Lesson 1.1: Overview of the Nervous System

• Your nervous system has three basic components: the brain, spinal cord, and neurons
• Despite its simple composition, the subparts of its components and how they coordinate are so complex that it is better to organize them into divisions.
• The two major divisions of our nervous system are the central nervous system and the peripheral nervous system.
• The central nervous system (CNS) is composed of the brain and the spinal cord. Together, they act as integrating command centers of the nervous system by interpreting information perceived by our sensory organs and issue responses based on our experiences and current conditions.
• The peripheral nervous system (PNS), on the other hand, is the division of the nervous system outside the CNS. It consists mainly of the nerves that extend from the brain and spinal cord. They link the body organs to the CNS via sensory receptors. They carry this through the transmission of nerve impulses or electrical messages.
• Those nerves that carry impulses to and from the brain are called cranial nerves
• while those that carry impulses to and from the spinal cord are called spinal nerves.
• Neurons are the basic units of the nervous system. They are also referred to as the nerve cells. They are highly specialized cells that transmit nerve impulses or electrical messages from one body part to another.
• All types of neurons, although structurally different from one another, have two common features: a cell body and its neural processes or extensions.
• The cell body, or soma, is the center of a neuron because it contains the nucleus and all its organelles.
• The arm-like neural processes of a neuron serve as passageways of impulses
• Processes that transmit electrical signals toward the cell body are called dendrites
• while those that generate impulses away from the cell body are called axons
• The axons branch out into hundreds to thousands of axon terminals, which are small vessels that contain the neurotransmitters or chemical messengers of signals
• When these neurotransmitters are stimulated by electric signals, they create an extracellular space called a synapse.
• The synapse connects the axon terminals of one neuron to the dendrites of another neuron.
• Axons and dendrites may be covered with myelin sheath, a whitish fatty material that insulates them and speeds up the transmission of nerve impulses.
• Neurons are grouped functionally based on the direction through which the nerve impulses travel relative to the central nervous system. There are three functional classification groups of neurons: sensory neurons, motor neurons, and interneurons.
• Sensory neurons, as their name suggests, carry impulses from sensory receptors incorporated in our internal organs and skin toward the CNS. They keep us informed about what is happening inside and outside of our bodies.
• Sensory neurons are also referred to as afferent neurons, which literally means “to go toward.”
• Motor neurons, on the other hand, carry impulses from the CNS to the effector organs like muscles and glands of our body. They carry the responses that our muscles will perform based on the stimulus the sensory neurons are receiving
• Motor neurons are also referred to as efferent neurons which means “away from.”
• Interneurons are the third functional classification group of neurons. They link the sensory and motor neurons together to keep the flow of impulses between the two different kinds of neurons. With this, they are also called association neurons
• The structural classification of neurons is based on the number of neural processes that extend from the cell body. There are three structural classification groups of neurons: multipolar, bipolar, and unipolar neurons.
• A neuron is a multipolar neuron if it has several dendrites and an axon. All motor neurons and interneurons have this kind of neural structure. Because of this, most neurons are structurally multipolar
• Neurons with one axon and one dendrite are called bipolar neurons. They are only found in sensory organs, like the ears and eyes, where they act as sensory receptors.
• Unipolar neurons consist of a single neural process that extends from the cell body in the form of an axon. This axon conducts nerve impulses both toward and away from the cell body. They are commonly found as sensory neurons in the peripheral nervous system.
• Because of the complexity of our nervous system, it needs reinforcement from supporting cells to carry out its functions.
• Neuroglia, which means “nerve glue,” are supporting cells that lump together to support and protect the delicate nerve cells.
• Neuroglial cells may structurally resemble neurons but they cannot transmit impulses. Each major division of the nervous system has distinct neuroglial cells.
• Astrocytes are star-shaped cells with numerous projections that insulate most of the neural tissues. They form a barrier between the capillaries and neurons in the brain to protect them from harmful substances that may be found in the blood. They are the most abundant type of neuroglial cells in the central nervous system.
• Microglia are spider-like cells that act as phagocytes to eliminate debris, which includes dead brain cells and bacteria, found wandering in the brain. With this function, they serve as the protectors of the central nervous system.
• Oligodendrocytes are the supporting cells that produce the insulative myelin sheaths of the neural processes in the central nervous system.
• Ependymal cells form a series of cells that line the cavities of the brain and the spinal cord. Specialized ependymal cells found in the brain produce the cerebrospinal fluid (CSF) which fills those cavities and forms a cushion that protects the central nervous system
• Schwann cells are specialized supporting cells that produce the myelin of neural processes outside the CNS. They wrap themselves around the axons and dendrites in a concentric fashion forming the myelin sheath. Their difference with the oligodendrocytes is that they have exposed membranes and cytoplasms.
• Schwann cells are also separated by gaps or indentations called the nodes of Ranvier which hasten the impulse propagation.
• Satellite cells are flattened supporting cells found wrapped around the cell bodies of sensory neurons in the peripheral nervous system. Their main function is to act as protective cushioning cells.
• Through the coordination of the components of the nervous system, with the neurons and supporting cells linking each of them together, the nervous system is able to carry out its functions that control all the processes in our body.
• Sensory Perceptions

  • They receive different internal and external stimuli all the time called sensory input. These stimuli will be gathered and converted into electrical signals which will be sent via the spinal cord to the processing center of the nervous system—the brain.
• Information Processing and Integration

  • These electrical signals are assessed in the brain through the process called integration. The organs that may be involved in responding to a certain stimulus will be tapped to coordinate what should be done at each moment.