Cellular Physiology of Nerve and Muscle II: Muscle Tissue Structure and Function

Topic 2: Cellular Physiology of Nerve and Muscle II

Overview

This section explores the cellular physiology of nerve and muscle, focusing on the structure and function of muscle tissue. The content is essential for understanding how muscles contract, the role of neurons in muscle activation, and the differences among muscle tissue types.

Muscles and Muscle Tissue

Learning Objectives

• Describe the microscopic structure of skeletal muscle fibers and explain the cellular mechanisms of excitation-contraction coupling.

• Describe the neuromuscular junction of skeletal muscle (motor unit) and its functional properties, including spatial and temporal summation, and types of contractions (isotonic & isometric).

• Associate various muscle types with their metabolism and fatigue, and their speed of contraction and rate of fatigue.

• Compare the properties of smooth muscle with those of skeletal muscle.

Overview of Muscle Tissue

Introduction to Muscle Tissue

Muscle tissue makes up nearly half of the body's mass and is specialized to transform chemical energy (ATP) into directed mechanical energy, enabling movement and force generation. Muscle tissue is excitable, meaning it can respond to stimuli and generate action potentials.

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched or extended.

• Elasticity: Ability to recoil to resting length after stretching.

Main Types of Muscle Tissue

There are three main types of muscle tissue in the human body, each with distinct structure and function:

Type	Location	Cell Shape & Appearance	Control	Key Features
Skeletal Muscle	Attached to bones or skin	Long, cylindrical, multinucleate, striated	Voluntary	Responsible for body movement, posture, and heat production
Cardiac Muscle	Walls of the heart	Branching, striated, usually uninucleate, intercalated discs	Involuntary	Pumps blood through the heart; rhythmic contractions
Smooth Muscle	Walls of hollow organs (e.g., stomach, intestines, blood vessels)	Spindle-shaped, non-striated, uninucleate	Involuntary	Moves substances through internal body channels

Muscle Fiber Structure

• Muscle Fiber: Also called a muscle cell or myocyte; elongated and multinucleated in skeletal muscle.

• Sarcomere: The basic contractile unit of striated muscle, composed of actin and myosin filaments.

• Myofibril: A bundle of actin and myosin filaments within the muscle fiber, responsible for contraction.

Neurons and Muscle Activation

• Neuron: A nerve cell that transmits electrical signals to muscle fibers, initiating contraction.

• Axon Terminal: The end of a neuron where neurotransmitters are released to stimulate muscle fibers at the neuromuscular junction.

• Neuromuscular Junction: The synapse between a motor neuron and a skeletal muscle fiber, essential for voluntary muscle contraction.

Required Readings

• Muscle: Chapter 9, pp. 279-316 (Sections 9.1-9.9)

Additional info:

• Excitation-contraction coupling refers to the sequence of events by which an action potential in the sarcolemma leads to muscle contraction.

• Isotonic contractions involve muscle changing length (concentric or eccentric), while isometric contractions involve tension without length change.

• Muscle fatigue is influenced by metabolic properties and the type of muscle fiber (slow-twitch vs. fast-twitch).