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An overview of the nervous system of the human body

Nervous System Anatomy Nervous Tissue The majority of the nervous system is tissue made up of two classes of cells: Neurons Neurons, also known as nerve cells, communicate within the body by transmitting electrochemical signals. Neurons look quite different from other cells in the body due to the many long cellular processes that extend from their central cell body. The cell body is the roughly round part of a neuron that contains the nucleus, mitochondria, and most of the cellular organelles.

Small tree-like structures called dendrites extend from the cell body to pick up stimuli from the environment, other neurons, or sensory receptor cells. Long transmitting processes called axons extend from the cell body to send signals onward to other neurons or effector cells in the body.

There are 3 basic classes of neurons: Also known as sensory neurons, afferent neurons transmit sensory signals to the central nervous system from receptors in the body.

Also known as motor neurons, efferent neurons transmit signals from the central nervous system to effectors in the body such as muscles and glands.

Interneurons form complex networks within the central nervous system to integrate the information received from afferent neurons and to direct the function of the body through efferent neurons. Each neuron in the body is surrounded by anywhere from 6 to 60 neuroglia that protect, feed, and insulate the neuron.

  • Each axon is wrapped in a connective tissue sheath called the endoneurium;
  • CSF is constantly produced at the choroid plexuses and is reabsorbed into the bloodstream at structures called arachnoid villi;
  • The olfactory nerve I carries scent information to the brain from the olfactory epithelium in the roof of the nasal cavity;
  • Integration takes place in the gray matter of the brain and spinal cord and is performed by interneurons;
  • Other neurons, known as efferent nerves, carry signals only from the central nervous system to effectors such as muscles and glands.

Because neurons are extremely specialized cells that are essential to body function and almost never reproduce, neuroglia are vital to maintaining a functional nervous system. Brain The braina soft, wrinkled organ that weighs about 3 pounds, is located inside the cranial cavity, where the bones of the skull surround and protect it. The brain and spinal cord together form the central nervous system CNSwhere information is processed and responses originate.

The brain, the seat of higher mental functions such as consciousness, memory, planning, and voluntary actions, also controls lower body functions such as the maintenance of respiration, heart rate, blood pressure, and digestion. Spinal Cord The spinal cord is a long, thin mass of bundled neurons that carries information through the vertebral cavity of the spine beginning at the medulla oblongata of the brain on its superior end and continuing inferiorly to the lumbar region of the spine.

The white matter of the spinal cord functions as the main conduit of nerve signals to the body from the brain. The grey matter of the spinal cord integrates reflexes to stimuli. Nerves Nerves are bundles of axons in the peripheral nervous system PNS that act as information highways to carry signals between the brain and spinal cord and the rest of the body. Each axon is wrapped in a connective tissue sheath called the endoneurium.

Individual axons of the nerve are bundled into groups of axons called fascicles, wrapped in a sheath of connective tissue called the perineurium. Finally, many fascicles are wrapped together in another layer of connective tissue called the epineurium to form a whole nerve.

Nervous System

The wrapping of nerves with connective tissue helps to protect the axons and to increase the speed of their communication within the body. Afferent, Efferent, and Mixed Nerves.

Some of the nerves in the body are specialized for carrying information in only one direction, similar to a one-way street. Nerves that carry information from sensory receptors to the central nervous system only are called afferent nerves. Other neurons, known as efferent nerves, carry signals only from the central nervous system to effectors such as muscles and glands.

Finally, some nerves are mixed nerves that contain both afferent and efferent axons. Mixed nerves function like 2-way streets where afferent axons act as lanes heading toward the central nervous system and efferent axons act as lanes heading away from the central nervous system. Extending from the inferior side of the brain are 12 pairs of cranial nerves. Each cranial nerve pair is identified by a Roman numeral 1 to 12 based upon its location along the anterior-posterior axis of the brain.

Each nerve also has a descriptive name e. The cranial nerves provide a direct connection to the brain for the special sense organs, muscles of the headneck, and shoulders, the heart, and the GI tract.

Extending from the left and right sides of the spinal cord are 31 pairs of spinal nerves. The spinal nerves are mixed nerves that carry both sensory and motor signals between the spinal cord and specific regions of the body. The 31 spinal nerves are split into 5 groups named for the 5 regions of the vertebral column. Thus, there are 8 pairs of cervical nerves, 12 pairs of thoracic nerves5 pairs of lumbar nerves5 pairs of sacral nervesand 1 pair of coccygeal nerves.

Each spinal nerve exits from the spinal cord an overview of the nervous system of the human body the intervertebral foramen between a pair of vertebrae or between the C1 vertebra and the occipital bone of the skull.

Meninges The meninges are the protective coverings of the central nervous system CNS. They consist of three layers: Made of dense irregular connective tissue, it contains many tough collagen fibers and blood vessels. It lines the inside of the dura mater and contains many thin fibers that connect it to the underlying pia mater. These fibers cross a fluid-filled space called the subarachnoid space between the arachnoid mater and the pia mater. Containing many blood vessels that feed the nervous tissue of the CNS, the pia mater penetrates into the valleys of the sulci and fissures of the brain as it covers the entire surface of the CNS.

CSF is formed from blood plasma by special structures called choroid plexuses. Newly created CSF flows through the inside of the brain in hollow spaces called ventricles and through a small cavity in the middle of the spinal cord called the central canal. CSF also flows through the subarachnoid space around the outside of the brain and spinal cord.

CSF is constantly produced at the choroid plexuses and is reabsorbed into the bloodstream at structures called arachnoid villi.

