Brain and Nervous System

Brain and Nervous System

Summary

The brain and nervous system receive, process and respond to stimuli from both outside and inside the body. The senses (e.g., touch, taste, sight, hearing and smell) provide information about the person’s environment. After the information is processed, the nervous system initiates a response that might include moving a limb, increasing hormone levels or increasing the person’s heart rate.

The nervous system is spread throughout the body, but it can be divided into two categories. The brain and spinal cord make up the central nervous system (CNS). The remainder, which include the nerves running to internal organs and extremities, is called the peripheral nervous system (PNS).

The nervous system functions in ways in which a person is both aware and unaware. Conscious thought and decisions to move are examples of nervous system processes of which a person is aware that occur in the CNS. Processes that involve no awareness include breathing, heart beating and digestion. Most of these are regulated through the PNS, though some also receive commands from the CNS.

Neurons are the basic cell of the nervous system. They receive and transmit information throughout the body. Each neuron consists of three main parts: the cell body, the dendrite and the axon. Nerve signals are received at the dendrites and travel through the axon to the next neuron or an effector, such as a muscle or gland. Before the signal can reach the effector it must cross a space called the synapse. This may be accomplished with electrical signals or through neurotransmitters, a type of chemical messenger.

The nervous system is complex and is susceptible to a number of different conditions and diseases. Some are caused by bacterial or viral infections, such as encephalitis, while others are caused by disruptions in chemical or electrical processes. Physical changes in the brain or neurons can also lead to some conditions (e.g., stroke, multiple sclerosis). Because of the importance of the brain and nervous system, any condition affecting these areas may produce extreme symptoms such as paralysis or loss of speech and life-threatening conditions that may affect breathing, cardiac function or digestion.

About the brain and nervous system

The brain and nervous system direct and initiate the actions and vital functions of the human body. The nervous system is a complex network of cells that receives, processes and responds to stimuli from outside and inside the body. Information about external conditions is provided by the sensory organs (e.g., eyes, ears, skin, nose, tongue). Though people are often unaware of it, the nervous system also gets information about what is happening inside the body. This includes monitoring a person’s blood chemistry and adjusting it through the heart rate, breathing and digestion.

Much information that travels through the nervous system is processed in the brain. Some signals are processed in other areas, such as the spinal cord or areas called ganglia to provide a quicker response. Many responses are so rapid that a person is barely aware of them, such as blinking an eye to repel dust. Other responses occur in reaction to body functions, such as responses of muscles and gastric enzymes in the digestive system after a meal.

The nervous system responds to the information it receives and processes through effectors, such as muscles and glands. Muscles have many functions. Some are controlled consciously and move the limbs and trunk and others work automatically to help control the lungs and pump blood. Glands, another type of effector, are located throughout the body. They release hormones and other chemicals to initiate or stop a response by other tissues.

The nervous system can be classified in multiple ways. Anatomy is one of the most basic methods. The central nervous system includes the brain and spinal cord. The peripheral nervous system includes all nerves that are not in the brain and spinal cord.

The nervous system can also be categorized by whether the functions are consciously controlled. The somatic nervous system includes those parts of the nervous system under conscious control. They include the nerves that initiate movement, such as flexing a joint. The five senses are also part of the somatic system. The autonomic nervous system regulates body functions that are not entirely under conscious control. A person can modify some of the functions, such as breathing. Other processes, such as reflexes, blood pressure and digestion, occur without thought.

The scope of the functions of the brain and nervous system highlights the potential severity of conditions and diseases that affect them. Any malfunction in an area of the brain or nerves may affect other systems or abilities such as movement. Research has helped improve scientific understanding of the brain and nervous system and its diseases, such as Alzheimer’s disease, Parkinson’s disease and muscular dystrophy. While much has been learned about functioning and some of these conditions, research continues to focus on improving treatment.

Parts of the brain and nervous system

The nervous system is a complex chain of organs, tissues and cells that occur throughout the human body. Parts of the nervous system can be divided to make them easier to understand. One division is the central nervous system (CNS), which includes the brain and spinal cord. The parts of the brain are characterized by their function, which is largely derived from the type, number and connections of neurons (nerve cells).

The cerebrum of the brain is divided into two cerebral hemispheres (right and left) that surround most of the other parts of the brain. The two hemispheres are largely separate, though they are linked by a group of nerves called the corpus callosum. Each hemisphere of the cerebrum is most involved in monitoring and controlling the opposite half of the body. For example, the right hemisphere is largely associated with the left side of body.

