Muscles & Muscle Innervation

Learning Objectives

• Understand how muscle is innervated by the nervous system.

• Describe the structure of muscle.

• Explain the mechanism of muscle contraction.

• Outline the steps that occur during transmission at the neuromuscular junction.

• Describe excitation-contraction coupling.

• Give an overview of the mechanisms of ATP generation in muscle.

• Understand the muscle length-tension relationship and how contractions are achieved.

The Motor System

Efferent Division of the Peripheral Nervous System

The efferent division of the peripheral nervous system (PNS) transmits signals from the central nervous system (CNS) to effector organs, including muscles. It is divided into somatic (motor) and autonomic branches.

• Somatic/Motor System: Controls voluntary movements by innervating skeletal muscle.

• Autonomic System: Regulates involuntary functions by innervating cardiac and smooth muscle, as well as glands.

The Somatic Motor System

Motor Neuron Pathway

Somatic motor neurons originate in the CNS and travel to skeletal muscle, where they form synapses at the neuromuscular junction (NMJ).

• A motor neuron travels from the CNS to the target skeletal muscle.

• At the neuromuscular junction, the neuron synapses with the muscle fiber.

• Action potentials sent down the neuron stimulate muscle contraction by activating the NMJ.

Muscle Types and Structure

Types of Muscle Tissue

There are three main types of muscle tissue in the human body: skeletal, cardiac, and smooth muscle. Each type has distinct structural and functional characteristics.

• Skeletal Muscle: Controlled by the somatic nervous system, attached to the skeleton, responsible for voluntary movements, and characterized by striations due to organized contractile proteins.

• Cardiac Muscle: Found in the heart, striated, involuntary, and influenced by the autonomic nervous system.

• Smooth Muscle: Found in walls of hollow organs (e.g., intestines, blood vessels), non-striated, involuntary, and regulated by the autonomic nervous system.

Striation: Refers to the banded appearance of skeletal and cardiac muscle due to the regular arrangement of actin and myosin filaments.

Structural Organization of Skeletal Muscle

Skeletal muscle is composed of bundles of muscle fibers, each containing myofibrils made up of repeating units called sarcomeres.

• Muscle Fiber: The basic cellular unit of muscle tissue.

• Myofibril: A long, cylindrical organelle within muscle fibers, composed of sarcomeres.

• Sarcomere: The functional contractile unit of muscle, defined by Z lines and containing overlapping thick (myosin) and thin (actin) filaments.

Comparison Table: Muscle Types

Key Terms

• Neuromuscular Junction (NMJ): The synapse between a motor neuron and a skeletal muscle fiber, where neurotransmission initiates muscle contraction.

• Action Potential: An electrical signal that travels along neurons and muscle fibers, triggering physiological responses.

• Striation: The striped appearance of muscle tissue due to the arrangement of contractile proteins.

Example

When you decide to move your arm, your brain sends an action potential down a somatic motor neuron. This neuron synapses at the neuromuscular junction of the biceps muscle, releasing neurotransmitters that cause the muscle fibers to contract, resulting in arm movement.

Additional info: These notes cover content relevant to Ch. 10 (Skeletal Muscle Tissue), Ch. 11 (Muscles of the Body), and Ch. 12 (Fundamentals of the Nervous System and Nervous Tissue) from a standard Anatomy & Physiology curriculum.