What connects the brain to the peripheral nervous system

The peripheral nervous system consists of the nerves that branch out from the brain and spinal cord. These nerves form the communication network between the CNS and the body parts. The peripheral nervous system is further subdivided into the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of nerves that go to the skin and muscles and is involved in conscious activities. The autonomic nervous system consists of nerves that connect the CNS to the visceral organs such as the heart, stomach, and intestines. It mediates unconscious activities.

Structure of a Nerve

A connective tissue sheath called the epineurium surrounds each nerve. Each bundle of nerve fibers is called a fasciculus and is surrounded by a layer of connective tissue called the perineurium. Within the fasciculus, each individual nerve fiber, with its myelin and neurilemma, is surrounded by connective tissue called the endoneurium. A nerve may also have blood vessels enclosed in its connective tissue wrappings.

Cranial Nerves

Twelve pairs of cranial nerves emerge from the inferior surface of the brain. All of these nerves, except the vagus nerve, pass through foramina of the skull to innervate structures in the head, neck, and facial region.

The cranial nerves are designated both by name and by Roman numerals, according to the order in which they appear on the inferior surface of the brain. Most of the nerves have both sensory and motor components. Three of the nerves are associated with the special senses of smell, vision, hearing, and equilibrium and have only sensory fibers. Five other nerves are primarily motor in function but do have some sensory fibers for proprioception. The remaining four nerves consist of significant amounts of both sensory and motor fibers.

Acoustic neuromas are benign fibrous growths that arise from the balance nerve, also called the eighth cranial nerve or vestibulocochlear nerve. These tumors are non-malignant, meaning that they do not spread or metastasize to other parts of the body. The location of these tumors is deep inside the skull, adjacent to vital brain centers in the brain stem. As the tumors enlarge, they involve surrounding structures which have to do with vital functions. In the majority of cases, these tumors grow slowly over a period of years. In other cases, the growth rate is more rapid and patients develop symptoms at a faster pace. Usually, the symptoms are mild and many patients are not diagnosed until some time after their tumor has developed. Many patients also exhibit no tumor growth over a number of years when followed by yearly MRI scans.

Spinal Nerves

Thirty-one pairs of spinal nerves emerge laterally from the spinal cord. Each pair of nerves corresponds to a segment of the cord and they are named accordingly. This means there are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve.

Each spinal nerve is connected to the spinal cord by a dorsal root and a ventral root. The cell bodies of the sensory neurons are in the dorsal root ganglion, but the motor neuron cell bodies are in the gray matter. The two roots join to form the spinal nerve just before the nerve leaves the vertebral column. Because all spinal nerves have both sensory and motor components, they are all mixed nerves.

Autonomic Nervous System

The autonomic nervous system is a visceral efferent system, which means it sends motor impulses to the visceral organs. It functions automatically and continuously, without conscious effort, to innervate smooth muscle, cardiac muscle, and glands. It is concerned with heart rate, breathing rate, blood pressure, body temperature, and other visceral activities that work together to maintain homeostasis.

The autonomic nervous system has two parts, the sympathetic division and the parasympathetic division. Many visceral organs are supplied with fibers from both divisions. In this case, one stimulates and the other inhibits. This antagonistic functional relationship serves as a balance to help maintain homeostasis.

divisions of the nervous system

table of contents

• Central Nervous System
• Peripheral Nervous System
• Divisions of the Nervous System
• Brain Structures

Neuroanatomy: the structure of the nervous system. To learn how the nervous system functions, you must learn how the nervous system is put together.

The nervous system can be divided into several connected systems that function together. Let's start with a simple division:

The nervous system is divided into the central nervous system and peripheral nervous system.

Let's break the central nervous system and the peripheral nervous system into more parts.

Central Nervous System

For brain weights of other animals, see brain facts and figures.

Did you know?

A stegosaurus dinosaur weighed approximately 1,600 kg but had a brain that weighed only approximately 70 grams (0.07 kg). Therefore, the brain was only 0.004% of its total body weight. In contrast, an adult human weighs approximately 70 kg and has a brain that weighs approximately 1.4 kg. Therefore, the human brain is about 2% of the total body weight. This makes the brain to body ratio of the human 500 times greater than that of the stegosaurus. See "My Brain is Bigger than Your Brain" for more about brain size.

Peripheral Nervous System

The peripheral nervous system is divided into two major parts: the somatic nervous system and the autonomic nervous system.

Somatic Nervous System

The picture on the left shows the somatic motor system. The cell body is located in either the brain or spinal cord and projects directly to a skeletal muscle.

