The internal structures inside a neuron include the cell surface, nucleus, and the cytoplasm and its organelles (rough endoplasmic reticulum, polyribosomes, mitochondria, and Golgi apparatus).
Cell surface-the surface or limiting membrane of the neuron plays a role in the initiation and transmission of signals. The plasma membrane is a double layer of phospholipid molecules. Protein molecules are embedded in this double-layered structure. Some transmembrane proteins provide hydrophilic channels through which inorganic ions can enter and leave the cell. Pumps are protein molecules of the cell membrane. A pump is known as the Na/K-ATPase transports potassium ions into the cell and sodium ions out of the cell, which results in a net negative charge within the cell. This contributes to the membrane potential. Receptors are protein molecules that respond to specific chemical stimuli.
Sodium (Na+) and chloride (Cl-) are the most abundant ions in the extracellular fluid, while potassium (K+) is the main positive ion inside the cell. There is a potential difference across the membrane: the inside is negative (-70 mV) with respect to the outside when the neuron is not conducting a signal. This resting membrane potential opposes the outward diffusion of K+ and the inward diffusion of Cl-. The ionic concentrations are maintained by the activity of the sodium pump. At rest, the cytoplasm is negative (-70 mV) with respect to the extracellular fluid. This difference is reversed to about +40 mV inside when an axon is sufficiently stimulated. This reversal, which is known as the action potential, is then propagated along the axon.
The nucleus and cytoplasm. The nucleus is in the center of the cell body. There is a prominent nucleolus. The cytoplasm of the cell body is dominated by organelles of protein synthesis (rough endoplasmic reticulum, polyribosomes), and cellular respiration (mitochondria), and Golgi apparatus. Neurofilaments are filamentous organelles, which are gathered into bundles to form neurofibrils. Microtubules are involved in the fast transport of protein molecules and small particles along the axons and dendrites. Microfilaments are molecules of the contractile protein actin. Small membrane-bound vesicles called lysosomes contain enzymes that catalyze the breakdown of large unwanted molecules.