based.

  • Type S (slow-twitch) motor units are inhibited during these activities.

Varying Contraction Strength with the All-or-None Law

1. Type of Motor Unit: Choice between slow (Type I) or fast (Type II).

2. Size of Motor Unit: Variation in the number of muscle fibers (few or many).

3. Number of Motor Units: Increasing the number of motor units activated can also vary strength.

Neuromuscular System

• Autonomic Nervous System (ANS): Plays an important role in homeostasis.

  • Innervates effector organs that are not under voluntary control, such as:

    • Cardiac (heart)

    • Digestive organs

    • Respiratory organs

• Two Sub-Sections of ANS:

  • Sympathetic Nervous System (SNS): Activates the "fight or flight" response.

  • Parasympathetic Nervous System (PNS): Responsible for the body's housekeeping functions.

Neuromuscular System

• Neurotransmitters:

  • Sympathetic Portion: Releases norepinephrine to excite an organ.

  • Parasympathetic Portion: Releases acetylcholine to inhibit the same organ.

• Sympathetic System: Works to activate an organ (e.g., increases heart rate).

• Parasympathetic System: Works to inhibit an organ (e.g., decreases heart rate).

• At rest, they are in balance...aka homeostasis.

• During exercise, the activity of the parasympathetic system decreases, leading to an increase in sympathetic system activity.

• After exercise stops, the sympathetic system activity diminishes, allowing the parasympathetic system to regain balance and restore normal function (homeostasis).

Neuromuscular System

• Example: Blood flow to muscles during exercise.

  • Exercise begins and takes place.

  • Sympathetic Nervous System (SNS): Increases cardiac output (Q) and redistributes blood to working muscles.

  • Exercise ends, SNS activity decreases and Parasympathetic Nervous System (PNS) activity increases, allowing the body (Q, blood flow, etc.) to return to resting state.