Autonomic Nervous System

This picture shows the general organization of the autonomic nervous system. The preganglionic neuron is located in either the brain or the spinal cord. This preganglionic neuron projects to an autonomic ganglion. The postganglionic neuron then projects to the target organ. Notice that the somatic nervous system has only one neuron between the central nervous system and the target organ while the autonomic nervous system uses two neurons.

The enteric nervous system is a third division of the autonomic nervous system that you do not hear much about. The enteric nervous system is a meshwork of nerve fibers that innervate the viscera (gastrointestinal tract, pancreas, gall bladder).

The following table shows how the nervous system can be divided. The bottom row of the table contains the names of specific areas within the brain.

Divisions of the Nervous System

From a top view, notice how the brain is divided into two halves, called hemispheres. Each hemisphere communicates with the other through the corpus callosum, a bundle of nerve fibers. (Another smaller fiber bundle that connects the two hemispheres is called the anterior commissure).

Some differences between the peripheral nervous system (PNS) and the central nervous system (CNS):

1. In the CNS, collections of neurons are called nuclei. In the PNS, collections of neurons are called ganglia.
2. In the CNS, collections of axons are called tracts. In the PNS, collections of axons are called nerves.

In the peripheral nervous system, neurons can be functionally divided in three ways:

1. Sensory (afferent) - carry information INTO the central nervous system from sense organs or motor (efferent) - carry information away from the central nervous system (for muscle control).
2. Cranial - connects the brain with the periphery or spinal - connects the spinal cord with the periphery.
3. Somatic - connects the skin or muscle with the central nervous system or visceral - connects the internal organs with the central nervous system.

Brain Structures

Cerebral Cortex

Functions:

• Thought
• Voluntary movement
• Language
• Reasoning
• Perception

The word "cortex" comes from the Latin word for "bark" (of a tree). This is because the cortex is a sheet of tissue that makes up the outer layer of the brain. The thickness of the cerebral cortex varies from 2 to 6 mm. The right and left sides of the cerebral cortex are connected by a thick band of nerve fibers called the "corpus callosum." In higher mammals such as humans, the cerebral cortex looks like it has many bumps and grooves. A bump or bulge on the cortex is called a gyrus (the plural of the word gyrus is "gyri") and a groove is called a sulcus (the plural of the word sulcus is "sulci"). Lower mammals, such as rats and mice, have very few gyri and sulci.

Cerebellum

• Movement
• Balance
• Posture

The word "cerebellum" is derived from the Latin word for "little brain." Located behind the brain stem, the cerebellum is similar to the cerebral cortex because it has hemispheres and a cortex that surrounds the hemispheres.

Brain stem

Functions:

• Breathing
• Heart Rate
• Blood Pressure

The brain stem refers to the area of the brain between the thalamus and spinal cord. Structures of the brain stem include the pons, medulla oblongta, tectum, reticular formation and tegmentum. The brain stem is important for maintaining basic life functions such as breathing, heart rate and blood pressure.

Hypothalamus

Functions:

• Body Temperature
• Emotions
• Hunger
• Thirst
• Circadian Rhythms

The hypothalamus is composed of several different areas and is located at the base of the brain. The hypothalamus is only 1/300 of the total brain weight. One function of the hypothalamus is the control of body temperature. The hypothalamus detects changes in body temperature and sends commands to adjust the temperature. For example, the hypothalamus can detect fever and respond by sending a command to expand capillaries in the skin. The expansion of the capillaries cools the blood and results in a drop in body temperature. The hypothalamus also controls the pituitary.

Thalamus

Functions:

• Sensory processing
• Movement

The thalamus receives sensory information from other areas of the nervous system and sends this information to the cerebral cortex. The thalamus is also important for processing information related to movement.

Limbic System

• Emotions
• Memory

The limbic system (or the limbic areas) is a group of structures that includes the amygdala, the hippocampus, mammillary bodies and cingulate gyrus. These areas are important for controlling the emotional response to a given situation. The hippocampus is also important for memory.

Hippocampus

Functions:

• Learning
• Memory

The hippocampus is one part of the limbic system that is important for memory and learning.

Basal Ganglia

Functions:

• Movement

The basal ganglia are a group of structures, including the globus pallidus, caudate nucleus, subthalamic nucleus, putamen and substantia nigra, that are important in coordinating movement.

Midbrain

Functions:

• Vision
• Audition
• Eye Movement
• Body Movement

The midbrain includes structures such as the superior and inferior colliculi and red nucleus. There are several other areas also in the midbrain.

Now that you have read about the areas of the brain, take a look at where these areas are located:

Check out the glossary for definitions of other brain areas.

Did you know?

John Adams (2nd President of the US) and his son, John Quincy Adams (6th President of the US), were both born in Braintree, Massachusetts.

Copyright © 1996-2020, Eric H. Chudler All Rights Reserved.

What is the link between the brain and peripheral nervous system?

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