Introduction to the Nervous System

Organization of the Nervous System

The nervous system is a network of cells called neurons that coordinate actions and transmit signals between different parts of the body.

Learning Objectives

Describe the organization of the nervous system

Key Takeaways

Key Points

  • Neurons (specialized cells of the nervous system ) send signals along thin fibers called axons and communicate with other cells by releasing chemicals called neurotransmitters at cell-cell junctions called synapses.
  • Glial cells are non-neuronal cells that provide support and nutrition in the nervous system.
  • In humans, the nervous system consists of both the central and peripheral nervous systems.
  • The human central nervous system contains the brain, spinal cord, and retina.
  • The peripheral nervous system consists of sensory neurons, clusters of neurons called ganglia, and nerves connecting them to each other and to the central nervous system.

Key Terms

  • sensory receptor: A nerve ending that recognizes stimulus in the internal or external environment of an organism.
  • peripheral nervous system: This system consists of the nerves and ganglia outside of the brain and spinal cord.
  • glia: Non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain and other parts of the nervous system.

Examples

A nervous system allows us to react to the changing environment around us.

The nervous system is an organ system that coordinates voluntary and involuntary actions and responses by transmitting signals between different parts of our bodies.

Neurons

Central to the functioning of the nervous system is an extensive network of specialized cells called neurons. Neurons feature many thin projecting fibers called axons, which penetrate deep into tissues. They are able to communicate with other cells by chemical or electrical means at synapses. Neuronal function is supported by neuroglia, specialized cells which provide nutrition, mechanical support, and protection.

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Major elements in neuron-to-neuron communication: Electrical impulses travel along the axon of a neuron. When this signal reaches a synapse, it provokes release of neurotransmitter molecules, which bind to receptor molecules located in the the target cell.

Divisions of the Nervous System

In most animals, including humans, the nervous system consists of two parts: central and peripheral. The central nervous system (CNS) is composed of the brain, spinal cord, and cerebellum. The peripheral nervous system (PNS) consists of sensory neurons, motor neurons, and neurons that communicate either between subdivisions of the PNS or connect the PNS to the CNS

This image depicts the major parts of the nervous system, including brain, cerebellum, spinal cord, intercostal nerves, subcostal nerve, lumbar plexus, sacral plexus, femoral nerve, pudendal nerve, sciatic nerve, muscular branches of femoral nerve, saphenous nerve, tibial nerve, superficial peroneal nerve, deep peroneal nerve, common peroneal nerve, ulnar nerve, obturator nerve, genitofemoral nerve, iliohypogastric nerve, median nerve, radial nerve, musculocutaneous nerve, and brachial plexus.

The Human Nervous System: The major organs and nerves of the human nervous system. The CNS is comprised of the brain, cerebellum and spinal cord. Remaining neurons, and associated cells, distributed throughout the body form the PNS.

The nervous system has three broad functions: sensory input, information processing, and motor output. In the PNS, sensory receptor neurons respond to physical stimuli in our environment, like touch or temperature, and send signals that inform the CNS of the state of the body and the external environment. This sensory information is then processed by the CNS, predominantly by the brain.

After information is processed, motor neurons return signals to the muscles and glands of the PNS, which responds with motor output. Central neurons, which in humans greatly outnumber the sensory and motor neurons, make all of their input and output connections with other neurons. The connections of these neurons form neural circuits that are responsible for our perceptions of the world and determine our behavior. Along with neurons, the nervous system relies on the function of other specialized cells called glial cells, or glia, that provide structural and metabolic support to the nervous system.

Functions of the Nervous System

The primary function of the nervous system is to coordinate and control the various body functions.

Learning Objectives

Describe the functions of the nervous system

Key Takeaways

Key Points

  • The nervous system is a highly integrated system. The nervous system has three overlapping functions based on sensory input, integration, and motor output.
  • At a more integrative level, the primary function of the nervous system is to control and communicate information throughout the body.

Key Terms

  • hormone: A molecule released by a cell or a gland in one part of the body that sends out messages affecting cells in other parts of the organism.
  • nervous system: The organ system that coordinates the activities of muscles, monitors organs, constructs and processes data received from the senses, and initiates actions.

The nervous system has three overlapping functions based on the sensory input,  integration, and motor output. The nervous system is a highly integrated system.

Sensory Input

Sensory input comes from the many sensory receptors that monitor changes occurring both inside and outside the body. The total sum of the information gathered by these receptors is called sensory input. The nervous system processes and interprets sensory input and decides what actions should be taken. The nervous system activates effector organs such as muscles and glands to cause a response called motor output.

Integration

At a more integrative level, the primary function of the nervous system is to control and communicate information throughout the body. It does this by extracting information from the environment using sensory receptors. This sensory input is sent to the central nervous system, which determines an appropriate response.

Motor Response

Once the response is activated, the nervous system sends signals via motor output to muscles or glands to initiate the response.

In humans, the sophistication of the nervous system allows for language, abstract representation of concepts, transmission of culture, and many other features of society that would not otherwise exist.

