Neurons and Synapses in the Nervous System

Ion Channels and Action Potential

• Ion channels are closed in the resting cell, making the plasma membrane less permeable.
• When ion channels are open, the membrane is more permeable to ions like K+.
• There are two types of K+ channels: one is always open, and the other is closed in the resting cell.
• Na+ channels are always closed in the resting cell and open if the Na+ channel is opened first, then the K+ channel will open.

Synapses

• A synapse is the functional connection between a neuron and a second cell.
• In the CNS, the other cell is also a neuron.
• In the PNS, the other cell may be a neuron or an effector cell within a muscle or gland.

Neurotransmitters and Synaptic Cleft

• Neurotransmitters (like acetylcholine) are released from presynaptic axon terminals.
• Neurotransmitters diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic cell membrane.
• Receptor proteins have high specificity for their neurotransmitter (ligand).

Glial Cells

• In the PNS, there are Schwann cells (form myelin sheaths) and satellite cells (support neuron cell bodies).
• In the CNS, there are oligodendrocytes (form myelin sheaths), microglia (phagocytose foreign material), astrocytes (regulate neuron environment), and ependymal cells (line ventricles and central canal).

Axon Regeneration

• When an axon in a peripheral nerve is cut, the distal portion degenerates and is phagocytosed by Schwann cells.
• Schwann cells form a regeneration tube, and neurotrophins (like NGF, BDNF, and GDNF) promote neuron growth.

Astrocytes

• Astrocytes take up K+ from the extracellular fluid.
• Astrocytes take up some neurotransmitters released from axon terminals.
• Astrocyte end-feet surrounding blood capillaries take up glucose from the blood.
• Astrocytes are needed for the formation of synapses in the CNS and induce the formation of the blood-brain barrier.

Nervous System

• The nervous system is composed of neurons (produce and conduct electrochemical impulses) and supporting cells (assist neuron functions).
• The nervous system has a central nervous system (CNS) and peripheral nervous system (PNS).
• Neurons are classified functionally and structurally, and supporting cells perform specialized functions.

Electrical Activity in Axons

• Permeability of the axon membrane to Na+ and K+ is regulated by gated channels.
• The flow of ions and changes in membrane potential constitute an action potential.
• The resting membrane potential (rmp) of the axon can be reduced (depolarization) or increased (hyperpolarization).

Ion Gating in Axons

• Ions like Na+, K+, and others pass through gated channels in the plasma membrane.
• Ion channels are said to be gated channels.

Explore the composition of the nervous system, including neurons that produce and conduct electrochemical impulses, and supporting cells that assist neuron functions. Learn about the classification of neurons and the specialized functions of supporting cells within the central and peripheral nervous systems.