The Nervous System

Topic: Biological Bases Of Behavior

The nervous system controls bodily function by gathering sensory input, integrating that information internally, and communicating proper motor output.

The nervous system allows organisms to sense, organize, and react to information in the environment. The basic unit of the nervous system is the neuron. Synapses form between the neurons, allowing them to communicate to other neurons or other systems in the body.

The general flow of information is that the peripheral nervous system (PNS) takes in information through sensory neurons, then sends it to the central nervous system (CNS) to be processed. After processing, the CNS “tells” the PNS what to do—what muscles to flex, whether the lungs need more oxygen, which limbs need more blood, any number of biological processes—and the PNS makes it happen through muscle control.

The nervous system can be divided into two major parts—the central nervous system (CNS) and the peripheral nervous system (PNS).

The central nervous system includes the spinal cord and the brain. The brain is the body’s main control center. The main function of the CNS is the integration and processing of sensory information. It synthesizes sensory input to compute an appropriate motor response, or output.

The peripheral nervous system includes a large system of nerves that are linked to the brain and spinal cord. It is comprised of sensory receptors, which process changes in internal and external stimuli and communicate that information to the CNS. The PNS can be further subdivided into the autonomic nervous system and the somatic nervous system.

The autonomic nervous system regulates involuntary actions such as internal-organ function and blood-vessel movement. The somatic nervous system controls voluntary movements such as those in the skin, bones, joints, and skeletal muscles.

The nervous system has three main functions: gathering sensory information from external stimuli, synthesizing that information, and responding to those stimuli. The CNS is mainly devoted to the “information synthesizing” function. During this step in the process, the brain and spinal cord decide on appropriate motor output, which is computed based on the type of sensory input. The CNS regulates everything from organ function to high-level thought to purposeful body movement. Thus, the CNS is commonly thought of as the control center of the body.

The CNS is comprised of the brain, brain stem, and spinal cord.

The brain is found in the cranial cavity and consists of the cerebrum and cerebellum. It houses the nerve centers responsible for coordinating sensory and motor systems in the body. The cerebrum, or the top portion for the brain, is the seat of higher-level thought. It is comprised of two hemispheres, each controlling the opposite side of the body. Each of these hemispheres is divided into four separate lobes:

– the frontal lobe, which controls specialized motor control, learning, planning, and speech;
– the parietal lobe, which controls somatic or voluntary sensory functions;
– the occipital lobe, which controls vision;
– the temporal lobe, which controls hearing and some other speech functions.

The cerebellum is located underneath the backside of the cerebrum, and governs balance and fine motor movements. Its main function is maintaining coordination throughout the body.

The brain stem is connected to the underside of the brain. It consists of the midbrain, pons, and medulla. The midbrain is found in between the hindbrain and the forebrain. It regulates motor function and allows motor and sensory information to pass from the brain to the rest of the body. The pons houses the control centers for respiration and inhibitory functions. The medulla also helps regulate respiration, as well as cardiovascular and digestive functioning.

The spinal cord connects the brain and brain stem to all of the major nerves in the body. Spinal nerves originate from the spinal cord and control the functions of the rest of the body. Impulses are sent from receptors through the spinal cord to the brain, where they are processed and synthesized into instructions for the rest of the body. This data is then sent back through the spinal cord to muscles and glands for motor output.

The human brain is composed of a right and a left hemisphere, and each participates in different aspects of brain function. A longitudinal fissure separates the human brain into two distinct cerebral hemispheres connected by the corpus callosum. The two sides resemble each other and each hemisphere’s structure is generally mirrored by the other side. Yet despite the strong anatomical similarities, the functions of each cortical hemisphere are distinct.

Broad generalizations are often made in popular psychology about one hemisphere having a broad label, such as “logical” for the left side or “creative” for the right. But although measurable lateral dominance occurs, most functions are present in both hemispheres. The extent of specialization by hemisphere remains under investigation. If a specific region of the brain or even an entire hemisphere is either injured or destroyed, its functions can sometimes be taken over by a neighboring region even in the opposite hemisphere, depending upon the area damaged and the patient’s age. When injury interferes with pathways from one area to another, alternative (indirect) connections may develop to communicate information with detached areas, despite the inefficiencies.

While many functions are lateralized, this is only a tendency. The implementation of a specific brain function significantly varies by individual. The areas of exploration of this causal or effectual difference of a particular brain function include gross anatomy, dendritic structure, and neurotransmitter distribution. The structural and chemical variance of a particular brain function, between the two hemispheres of one brain or between the same hemisphere of two different brains, is still being studied. Short of having a hemispherectomy (removal of a cerebral hemisphere), no one is a “left-brain only” or “right-brain only” person.

