Neurons, Synapses, and the Nervous System
T.2.1 Neurons: Electrical Properties
Neurons are the cells of the nervous system. They are connected and are responsible for transmitting information from sensory systems to the nervous system (brain and spinal cord). They prepare responses sent to the effector organs (muscles and glands).
Structure of Neurons: They consist of a cell body, neuronal endings called dendrites, and a longer extension called an axon. The axons of neurons can associate with attached cells called Schwann cells with a lipid coating called myelin, forming myelin sheaths. The nodes of Ranvier are the spaces between two Schwann cells, forming pods.
Electrical Properties: The nerve impulse originates in the terminal part of the dendrite, reaches the cell body, and then passes to the axon, which connects with the next neuron. The nerve impulse is due to changes in the membrane potential. (The membrane potential is the difference in charges that can be measured between the inside and outside of the neuron’s membrane. Inside, a negative charge builds up, and through a chemical process that involves sodium ions and potassium ions, power transmission between neurons is achieved).
T.2.2 Nerves and Synapses
The junction between neurons is not anatomical, but rather functional. It can occur between two neurons, between a neuron and a gland, or between a neuron and a muscle. A synapse is the nerve impulse that goes between neurotransmitters. It is chemical because a substance is found at the end of axons within vesicles called synaptic vesicles.
Stages of Synapse:
- Neurotransmitter release: When the action potential reaches the end of a presynaptic neuron, it causes the opening of calcium channels that stimulate neurotransmitter-loaded vesicles. These fuse with the cell membrane and expel the neurotransmitter into the synaptic space.
- Neurotransmitter Recognition: The synaptic space is very small, and neurotransmitters rapidly diffuse to the postsynaptic neuron. Once they have submitted the information, they are removed or reabsorbed.
- Response of the postsynaptic neuron: The neuron receptors, once stimulated, cause the flow of ions, modifying the potential and thus producing the action potential. This potential is transmitted along the neuron to finally produce a new synapse.
T.2.3 Central Nervous System (Brain and Spinal Cord)
It consists of millions of connected neurons, glial cells, collagen fibers, and blood vessels. The cell bodies form the gray matter, which occupies the peripheral part of the brain and the central part of the spinal cord. The extensions of the cell bodies, dendrites, and axons form the white matter. This color is due to the myelin sheaths. The situation of the white matter is the opposite of the gray matter. The brain and spine are very delicate structures, so one is protected by the skull and the other by the spine. In addition, there are three protective layers called meninges: the dura mater, the arachnoid mater, and the pia mater. Between them, there is a liquid called cerebrospinal fluid.
Brain: It is the organ that receives the information collected by the peripheral nervous system, processes it, and responds. It joins the spinal cord through the spinal canal. The brain includes:
- Brain: cerebral hemispheres, diencephalon (thalamus, hypothalamus, and pituitary)
- Cerebellum
- Medulla oblongata
Brain (cerebral hemispheres): These are two lateral masses, left and right, that are covered by gray matter, forming the cerebral cortex. The white matter occupies the central part, which is the point where the sensory and motor pathways cross. The cortex has a folded form, where there are some fissures that separate the hemispheres into four lobes: frontal, occipital, parietal, and temporal. On the surface of the cortex, three regions are distinguished:
- Sensory Area: Receives information from the five senses and is related to the occipital region.
- Association Area: Is in charge of analyzing the information that reaches the brain and subsequently developing responses. This zone is associated with abilities such as memory, thinking, and decision-making capacity.
- Motor area: Sends responses to the muscles and glands.
Brain (diencephalon): This consists of the:
- Thalamus: Controls all sensations except smell, which is associated with the pituitary membrane.
- Hypothalamus: This is below the thalamus and is responsible for monitoring the autonomic nervous system, controlling autonomous movements, regulating body temperature, emotions, and endocrine functions such as hunger, thirst, and sexual instinct.
- Pituitary gland: A gland that secretes hormones (growth hormone, prolactin, and other hormones such as those that control the ovaries and pancreas).
Cerebellum: It is divided into two cerebellar hemispheres joined together by a central part called the vermis. As in the cerebral hemispheres, gray matter is on the outside and white on the inside. It is responsible for monitoring balance.
Medulla Oblongata: Connects the brain with the spinal cord and is responsible for monitoring reflex activities such as the heart and respiratory muscles.
Spinal Cord: Located inside the spine, it connects to the skull through a hole in the skull base called the foramen magnum. The medulla consists of gray matter in the central part and white matter on the outside. We distinguish two regions:
- Anterior Region: Motor region, has two motor neuronal bodies from which the nerves supplying the muscles leave.
- Posterior Region: Sensory region, is the seat of the axons of sensory neurons.
It has two main functions: processing and sending information to the brain and processing elementary responses (reflex action).
T.2.4 Peripheral Nervous System
Responsible for sending information to the central nervous system and receiving/executing the orders that it transmits. We distinguish two types of peripheral nerves:
- Cranial Nerves: They are found in the skull and are responsible for sensory and motor functions in that area.
- Spinal nerves: They are formed by the junction of the anterior and posterior nerves of the spinal cord and are responsible for innervating the skeletal muscle system.
T.2.5 Autonomic Nervous System
Responsible for reviewing involuntary movements, which are those that correspond to the functioning of the organs. It consists of two antagonistic neural networks that cannot be given at the same time. This allows physiological equilibrium.
- Sympathetic: It acts during a state of emergency, accelerates the heart rate, respiratory rate, sweat glands, and energy consumption considerably (e.g., during a test, etc.).
- Parasympathetic: It produces an effect contrary to the above; it acts in relaxed situations.