Cells of the CNS & PNS

Ependymal cells bind to the ventricles in the brain.
- These cells are integral to the formation and circulation of cerebrospinal fluid (CSF).
- The ventricles are fluid-filled chambers in the brain, and CSF occupies these spaces.
CSF is produced by ependymal cells.
- The fluid circulates through a system of four ventricles.
- The circulation promotes a consistent movement of CSF throughout the ventricular system.
Ependymal cells are ciliated, which aids in distributing and circulating CSF.
Ependymal cells serve dual functions:
- Produce CSF
- Aid in the circulation of CSF

Microglial cells are another type of glial cell.
- They contribute to the defense and immune response in the central nervous system (CNS).
- Similar to other defense cells in the body, microglial cells have extensions that interact with neuronal structures.

Oligodendrocytes are central nervous system glial cells.
- They provide myelin coverage for axons.
- Myelin enhances conduction velocity of action potentials along axons by insulating them.
- The insulating effect of myelin prevents electrical signals from escaping and ensures rapid signal transmission.

There are additional glial cells in the peripheral nervous system (PNS), including:
- Satellite cells
- Schwann cells (neurolemmocytes)

Satellite cells surround the cell bodies of neurons within ganglia.
- They serve two primary functions:
1. Provide nutrients to neurons.
2. Remove metabolic waste generated by neurons.
- Act as insulation to prevent action potentials from affecting surrounding cells, ensuring clear signal transmission to the spinal cord without interference.

Schwan cells are responsible for myelinating axons in the peripheral nervous system:
Each Schwann cell can envelop segments of an axon.
- Schwann cells wrap tightly around axons to form the myelin sheath.
- The myelin sheath is critical for increasing the speed of nerve impulse conduction.

Myelination is the process of forming the myelin sheath around axons.
- Myelin is primarily composed of lipids, which are responsible for its characteristic white appearance.
- As Schwann cells or oligodendrocytes wrap around an axon, the extensions of their membranes spiral around the axon, resulting in multiple layers of plasma membrane.
- The cytoplasmic components of the cell are pushed outward during this process, leading to a thin layer known as the neurolemma surrounding the myelin sheath.
Myelinated axons have nodes of Ranvier (neurofibril nodes), which are gaps between adjacent myelinating cells, allowing for rapid conduction of action potentials via saltatory conduction.

Oligodendrocytes (CNS):
- Extensions wrap around multiple axons, providing myelin to many axons at once.
- Do not form a neurolemma as part of the myelin sheath.
Schwann cells (PNS):
- Each cell wraps around a single segment of a single axon.
- Form a neurolemma which consists of cell content surrounding the myelin sheath.
- Each Schwann cell covers approximately one millimeter of an axon.

In the PNS, unmyelinated axons are not completely enveloped by Schwann cells, but those cells provide some structural support.
- The axons are positioned in trough-like depressions of the Schwann cell membrane.
- In the CNS, unmyelinated axons lack association with oligodendrocytes.

The synapse is a junction where a neuron communicates with another neuron or effector cell (e.g., muscle cell).
At the synapse:
- The presynaptic neuron releases a neurotransmitter in response to an action potential.
- Neurotransmitters bind to receptors on the postsynaptic neuron, causing changes in membrane potential.
Dendrites of the postsynaptic neuron are typically where synapses occur, but synapses can also form on cell bodies.
Synaptic delay is the time it takes from the release of neurotransmitters to their binding to receptors (typically about 0.5 milliseconds).

Resting neurons have a resting membrane potential of approximately -70 mV.
- This is established by ion concentrations across the membrane, particularly sodium (Na+) and potassium (K+).
- Sodium-potassium pumps actively transport Na+ out of the cell and K+ into the cell, creating a concentration gradient.
Other ions such as calcium (Ca2+) and chloride (Cl-) also play a role in establishing resting potential.
The movement of K+ through leak channels contributes significantly to the resting potential, with concentration gradients favoring K+ efflux.
Na+ leak channels also allow some Na+ to enter, reducing negativity inside the cell (to about -67 mV).

The resting membrane potential is determined by both the concentration gradients of the ions and the permeability of the membrane to those ions.
Equilibrium is reached between the concentration gradient pulling K+ out and the electrical gradient attracting K+ back into the cell.
- An equilibrium potential for K+ is about -90 mV, but the actual resting potential is -70 mV due to the influence of Na+ ions.

Neurons establish their resting membrane potential similarly to muscle cells.
The exact mechanisms and channels involved in maintaining these potentials in neurons mirror those in muscle tissue.