Early Animal Evolution: From Filter Feeders to Nervous Systems

The earliest marine creatures, akin to modern sponges, thrived by filtering water for sustenance, devoid of sensory perception or response mechanisms. Gradually, diffuse nerve nets emerged in jellyfish-like organisms, enabling basic touch and chemical detection but lacking precise spatial discrimination.

Approximately 550 million years ago, the theoretical urbilaterian emerged, exhibiting bilateral symmetry and concentrated sensory and nervous structures at its anterior end. This ancestral form is believed to have given rise to various lineages, including vertebrates, worms, and insects, despite lacking fossil evidence.

Early scholars, like the Ancient Greeks, speculated on the brain's influence on muscles, attributing nerve function to "animal spirits." Later, the discovery of animal electricity by Luigi Galvani in 1791, through studies on frogs, revealed the role of electrical currents in nerve-muscle interactions. Julius Bernstein's 1902 proposition on nerve cell currents furthered understanding, attributing them to voltage differences caused by charged particle distribution.

According to the neuron doctrine, neurons function as discrete units, separated by synapses, physical gaps between neurons and muscles. Electrical impulses facilitate long-distance communication within neurons, while neurotransmitters transmit messages across synapses. The release of neurotransmitters in response to electrical impulses facilitates communication between nerves and muscles.

The illustration portrays neurotransmitter-mediated message transmission between two neurons across a synaptic gap.

Overview of Synaptic Transmission and Neurotransmitters in Neurons

Nerve impulses are electrical signals that travel through neurons along complex pathways. These signals move from one neuron to another through synapses, specialized junctions where communication between neurons occurs. The process of transmitting a nerve impulse across a synapse involves chemical messengers called neurotransmitters.

The Role of Synapses in Nerve Impulse Transmission

A synapse is the junction between two neurons, separated by a small gap called the synaptic cleft. When a nerve impulse reaches the end of a neuron (presynaptic neuron), it triggers the release of neurotransmitters, which then transmit the signal to the next neuron (postsynaptic neuron). This process generates new action potentials, allowing the impulse to continue traveling.

Neurotransmitter Substances

Neurotransmitters are chemicals that facilitate communication between neurons. Some of the key neurotransmitters include:

• Acetylcholine – Stimulates skeletal muscle contractions and is the primary neurotransmitter for synapses outside the central nervous system (CNS).
• Monoamines – A group of neurotransmitters derived from amino acids, including:
• Epinephrine – Involved in the fight-or-flight response.
• Norepinephrine – Regulates alertness and blood pressure.
• Dopamine – Plays a role in pleasure, motivation, and movement control.
• Serotonin – Affects mood, appetite, and sleep.

Steps of Synaptic Transmission

The transmission of nerve impulses across a synapse occurs in several steps:

1. Arrival of Action Potential: A nerve impulse (red arrow) reaches the synaptic knob at the end of the presynaptic neuron.
2. Vesicle Fusion: The impulse triggers chemical changes, causing neurotransmitter vesicles to fuse with the plasma membrane of the presynaptic neuron.
3. Release of Neurotransmitters: The vesicles release neurotransmitter molecules (green) into the synaptic cleft.
4. Neurotransmitter Binding: The neurotransmitters diffuse across the cleft and bind to receptor molecules on the postsynaptic neuron’s plasma membrane.
5. Opening of Ion Channels: Binding activates ion channels, allowing ions to enter the postsynaptic neuron, leading to the formation of new action potentials.
6. Neurotransmitter Removal: After transmission, neurotransmitters are either broken down by enzymes in the synaptic cleft, reabsorbed into the presynaptic neuron, or taken up by surrounding support cells.

Final Thoughts

Synaptic transmission is essential for communication within the nervous system. Neurotransmitters play a crucial role in regulating brain activity, muscle movement, and various bodily functions. Understanding how nerve impulses travel and how neurotransmitters function helps in diagnosing and treating neurological disorders.

Neurons Communicate At Synapses