The cerebrum has several parts. The cerebral cortex is the wrinkled outer layer of the brain. The wrinkles provide a structural advantage by allowing more cells to fit into the available space. The cerebral cortex is largely made of gray matter, which consists of nerve cell bodies and extensions called dendrites. Much of the brain’s cognitive ability occurs in the cerebral cortex. Several sections called lobes make up the cortex and each exists on both the right and left hemispheres of the cerebrum. The lobes are the frontal (involved in planning future action and controlling movement), the parietal (involved with touch, sensing pain and spatial relationships), the occipital (involved with vision) and the temporal (involved with hearing, learning, memory and emotions).

Two clusters called basal ganglia are near the center of the cerebrum. They are among the many parts of the nervous system involved in regulating muscle movement. The basal ganglia also have a role in memory.

Two other portions of the cerebrum are in the temporal lobe. The hippocampus is deep inside the temporal lobe and is involved in memory, learning and regulating emotions. The amygdala is also a portion of the temporal lobe and is involved in regulating emotions and social behavior. It helps coordinate the hormonal response of other tissues.

The hemispheres of the cerebrum surround the diencephalon. It has two parts, the thalamus and the hypothalamus. The thalamus is a gateway that blocks or enhances information before it reaches the cerebral cortex. The hypothalamus is part of the autonomic nervous system, which regulates the actions of the internal organs that are not controlled by conscious thought (e.g., heartbeat, digestion).

Below the cerebrum and at the back of the brain is the cerebellum, which is involved in movement and learning motor skills. It also helps control the force and range of muscular contractions.

The brain stem is in the center of the brain, in front of the cerebellum and surrounded by the temporal lobe. The brain stem receives sensory data from the skin and muscles of the head and also controls the signals that trigger movement of the head and neck. The brain stem regulates a person’s level of arousal and awareness, and a severe injury to this part of the brain may lead to a coma. The parts of the brain stem are:

  • Midbrain. The top of the brain stem is the midbrain. This area helps controls some sensory and motor function, especially movement of the eyes. It is also involved in coordinating reflex reactions to visual and auditory stimuli.
  • Pons. This tissue, directly below the midbrain, conveys information from the cerebrum to the cerebellum. It is also involved in respiration, taste and sleep.
  • Medulla oblongata. This part of the brain stem joins the pons to the spinal cord. It is involved in monitoring and controlling autonomic functions, such as digestion, breathing and heart rate.

There are other ways that the brain may be divided. Some physicians may refer to the hindbrain, midbrain and forebrain. The hindbrain includes the medulla, pons and cerebellum. The diencephalon and cerebrum make up the forebrain. The midbrain is the section above the pons in the brain stem.

The spinal cord is the other major portion of the central nervous system. It runs vertically along the middle of the back. The spinal cord is surrounded by a chain of dense, protective bones called vertebrae. The nerves of the spinal cord receive and process information from the senses as well as many internal organs. Some nerves also run from the brain through the spinal cord to muscles. These nerves control muscle movement in the limbs and trunk of the body. Clusters of nerves called ganglia occur at points along the spinal cord to allow information to be processed and responded to without traveling the entire distance to the brain.

The meninges are three layers of tissue that surround the brain and spinal cord. They are between the nerves of the CNS and the surrounding bones of the skull or vertebrae. The meninges contain veins that carry waste products away from the nerve cells.

The peripheral nervous system (PNS) includes the nerves of the body outside the central nervous system. These nerves run from the brain and spinal cord to the muscles, glands and vital organs. Some information received by the PNS is processed through ganglia. This allows the body to respond more rapidly by not involving the CNS.

Nerve anatomy

Each section of the nervous system is composed of nerves and their support cells. Scientists estimate the body has approximately 100 billion nerve cells, or neurons, to perform a variety of functions. These neurons carry nerve signals between the sensory organs (e.g., eyes, skin), information processing structures (e.g., brain, ganglia) and effector tissues (e.g., muscles, glands).

Each nerve performs its functions with the same basic parts. Each neuron contains a central portion called the cell body, which contains the vital parts of the neuron such as its nucleus. One end of the cell body contains the axon, a long extension that carries nerve impulses to other neurons. The nerve cell body also contains one or more projections called dendrites, which receive signals from other cells or other nerves.

Dendrites may project from the cell body as several branches, one main trunk with smaller branches diverting off it, or a combination. The branches can be extensive and some neurons may have approximately 150,000 dendrite contacts with other cells.

