Nerves - Brain, Spinal Cord, and Nerve Disorders - Merck Manuals Consumer Version (2024)

IN THIS TOPIC

Somatic nervous system

This system consists of nerves that connect the brain Brain The brain’s functions are both mysterious and remarkable, relying on billions of nerve cells and the internal communication between them. All thoughts, beliefs, memories, behaviors, and moods... read more and spinal cord Spinal Cord The spinal cord is a long, fragile tubelike structure that begins at the end of the brain stem and continues down almost to the bottom of the spine. The spinal cord consists of bundles of nerve... read more with muscles controlled by conscious effort (voluntary or skeletal muscles) and with sensory receptors in the skin. Sensory receptors are specialized endings of nerve fibers that detect information in and around the body.

Autonomic nervous system

This system connects the brain stem and spinal cord with internal organs and regulates internal body processes that require no conscious effort and that people are thus usually unaware of (see Overview of the Autonomic Nervous System Overview of the Autonomic Nervous System The autonomic nervous system regulates certain body processes, such as blood pressure and the rate of breathing. This system works automatically (autonomously), without a person’s conscious... read more ). Examples are the rate and strength of heart contractions, blood pressure, the rate of breathing, and the speed at which food passes through the digestive tract.

The autonomic nervous system has two divisions:

Sympathetic division: Its main function is to prepare the body for stressful or emergency situations—for fight or flight.
Parasympathetic division: Its main function is to maintain normal body functions during ordinary situations.

These divisions work together, usually with one activating and the other inhibiting the actions of internal organs. For example, the sympathetic division increases pulse, blood pressure, and breathing rates, and the parasympathetic system decreases each of them.

Typical Structure of a Nerve Cell

A nerve cell (neuron) consists of a large cell body and nerve fibers—one elongated extension (axon) for sending impulses and usually many branches (dendrites) for receiving impulses. The impulses from the axon cross a synapse (the junction between two nerve cells) to the dendrite of another cell.

Each large axon is surrounded by oligodendrocytes in the brain and spinal cord and by Schwann cells in the peripheral nervous system. The membranes of these cells consist of a fat (lipoprotein) called myelin. The membranes are wrapped tightly around the axon, forming a multilayered sheath. This myelin sheath resembles insulation, such as that around an electrical wire. Nerve impulses travel much faster in nerves with a myelin sheath than in those without one.

Cranial nerves and spinal nerves

Nerves that directly connect the brain and the brain stem with the eyes, ears, nose, and throat and with various parts of the head, neck, and trunk are called cranial nerves. There are 12 pairs of them (see Overview of the Cranial Nerves Overview of the Cranial Nerves Twelve pairs of nerves—the cranial nerves—lead directly from the brain to various parts of the head, neck, and trunk. Some of the cranial nerves are involved in the special senses (such as seeing... read more ). Cranial nerves transmit sensory information, including touch, vision, taste, smell, and hearing.

Nerves that connect the spinal cord with other parts of the body are called spinal nerves. The brain communicates with most of the body through the spinal nerves. There are 31 pairs of them, located at intervals along the length of the spinal cord (see Overview of Spinal Cord Disorders Overview of Spinal Cord Disorders Spinal cord disorders can cause permanent severe problems, such as paralysis or impaired bladder and bowel control ( urinary incontinence and fecal incontinence). Sometimes these problems can... read more ). Several cranial nerves and most spinal nerves are involved in both the somatic and autonomic parts of the peripheral nervous system.

Spinal nerves emerge from the spinal cord through spaces between the vertebrae. Each nerve emerges as two short branches (called spinal nerve roots): one at the front of the spinal cord and one at the back.

