Overview of the nervous system

Neurons

Neurons, or nerve cell, are the main structural and functional units of the nervous system. Every neuron consists of a body (soma) and a number of processes (neurites). The nerve cell body contains the cellular organelles and is where neural impulses (action potentials) are generated. The processes stem from the body, they connect neurons with each other and with other body cells, enabling the flow of neural impulses. There are two types of neural processes that differ in structure and function; 

• Axons are long and conduct impulses away from the neuronal body. 
• Dendrites are short and act to receive impulses from other neurons, conducting the electrical signal towards the nerve cell body.

Every neuron has a single axon, while the number of dendrites varies. Based on that number, there are four structural types of neurons; multipolar, bipolar, pseudounipolar and unipolar. 

Learn more about the neurons in our study unit:

How do neurons function?

The morphology of neurons makes them highly specialized to work with neural impulses; they generate, receive and send these impulses onto other neurons and non-neural tissues. 

There are two types of neurons, named according to whether they send an electrical signal towards or away from the CNS;

• Efferent neurons (motor or descending) send neural impulses from the CNS to the peripheral tissues, instructing them how to function. 
• Afferent neurons (sensory or ascending) conduct impulses from the peripheral tissues to the CNS. These impulses contain sensory information, describing the tissue's environment.

The site where an axon connects to another cell to pass the neural impulse is called a synapse. The synapse doesn't connect to the next cell directly. Instead, the impulse triggers the release of chemicals called neurotransmitters from the very end of an axon. These neurotransmitters bind to the effector cell’s membrane, causing biochemical events to occur within that cell according to the orders sent by the CNS.

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Glial cells

Glial cells, also called neuroglia or simply glia, are smaller non-excitatory cells that act to support neurons. They do not propagate action potentials. Instead, they myelinate neurons, maintain homeostatic balance, provide structural support, protection and nutrition for neurons throughout the nervous system. 

This set of functions is provided for by four different types of glial cells;

• Myelinating glia produce the axon-insulating myelin sheath. These are called oligodendrocytes in the CNS and Schwann cells in the PNS. Remember these easily with the mnemonic "COPS" (Central - Oligodendrocytes; Peripheral - Schwann)
• Astrocytes (CNS) and satellite glial cells (PNS) both share the function of supporting and protecting neurons. 
• Other two glial cell types are found in CNS exclusively; microglia are the phagocytes of the CNS and ependymal cells which line the ventricular system of the CNS. The PNS doesn’t have a glial equivalent to microglia as the phagocytic role is performed by macrophages.

Most axons are wrapped by a white insulating substance known as a myelin sheath, which is produced by oligodendrocytes and Schwann cells. Myelin encloses an axon segmentally, leaving interruptions between the segments known as myelin sheath gaps (a.ka. nodes of Ranvier). The neural impulses propagate through the myelin sheath gaps only, skipping the myelin sheath. This significantly increases the speed of neural impulse propagation. 

White and gray matter

The white color of myelinated axons is distinguished from the gray colored neuronal bodies and dendrites. Based on this, nervous tissue is divided into white matter and gray matter, both of which has a specific distribution; 

• White matter comprises the outermost layer of the spinal cord and the inner part of the brain.
• Gray matter is located in the central part of the spinal cord, outermost layer of the brain (cerebral cortex), and in several subcortical nuclei of the brain deep to the cerebral cortex.

Master the histology of nervous tissue with our customizable quiz: We got you covered with neurons, nerves and ganglia!

Cranial nerves

Cranial nerves are peripheral nerves that emerge from the cranial nerve nuclei of the brainstem and spinal cord. They innervate the head and neck. Cranial nerves are numbered one to twelve according to their order of exit through the skull fissures. Namely, they are: olfactory nerve (CN I), optic nerve (CN II), oculomotor nerve (CN III), trochlear nerve (CN IV), trigeminal nerve (CN V), abducens nerve (VI), facial nerve (VII), vestibulocochlear nerve (VIII), glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI), and hypoglossal nerve (XII). These nerves are motor (III, IV, VI, XI, and XII), sensory (I, II and VIII) or mixed (V, VII, IX, and X).

Among many strategies for learning cranial nerves anatomy, our experts have determined that one of the most efficient is through interactive learning. Check out Kenhub’s interactive cranial nerves quizzes and labeling exercises to cut your studying time in half.

Jump right into our cranial nerves quiz in multiple difficulty levels:

Or learn more about the cranial nerves in this study unit.

