Nervous System - Structure and Functions of Cells

This section focuses on how an action potential travels within a single neuron, from the cell body down the axon to the terminal buttons.
The action potential triggers the release of neurotransmitters.
Synaptic transmission (communication between neurons) will be discussed in the next section.
An action potential involves changes in the axon membrane, allowing ions to move between the axon's interior and the surrounding fluid, creating electrical currents.

LO 2.5: Compare neural communication in a withdrawal reflex with and without inhibition of the reflex.
A simple neural circuit consisting of three neurons and a muscle is used to illustrate a withdrawal reflex.

A sensory neuron detects a painful stimulus (e.g., touching a hot object).
The sensory neuron sends messages to its terminal buttons in the spinal cord.
The terminal buttons release a neurotransmitter that excites an interneuron.
The interneuron sends messages down its axon.
The interneuron's terminal buttons release a neurotransmitter that excites a motor neuron.
The motor neuron sends messages down its axon, which joins a nerve and travels to a muscle.
The motor neuron's terminal buttons release a neurotransmitter, causing the muscle cells to contract and the hand to move away from the hot object.

Inhibitory synapses can counteract excitatory effects.
Example: Holding a hot drink. The pain from the heat triggers a withdrawal reflex, but the brain inhibits this reflex to prevent dropping the cup.
Neural circuits in the brain recognize the potential disaster of dropping the cup and send information to the spinal cord.
An axon from a neuron in the brain reaches the spinal cord and forms synapses with an inhibitory interneuron.
When the neuron in the brain is active, it excites the inhibitory interneuron.
The inhibitory interneuron releases an inhibitory neurotransmitter, decreasing the activity of the motor neuron and blocking the withdrawal reflex.
This demonstrates a contest between two competing tendencies: dropping the cup (withdrawal reflex) and holding onto it (inhibition from the brain).
The figure 2.13 shows how this information reaches the spinal cord.

LO 2.2: Describe the structures of a neuron, including their general function.
Neurons consist of four basic structures: soma, dendrites, axon, and terminal buttons.
- Soma: Contains the nucleus and organelles.
- Dendrites: Branched structures attached to the soma that receive messages from other neurons.
- Axon: A long, thin extension of the soma that conveys an electrical message to the terminal buttons.
- Terminal Buttons: Extensions of the axon that convert electrical messages into chemical messages by releasing neurotransmitters into the synapse.
Other important structures include the cell membrane, cytoskeleton, and cytoplasm, which provide shape and support, and internal organelles (nucleus, endoplasmic reticulum, Golgi apparatus, and mitochondria) that help the cell survive.

LO 2.3: Differentiate functions of supporting cells of the central and peripheral nervous systems.
In the CNS, astrocytes, oligodendrocytes, and microglia support neurons by creating a conducive environment, providing myelin, and activating immune responses.
In the PNS, Schwann cells provide myelin and assist with neural regeneration.

LO 2.4: Discuss the features and importance of the blood-brain barrier.
The blood-brain barrier protects the CNS by selectively permitting only certain substances to enter.
It is made of capillary walls and regulates the composition of fluids in the brain, protecting neuronal transmission.
The area postrema has a more permeable blood-brain barrier, allowing neurons in this region to detect toxic substances in the blood.
The blood–brain barrier is effective in preventing pathogens from entering the brain, it can also prevent therapeutic molecules from entering.

LO 2.6: Contrast the changes in electrical potential within a neuron when it is experiencing resting potential, hyperpolarization, depolarization, and an action potential.
Electrical events can be measured to understand the message conducted along the axon.
Microelectrodes are used to record changes in electrical activity across the axon membrane.

When a microelectrode is inserted into an axon at rest, it detects a negative charge inside the membrane.
Most neurons have a resting potential of approximately $-70 mV$ , meaning they are 70 units more negatively charged inside the axon compared to outside.
The membrane potential is the difference in charge (positive or negative) across the membrane.
When the neuron is at rest, the membrane potential remains at approximately $-70mV$ .

It is possible to artificially simulate messages by applying electrical charge to neurons.
If a negative charge is applied, the inside of the axon can become more negative (e.g., $-80 mV$ ).
When the inside of an axon becomes more negative relative to the outside, it is hyperpolarized.

If a positive charge is applied, the inside of the axon can become more positive (e.g., $-50mV$ ).
When the inside of the axon becomes more positive, the neuron is depolarized.

Each neuron has a threshold of excitation or a set point for depolarization to trigger the action potential.
The action potential is a burst of rapid depolarization followed by hyperpolarization.
This spread of depolarization followed by hyperpolarization begins at the point where the soma meets the axon and propagates to the end of the terminal buttons, triggering neurotransmitter release.

The giant squid axon (about $0.5 mm$ in diameter) is much larger than mammalian axons, making it easier to study electrical activity in an axon.
Much of our knowledge about electrical potentials in the axon comes from studies of the giant squid axon.