Anatomy and Physiology – Nervous System

Anatomy and Physiology – Nervous System

• Nervous systems:
• The nervous system is a complex collection of nerves and specialized cells
• known as neurons that transmit signals between different parts of the body.
• It is essentially the body‘s electrical wiring. Nerves are cylindrical bundles
• of fibers that start at the brain and central cord and branch out to every other
• part of the body. Every minute of every day, your nervous system is sending
• and receiving countless messages about what is happening both inside and
• around your body. Right now, your nervous system is receiving sensory
• input from your eyes about the words on the screen, from your ears about the
• sound of the computer, from your skin about the feel of your clothes, etc. At
• the same time, your brain is receiving information from sensors that monitor
• your heartrate, blood pressure, levels of oxygen and the contents of your
• stomach and intestines. Your brain then interprets all of these signals, which
• allows for an understanding of the words on the screen, the recognition of
• the noise as computer noise, and the development of motor responses such
• as moving your eyeballs, changing positions in your chair, and decreasing or
• increasing your heartrate and digestion. In short, your nervous system
• coordinates all the activities of your body. This module will provide a
• general overview of the nervous system as a whole. A word of caution: A
• system capable of so many sophisticated and complicated functions has to be
• extremely complex. One module cannot possibly present all the information
• about the nervous system, and it will probably take a few trips through the
• nervous system before the pieces fall into place, so don't despair if you're a
• bit confused. An Introduction to Nervous Systems presents the principles of
• neurobiology from an evolutionary perspective—from single–celled
• organisms to complex invertebrates such as flies—and is ideal for use as a
• supplemental textbook. Greenspan describes the mechanisms that allow
• behavior to become ever more sophisticated—from simple avoidance
• behavior of Paramecium through to the complex cognitive behaviors of the
• honeybee—and shows how these mechanisms produce the increasing neural
• complexity found in these organisms. The book ends with a discussion of
• what is universal about nervous systems and what may be required,
• neurobiologically, to be human. This novel and highly readable presentation
• of fundamental principles of neurobiology is designed to be accessible to
• undergraduate and graduate students not already steeped in the subject.
• The Central nervous system is made up of the brain and spinal cord and The
• Peripheral nervous system is made up of the Somatic and the
• Autonomic nervous systems. The Central nervous system. The
• central nervous system is divided into two major parts: the brain and the
• spinal cord. Thenervous system has two major parts: the central nervous
• system(CNS) and the peripheral nervous system (PNS). The
• central systemis the primary control center for the body and is composed of
• the brain and spinal cord. he nervous system has three main functions:
• gathering sensory input, integrating data, and forming motor output.
• Neurons sense external stimuli through receptors in the body, providing the
• first major function of the nervous system—gathering sensory input. The
• nervous system consists of the brain, spinal cord, and a complex network of
• neurons. This system is responsible for sending, receiving, and interpreting
• information from all parts of the body. The nervous system monitors and
• coordinates internal organ function and responds to changes in the external
• environment.
• The nervous system is a complex network of nerves and cells that carry
• messages to and from the brain and spinal cord to various parts of the body.
• The nervous system includes both the Central nervous system and Peripheral
• nervous system. The Central nervous system is made up of the brain and
• spinal cord and The Peripheral nervous system is made up of the Somatic
• and the Autonomic nervous systems.
• In vertebrates the system has two main divisions, the central and the
• peripheral nervous systems. The central nervous system consists of
• the brain and spinal cord. Linked to these are the cranial, spinal, and
• autonomic nerves, which, with their branches, constitute the peripheral
• nervous system. The brain might be compared to a computer and its memory
• banks, the spinal cord to the conducting cable for the computer's input and
• output, and the nerves to a circuit supplying input information to the cable
• and transmitting the output to muscles and organs.
• The nervous system is built up of nerve cells, called neurons, which are
• supported and protected by other cells. Of the 200 billion or so neurons
• making up the human nervous system, approximately half are found in the
• brain. From the cell body of a typical neuron extend one or more outgrowths
• (dendrites), threadlike structures that divide and subdivide into ever smaller
• branches. Another, usually longer structure called the axon also stretches
• from the cell body. It sometimes branches along its length but always
• branches at its microscopic tip. When the cell body of a neuron is chemically
• stimulated, it generates an impulse that passes from the axon of one neuron
• to the dendrite of another; the junction between axon and dendrite is called
• a synapse. Such impulses carry information throughout the nervous system.
• Electrical impulses may pass directly from axon to axon, from axon to
• dendrite, or from dendrite to dendrite.
• So-called white matter in the central nervous system consists primarily of
• axons coated with light-colored myelin produced by certain neuroglial cells.
• Nerve cell bodies that are not coated with white matter are known as gray
• matter. Nonmyelinated axons that are outside the central nervous system are
• enclosed only in a tubelike neurilemma sheath composed of Schwann cells,
• which are necessary for nerve regeneration. There are regular intervals along
• peripheral axons where the myelin sheath is interrupted. These areas, called
• nodes of Ranvier, are the points between which nerve impulses, in
• myelinated fibers, jump, rather than pass, continuously along the fiber (as is
• the case in unmyelinated fibers). Transmission of impulses is faster in
• myelinated nerves, varying from about 3 to 300 ft (1–91 m) per sec.
• Both myelinated and unmyelinated dendrites and axons are termed nerve
• fibers; a nerve is a bundle of nerve fibers; a cluster of nerve cell bodies
• (neurons) on a peripheral nerve is called a ganglion. Neurons are located
• either in the brain, in the spinal cord, or in peripheral ganglia. Grouped and
• interconnected ganglia form a plexus, or nerve center. Sensory (afferent)
• nerve fibers deliver impulses from receptor terminals in the skin and organs
• to the central nervous system via the peripheral nervous system. Motor
• (efferent) fibers carry impulses from the central nervous system to effector
• terminals in muscles and glands via the peripheral system.
• The peripheral system has 12 pairs of cranial nerves: olfactory, optic,
• oculomotor, trochlear, trigeminal, abducent, facial, vestibulo-cochlear
• (formerly known as acoustic), glossopharyngeal, vagus, spinal accessory,
• and hypoglossal. These have their origin in the brain and primarily control
• the activities of structures in the head and neck. The spinal nerves arise in
• the spinal cord, 31 pairs radiating to either side of the body: 8 cervical, 12

thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

Autonomic Nervous System

• The autonomic nerve fibers form a subsidiary system that regulates the iris
• of the eye and the smooth-muscle action of the heart, blood vessels, glands,
• lungs, stomach, colon, bladder, and other visceral organs not subject to
• willful control. Although the autonomic nervous system's impulses originate
• in the central nervous system, it performs the most basic human functions
• more or less automatically, without conscious intervention of higher brain
• centers. Because it is linked to those centers, however, the autonomic system
• is influenced by the emotions; for example, anger can increase the rate of
• heartbeat. All of the fibers of the autonomic nervous system are motor
• channels, and their impulses arise from the nerve tissue itself, so that the
• organs they innervate perform more or less involuntarily and do not require
• stimulation to function.
• Autonomic nerve fibers exit from the central nervous system as part of other
• peripheral nerves but branch from them to form two more subsystems: the
• sympathetic and parasympathetic nervous systems, the actions of which
• usually oppose each other. For example, sympathetic nerves cause arteries to
• contract while parasympathetic nerves cause them to dilate. Sympathetic
• impulses are conducted to the organs by two or more neurons. The cell body
• of the first lies within the central nervous system and that of the second in an
• external ganglion. Eighteen pairs of such ganglia interconnect by nerve
• fibers to form a double chain just outside the spine and running parallel to it.
• Parasympathetic impulses are also relayed by at least two neurons, but the

cell body of the second generally lies near or within the target organ.