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Major elements in neuron-to-neuron communication: Electrical impulses travel along the axon of a neuron. When this signal reaches a synapse, it provokes release of neurotransmitter molecules, which bind to receptor molecules located in the the target cell.

Subdivisions of the Nervous System

The CNS includes the brain and spinal cord, while the PNS is a network of nerves linking the body to the brain and spinal cord.

Learning Objectives

Describe the subdivisions of the nervous system

Key Takeaways

Key Points

  • The nervous system is often divided into components called gray matter and white matter. Gray matter contains a relatively high proportion of neuron cell bodies and white matter is composed mainly of axons.
  • The peripheral nervous system is subdivided into nerves, the autonomic system, and the somatic system. The autonomic nervous system is further subdivided into the parasympathetic and sympathetic nervous systems.
  • The enteric nervous system is an independent subsystem of the peripheral nervous system.
  • The central nervous system includes the brain and spinal cord and has various centers that integrate of all the information in the body. These can be subdivided into lower centers that carry out essential body functions and higher centers that control more sophisticated information processing.

Key Terms

  • gray matter: A major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astroglia and oligodendrocytes), and capillaries.
  • central nervous system: In vertebrates, the part of the nervous system comprising the brain, brainstem, and spinal cord.
  • white matter: A region of the central nervous system containing myelinated nerve fibers and no dendrites.
  • peripheral nervous system: This system consists of the nerves and ganglia outside of the brain and spinal cord.

The nervous system is comprised of two major subdivisions, the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System

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The Central Nervous System: The central nervous system (2) is a combination of the brain (1) and the spinal cord (3).

The CNS includes the brain and spinal cord along with various centers that integrate all the sensory and motor information in the body. These centers can be broadly subdivided into lower centers, including the spinal cord and brain stem, that carry out essential body and organ-control functions and higher centers within the brain that control more sophisticated information processing, including our thoughts and perceptions. Further subdivisions of the brain will be discussed in a later section.

Gray Matter and White Matter

The nervous system is often divided into components called gray matter and white matter. Gray matter, which is gray in preserved tissue but pink or light brown in living tissue, contains a relatively high proportion of neuron cell bodies. Conversely, white matter is composed mainly of axons and is named because of the color of the fatty insulation called myelin that coats many axons. White matter includes all of the nerves of the PNS and much of the interior of the brain and spinal cord. Gray matter is found in clusters of neurons in the brain and spinal cord and in cortical layers that line their surfaces.

By convention, a cluster of neuron cell bodies in the gray matter of the brain or spinal cord is called a nucleus, whereas a cluster of neuron cell bodies in the periphery is called a ganglion. However, there are a few notable exceptions to this rule, including a part of the brain called the basal ganglia, which will be discussed later.

Peripheral Nervous System

The PNS is a vast network of nerves consisting of bundles of axons that link the body to the brain and the spinal cord. Sensory nerves of the PNS contain sensory receptors that detect changes in the internal and external environment. This information is sent to the CNS via afferent sensory nerves. Following information processing in the CNS, signals are relayed back to the PNS by way of efferent peripheral nerves.

Autonomic and Somatic Nervous Systems

The PNS is further subdivided into the autonomic nervous system (ANS) and the somatic nervous system. The autonomic system has involuntary control of internal organs, blood vessels, and smooth and cardiac muscles. The somatic system has voluntary control of our movements via skeletal muscle.

As mentioned, the autonomic nervous system acts as a control system and most functions occur without conscious thought. The ANS affects heart rate, digestion, respiratory rate, salivation, perspiration, pupil diameter, urination, and sexual arousal. While most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind. The ANS is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS).

Parasympathetic and Sympathetic Nervous Systems

Broadly, the parasympathetic system is responsible for stimulation of “rest-and-digest” activities that occur when the body is at rest, including sexual arousal, salivation, lacrimation (tears), urination, digestion, and defecation. The sympathetic nervous syste is responsible for stimulating activities associated with the “fight-or-flight” response: mobilizing the systems of the body for escape or attacking sources of danger. In truth, the functions of both the parasympathetic and sympathetic nervous systems are not so straightforward, but this division is a useful rule of thumb.

The enteric nervous system (ENS) controls the gastrointestinal system and is sometimes considered part of the autonomic nervous system. However, it is sometimes considered an independent system because it can operate independently of the brain and the spinal cord.

This diagram details the parts of the central nervous system, including brain, spinal cord, brachial plexus, musculocutaneous nerve, radial nerve, median nerve, iliohypogastric nerve, genitofemoral nerve, obturator nerve, ulnar nerve, common peroneal nerve, deep peroneal nerve, superficial peroneal nerve, tibial nerve, saphenous nerve, muscular branches of femoral nerve, sciatic nerve, pudendal nerve, femoral nerve, sacral plexus, lumbar plexus, subcostal nerve, intercostal nerve, cerebellum.

The Nervous System of a Vertebrate: The brain and the spinal cord are the central nervous system (CNS) (shown in yellow). The left-right pair of cranial nerves, spinal nerves, and ganglia make up the peripheral nervous system (shown in dark gold).