The lateralization of brain function is the tendency for some neural functions or cognitive processes to be specialized to one side of the brain or the other. Language functions such as grammar, vocabulary and literal meaning are typically lateralized to the left hemisphere, especially in right-handed individuals. While language production is left-lateralized in up to 90% of right-handed subjects, it is more bilateral or even right-lateralized in approximately 50% of left-handers. In contrast, prosodic language functions, such as intonation and accentuation, often are lateralized to the right hemisphere of the brain. The processing of visual and auditory stimuli, spatial manipulation, facial perception, and artistic ability are represented bilaterally, but may show right-hemisphere dominance. Numerical estimation, comparison, and online calculation depend on bilateral parietal regions.

Psychologists study the brain with many different methods. Neuroimaging, or brain scanning, includes the use of various techniques to directly or indirectly image the structure, function, or pharmacology of the brain. It is a relatively new discipline within medicine, neuroscience, and psychology. Physicians who specialize in the performance and interpretation of neuroimaging in the clinical setting are known as neuroradiologists.

Neuroimaging falls into two broad categories:

1. Structural imaging, which deals with the structure of the brain and the diagnosis of large-scale intracranial disease (such as a tumor), as well as injury. Structural imaging includes MRI and CAT scans.
2. Functional imaging, which is used to diagnose metabolic diseases and lesions on a finer scale (such as Alzheimer’s disease), and also for neurological and cognitive-psychology research. Functional imaging allows the brain’s information processing to be visualized directly, because activity in the involved area of the brain increases metabolism and “lights up” on the scan. Functional imaging includes EEG, PET and fMRI scans.

The most common types of brain scans are CAT, EEG, PET, MRI, and fMRI.

Computerized tomography (CT), or computerized axial tomography (CAT), scans use x-rays from multiple angles to construct detailed images of internal structures.

Electroencephalography (EEG) is used to show brain activity in certain psychological states, such as alertness or drowsiness. It is useful in the diagnosis of seizures and other medical problems that involve an overabundance or lack of activity in certain parts of the brain.

To prepare for an EEG, electrodes are placed on the face and scalp. After placing each electrode in the right position, the electrical potential of each electrode can be measured. According to a person’s state (waking, sleeping, etc.), both the frequency and the form of the EEG signal differ. Patients who suffer from epilepsy show an increase of the amplitude of firing visible on the EEG record. The disadvantage of EEG is that the electric conductivity —and therefore the measured electrical potentials—may vary widely from person to person and also over time, due to the natural conductivities of other tissues such as brain matter, blood, and bones. Because of this, it is sometimes unclear exactly which region of the brain is emitting a signal.

Positron emission tomography (PET) scans measure the binding of a radioactive “tracer” molecule in the brain.  The most common tracer is a modified glucose molecule. The modified glucose tracer is injected into the blood, and is taken up preferentially by active neurons in the brain. The radioactive decay of the tracer can then be visualized by detectors and used to create an image (or video) of the active areas of the brain. However, with PET scans, we can only locate generalized areas of brain activity and not specific locations. In addition, PET scans are costly and invasive, making their use limited. However, they can be used in some forms of medical diagnosis, including for Alzheimer’s.

Magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI) scans are the form of neural imaging most directly useful to the field of psychology.

An MRI uses strong magnetic fields to align spinning atomic nuclei (usually hydrogen protons) within body tissues, then disturbs the axis of rotation of these nuclei and observes the radio frequency signal generated as the nuclei return to their baseline status. Through this process, an MRI creates an image of the brain structure. MRI scans are noninvasive, pose little health risk, and can be used on infants and in utero, providing a consistent mode of imaging across the development spectrum. One disadvantage is that the patient has to hold still for long periods of time in a noisy, cramped space while the imaging is performed. Functional MRI scans measure oxygen levels in specific areas of the brain, as a measurement of brain activity.


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Key Points

• The nervous system is the body’s main communication system; it gathers, synthesizes, and uses data from the environment.

• The most basic unit of the nervous system is the neuron, which serves as both a sensor and communicator of internal and external stimuli.

• The nervous system can be broken down into two major parts—the central nervous system and the peripheral nervous system.

• The central nervous system, the main data center of the body, includes the brain and spinal cord.

• The peripheral nervous system includes all of the neurons that sense and communicate data to the central nervous system.

• The peripheral nervous system can be further divided into the autonomic system, which regulates involuntary actions, and the somatic system, which controls voluntary actions.

