Introduction to the Nervous System and Neurons

I. Introduction

Overview of class structure for exam preparation, including a timetable for review sessions and highlighting key topics in each session. Recognition of nervousness around lab practicals, emphasizing the importance of practice, familiarization with equipment, and building confidence through mock exams.

II. Structure of Muscle Types

A. Types of Muscle

1. Striated Muscle (Skeletal Muscle)
   

   • Multinucleated and voluntary, enabling conscious control of muscle movements.

   • Responsible for movement of bones at joints under conscious control, essential for locomotion, balance, and posture.

   • Muscle fibers are long and cylindrical with a banded appearance, due to the organized arrangement of actin and myosin filaments, which allows for the sliding filament mechanism of contraction.

   • Capable of hypertrophy in response to strength training, adapting by increasing fiber size and number with consistent exercise, leading to enhanced muscle strength and endurance over time.

2. Cardiac Muscle
   

   • Involuntary with one nucleus, structurally and functionally adapted for continuous activity; found exclusively in the heart.

   • Associated with heart function, coordinating rhythmic contractions that are vital for efficient blood circulation throughout the body.

   • Features intercalated discs, which contain gap junctions and desmosomes, allowing for synchronized contractions, fast electrical signal transmission, and structural integrity during heartbeats.

   • Resistant to fatigue, enabling continuous function due to a high density of mitochondria, abundant blood supply, and unique energy metabolism that relies predominantly on aerobic respiration.

3. Smooth Muscle
   

   • Involuntary; found in hollow organs such as the esophagus, intestines, ureters, blood vessels, and respiratory passages.

   • Non-striated fibers allowing for slow, sustained contractions necessary for functions such as digestion, blood flow regulation, and airway management.

   • Adapts tone based on internal demands, enabling responses to physiological changes (e.g., increased blood flow during exercise, decreased flow during rest) and facilitating processes like peristalsis in the digestive system.

III. Key Histological Features

A. Structural Differences among Muscle Types

1. Skeletal muscle allows quick, powerful actions such as sprinting, enabling rapid energy expenditure and explosive movements.

2. Cardiac muscle supports rhythmic contractions vital for life, ensuring the heart pumps blood efficiently during various physiological states (resting, active, stressed).

3. Smooth muscle provides controlled, slow movements for bodily functions, helping regulate blood pressure through vasoconstriction and vasodilation, adjusting to the body's needs or environmental changes.

IV. Nervous Tissue and Communication

A. Nervous System Overview

1. Connection between the brain and muscles via nervous tissue for voluntary (skeletal) and involuntary (smooth and cardiac) actions; enables complex behaviors and physiological regulation.

2. Divided into Central (CNS: brain and spinal cord) and Peripheral (PNS: nerves throughout the body) systems, facilitating rapid communication and coordination across body functions.

3. PNS carries sensory information to the CNS and relays motor commands back to the muscles and glands, playing crucial roles in reflex responses, voluntary movements, and sensory processing.

B. Neuromuscular Junction

1. Site of communication between nerve endings and skeletal muscles, essential for initiating voluntary muscle contractions and maintaining tone.

2. Acetylcholine release from the nerve terminal into the synaptic cleft initiates muscle action potentials, which lead to a cascade of events resulting in muscle contraction through calcium ion release and actin-myosin interaction.

C. Sensory and Motor Nerves

1. Sensory Nerves
   

   • Carry information from sensory receptors (e.g., skin, eyes, ears) to the CNS, enabling perception and response to environmental stimuli such as pain, temperature, and pressure changes.

   • Examples include touch sensations from skin, visual input from the retina, and proprioceptive feedback regarding body position and movement awareness from joints and muscles.

2. Motor Neurons
   

   • Carry commands from CNS to muscles, facilitating voluntary and reflex actions critical for movement, coordination, and behavior.

   • Key for rapid reflex actions (e.g., knee-jerk response), enabling immediate reaction to harmful stimuli and facilitating complex motor tasks such as writing or playing sports.

V. Neuron Structure and Function

A. Neuron Anatomy

1. Dendrites
   

   • Collect electrical signals and information from other neurons, increasing the neuron's surface area for synaptic connections and enhancing information processing capabilities.

2. Soma (Cell Body)
   

   • Contains the nucleus and organelles; integrates incoming signals and determines whether to generate an action potential based on summation of excitatory and inhibitory inputs.

3. Axon
   

   • Transmits electrical signals away from the cell body (axon potential); can be myelinated to drastically increase conduction speed and efficiency of signal transmission; longer axons correlate with longer distances between neurons.

4. Myelin Sheath
   

   • Insulation formed by glial cells (e.g., oligodendrocytes in the CNS, Schwann cells in the PNS); enhances signal conduction, protecting the axon from degradation, and allows for nodes of Ranvier to facilitate saltatory conduction, significantly speeding up signal transmission.

B. Supporting Cells

1. Neuroglia
   

   • Non-neuronal cells that support and protect neurons; play essential roles in maintaining homeostasis, forming myelin, providing metabolic support, and responding to injury and inflammation within the nervous system.

VI. Action Potential and Neural Communication

A. Resting Membrane Potential

• Typical value: -70 mV, indicating a polarized state of the neuron, essential for maintaining the neuron's readiness to fire upon stimulation.

B. Summary of Action Potential Generation

1. A stimulus leads to depolarization, altering the membrane potential, making the inside of the neuron more positive in response.

2. When the threshold is reached, sodium ion channels open, causing a rapid influx of sodium ions and a significant, rapid change in membrane potential known as depolarization.

3. Repolarization occurs as potassium ions exit the cell; this restores a negative membrane potential after the peak of the action potential, re-establishing the ionic gradients.

4. Hyperpolarization may occur due to excessive potassium exit, temporarily increasing negativity and necessitating recovery time to return to the resting potential.

C. Recovery to Resting State

1. Sodium-Potassium Pump restores ion concentrations (3 Na+ out and 2 K+ in), essential for the neuron's readiness to fire again and for maintaining osmotic balance across the membrane.

VII. Importance of Neuronal Function

A. Relation to bodily responses and reflexes, ensuring effective interaction with the environment during motor tasks and behavior, reinforcing the significance of neural pathways in daily living.
B. Maintenance of ionic gradients for neuron function is vital for coordination, perception, and cognition, supporting overall body function and health by enabling complex behaviors and responses to stimuli.

VIII. Conclusion

A. Summary of communication processes and muscle control, highlighting their interdependence and complexity in maintaining homeostasis and facilitating responses to internal and external changes.
B. Recognition of upcoming practicals, emphasizing the need for understanding neural and muscular interactions for successful performance in exams and real-life situations.
C. Integration of muscle types and neural structures is crucial for survival and interaction with the environment, underscoring the importance of these concepts in both anatomy and physiology education.