Neuronal Communication

Structure:
- Soma: The cell body, containing the nucleus and other organelles.
- Axon: A long, slender projection of a neuron that conducts electrical impulses away from the cell body.
- Dendrites: Branch-like structures that extend from the cell body and receive signals from other neurons.
- Pre-synaptic terminals: Specialized endings of the axon that release neurotransmitters to communicate with other neurons.
- Myelin: A fatty substance that insulates the axon and speeds up the transmission of electrical signals.
Types:
- Multipolar: Typical motor neuron; characterized by having one axon and multiple dendrites.
- Bipolar: Typical special sensory neuron; characterized by having one axon and one dendrite.
- Unipolar: Typical somatosensory neuron; characterized by having a single process extending from the cell body, which then branches into two axons.

Grey Matter: Areas of the central nervous system (CNS) that are rich in neuronal cell bodies; primarily found in the cortex of the brain and spinal cord.
White Matter: Areas of the CNS that are mainly composed of myelinated axons; responsible for transmitting signals between different regions of the brain and spinal cord.

Ganglia: Collections of cell bodies outside the CNS, often involved in relaying sensory and motor information.
Nuclei: Collections of cell bodies within the CNS, serving specific functions such as relaying information or controlling specific processes.

Action potentials (APs) are electrical signals propagating along axons.
AP is a rapid, transient depolarization of the cell membrane.
Sodium channels open, and sodium flows into the cell, reducing resting membrane potential.
If the reduction reaches threshold, an AP results; this threshold is the level of depolarization required to trigger an action potential.

Axons of many neurons are covered in myelin (= myelinated axons).
Myelin is produced by oligodendrocytes (in the central nervous system) and Schwann cells (in the peripheral nervous system).
Myelin is an insulator that increases conduction speed along the axon by allowing saltatory conduction (the action potential jumps from one node of Ranvier to the next).

A $\alpha$ : Proprioception, motor (12 - 20 $\mu$ m, 70 - 120 m/s, myelinated)
A $\beta$ : Touch (5 - 12 $\mu$ m, 30 - 70 m/s, myelinated)
A $\gamma$ : Motor (muscle spindles) (3 – 6 $\mu$ m, 15 – 30 m/s, myelinated)
A $\delta$ : Pain, temp, touch (2 - 5 $\mu$ m, 12 - 30 m/s, myelinated but thin)
B: Autonomic NS (<3 $\mu$ m, 3 - 15 m/s, myelinated)
C: Pain, autonomic NS (0.3 – 1.3 $\mu$ m, 0.5 – 2.3 m/s, unmyelinated)

Neurotransmitters: Chemicals released from an axon terminal; act on the postsynaptic membrane.
- Can be inhibitory or excitatory.
- Excitatory: Binding causes depolarization; if the membrane reaches threshold, an AP results.
- Inhibitory: Binding causes further polarization, moving the membrane away from threshold, reducing the chance of an AP.

Convergence: Multiple pre-synaptic neurons converge onto a single post-synaptic neuron.
Divergence: A single pre-synaptic neuron synapses with multiple post-synaptic neurons.

A specialized synapse between a motor neuron and a muscle fiber; where the motor neuron transmits a signal to the muscle fiber to cause contraction.