The human nervous system is a system of organs made up of the brain, spinal cord, and a massive network of nerves. This organ system is vital: we need our nervous systems to allow us to move, respond to sensory information, and much more! Read on to learn more about the nervous System.
What’s the Human Nervous System Made Of?
The human nervous system is made up of two distinct categories: the central nervous system, which is made up of the brain and the spinal cord, and the peripheral nervous system, which is made up of a massive network of nerves that branch off the spinal cord and extend throughout our entire bodies.
The Central Nervous System (CNS)
The central nervous system consists of two main parts: the brain and the spinal cord. Let’s dive a little bit deeper into each of these sections:
The brain is an extremely complex organ that controls our thoughts, memory, emotion, touch, motor skills, vision, breathing, temperature, hunger, and every process that regulates our body. The brain consists of four main parts:
- The brain stem
The brain stem is another key aspect of the body’s central nervous system. It is located between the pons, a mass of transverse nerve fibers connecting the brain stem with the cerebellum and the spinal cord.
- The cerebrum
The cerebrum makes up most of the brain and is supported by the brain stem. It can be divided into two distinct hemispheres, each of which controls the activities of the opposite side of the body.
The hemispheres are then divided further into four lobes, known as:
- Frontal lobe
- Temporal lobes
- Parietal lobe
- Occipital lobe
- The cerebellum
The cerebellum is located behind and below the cerebrum. It coordinates voluntary movements and motor skills such as balance, coordination, and posture.
- The diencephalon
The diencephalon is the part of the brain that connects the midbrain to the forebrain. It is located deep within the brain and contains the epithalamus, thalamus, subthalamus, and hypothalamus.
You may have heard the word ‘matter’ used when talking about the brain. The brain contains two types of matter: grey matter and white matter. Both of these types of matter work together to help the brain function. For instance, grey matter is responsible for receiving and storing the body’s impulses, while white matter carries these impulses to and from the grey matter.
The spinal cord
The spinal cord is the second component of the body’s central nervous system. The spinal cord is divided into 31 smaller segments, each with a pair of spinal nerves. Both motor and sensory nerves can be found within the spinal cord.
Other parts of the central nervous system
Aside from the brain and spinal cord, other body parts are included within the central nervous system. The meninges, for example, are the three layers of membranes that cover the brain and the spinal cord. The outermost layer of the meninges is called the dura mater, the middle layer is called the arachnoid, and the innermost layer is called the pia mater. The function of the meninges is to protect the brain and the spinal cord by forming a barrier against bacteria and other microorganisms.
The cerebrospinal fluid (CSF) is also part of the central nervous system. This fluid circulates the brain and spinal cord, protecting and nourishing them.
The Peripheral Nervous System
The second part of the human nervous system is the peripheral nervous system. The peripheral nervous system is made up of two parts:
- Somatic Nervous System
- Autonomic nervous system
Somatic nervous system
Within the somatic nervous system is a range of peripheral nerve fibers that pick up sensory information from the peripheral organs, i.e., organs far away from the brain. These fibers then carry this sensory information to the central nervous system.
The somatic nervous system also contains motor nerve fibers from the brain and transports messages for movement and necessary action to the skeletal muscles.
Autonomic Nervous System
The autonomic nervous system is another part of the human nervous system. It contains three parts:
- The sympathetic nervous system is responsible for the body’s rapid involuntary response to dangerous or stressful situations.
- The parasympathetic nervous system controls bodily functions when a person is at rest.
- The enteric nervous system: This has many functions, including controlling motor functions, local blood flow, mucosal transport and secretions, and modulating immune and endocrine functions.
The autonomic nervous system controls the nerves of the body’s inner organs, which humans have no conscious control over. These inner organs include the heartbeat, digestion, and breathing, excluding conscious breathing.
The enteric nervous system is an extremely complex network of nerve fibers that supply the abdomen’s organs, like the gastrointestinal tract, pancreas, and gall bladder, with nerves. There are almost 100 million nerves in the enteric nervous system.
The entire human nervous system is built on the back of a specialized neuron cell.
As you can see in this diagram, neurons have a long, tail-like extension called an axon, and they end in tiny branching protrusions called dendrites. These two parts allow neurons to communicate with each other, even over long distances. In addition, big bundles of axons are found throughout the body, helping to organize signals from any individual body part, which forms our “nerves.” Our brains contain 86 billion neurons on average – a staggering amount of cells organized into smaller structures to manage the different processes we need to survive.
The body has different types of neurons for different tasks: motor neurons transmit messages from the brain to the muscles to generate movement. Meanwhile, sensory neurons detect light, sound, odor, taste, pressure, and heat and send messages about those things to the brain. Other parts of the nervous system control involuntary movements that are still essential for keeping our bodies alive, such as keeping a regular heartbeat, releasing hormones like adrenaline, opening the pupil in response to light, and regulating the digestive system.
How Does the Nervous System Work?
Let’s start by explaining how messages travel between our neurons, allowing messages to travel throughout the body. When a neuron sends a message to another neuron, it sends an electrical signal down the length of its axon. At the end of the axon, the electrical signal changes to a chemical signal. The axon then releases the chemical signal with chemical messengers called neurotransmitters into the synapse —the space between the end of an axon and the tip of a dendrite from another neuron. The neurotransmitters move the signal through the synapse to the neighboring dendrite, which converts the chemical signal back into an electrical signal. The electrical signal then travels through the neuron and goes through the same conversion processes as it moves to neighbor neurons, allowing messages to travel throughout the body. Although it sounds complicated, this can all happen in a fraction of a second!
The messages are transmitted from the peripheral nerves into the body and then to the Spinal Cord. The Spinal Cord has an extremely large amount of neurons that carry messages back to the brain and organize all the input from nerves all over the body into one channel of information – think of it as a big motorway that lots of smaller roads filter into, taking traffic directly to a single destination, and allowing traffic from that destination to get to remote places quickly. Once the message has arrived at the brain, the brain can analyze and make sense of the information transmitted from the peripheral nervous system, work out a response, and then send out instructions with the appropriate response.
It’s easier to make sense of this remarkable process with an example. Say, for instance, that you’re moving your hand toward a hot stove. Sensory neurons in your hand will detect the heat and send the signal back through the network of nerves in your hand to increasingly large nerve bundles until the message telling the brain that your hand is about to touch something hot reaches the spinal cord. Then, from the spinal cord, the information is transferred up the spinal cord to your brain, where the brain recognizes the message and forms a response: move your hand away from the heat.
The brain then sends this message through the neurons in the brain back to the spinal cord, and through the spinal cord, to the motor neurons in hand. These motor neurons then instruct the muscles in the hand and arm to pull away from the hot stove, preventing you from getting burned. All of this happens in just a few seconds, and you don’t need to think about it consciously!
Why Do We Need a Nervous System?
The nervous system plays a vital role within the body since it effectively controls everything that happens within the body. The brain coordinates all bodily functions, from breathing to movement to complex activities like solving complicated mathematics puzzles. The nerves’ network carries information and instructions from the brain to our organs and limbs and carries sensory information back to the brain.
Both are essential for our continued survival: without our nervous systems, we’d be unaware of the world around us, unable to sense anything, move, respond to stimuli, or even breathe!