Cerebrospinal fluid provides several vital functions to the central nervous system: CSF absorbs shocks between the brain and skull and between the spinal cord and vertebrae. This shock absorption protects the CNS from blows or sudden changes in velocity, such as during a car accident. The brain and spinal cord float within the CSF, reducing their apparent weight through buoyancy.

The brain is a very large but soft organ that requires a high volume of blood to function effectively.

  • The wrapping of nerves with connective tissue helps to protect the axons and to increase the speed of their communication within the body;
  • Extending from the left and right sides of the spinal cord are 31 pairs of spinal nerves;
  • Finally, many fascicles are wrapped together in another layer of connective tissue called the epineurium to form a whole nerve.

The reduced weight in cerebrospinal fluid allows the blood vessels of the brain to remain open and helps protect the nervous tissue from becoming crushed under its own weight. CSF helps to maintain chemical homeostasis within the central nervous system. It contains ions, nutrients, oxygen, and albumins that support the chemical and osmotic balance of nervous tissue.

CSF also removes waste products that form as byproducts of cellular metabolism within nervous tissue. What are known as the special senses—vision, taste, smell, hearing, and balance—are all detected by specialized organs such as the eyestaste budsand olfactory epithelium. Sensory receptors for the general senses like touch, temperature, and pain are found throughout most of the body. All of the sensory receptors of the body are connected to afferent neurons that carry their sensory information to the CNS to be processed and integrated.

These signals are then passed on to the central nervous system CNS for further processing by afferent neurons and nerves. The process of integration is the processing of the many sensory signals that are passed into the CNS at any given time.

These signals are evaluated, compared, used for decision making, discarded or committed to memory as deemed appropriate. Integration takes place in the gray matter of the brain and spinal cord and is performed by interneurons. Many interneurons work together to form complex networks that provide this processing power. Once the networks of interneurons in the CNS evaluate sensory information and decide on an action, they stimulate efferent neurons.

Efferent neurons also called motor neurons carry signals from the gray matter of the CNS through the nerves of the peripheral nervous system to effector cells. The effector may be smooth, cardiac, or skeletal muscle tissue or glandular tissue. The effector then releases a hormone or moves a part of the body to respond to the stimulus. Did you know that DNA testing can help you discover your genetic risk of acquiring certain health conditions that affect the organs of our nervous system?

The CNS is also responsible for the higher functions of the nervous system such as language, creativity, expression, emotions, and personality.

The brain is the seat of consciousness and determines who we are as individuals. Peripheral Nervous System The peripheral nervous system PNS includes all of the parts of the nervous system outside of the brain and spinal cord. These parts include all of the cranial and spinal nerves, ganglia, and sensory receptors. The SNS is the only consciously controlled part of the PNS and is responsible for stimulating skeletal muscles in the body.

The ANS controls subconscious effectors such as visceral muscle tissue, cardiac muscle tissue, and glandular an overview of the nervous system of the human body.

There are 2 divisions of the autonomic nervous system in the body: The sympathetic division increases respiration and heart rate, releases adrenaline and other stress hormones, and decreases digestion to cope with these situations.

The parasympathetic works to undo the work of the sympathetic division after a stressful situation.

Human nervous system

Among other functions, the parasympathetic division works to decrease respiration and heart rate, increase digestion, and permit the elimination of wastes. The ENS receives signals from the central nervous system through both the sympathetic and parasympathetic divisions of the autonomic nervous system to help regulate its functions.

Action Potentials Neurons function through the generation and propagation of electrochemical signals known as action potentials APs. An AP is created by the movement of sodium and potassium ions through the membrane of neurons.

See Water and Electrolytes. At rest, neurons maintain a concentration of sodium ions outside of the cell and potassium ions inside of the cell. This concentration is maintained by the sodium-potassium pump of the cell membrane which pumps 3 sodium ions out of the cell for every 2 potassium ions that are pumped into the cell.

The ion concentration results in a resting electrical potential of -70 millivolts mVwhich means that the inside of the cell has a negative charge compared to its surroundings. Sodium carries a positive charge that causes the cell to become depolarized positively charged compared to its normal negative charge. The depolarization of the cell is the AP that is transmitted by the neuron as a nerve signal.

  • The accessory nerve XI controls the movements of the shoulders and neck;
  • White matter is specialized for carrying information quickly through the brain and spinal cord;
  • White matter is specialized for carrying information quickly through the brain and spinal cord;
  • Functions of the Cranial Nerves Each of the 12 cranial nerves has a specific function within the nervous system;
  • Because neurons are extremely specialized cells that are essential to body function and almost never reproduce, neuroglia are vital to maintaining a functional nervous system;
  • Mixed nerves function like 2-way streets where afferent axons act as lanes heading toward the central nervous system and efferent axons act as lanes heading away from the central nervous system.

The positive ions spread into neighboring regions of the cell, initiating a new AP in those regions as they reach -55 mV. The AP continues to spread down the cell membrane of the neuron until it reaches the end of an axon. The loss of potassium along with the pumping of sodium ions back out of the cell through the sodium-potassium pump restores the cell to the -55 mV resting potential.

At this point the neuron is ready to start a new action potential. Synapses A synapse is the junction between a neuron and another cell. Synapses may form between 2 neurons or between a neuron and an effector cell. There are two types of synapses found in the body: The axon terminal is separated from the next cell by a small gap known as the synaptic cleft. When an AP reaches the axon terminal, it opens voltage-gated calcium ion channels. Calcium ions cause vesicles containing chemicals known as neurotransmitters NT to release their contents by exocytosis into the synaptic cleft.