Autonomic Nervous System

• Another part of the nervous system is the Autonomic Nervous System. It has
• three parts:
• the sympathetic nervous system
• the parasympathetic nervous system
• the enteric nervous system
• This nervous system controls the nerves of the inner organs of the body on
• which humans have no conscious control. This includes the heartbeat,
• digestion, breathing (except conscious breathing) etc.
• The nerves of the autonomic nervous system enervate the smooth
• involuntary muscles of the (internal organs) and glands and cause them to
• function and secrete their enzymes etc.
• The Enteric nervous system is the third part of the autonomic nervous
• system. The enteric nervous system is a complex network of nerve fibers
• that innervate the organs within the abdomen like the gastrointestinal tract,
• pancreas, gall bladder etc. It contains nearly 100 million nerves.
• Somatic nervous system
• The somatic nervous system consists of peripheral nerve fibers that pick up
• sensory information or sensations from the peripheral or distant organs
• (those away from the brain like limbs) and carry them to the central nervous
• system.
• These also consist of motor nerve fibers that come out of the brain and take
• the messages for movement and necessary action to the skeletal muscles. For
• example, on touching a hot object the sensory nerves carry information
• about the heat to the brain, which in turn, via the motor nerves, tells the
• muscles of the hand to withdraw it immediately.
• The whole process takes less than a second to happen. The cell body of the
• neuron that carries the information often lies within the brain or spinal cord
• and projects directly to a skeletal muscle.
• The nervous system consists of the brain, spinal cord, sensory organs, and all
• of the nerves that connect these organs with the rest of the body. Together,
• these organs are responsible for the control of the body and communication
• among its parts. The brain and spinal cord form the control center known as
• the central nervous system (CNS), where information is evaluated and
• decisions made. The sensory nerves and sense organs of the peripheral
• nervous system (PNS) monitor conditions inside and outside of the body and
• send this information to the CNS. Efferent nerves in the PNS carry signals
• from the control center to the muscles, glands, and organs to regulate their

functions.

Nervous Tissue

• The majority of the nervous system is tissue made up of two classes of cells:
• neurons and neuroglia.
• 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 effect or cells in the body.
• The smallest worker in the nervous system is the neuron. For each of the
• chain of impulses there is one preganglionic neuron, or one before the cell
• body or ganglion, that is like a central controlling body for numerous
• neurons going out peripherally.
• The preganglionic neuron is located in either the brain or the spinal cord. In
• the autonomic nervous system this preganglionic neuron projects to an
• autonomic ganglion. The postganglionic neuron then projects to the target
• organ.
• In the somatic nervous system there is only one neuron between the central
• nervous system and the target organ while the autonomic nervous system
• uses two neurons.
• There are 3 basic classes of neurons: afferent neurons, efferent neurons, and
• inter neurons.
• Afferent neurons. Also known as sensory neurons, afferent neurons transmit
• sensory signals to the central nervous system from receptors in the body.
• Efferent neurons. 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. 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.
• Neuroglia. Neuroglia, also known as glial cells, act as the ―helper‖ cells of
• the nervous system. Each neuron in the body is surrounded by anywhere
• from 6 to 60 neuroglia that protect, feed, and insulate the neuron. 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.
• The brain, a 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 approximately 100 billion neurons of the brain form the main control
• center of the body. The brain and spinal cord together form the central
• nervous system (CNS), where 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.
• The brain lies within the skull and is shaped like a mushroom. The brain
• consists of four principal parts:
• the brain stem
• the cerebrum
• the cerebellum
• the diencephalon
• The brain weighs approximately 1.3 to 1.4 kg. It has nerve cells called the
• neurons and supporting cells called the glia.
• There are two types of matter in the brain: grey matter and white
• matter. Grey matter receives and stores impulses. Cell bodies of neurons
• and neuroglia are in the grey matter. White matter in the brain carries
• impulses to and from grey matter. It consists of the nerve fibers (axons).
• The brain stem
• The brain stem is also known as the Medulla oblongata. It is located between
• the pons and the spinal cord and is only about one inch long.
• The cerebrum
• The cerebrum forms the bulk of the brain and is supported on the brain
• stem. The cerebrum is divided into two hemispheres. Each hemisphere
• controls the activities of the side of the body opposite that hemisphere.
• The hemispheres are further divided into four lobes:
• Frontal lobe
• Temporal lobes
• Parietal lobe
• Occipital lobe
• The cerebellum
• This is located behind and below the cerebrum.
• The diencephalon
• The diencephalon is also known as the fore brain stem. It includes the
• thalamus and hypothalamus. The thalamus is where sensory and other
• impulses go and coalesce.
• The hypothalamus is a smaller part of the diencephalon
• Other parts of the brain
• Other parts of the brain include the midbrain and the pons:
• the midbrain provides conduction pathways to and from higher and
• lower centers
• the pons acts as a pathway to higher structures; it contains conduction

pathways between the medulla and higher brain centers

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. In the lumbar region, the spinal
• cord separates into a bundle of individual nerves called thecauda
• equina (due to its resemblance to a horse‘s tail) that continues inferiorly to
• thesacrum and coccyx. 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. 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 oneway 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.

