Our bodies couldn’t operate without the nervous system – the complex network that coordinates our actions, reflexes, and sensations. Broadly speaking, the nervous system is organised into two main parts. The central nervous system (CNS) and the peripheral nervous system (PNS).
The CNS consists of the brain and spinal cord. The brain is protected by the skull (the cranial cavity) and the spinal cord travels from the back of the brain, down the center of the spine, stopping in the lumbar region of the lower back. The brain and spinal cord are both housed within a protective triple-layered membrane called the meninges.
The central nervous system has been thoroughly studied by anatomists and physiologists, but it still holds many secrets. It controls our thoughts, movements, emotions, and desires. It also controls our breathing, heart rate, the release of some hormones, body temperature, and much more.
The retina, optic nerve, olfactory nerves, and olfactory epithelium are sometimes considered to be part of the CNS alongside the brain and spinal cord. This is because they connect directly with brain tissue without intermediate nerve fibres.
Pritish Kumar Halder gives a brief Illustration of the Central nervous system:
The brain is the most complex organ in the human body; the cerebral cortex contains an estimated 15–33 billion neurons. Each of which is connected to thousands of other neurons.
In total, around 100 billion neurons and 1,000 billion glial (support) cells make up the human brain. Our brain uses around 20 percent of our body’s total energy.
The brain is the central control module of the body and coordinates activity. From physical motion to the secretion of hormones, the creation of memories, and the sensation of emotion. To carry out these functions, some sections of the brain have dedicated roles. However, many higher functions — reasoning, problem-solving, creativity — involve different areas working together in networks.
The brain is roughly split into four lobes:
Temporal lobe (yellow): Important for processing sensory input and assigning it emotional meaning. It is also involved in laying down long-term memories. Some aspects of language perception are also housed here.
Occipital lobe (green): Visual processing region of the brain, housing the visual cortex.
Parietal lobe (purple): The parietal lobe integrates sensory information including touch, spatial awareness, and navigation. Touch stimulation from the skin is ultimately sent to the parietal lobe. It also plays a part in language processing.
Frontal lobe (blue): Positioned at the front of the brain. The frontal lobe contains the majority of dopamine-sensitive neurons. And is involved in attention, reward, short-term memory, motivation, and planning.
Basal ganglia: involved in the control of voluntary motor movements, procedural learning, and decisions about which motor activities to carry out. Diseases that affect this area include Parkinson’s disease and Huntington’s disease.
Cerebellum: mostly involved in precise motor control, but also in language and attention. If the cerebellum is damaged, the primary symptom is disrupted motor control, known as ataxia.
Broca’s area: this small area on the left side of the brain (sometimes on the right in left-handed individuals) is important in language processing. When damaged, an individual finds it difficult to speak but can still understand speech. Stuttering is sometimes associated with an underactive Broca’s area.
Corpus callosum: a broad band of nerve fibers that join the left and right hemispheres. It is the largest white matter structure in the brain and allows the two hemispheres to communicate. Dyslexic children have smaller corpus callosums; left-handed people, ambidextrous people, and musicians typically have larger ones.
Medulla oblongata: extending below the skull, it is involved in involuntary functions, such as vomiting, breathing, sneezing, and maintaining the correct blood pressure.
Hypothalamus: sitting just above the brain stem and roughly the size of an almond, the hypothalamus secretes a number of neurohormones and influences body temperature control, thirst, and hunger.
Thalamus: positioned in the center of the brain, the thalamus receives sensory and motor input and relays it to the rest of the cerebral cortex. It is involved in the regulation of consciousness, sleep, awareness, and alertness.
Amygdala: two almond-shaped nuclei deep within the temporal lobe. They are involved in decision-making, memory, and emotional responses; particularly negative emotions.
The spinal cord, running almost the full length of the back, carries information between the brain and body, but also carries out other tasks. From the brainstem, where the spinal cord meets the brain, 31 spinal nerves enter the cord.
Along its length, it connects with the nerves of the peripheral nervous system (PNS) that run in from the skin, muscles, and joints.
Motor commands from the brain travel from the spine to the muscles and sensory information travels from the sensory tissues — such as the skin — toward the spinal cord and finally up to the brain.
The spinal cord contains circuits that control certain reflexive responses, such as the involuntary movement your arm might make if your finger was to touch a flame. The circuits within the spine can also generate more complex movements such as walking. Even without input from the brain, the spinal nerves can coordinate all of the muscles necessary to walk. For instance, if the brain of a cat is separated from its spine so that its brain has no contact with its body, it will start spontaneously walking when placed on a treadmill. The brain is only required to stop and start the process, or make changes if, for instance, an object appears in your path.
White and gray matter
The CNS can be roughly divided into white and gray matter. As a very general rule, the brain consists of an outer cortex of gray matter and an inner area housing tract of white matter.
Both types of tissue contain glial cells, which protect and support neurons. White matter mostly consists of axons (nerve projections) and oligodendrocytes — a type of glial cell — whereas gray matter consists predominantly of neurons.
Central glial cells
Also called neuroglia, glial cells are often called support cells for neurons. In the brain, they outnumber nerve cells 10 to 1. Without glial cells, developing nerves often lose their way and struggle to form functioning synapses.
Glial cells are found in both the CNS and PNS but each system has different types. The following are brief descriptions of the CNS glial cell types:
Astrocytes: these cells have numerous projections and anchor neurons to their blood supply. They also regulate the local environment by removing excess ions and recycling neurotransmitters.
Oligodendrocytes: responsible for creating the myelin sheath — this thin layer coats nerve cells, allowing them to send signals quickly and efficiently.
Ependymal cells: lining the spinal cord and the brain’s ventricles (fluid-filled spaces). These create and secrete cerebrospinal fluid (CSF) and keep it circulating using their whip-like cilia.
Radial glia: act as scaffolding for new nerve cells during the creation of the embryo’s nervous system.
The cranial nerves are 12 pairs of nerves that arise directly from the brain and pass through holes in the skull rather than traveling along the spinal cord. These nerves collect and send information between the brain and parts of the body – mostly the neck and head. Of these 12 pairs, the olfactory and optic nerves arise from the forebrain and are considered part of the central nervous system:
Olfactory nerves (cranial nerve I): transmit information about odors from the upper section of the nasal cavity to the olfactory bulbs on the base of the brain.
Optic nerves (cranial nerve II): carry visual information from the retina to the primary visual nuclei of the brain. Each optic nerve consists of around 1.7 million nerve fibers.