Spinal nerves

Spinal nerves emerge from the segments of the spinal cord. They are numbered according to their specific segment of origin. Hence, the 31 pairs of spinal nerves are divided into 8 cervical pairs, 12 thoracic pairs, 5 lumbar pairs, 5 sacral pairs, and 1 coccygeal spinal nerve. All spinal nerves are mixed, containing both sensory and motor fibers.

Spinal nerves innervate the entire body, with the exception of the head. They do so by either directly synapsing with their target organs or by interlacing with each other and forming plexuses. There are four major plexuses that supply the body regions; 

• Cervical plexus (C1-C4) - innervates the neck 
• Brachial plexus (C5-T1) - innervates the upper limb 
• Lumbar plexus (L1-L4) - innervates the lower abdominal wall, anterior hip and thigh 
• Sacral plexus (L4-S4) - innervates the pelvis and the lower limb

Want to learn more about the spinal nerves and plexuses? Check out our resources.

Ganglia

Ganglia (sing. ganglion) are clusters of neuronal cell bodies outside of the CNS, meaning that they are the PNS equivalents to subcortical nuclei of the CNS. Ganglia can be sensory or visceral motor (autonomic) and their distribution in the body is clearly defined.

Dorsal root ganglia are clusters of sensory nerve cell bodies located adjacent to the spinal cord. They are a component of the posterior root of a spinal nerve.

Autonomic ganglia are either sympathetic or parasympathetic. Sympathetic ganglia are found in the thorax and abdomen, grouped into paravertebral and prevertebral ganglia. Paravertebral ganglia lie on either side of vertebral column (para- means beside), comprising two ganglionic chains that extend from the base of the skull to the coccyx, called sympathetic trunks. Prevertebral ganglia (collateral ganglia, preaortic ganglia) are found anterior to the vertebral column (pre- means in front of), closer to their target organ. They are further grouped according to which branch of abdominal aorta they surround; celiac, aorticorenal, superior and inferior mesenteric ganglia.

Parasympathetic ganglia are found in the head and pelvis. Ganglia in the head are associated with relevant cranial nerves and are the ciliary, pterygopalatine, otic and submandibular ganglia. Pelvic ganglia lie close to the reproductive organs comprising autonomic plexuses for innervation of pelvic viscera, such as prostatic and uterovaginal plexuses.

Find everything about ganglia needed for your neuroanatomy exam here.

Sympathetic nervous system

The sympathetic system (SANS) adjusts our bodies for situations of increased physical activity. Its actions are commonly described as the “fight-or-flight” response as it stimulates responses such as faster breathing, increased heart rate, elevated blood pressure, dilated pupils and redirection of blood flow from the skin, kidneys, stomach and intestines to the heart and muscles, where it’s needed. 

Sympathetic nerve fibers have a thoracolumbar origin, meaning that they stem from the T1-L2/L3 spinal cord segments. They synapse with prevertebral and paravertebral ganglia, from which the postsynaptic fibers travel to supply the target viscera.

Parasympathetic nervous system

The parasympathetic nervous system (PSNS) adjusts our bodies for energy conservation, activating “rest and digest” or “feed and breed” activities. The nerves of the PSNS slow down the actions of cardiovascular system, divert blood away from muscles and increase peristalsis and gland secretion. 

Parasympathetic fibers have craniosacral outflow, meaning that they originate from the brainstem (cranio-) and S2-S4 spinal cord segments (-sacral). These fibers travel to thoracic and abdominal organs, where they synapse in ganglia located close to or within the target organ.

Enteric nervous system

Enteric nervous system comprises the SANS and PANS fibers that regulate the activity of the gastrointestinal tract. This system is made of parasympathetic fibers of the vagus nerve (CN X) and sympathetic fibers of the thoracic splanchnic nerves. These fibers form two plexuses within the wall of the intestinal tube which are responsible for modulating intestinal peristalsis, i.e. propagation of consumed food from esophagus to rectum;

• Submucosal plexus (of Meissner) found in the submucosa of the intestines and contains only parasympathetic fibers
• Myenteric plexus (of Auerbach) located in the muscularis externa of intestines, containing both sympathetic and parasympathetic nerve fibers

Mnemonic

You can easily remember these two plexuses using a simple mnemonic! ' SMP & MAPS', which stands for:

• Submucosal
• Meissner's
• Parasympathetic

• Myenteric
• Auerbach's
• Parasympathetic
• Sympathetic