Nervous system anatomy and physiology

The nervous system is involved in nearly everything we do - from how we see, to how we walk and talk.

The nervous system is divided into the central nervous system, so the brain and the spinal cord, and the peripheral nervous system, which is further divided into the somatic and the autonomic nervous systems.

Broadly speaking, the nervous system can be split into an afferent and an efferent division.

The afferent division brings sensory information from the outside into the central nervous system, and includes visual receptors, auditory receptors, chemoreceptors, and somatosensory or touch receptors.

On the other hand, the efferent division brings motor information from the central nervous system to the periphery, ultimately resulting in contraction of skeletal muscles to trigger movement through the somatic nervous system, as well as contraction of the smooth muscles to trigger activity of the internal organs through the autonomic nervous system.

The nervous system is made up of two main types of cells: neurons and glial cells.

Neurons are the main cells of the nervous system. They’re composed of a cell body, which contains all the cell’s organelles, and when there’s a group of neuron cell bodies that are next to each other in the central nervous system, the whole thing is called a nucleus, while a group of neuron cell bodies that are located outside of the central nervous system is called a ganglion.

Neurons have nerve fibers that extend out from the neuron cell body- these are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons.

Where two neurons come together is called a synapse, and that’s where one end of an axon releases neurotransmitters, further relaying the signal to the dendrites or directly to the cell body of the next neuron in the series.

To trigger the release of neurotransmitters, neurons use an electrical signal that races down the axon, known as the action potential.

To help speed up that electrical signal - the axons are intermittently wrapped by a fatty protective sheath called myelin, which comes from glial cells like oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system.

Another type of glial cells are called astrocytes, and they’re only present in the central nervous system.

Astrocytes give structural and metabolic support to neurons, as well as act as resident immune cells, and help seal and nourish the blood-brain barrier.

The blood-brain barrier consists of tight junctions that connect endothelial cells that line the capillaries in the brain. These tight junctions seal off the space between the endothelial cells, and they’re surrounded by basement membrane as well as astrocytes which further strengthen the barrier.

Think of the blood-brain barrier as the brain’s bouncer, a highly selective membrane that turns bacteria and other large, shady-looking molecules that are floating around in the blood away at the door, while letting in nutrients like water, oxygen, glucose, and smaller, fat-soluble molecules.

The brain has a few regions - the most obvious is the cerebrum, which is divided into two cerebral hemispheres.

The right cerebral hemisphere receives afferent fibers and sends efferent fibers to the left side of your body, while the left cerebral hemisphere receives afferent fibers and sends efferent fibers to the right side of the body.

If we look at a cross section of the cerebrum, the outermost area is the grey matter or cerebral cortex and is made up of billions of neuron cell bodies, and the innermost area is the white matter and is made up of the axons that come off of all of those neurons.

The cerebral cortex is divided into the frontal lobe, parietal lobe, temporal lobe, and the occipital lobe.

The frontal lobe controls movement, and executive function, which is our ability to make decisions.

The parietal lobe processes sensory information, which lets us locate exactly where we are physically and guides movements in a three-dimensional space.

The temporal lobe plays a role in hearing, smell, and memory, as well as visual recognition of faces and languages.

The temporal lobe surrounds and communicates with the hippocampus and helps send information from short-term to long-term memory.

Finally, there’s the occipital lobe, which is primarily responsible for vision.

Within the white matter there are deeper structures that are subcortical - below the cortex - like the internal capsule, which is like a highway that allows information to flow through neurons that are going to and from the cerebral cortex.

There’s also the basal ganglia, which are actually two deep structures - the pallidum and the striatum, with the striatum further divided into the caudate nucleus and the putamen.

The striatum receives input from the cerebral cortex about a desired movement, and then it sends output to the other basal ganglia structures to control smooth movement by inhibiting undesired movements.

As an example, when you walk, you have to move one leg at a time - so when one leg steps forward, the other leg gets inhibited by the basal ganglia, so that it’s stationary - and that prevents you from falling!

Next, there’s the diencephalon, which is composed of an upper part called the thalamus and a lower part called the hypothalamus.

The thalamus is a collection of nuclei - so millions of nerve cell bodies - that process the sensory information coming in from the body to the cerebral cortex, as well as the motor information going from the cerebral cortex to the body.

The hypothalamus is a small region that does a variety of things like regulate the body temperature, the sleep and wake cycle, and eating and drinking. To help do all of this, the hypothalamus regulates the release of the major endocrine hormones.

The hypothalamus sends signals to the pituitary, which is a pea-sized gland, that hangs by a stalk from the base of the brain and has two parts - the anterior and posterior pituitary.

The pituitary gland produces and secretes hormones when it receives signals the hypothalamus. Together, they form the hypothalamic-pituitary axis.

Next, there’s the cerebellum, which sits down at the base of the skull.

The cerebellum helps with coordinating movement, precision, and balance.

The cerebellum receives sensory input about body position from the spinal cord and receives motor input from the brain, and integrates them together to help fine-tune motor activity and store it as muscle memory. An example is riding a bicycle, something you typically can do pretty easily, even if you haven’t used a bike in a while.

And finally there’s the brainstem, which is located right in front of the cerebellum.