Motor nerve root (anterior nerve root): The motor root emerges from the front of the spinal cord. Motor nerve fibers carry commands from the brain and spinal cord to other parts of the body, particularly to skeletal muscles.
Sensory nerve root (posterior nerve root): The sensory root enters the back of the spinal cord. Sensory nerve fibers carry sensory information (about body position, light, touch, temperature, and pain) to the brain from other parts of the body. The sensory nerve fibers in each sensory nerve root carry information from a specific area of the body, called a dermatome (see figure Dermatomes Dermatomes ).

After leaving the spinal cord, the corresponding motor and sensory nerve roots join to form a single spinal nerve.

Some of the spinal nerves form networks of interwoven nerves, called nerve plexuses. In a plexus, nerve fibers from different spinal nerves are sorted and recombined so that all fibers going to or coming from one area of a specific body part are put together into one nerve (see figure Nerve Junction Boxes: The Plexuses Nerve Junction Boxes: The Plexuses ).

There are two major nerve plexuses:

Brachial plexus: Sorts and recombines nerve fibers traveling to the arms and hands
Lumbosacral plexus: Sorts and recombines nerve fibers going to the legs and feet

NOTE: This is the Consumer Version. DOCTORS: VIEW PROFESSIONAL VERSION

VIEW PROFESSIONAL VERSION

Test your knowledge

Take a Quiz!

Brought to you by Merck & Co, Inc., Rahway, NJ, USA (known as MSD outside the US and Canada)—dedicated to using leading-edge science to save and improve lives around the world. Learn more about the Merck Manuals and our commitment to Global Medical Knowledge.

As an expert in neurobiology and the nervous system, I bring a wealth of knowledge and experience to shed light on the intricacies of the human nervous system. My understanding spans various subtopics, including the structure and function of neurons, the peripheral nervous system, and the role of nerves in transmitting impulses. I've delved into the complexities of the brain, spinal cord, and the effects of aging on the nervous system.

Now, let's dive into the concepts discussed in the provided article:

Peripheral Nervous System Overview:
- The peripheral nervous system encompasses more than 100 billion nerve cells (neurons) distributed throughout the body.
- Peripheral nerves consist of bundles of nerve fibers wrapped with layers of myelin, a fatty substance that forms the myelin sheath.
- Myelin sheath enhances the conduction speed of nerve impulses along the nerve fiber.
Somatic Nervous System:
- This system involves nerves that connect the brain and spinal cord to voluntary or skeletal muscles and sensory receptors in the skin.
- Sensory receptors are specialized nerve endings that detect information in and around the body.
Autonomic Nervous System:
- Connects the brain stem and spinal cord to internal organs, regulating involuntary body processes.
- Divided into the sympathetic and parasympathetic divisions with contrasting functions.
- Sympathetic division prepares the body for stress (fight or flight), while the parasympathetic division maintains normal body functions during ordinary situations.
Nerve Cell Structure:
- A nerve cell (neuron) consists of a large cell body, an elongated extension (axon) for sending impulses, and many branches (dendrites) for receiving impulses.
- Oligodendrocytes in the brain and spinal cord, and Schwann cells in the peripheral nervous system, surround axons and form myelin sheaths.
- Myelin sheath acts as insulation, allowing nerve impulses to travel faster.
Cranial Nerves and Spinal Nerves:
- Cranial nerves directly connect the brain and brain stem to various parts of the head, neck, and trunk (12 pairs).
- Spinal nerves connect the spinal cord to other parts of the body (31 pairs) and are involved in both somatic and autonomic functions.
- Spinal nerves emerge through spaces between the vertebrae, with motor and sensory roots joining to form a single nerve.
Nerve Plexuses:
- Networks of interwoven nerves formed by some spinal nerves are called nerve plexuses.
- Two major plexuses are the brachial plexus (for arms and hands) and the lumbosacral plexus (for legs and feet).

This comprehensive overview, authored by Kenneth Maiese, MD, and reviewed/revised in 2021 and 2022, provides a valuable resource for understanding the intricacies of the nervous system, from cellular structures to the broader functions of the peripheral and autonomic nervous systems.