The nervous system relies on neurons, specialized cells that communicate through electrical and chemical signals, forming complex networks. A neuron’s cell membrane is a dynamic boundary that actively participates in generating, transmitting, and receiving signals. Its unique properties allow neurons to maintain distinct internal environments and interact with their surroundings, which is fundamental to nervous system function.
Building Blocks of the Neuron Membrane
The neuron’s cell membrane is composed of a lipid bilayer, made of phospholipids. Each phospholipid has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails, which arrange into two layers with tails facing inward and heads facing watery environments. This bilayer acts as a semi-permeable barrier, regulating substance passage.
Embedded within this lipid bilayer are various membrane proteins, performing specialized functions. Integral proteins span the entire membrane, while peripheral proteins are loosely associated with either the inner or outer surface. These proteins serve diverse roles, acting as channels for specific molecules, pumps to actively transport ions, or receptors to bind chemical messengers.
Carbohydrates are also present on the outer surface of the neuronal membrane, forming a coat called the glycocalyx. These carbohydrate chains are attached to either lipids (forming glycolipids) or proteins (forming glycoproteins). They play a part in cell-to-cell recognition and adhesion, helping neurons identify and interact with other cells.
Generating Electrical Signals
The neuron membrane maintains an electrical potential difference, known as the resting membrane potential, typically ranging from -50 to -75 millivolts (mV). This negative charge is established by an unequal distribution of ions, particularly sodium (Na+), potassium (K+), and chloride (Cl-), across the membrane. The membrane is selectively permeable, allowing potassium ions to leak out through specific “leak channels” more readily than sodium ions leak in.
The sodium-potassium pump, an active transport protein, maintains these ion gradients. This pump uses energy from ATP to actively move three sodium ions out of the cell for every two potassium ions it brings in. This pumping ensures higher sodium concentrations outside and higher potassium concentrations inside, contributing to the negative resting potential.
When a neuron receives a sufficient stimulus, its membrane potential can rapidly change, generating an action potential. Depolarization begins when voltage-gated sodium channels in the membrane open, allowing rapid influx of sodium ions into the cell. This influx causes the inside of the membrane to become temporarily positive, reaching a peak around +30 to +40 mV.
Following depolarization, repolarization occurs as voltage-gated potassium channels open, allowing potassium ions to flow out of the cell. This efflux of positive charge quickly restores the negative membrane potential. Often, the membrane briefly hyperpolarizes, becoming more negative than the resting potential, before returning to its stable resting state. This sequence allows the action potential to propagate along the neuron’s axon, transmitting the electrical signal.
Facilitating Communication Between Neurons
Neurons communicate at specialized junctions called synapses. The presynaptic membrane, located at the end of the transmitting neuron’s axon, releases chemical messengers.
When an action potential arrives at the presynaptic terminal, it triggers the opening of voltage-gated calcium channels on this membrane. The influx of calcium ions causes synaptic vesicles, sacs filled with neurotransmitters, to fuse with the presynaptic membrane. This fusion releases the neurotransmitters into the synaptic cleft, the gap between neurons.
These neurotransmitters diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic membrane. The binding causes ion channels on the postsynaptic membrane to open or close, altering ion flow. This change can either depolarize the postsynaptic membrane, making it more likely to generate an action potential (an excitatory postsynaptic potential), or hyperpolarize it, making it less likely to fire (an inhibitory postsynaptic potential).
Beyond chemical synapses, some neurons communicate via electrical synapses through structures called gap junctions. Here, the membranes of two neurons are physically connected, allowing ions and small molecules to pass directly between them. This direct connection enables rapid, synchronized electrical signaling between cells.
The Membrane’s Role Beyond the Neuron
The neuronal membrane interacts extensively with other cell types in the nervous system, particularly glial cells. Glial cells, such as astrocytes, oligodendrocytes, and Schwann cells, provide support and modulate neuronal function. Their membranes are closely associated with neuronal membranes, forming an integrated system.
Oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system form myelin sheaths around axons. These myelin sheaths are extensions of the glial cell membranes, wrapping concentrically around the axon to insulate it. This insulation increases the speed of electrical signal transmission along the axon through saltatory conduction, where the action potential “jumps” between unmyelinated Nodes of Ranvier.
Astrocytes, another type of glial cell, closely associate with neuronal synapses, influencing the synaptic environment. Their membranes contain transporters that regulate the uptake of neurotransmitters from the synaptic cleft, helping clear excess neurotransmitters and maintain ion balance. Astrocytes also contribute to the blood-brain barrier, a protective interface regulating substance movement between blood and brain. This barrier is formed by specialized endothelial cells with tight junctions, reinforced by astrocyte end-feet that ensheath the capillaries, controlling what enters the neuronal environment.