While all neurons may have the same general structure, scientists have identified approximately 1,000 types. In general, they can be grouped into three classes. These are:

  • Sensory neurons. Transmit information from the sensory tissues to other parts of the nervous system for processing. They allow for the perception of a person’s environment and coordinated movement. Some sensory neurons end with nociceptors in the skin or internal tissues. The nociceptors enable the body to quickly detect pain, transmit the information to the central nervous system, and produce a reaction.

  • Motor neurons. Communicate signals from the brain and spinal cord to the muscles and glands to produce a result. One motor neuron may communicate with multiple muscle fibers, which generates movement with a single nerve message.

  • Interneurons. Neurons that are neither a sensory or motor neuron are classified as an interneuron. They convey signals around the body and are the most common type of neuron.

In addition to the neurons, the nervous system also includes glial cells (also known as glia). Scientists believe there are 10 to 50 times more glial cells than neurons in the body.

Glial cells provide support to keep neurons functioning efficiently. Among their many roles, they provide structural support to the neurons and the brain. Certain types of glia called astrocytes form part of the blood-brain barrier, which regulates materials entering the brain. They allow nutrients to get to the neurons, while blocking toxins and infectious agents. They are also involved in nerve communication by disposing of neurotransmitters after they have been used. Two types of glial cells called oligodendrocytes and Schwann cells produce myelin, which is found on the axons of some nerves.

Nerve function

Nerves enable rapid communication among multiple parts of the body. Signals are initiated in sensory organs (e.g., eyes, nose, skin) or from pain-detecting dendrites called nociceptors

Nerve signals begin at the dendrite as an electrical impulse that travels the length of the neuron. These signals, also known as action potentials, occur when particles (ions) with opposite charges switch between inside and outside of the cell. The signal can only move toward the axon and may be incredibly fast. Some signals travel at rates up to 330 feet per second (100 meters per second).

After the signal reaches the axon end of the cell, it must cross a synapse, or space, between the neuron and another cell, such as a muscle fiber, gland or another neuron. Signals may cross the synapse through chemical or electrical signals. Electrical signals cross the synapse rapidly. Chemical signals use the messengers called neurotransmitters to cross the synapse to the next cell. Neurotransmitters allow for more variety in the nerve signal and a stronger reaction from the responding tissue.

There are many types of neurotransmitters. Some are present throughout the body and may perform multiple functions. Some neurotransmitters include acetylcholine, dopamine, norepinephrine and serotonin. Deficiencies or excesses of neurotransmitters contribute to many neurological and mental health conditions.

Brain and nervous system conditions

Many medical conditions and disorders are related to the brain and nervous system. These diseases may range from mild to severe. Some conditions include:

  • Alzheimer’s disease. This progressive, degenerative neurological disorder is the most common cause of dementia among people over the age of 65, according to the National Institute of Neurological Disorders and Stroke (NINDS). It occurs when neurons in the brain die or break connections with other neurons.
  • Multiple sclerosis. This autoimmune disorder is characterized by immune system attacks on the myelin of nerves in the brain and spinal cord. The myelin is replaced with scar tissue, which slows or blocks nerve signals. It may result in loss of vision, mobility and sensation.
  • Encephalitis. Viruses and bacteria sometimes cause infection and inflammation of the brain. Patients may experience severe headaches that gradually progress into additional symptoms.
  • Epilepsy. This is one of the most common brain disorders and is characterized by recurrent seizures. They result from sudden changes in the electrical activity of the brain.
    Seizures
  • Stroke. A blood clot or bleeding can lead to oxygen deprivation in parts of the brain. The resulting death of nerve cells can have lasting consequences, including difficulty speaking, emotional problems and paralysis.
  • Parkinson’s disease. Deficiency in the production of dopamine, a neurotransmitter, causes the symptoms of Parkinson’s disease. This limits a patient’s movement and ability to balance.

Questions for your doctor

Preparing questions in advance can help patients have more meaningful discussions with healthcare professionals regarding their condition. Patients may wish to ask their doctor the following questions about the brain and nervous system:

  1. What can I do to maintain the health of my brain and nervous system?
  2. What are some examples of diseases of the brain or nervous system?
  3. Am I at risk for a brain or nervous system condition?
  4. Are there any symptoms that may be early indicators of a nervous system condition or disease?
  5. Are brain and nervous system diseases contagious?
  6. Can brain and nervous system conditions be passed on to my children?
  7. How will a brain or nervous system condition affect my ability to live a normal life?
  8. Will my current medications affect my brain or nervous system?
  9. Are there supplements that may affect the health of my brain or nervous system?
  10. Should I see a specialist about my condition?
  11. Can diseases in other body systems affect the brain or nervous system?
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