Nervous System

ntroduction

The nervous system is our body's master control and communication system. Together with the endocrine system, it is responsible for regulating and maintaining homeostasis.

Content

1. The nervous system has three major functions:
    It uses sensory receptors to monitor changes occurring both outside and inside the body.

2. It processes and interprets the sensory information it receives and makes decisions about what to do at any given time (integration).

3. Generates a response (motor output) by activating effector organs such as muscles and glands.
   Neurons and supporting cells.

Nervous tissue is highly cellular and is essentially composed of two types of cells: neurons and supporting cells

Neurons:

These are nerve cells that transmit electrical signals, are the functional unit of the nervous system, are very long-lived, and are amitotic (Fig. 4.1).

                                                               Fig. 4.1 Diagram of a neuron (Vázquez, R. 2015).

Reflex Arc
The reflex arc is formed by a receptor, which are the sensitive or afferent neurons, an integration center (spinal cord), and motor or efferent neurons. These components are connected by synapses (Fig. 4.2).

In a space called a synapse  neurons communicate with each other by exchanging chemical messengers (neuroreceptors).

Figure 4.3 When a synaptic vesicle is stimulated, neurotransmitter is released into the space between neurons. The neurotransmitter diffuses and binds to receptors on the postsynaptic cell. In most cases, the binding of the transmitter to the receptors produces a change in the resting potential of the postsynaptic cell, known as the postsynaptic potential (Vazquez, R. 2015).

                                                               Figure 4.4 Nervous System.

The nervous system is divided into two parts: central and peripheral, each of which has two subdivisions. The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of motor neurons and sensory neurons.

The central nervous system (CNS) receives and processes information and initiates an action. The brain receives and processes sensory information, initiates responses, stores memories, and generates thoughts and emotions. The spinal cord conducts signals to and from the brain and controls reflex activity.

The peripheral nervous system (PNS) transmits signals between the CNS and the rest of the body. It is made up of a series of neurons (motor and sensory). Motor neurons carry signals from the CNS that control the activities of muscles and glands. Sensory neurons carry signals from the CNS to the sensory organs.
The somatic nervous system controls voluntary movement by activating skeletal muscles.

The autonomic nervous system controls involuntary responses by acting on organs, glands, and smooth muscles. The sympathetic division prepares the body for tense or energetic situations such as flight or combat. The parasympathetic dominates during times of rest. It controls maintenance activities (Dienhart, 1981).

Figure 4.5 Diagram of the central and peripheral nervous systems (Wikimedia Commons, 2023).

The brain is made up of several specialized parts that perform specific functions. It consists of three main parts: the cerebrum, the midbrain, and the cerebellum.
The cerebrum is the largest part of the brain. It is made up of four areas called lobes and has two halves called hemispheres.

The medulla oblongata is made up of nerve cells. It controls automatic functions such as breathing, swallowing, blood pressure, heart rate, and others. The pons, located above the bulb, influences the transitions between sleep and wakefulness and between the rhythm and pattern of breathing.
The cerebellum coordinates body movements and is involved in motor learning.

The midbrain contains a center that controls reflex eye movements, retransmission of auditory signals, and part of the reticular formation (interconnected neurons found in the medulla oblongata, midbrain, and cerebellum), and the activities of the reticular formation allow us to concentrate and read when exposed to stimuli such as music, noise such as television, etc.

The forebrain (also called the cerebrum) includes the thalamus, the limbic system, and the cerebral cortex.
The thalamus transmits sensory information from throughout the body to the limbic system and the cerebral cortex.
The limbic system includes the hypothalamus, amygdala, and hippocampus, as well as regions near the cerebral cortex. Together, these structures produce our most basic emotions, impulses, and behaviors, such as anger, hunger, thirst, and even memories.
The hypothalamus contains neurons, some of which are neurosecretory neurons that release hormones into the blood; others control the release of hormones from the pituitary gland; it acts as a coordinating center that maintains homeostasis.

The amygdala produces feelings of pleasure, punishment, or sexual arousal when stimulated.
The hippocampus produces emotions such as anger and sexual arousal. It plays an important role in the formation of long-term memory and is, therefore associated with learning.

The cerebral cortex consists of two halves: the left hemisphere and the right hemisphere. Both parts are connected by a band of axons called the corpus callosum. The cortex is folded into convolutions. The cortex is dominated by bodies of neurons that give it a gray color and where it gets its name "gray matter".  These neurons receive sensory information, process it, store some of it in memory for later use, control voluntary movements, and allow us to plan and think. The cortex is divided into 4 anatomical regions: the frontal lobe, parietal lobe, temporal lobe, and occipital lobe.

The frontal lobe regulates voluntary movement, intellect, memory, language, complex reasoning such as decision-making. The parietal lobe is associated with touch, speech, reading, etc. The temporal lobe regulates long-term memory, facial recognition, hearing. The occipital lobe regulates vision, local orientation and shape perception.

The spinal cord is protected by the spine or backbone. It extends from the base of the brain to the base of the back. It is formed by spinal nerves that carry messages between the brain and the rest of the body, integrates the reflex arc, regulates basic walking and running movements.