Nervous and muscle Anatomy (short version)
Our body is made of cells. Each cell must communicate with others to be what we are, a living organism. Communication between cells is done in different ways. Cells communicate through chemical signals. Hormones, enzymes and neurotransmitters, are the sensors of the cells, the connection with the “outer world”. Every cell has receptors that detect different signals. Signaling molecules come in multiple forms and shapes. Signaling happens in more than one way. It is within the cell itself and cells send messages to neighbors or other cells a distance away. The way these signals are transferred is:
Chemical compounds (example: nutrients and toxins)
Electrical impulses (example: neurotransmitters inducing electrical signals along nerves) Mechanical stimuli (example: stretching of the stomach to signal you are full)
Chemical signaling is divided into four pathways, depending on the distance that needs to be bridged.
Autocrine Signaling = Internal communication. In autocrine signaling, the cell releases a chemical signal that binds to a receptor within the cell transferring a specific message.
Paracrine Signaling = two cell communication. For example when signaling occurs across the gap between two neurons by neurotransmitters.
Endocrine Signaling = send messages across long distances. Signals that originate in one part of the body and travel to their target through the bloodstream are called hormones. The hormone then sets specific cells or tissues into action.
Direct-Contact Signaling = Gap junctions are filled with water and allow small signaling molecules to travel across the channel. This is cell signaling through direct contact. It allows groups of cells to respond to a signal that only one cell received. It is not a typical human method
Electrical and Mechanical Signaling
Many cells respond to electrical or mechanical signals. Two examples of this are the regulation of the heartbeat (electrical) and the signaling for muscle growth following exercise (mechanical).
The heart beats like a wave. This is a defined beating pattern, initiated and synchronized by electrical signals.
Mechanical signals in muscle cells lead to growth and strength gains. When muscle cells are stretched—otherwise deformed or damaged—calcium ions flood into the muscle cell. This flux of calcium ions is the intermediary, changing the mechanical signal into a chemical one. The presence of calcium ions signals a number of cell signaling pathways inside of the muscle, including hormones responsible for muscle growth. This is external energy transfer into biological energy (ion-based).
How Cells Recognize and Respond to Signals
Protein receptors help cells recognize received signals. Receptors are located both inside and outside the cell or situated into a cellular membrane. Signaling occurs when specific molecules bind to their receptors. There are two classes of receptors: intracellular and cell-surface receptors.
Intracellular receptors are located inside the cell. Signal molecules travel through pores in the cell’s membrane to reach this type of receptor and elicit a response.
Cell-surface receptors are embedded in the cell’s membrane. They bind signaling molecules on the outside of the cell and relay the message internally.
The central nervous system comprises the brain (cerebrum, cerebellum, and brainstem) and the spinal cord. There are over 100 trillion neural connections in the average human brain, though the number and location can vary. A synapse gives a command to the cell and the entire communication process typically takes only a fraction of a millisecond. Signals travel along an alpha motor neuron in the spinal cord 431 km/h the fastest transmission in the human body. The peripheral nervous system includes sensory neurons, which link sensory receptors on the body surface and specialized receptor structures such as the ear, with processing circuits in the central nervous system. Our nervous system is made up of two classes of cells: neurons and neuroglia.