Cell-to-cell communication
Cells coordinate through electrical signals, chemical messengers and direct connections; a signal acts only on cells that carry its receptor protein.
Trillions of cells act as one body because they signal to each other. Some signals are electrical, spreading along a nerve or through the gap junctions that link heart muscle cells. Most are chemical: a cell releases a messenger, and it acts on any cell that carries a matching receptor protein, whether that cell is next door or across the body.
The receptor protein, not the messenger, decides what happens. The same messenger can speed one tissue and slow another because the two carry different receptor proteins. That is why one nerve signal can raise your heart rate and narrow your skin's blood vessels at the same time, and why a drug that blocks one kind of receptor protein can have such a precise effect.
Where it shows up
Cells
Tissues
Signals, repair and control
- The resting membrane potential
- Electrical signals: graded potentials and action potentials
- Chemical messengers and receptors
- Glands: endocrine and exocrine
- Homeostasis and feedback loops
Muscle tissue
Nervous tissue and neural signaling
The brain and spinal cord
Nerves, reflexes and pathways
Special senses
The autonomic nervous system
- Sympathetic and parasympathetic divisions
- Autonomic neurotransmitters and receptors
- Autonomic control and visceral reflexes
The endocrine system
- Hormones and how they act
- Hypothalamus and pituitary gland
- Thyroid and parathyroid glands
- Adrenal glands
- Other endocrine organs
The cardiovascular system
- Cardiac muscle cells and their action potential
- The conduction system and pacemaker cells
- The electrocardiogram
- Cardiac output and its control
- Short-term blood pressure regulation
Lymphatic and immune systems
- Innate immunity
- Antigens, lymphocytes and antigen presentation
- T cells and cell-mediated immunity
- B cells and antibodies
The respiratory system
The digestive system
The reproductive system
- Spermatogenesis and male hormones
- Oogenesis and the ovarian cycle
- Hormonal control of the ovarian and uterine cycles