Nerve cells, also known as neurons, are specialized cells that transmit information throughout the body. At rest, the inside of a neuron contains structures and chemicals that allow it to send and receive signals rapidly.
The Resting Membrane Potential
The membrane surrounding a resting neuron is polarized – it has a negative charge on the inside relative to the outside. This is called the resting membrane potential and is created by differences in the concentrations of ions across the membrane.
Inside the neuron, there are high levels of potassium ions (K+) and negatively charged proteins. Outside, there are high levels of sodium ions (Na+), chloride ions (Cl-), and positively charged proteins. This ion distribution is maintained by the sodium-potassium pump, which uses ATP to pump Na+ out and K+ in.
The resting potential typically ranges from -40 to -80 millivolts (mV), meaning the inside of the neuron is 40-80 mV more negative than the outside. This polarization is critical for generating electrical signals called action potentials.
The Cytoplasm
The cytoplasm inside the neuron contains organelles and molecules involved in cell functions:
- Mitochondria – produce ATP for energy
- Endoplasmic reticulum – synthesizes proteins
- Golgi apparatus – processes and packages proteins
- Microtubules – provide structural support and transport
- Neurotransmitters – chemical messengers stored in vesicles
- Buffers – regulate pH
The cytoplasm has a viscous, jelly-like consistency that allows molecules and organelles to move smoothly throughout the neuron.
The Nucleus
The nucleus contains the neuron’s DNA and directs protein synthesis and cell activities. The DNA is packaged into chromosomes that contain genes encoding proteins essential for neuronal function.
When a neuron is activated, immediate early genes in the nucleus are transcribed to produce proteins enabling downstream changes in the neuron.
The Dendrites
Dendrites are thread-like extensions that receive signals from other neurons. Small protrusions on dendrites called dendritic spines receive input from axon terminals.
Dendrites contain voltage-gated ion channels that allow ions to flow in and out, altering the membrane potential in response to neurotransmitters released by axon terminals. This excitatory or inhibitory input is integrated in the dendrites before being propagated to the cell body.
The Axon
The axon is a long, tubular extension that transmits signals away from the cell body toward target cells. Axons can range from less than a millimeter to over a meter long in humans.
Axons lack dendritic spines but have voltage-gated sodium and potassium channels concentrated at the initial segment near the cell body, where action potentials are generated. The flow of ions through these channels allows rapid transmission of signals down the axon.
The distal end of the axon branches into terminal boutons that contact and communicate with other neurons.
Key Takeaways
- The resting potential across the neuronal membrane enables signal transmission.
- The cytoplasm contains organelles and molecules that support cell function.
- The nucleus directs protein synthesis and neuronal activity.
- Dendrites receive and integrate signals from other neurons.
- Axons transmit signals to target cells.
In summary, the inside of a neuron at rest contains polarized membranes, negatively charged proteins and ions, cytoplasm with organelles, a nucleus, dendrites to receive input, and an axon to transmit signals. The specialized structures and chemicals allow neurons to rapidly send and receive signals and drive communication in the nervous system.