Nervous System: Neuron Types and CNS/PNS

• In the Central Nervous System (CNS), clusters of cell bodies are called nuclei, and bundles of axons are called tracts.
• In the Peripheral Nervous System (PNS), clusters of cell bodies are called ganglia, and bundles of axons are called nerves.

Neuron Types

• Neurons receive, process, and transmit electrochemical signals, enabling communication between neurons and other body parts like muscles and glands.
• Unipolar neurons have one extension (axon) from the cell body and primarily carry sensory information from the body to the spinal cord/brain.
• Bipolar neurons have one axon and one dendrite extending from the cell body and are involved in specialized sensory functions.
• Multipolar neurons have one axon and many dendrites and process and transmit information in the brain and spinal cord, such as motor neurons controlling muscle movement.
• Interneurons connect with other neurons within the brain and spinal cord, aiding in information processing and integration, like in reflexes.

Glial Cell Types

• Glial cells act as support cells in the nervous system, that unlike neurons, help them without sending electrical signals.
• Oligodendrocytes create myelin in the CNS, that insulates to speed up electrical signal transmission.
• Schwann cells produce myelin for neurons in the PNS. Unlike oligodendrocytes, each Schwann cell myelinates only one neuron.
• Microglia, found in the CNS, function as the brain's immune system by clearing dead cells, fighting infections, and monitoring for damage.
• Astrocytes, located in the CNS, regulate blood flow, maintain the blood-brain barrier (protecting the brain from harmful substances), provide nutrients to neurons, and aid in repair after injury.

Neuron Communication

• The resting membrane potential, at -70mV, is the voltage difference across the neuron's membrane when it's at rest.
• Signals are generated by altering the resting membrane potential.
• Signals travel through the neuron and across the synapse to the next neuron.

Postsynaptic Potentials

• Postsynaptic potentials vary in duration
• Postsynaptic potentials are graded potentials
• Postsynaptic potentials decrease over time (decremental)
• Excitatory postsynaptic potentials (EPSP) cause depolarization (e.g., -70 to -67mV), increasing the likelihood of a neuron firing.
• Inhibitory postsynaptic potentials (IPSP) cause hyperpolarization (e.g., -70 to -72mV), decreasing the likelihood of a neuron firing.

Action Potentials

• A neuron fires when the balance of IPSPs and EPSPs at the axon initial segment reaches the threshold of excitation to polarize the membrane.
• The threshold of excitation is approximately -65mV and triggers an action potential (AP).
• The action potential (AP) lasts about 1 millisecond and reverses the membrane potential from -70mV to +50mV.
• Neurons integrate incoming EPSP and IPSP signals to decide whether to fire, which is called integration

Neural Integration

• Spatial summation occurs when simultaneous EPSPs or IPSPs happen in different areas of the receptive membrane.
• Temporal summation occurs when EPSPs or IPSPs occur in quick succession over time

Depolarization and Repolarization

• During depolarization, Na+ rushes in, making the neuron positive (+50mV), which marks the rising phase of the action potential.
• K+ channels open after sodium entry, making the neuron more negative again.
• During repolarization, the neuron returns to its resting state as K+ exits, reducing the positive charge back to -70mV.
• Hyperpolarization happens when too much K+ leaves, causing the neuron to become extra negative, which briefly prevents immediate firing.

Refractory Periods

• The absolute refractory period (1-2ms) is a time when the neuron cannot fire again under any circumstances.
• The relative refractory period is a time when the neuron can fire, but only with a stronger stimulus.

Action Potential Movement

• Action potentials move in one direction, from the cell body to the terminal buttons.
• High-intensity stimulation results in fast firing (up to 1000 times per second), while low-intensity stimulation causes slow firing.

Conduction Types

• Orthodromic conduction refers to the normal direction of action potential movement, from the cell body to the terminal buttons.
• Antidromic conduction refers to backward action potential movement and can be triggered experimentally.