Nervous System and Neuron Types

• The brain and nervous system comprise approximately 100 billion neurons, which are specialized nerve cells that transmit signals.

Types of Neurons

• Sensory neurons (sensory receptors) transmit neural information from receptors in the PNS to the CNS, relaying sensory information from the five senses.
• Interneurons (relay neurons) transmit neural information within the spinal cord and brain, connecting sensory and motor neurons, and are exclusively found in the CNS.
• Motor neurons transmit neural information from the CNS to the PNS, designed to initiate a response in an effector, such as a muscle or gland.

Neuron Structures

• Key neuron structures include dendrites, cell body (soma), axon, myelin sheath, axon terminals, and the synapse.
• The synapse is the junction between neurons, including the axon terminal (of the sending neuron), the synaptic cleft (gap), and the dendrite (of the receiving neuron).
• Dendrites are sensory receptors that receive stimuli (for sensory neurons) or neurotransmitters from neighboring neurons (for interneurons or motor neurons).
• The cell body (soma) contains most of the cell’s organelles, including the nucleus.
• The nucleus is the modulator (control center) of the cell and contains its genetic material.
• The axon transfers electrical impulses from the cell body to the synapse, with grouped axons forming nerves.
• Axon terminals are located at the end of the axon and contain vesicles holding neurotransmitters, which are released upon receiving an electrical impulse.
• The myelin sheath is an insulating layer of Schwann cells covering the axon, which helps maintain electrical signals inside the cell for faster transmission.
• Synapses facilitate the passage of electrical messages between neurons that do not physically touch.
• An electrical impulse triggers the diffusion of chemical signals called neurotransmitters across the synaptic cleft, which then bind to receptors on the dendrites of neighboring neurons.
• Schwann cells form the myelin sheath.

Action Potential

• An action potential, or neural impulse, travels along a neuron.
• Once the action potential reaches the axon terminal, it relies on neurotransmitters to pass the signal to the next neuron.
• When a neuron is not sending a neural impulse, it is at rest.
• Each neuron needs to reach a minimum stimulation level, known as the threshold potential, to activate an action potential.
• An action potential begins and moves along the axon like a wave when enough neurotransmitters arrive and the threshold is reached.
• If the threshold is not reached, no action potential can start, following the ‘all or none’ principle.
• Once triggered, the action potential is self-sustaining and proceeds to the end of the axon without further stimulation.
• The speed of an action potential varies, ranging from 1 to over 100 meters per second, depending on the axon's diameter and the presence of a myelin sheath.
• A larger diameter and the presence of a myelin sheath increase the speed of the impulse.

Communication Between Neurons: The Synapse

• When neurons communicate, an action potential traveling down the axon to synapses at axon terminals causes neurotransmitters to be released from synaptic vesicles.
• The presynaptic neuron refers to this part of the synapse.
• Neurotransmitters then move across the synaptic cleft (the gap between neurons).
• Scientists have identified over 100 different neurotransmitters in the human brain.
• Neurotransmitters bind to receptor sites on the dendrite of the postsynaptic neuron.
• Receptor sites convert the information back into electrical signals, which are then transmitted to the cell body and along the axon of the post-synaptic neuron.
• This process continues until the last neuron connects to a muscle or gland, causing a response.
• Neurotransmitter messages can be either excitatory or inhibitory.
• Excitatory neurotransmitters increase the likelihood of an action potential being sent, while inhibitory neurotransmitters decrease this likelihood.
• Both are important for initiating or stopping responses at the effector site.

Neurotransmitters

• Dopamine acts within the brain on pathways associated with motor functions and emotional arousal and motivation.
• A lack of dopamine-producing cells can cause Parkinson’s disease, characterized by tremors, stiffness, and uncoordinated movements.
• Acetylcholine transmits messages from motor neuron axon terminals to skeletal muscle.
• Curare, a plant-based toxin, prevents acetylcholine from binding to the post-synaptic neuron, causing paralysis.
• Serotonin, produced in the intestine and CNS, regulates appetite, mood, memory, and behavior.
• Low serotonin levels are theorized to be linked to depression.
• Glutamate, a neurotransmitter in the CNS, is involved in memory and learning.
• High glutamate levels are associated with depression, anxiety, and ADHD symptoms, whereas low levels are linked to insomnia, lack of concentration, and low energy levels.