A New Learner’s Guide to Muscle and Nervous Tissues

Introduction: The Partnership of Action

Every conscious thought, every reflex, every heartbeat, and every breath is the result of an exquisite partnership between two tissues: the command and the action. Nervous tissue is the body’s master communication network, receiving, processing, and sending electrical signals. Muscle tissue is the machinery that carries out the orders, converting those signals into physical force and movement. In this section, we will briefly explores this fundamental relationship. If you are interested to know more in-depth, please feel free to review Cardiac Muscle Tissue, and Neuromuscular Control of Motion chapters!

The Action – Muscle Tissue

Muscle tissue is specialized for one universal function: contraction. Its cells are packed with proteins (actin and myosin) that slide past each other to generate force (for more details, please review The Neuromuscular Junction and Nervous Control of Skeletal Muscle). Based on structure and control, we classify muscle into three types.

* Unique to cardiac muscle, these are dark staining lines that represent specialized junctions between cells. They contain desmosomes for structural strength and gap junctions that allow electrical impulses to spread instantly from cell to cell, so the heart muscle contracts as a single, coordinated unit!

The Command – Nervous Tissue

Nervous tissue is the communication system. Its function is to sense internal and external environments, integrate information, and command a response, often by signaling muscle tissue or glands.

1. Neurons: The Signaling Units


These are the excitable cells that conduct electrical impulses. They are amitotic (do not divide) and have a high metabolic rate. Their structure is key to their function:

• Cell Body (Soma): The control center, containing the nucleus and most organelles. It performs essential metabolic functions.
• Dendrites: Short, branched processes that are the main receptive sites for incoming signals from other neurons. They conduct electrical signals toward the cell body.
• Axon: A single, long process that is the conducting component of the neuron. It generates nerve impulses and conducts them away from the cell body toward other neurons, muscles, or glands.
  • The axon may be insulated by a myelin sheath, which dramatically increases the speed of signal conduction. (What would happen if axons are no longer protected by myelin sheath? You can find your answer after reviewing this chapter!)
  • The axon ends in axon terminals, which release neurotransmitters to communicate with the next cell.

2. Neuroglia (Glial Cells): The Support Staff


These cells support, protect, and insulate neurons. They are far more numerous than neurons.

• In the Central Nervous System (CNS): Astrocytes (support, regulate environment), Microglia (immune defense), Ependymal cells (produce cerebrospinal fluid), Oligodendrocytes (myelinate CNS axons).
• In the Peripheral Nervous System (PNS): Satellite cells (support cell bodies), Schwann cells (myelinate PNS axons).

The Connection – Where Command Meets Action

The fundamental link between the nervous system and the muscular system is the Neuromuscular Junction (NMJ). This is a specialized synapse between the axon terminal of a motor neuron (a neuron that controls muscle) and the membrane of a skeletal muscle fiber. The following information is a very condensed summary. For more detailed description, please feel free to review The Neuromuscular Junction and Nervous Control of Skeletal Muscle chapter.

• Process: When a nerve impulse reaches the axon terminal, it triggers the release of the neurotransmitter acetylcholine (ACh).
• Result: ACh diffuses across the synaptic cleft and binds to receptors on the muscle fiber, triggering an electrical event that culminates in muscle contraction.
• Significance: The NMJ is the specific point where an electrical signal in the nervous system is translated into an electrical (and then mechanical) event in the muscular system. Every voluntary movement you make begins here.

Nervous and muscle tissues represent the ultimate expression of “Form Follows Function.” Neurons are structured like wires with input and output ends for communication. Muscle cells are packed with contractile proteins and are shaped for pulling. Their partnership, exemplified by the neuromuscular junction, is the physical basis for all behaviour, from a sprint to a heartbeat!