Nervous Impulse Transmission: Synapse and Invertebrates
Nervous Impulse Transmission
From Neuron to Neuron: Synapse
Neurons are independent; they are not in physical contact with each other (synaptic cleft). The synapse is the process of functional communication between two neurons.
- Presynaptic Area: This is the axon of the neuron from which the information arrives.
- Postsynaptic Area: This is the specialized part of another neuron to which the information is directed.
- Synaptic Cleft: This is the space separating the two areas.
The transmission of nerve impulses across the synapse is carried out through special chemical substances called neurotransmitters. These can act as activators or inhibitors. These substances fill synaptic vesicles found in abundance in the terminal buttons located in presynaptic axons. Some neurotransmitters include acetylcholine, norepinephrine, serotonin, dopamine, and endorphins.
The arrival of a nerve impulse to the terminal buttons through the axon causes the emptying of synaptic vesicles: the neurotransmitter passes into the synaptic cleft and diffuses to the postsynaptic membrane. There, it binds to specific receptors. Once acted, neurotransmitters are inactivated enzymatically to stop the stimulation; these enzymes are produced by the membranes of postsynaptic neurons.
CHEMISTRY: The arrival of the impulse to the terminal buttons is due to an electrical signal, which becomes chemical at the synapse and returns to being electrical in the postsynaptic neuron. The message is electrochemical.
ELECTRICITY: The synaptic cleft is narrower at the synapse, so the momentum produces sufficient postsynaptic membrane depolarization, without the intervention of neurotransmitters. (Invertebrates)
From Neuron to Effector Organs
It is similar, through neurotransmitters that are released into the organs (muscles or glands). Effectors respond to the presence of the neurotransmitter: if it is a muscle, it contracts; if it is a gland, secretion is produced.
Invertebrate Nervous System
Greater Complexity:
- It tends to polarize and direct currents through unidirectional nerve circuits.
- Larger diameter nerve fibers increase (conduction velocity increases).
- A greater number of nerve cells are found, forming ganglia.
- Cephalization occurs due to the concentration of neurons in the anterior region (head).
Coelenterates
Radial symmetry, network of nerve cells connected by synapses. Any stimulus is transmitted in all directions. They are the first to have sensory organs (statocysts for equilibrium).
Platyhelminthes
Bilateral symmetry, have a pair of ganglia in the anterior region from which two nerve cords extend along the body through distributed neurons.
Annelids
Present ventral ganglion chains and a pair of ganglia for each segment of the body. When ganglion chains reach the pharynx, they form the periesophageal collar around it and join back to the head where they reach the cerebral ganglia that occupy the ventral position.
Mollusca
They have ganglion concentrations in the head, foot, and mantle. Cephalopods have a more complex nervous system; the central nervous system concentrates on the head, and the optic lobes acquire great development.
Arthropods
Increased ganglion concentration in the cephalic region, related to the development of the sense organs.
Echinoderms
Primitive nervous system with radial symmetry, formed by a periesophageal ring around the pharynx that connects to radial nerve cords.