• The central nervous system (CNS) and the peripheral nervous system (PNS) comprise the entirety of the body’s nervous system, which regulates and maintains its most basic functions.

• The CNS is the main control center of the body—it takes in sensory information, organizes and synthesizes this input, then provides instructions for motor output to the rest of the body.

• The CNS is made up of the brain and spinal cord.

• The brain is the main data center of the body, consisting of the cerebrum (which regulates higher-level functioning such as thought) and the cerebellum (which maintains coordination).

• The brain stem includes the midbrain, pons, and medulla, and controls lower-level functioning such as respiration, digestion and heart rate.

• The spinal cord connects the brain and the body’s main receptors, and serves as a conduit for sensory input and motor output.

• The peripheral nervous system (PNS) provides the connection between internal or external stimuli and the central nervous system to allow the body to respond to its environment.

• The PNS is made up of different kinds of neurons, or nerve cells, which communicate with each other through electric signaling and neurotransmitters.

• The PNS can be broken down into two systems: the autonomic nervous system, which regulates involuntary actions such as breathing and digestion, and the somatic nervous system, which governs voluntary action and body reflexes.

• The somatic nervous system coordinates voluntary physical action. It is also responsible for our reflexes, which do not require brain input.

• The corpus collosum connects the hemispheres of the brain.

• Functional lateralization often varies between individuals.

• Neuroimaging, or brain scanning, includes the use of various techniques to either directly or indirectly image the structure, function, or pharmacology of the brain.

• Neuroimaging falls into two broad categories: structural imaging and functional imaging.

• Electroencephalography (EEG) is used to show brain activity under certain psychological states, such as alertness or drowsiness.

• Positron emission tomography (PET) scans show brain processes by using the sugar glucose in the brain to illustrate where neurons are firing.

• Magnetic resonance imaging (MRI) scans use echo waves to discriminate among grey matter, white matter, and cerebrospinal fluid.

Key Terms

neuron: A cell of the nervous system, which conducts nerve impulses; consisting of an axon and several dendrites. Neurons are connected by synapses.

neurotransmitters: A chemical messenger that carries, boosts, and balances signals between neurons, or nerve cells, and other cells in the body.

forebrain: The most forward part of the physical brain. The forebrain separates into the diencephalon (composed of the thalamus, hypothalamus, subthalamus, epithalamus and pretectum) and the telencephalon which develops into the cerebrum. The cerebrum itself consists of the cerebral cortex, with its underlying white matter, and the basal ganglia. The forebrain controls (amongst many other functions) body temperature, reproductive functions, eating, sleeping, and emotional display.

midbrain: A portion of the brain located just above the medulla and pons and contains basic vision and hearing functions; it also is the input center for muscle movement.

hindbrain (also formally known as the rhombencephalon): The portion of the brain that contains the pons, cerebellum, and medulla, and is responsible for regulating basic human functions.

central nervous system: In vertebrates, the part of the nervous system comprising the brain, brainstem, and spinal cord.

peripheral nervous system: The part of the nervous system comprising a large system of nerves that are linked to the brain and spinal cord; this system is divided into the autonomic and somatic nervous systems.

cerebrum: In humans it is the largest part of the brain and is the seat of motor and sensory functions, as well as the higher mental functions such as consciousness, thought, reason, emotion, and memory.

spinal cord: A thick, whitish cord of nerve tissue that is a major part of the central nervous system. It extends from the brain stem through the spine, with nerves branching off to various parts of the body.

cerebellum: Part of the hindbrain in vertebrates. In humans it lies between the brainstem and the cerebrum, and plays an important role in sensory perception, motor output, balance, and posture.

brain stem: The part of the brain that connects the spinal cord to the forebrain and cerebrum.

somatic nervous system: The part of the peripheral nervous system that transmits signals from the central nervous system to skeletal muscle and from receptors of external stimuli to the central nervous system, thereby mediating sight, hearing, and touch.

autonomic nervous system: The part of the nervous system that regulates the involuntary activity of the heart, intestines, and glands, including digestion, respiration, perspiration, metabolism, and blood-pressure modulation.

corpus collosum: A wide, flat bundle of neural fibers beneath the cortex that connects the left and right cerebral hemispheres and facilitates interhemispheric communication.

lateralization: Localization of a function such as speech to the right or left side of the brain.

hemisphere: Either of the two halves of the cerebrum..

conductivity: The ability of a material to conduct electricity, heat, fluid, or sound.

Magnetic field: A condition in the space around a magnet or electric current in which there is a detectable magnetic force and two magnetic poles are present.

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