By Elizabeth A. Murray, Mount St. Joseph University
The anatomy of the nervous system is primarily divided into two main components: the central nervous system and the peripheral nervous system. The central nervous system, often abbreviated as the CNS, consists of two and only two organs: the brain and the spinal cord. The peripheral nervous system, or PNS, on the other hand, is made up of all other nerve tissue; this could be summarized as the body’s nerves and structures, called ganglia. Let’s unpack that a bit more.
Central and Peripheral Nervous System
The brain is a mass of neural tissue housed inside the cranial cavity of the skull. The spinal cord is connected to the brain and housed within the vertebral column—the bones and structures that make up the spine. So, the skull and vertebrae protect and safeguard the two organs of the central nervous system, but each of those CNS organs has nerves extending off of it that exit the cranium or leave the spine to form the peripheral nervous system.
The nerves attached to the brain are called cranial nerves, and nerves extending from the spinal cord are known as spinal nerves. The other component of the peripheral nervous system, the ganglia, are limited areas where the nuclei of some nerve cells are found outside the central nervous system. That will make more sense if we first understand the nerve cells, called neurons.
Neurons
Most neurons have essentially three parts: first, a receiving end that has structures called dendrites that bring messages into the cell; secondly, a region called the cell body, that contains the nucleus of the neuron; third, an extension that is the transmitting end of the nerve cell, called an axon—or sometimes called a nerve fiber.
The axons release chemicals called neurotransmitters, and when we talk about neuromuscular relationships, those neurotransmitters can either stimulate or inhibit the next cell or cells in the line—which would be a group of skeletal muscle cells. The cranial and spinal nerves are bundles of these axons, wrapped in connective tissue, and traveling together outside the central nervous system. And while most neuron cell bodies are within the brain and spinal cord, there are a few isolated regions outside the central nervous system where cell bodies cluster in areas called ganglia. These nerves and ganglia together make up the peripheral nervous system.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Physiological Subdivisions of the Nervous System
Next come the physiological subdivisions of the nervous system. One subdivision is referred to as the somatic nervous system—abbreviated SNS—it controls voluntary skeletal muscles and conveys most of our conscious sensations to the brain. The other physiological subdivision is the autonomic nervous system, or ANS, that controls involuntary cardiac and smooth muscle, as well as many glands of the body.
In terms of functionality, much of the somatic nervous system can be divided into sensory pathways and motor pathways. Sensory pathways bring information into the central nervous system from the periphery, like from sensory receptors in our skin, from our joints and muscles, as well as from other internal organs. Sensory pathways are the input to the central nervous system—and are also known as afferent pathways. Motor pathways, on the other hand, carry commands from the central nervous system—either from the brain or spinal cord—and these commands are passed to nerves of the peripheral nervous system.
With respect to neuromuscular relationships, these motor nerves will either stimulate the skeletal muscle to contract, or it may inhibit the muscle, telling it to relax. These outgoing motor signals are also known as efferent pathways. The motor neurons themselves can be called efferent neurons, and sensory neurons are also known as afferent neurons.
What We Feel in Our Brain
So, let’s take an example: In order for us to feel something with our hands, receptors in the skin are stimulated by touch, and pass a signal along an afferent peripheral neuron, within a spinal nerve connected to the spinal cord. There, a second neuron in the pathway will carry that message up the spinal cord in areas called ascending tracts, that will eventually lead to brain regions where we perceive that sensation.
In fact, we don’t feel in our hands. It’s more accurate to say that we feel with our hands, but we feel in our brain. And for us to move our hand consciously, areas of our brain send messages down descending tracts of the spinal cord, and out one or more spinal nerves—full of axons of efferent neurons—that will lead to the muscles that control our hand.
Muscle Memory and Reflex Activity
The brain initiates our voluntary movements, and even generates defined and coordinated pathways for our collective actions that some call muscle memory. The memory isn’t in our muscles; the memory is a coordinated series of neural pathways that we store in our brain. These allow us to remember and perform activities like how to steer the car, dance the twist and pitch a baseball.
So, what about reflex activity, like pulling our hand away from a hot stove before we even feel it in our brain? Reflex activity uses many of the same neurons and the same nerves but operates on shorter pathways. Those that control reflexes of the upper limb are coordinated in the spinal cord to cause unplanned movement before we even realize it. That doesn’t make these muscles involuntary; they are still voluntary skeletal muscles. It’s just a protective mechanism of the body that utilizes a shorter, quicker, and largely involuntary pathway to those muscles.
Lastly, we also have receptors in our skeletal muscles, our tendons, and our joints that also act reflexively to aid our balance during coordinated muscular activities like running, bowling, dancing, and yoga.
Common Questions about the Anatomy of the Nervous System
The spinal cord is connected to the brain and housed within the vertebral column—the bones and structures that make up the spine.
The somatic nervous system controls voluntary skeletal muscles and conveys most of our conscious sensations to the brain.
Muscle memory isn’t in our muscles; the memory is a coordinated series of neural pathways that we store in our brain.
