Nerve Impulse Transmission and Nervous System Evolution
Transmission of Nerve Impulses from Neuron to Effector Organs
The transmission of nerve impulses to effector organs is conducted similarly to what happens at the synapse. Neurotransmitters are released into the effector organs. Thus, effectors respond to the presence of the neurotransmitter, causing a muscle to contract or producing a secretion in the case of a gland.
Invertebrate Nervous System
As one ascends the evolutionary scale, the nervous system acquires greater complexity:
- It tends to direct nerve currents through unidirectional channels.
- Nerve fibers increase in diameter, which increases conduction velocity.
- There is a greater number of nerve cells, forming nodes.
- Cephalization occurs due to the concentration of neurons in the head.
The nervous system is located in a ventral position. Evolution in some groups of animals:
- Radial symmetry and coelenterates have a network of nerve cells evenly distributed and connected by synapses. Any stimulation of a nerve cell is transmitted in all directions, so the whole body reacts to the stimulus (diffuse nerve plexus).
- Flatworms have bilateral symmetry and a pair of nodes in the anterior region (primitive brain) from which two nerve cords extend along the body. Neurons distributed at regular intervals send nerve branches to different tissues. The appearance of two nervous systems, the central and peripheral, characterizes all more complex animals over the course of evolution. This dual system has the advantage that each stimulus of a specific part of the body causes an individual response that does not affect the whole animal.
- Annelids have lymph node chains in a ventral position and a pair of nodes for each body segment. Lymph node chains, when they reach the pharynx, form the periesophageal collar, and join again to reach the head, giving rise to the cerebral ganglia, which occupy a dorsal position.
- Mollusks have nodal concentrations in various parts of the body, like the head, foot, and mantle. Within them, cephalopods such as octopus and squid have a more complex nervous system.
- Arthropods have a concentration of nodes in the region related to cephalization and a great development of the sense organs.
- Echinoderms are animals with radial symmetry, and their nervous system is quite primitive. It consists of a periesophageal ring around the pharynx that connects to the radial nerve cords.
Vertebrate Nervous System
It is available in a dorsal position. It originates during embryonic development from a layer of the embryo called the ectoderm, which forms the neural tube by invagination. The front of this tube expands and causes the brain, while the posterior portion results in the spinal cord. The nervous system is divided into the peripheral and central nervous systems. The first is the nerve centers of integration, control, and coordination, while the second is formed by connections with receptors and effectors.
Encephalopathy
The neural tube develops into three primary vesicles: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). Then the hindbrain and forebrain are divided again and constitute five secondary vesicles.
Forebrain
It is divided into two:
- Telencephalon: It is the most anterior and contains a pair of olfactory lobes and the brain. In birds and mammals, it is divided into two lateral lobes (cerebral hemispheres), and in humans, it reaches its maximum development with cerebral gyri and sulci. Both hemispheres are connected by a set of fibers within the corpus callosum. The gray matter is called the cerebral cortex. Its function is to centralize sensory information and control voluntary movements, as well as memory and intelligence.
- Diencephalon: Composed of the thalamus, which is a very important passageway of information and the central interpretation of many sensory stimuli that go to the brain, and the hypothalamus, which is shown below and regulates many functions within the body, such as the sensation of thirst, sex drive, and sleep. Part of the hypothalamus is the pituitary stalk, which connects to a gland, the pituitary. The roof of the diencephalon is the epithalamus, which contains the pineal gland.
Midbrain
The central core is more important in lower vertebrates. It contains the optical lobes, which, except in mammals, are the centers of vision. In mammals, this area loses its importance, and the optic fibers and auditory nerves reach the colliculi dorsally.
Hindbrain
It is divided into two parts:
- Metencephalon: It makes up the center of postural reflexes and motor coordination. It produces the cerebellum.
- Myelencephalon: Also called the medulla oblongata, it automatically controls many vital activities of the viscera and contains nerve centers that regulate heartbeat and breathing rate. The left side of the brain controls the activities and gets information on the right side of the body and vice versa.