• Cranial Nerves. 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.g. olfactory, optic, etc.) that identifies
• its function or location. The cranial nerves provide a direct connection to the
• brain for the special sense organs, muscles of the head, neck, and shoulders,
• the heart, and the GI tract.
• Spinal Nerves. 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 nerves, 5 pairs oflumbar nerves, 5 pairs of sacral nerves,
• and 1 pair of coccygeal nerves. Each spinal nerve exits from the spinal cord
• through 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: the dura mater, arachnoid mater, and pia
• mater
• Dura mater. The dura mater, which means ―tough mother,‖ is the thickest,
• toughest, and most superficial layer of meninges. Made of dense irregular
• connective tissue, it contains many tough collagen fibers and blood vessels.
• Dura mater protects the CNS from external damage, contains the
• cerebrospinal fluid that surrounds the CNS, and provides blood to the
• nervous tissue of the CNS.
• Arachnoid mater. The arachnoid mater, which means ―spider-like
• mother,‖ is much thinner and more delicate than the dura mater. 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.
• Pia mater. The pia mater, which means ―tender mother,‖ is a thin and
• delicate layer of tissue that rests on the outside of the brain and spinal cord.
• 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.

Cerebrospinal Fluid

• The space surrounding the organs of the CNS is filled with a clear fluid
• known as cerebrospinal fluid (CSF). CSF is formed from blood plasma by
• special structures called choroid plexuses. The choroid plexuses contain
• many capillaries lined with epithelial tissue that filters blood plasma and
• allows the filtered fluid to enter the space around the brain.
• 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 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.

Sense Organs

• All of the bodies‘ many sense organs are components of the nervous system.
• What are known as the special senses—vision, taste, smell, hearing, and
• balance—are all detected by specialized organs such as the eyes, taste buds,
• and 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.
• Functions of the Nervous System
• The nervous system has 3 main functions: sensory, integration, and motor.
• Sensory. The sensory function of the nervous system involves collecting
• information from sensory receptors that monitor the body‘s internal and
• external conditions. These signals are then passed on to the central nervous
• system (CNS) for further processing by afferent neurons (and nerves).
• Integration. 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.
• Motor. 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.

Divisions of the Nervous System

Central Nervous System

• The brain and spinal cord together form the central nervous system, or CNS.
• The CNS acts as the control center of the body by providing its processing,
• memory, and regulation systems. The CNS takes in all of the conscious and
• subconscious sensory information from the body‘s sensory receptors to stay
• aware of the body‘s internal and external conditions. Using this sensory
• information, it makes decisions about both conscious and subconscious
• actions to take to maintain the body‘s homeostasis and ensure its survival.
• 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.

Somatic Nervous System

• The somatic nervous system (SNS) is a division of the PNS that includes all
• of the voluntary efferent neurons. The SNS is the only consciously
• controlled part of the PNS and is responsible for stimulating skeletal muscles

in the body.

Autonomic Nervous System

• The autonomic nervous system (ANS) is a division of the PNS that includes
• all of the involuntary efferent neurons. The ANS controls subconscious
• effectors such as visceral muscle tissue, cardiac muscle tissue, and glandular
• tissue.
• There are 2 divisions of the autonomic nervous system in the body: the
• sympathetic and parasympathetic divisions.
• Sympathetic. The sympathetic division forms the body‘s ―fight or flight‖
• response to stress, danger, excitement, exercise, emotions, and
• embarrassment. The sympathetic division increases respiration and heart
• rate, releases adrenaline and other stress hormones, and decreases digestion
• to cope with these situations.
• Parasympathetic. The parasympathetic division forms the body‘s ―rest and
• digest‖ response when the body is relaxed, resting, or feeding. The
• parasympathetic works to undo the work of the sympathetic division after a
• stressful situation. Among other functions, the parasympathetic division
• works to decrease respiration and heart rate, increase digestion, and permit

the elimination of wastes.

Enteric Nervous System

• The enteric nervous system (ENS) is the division of the ANS that is
• responsible for regulating digestion and the function of the digestive organs.
• 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. However, the ENS mostly works
• independently of the CNS and continues to function without any outside
• input. For this reason, the ENS is often called the ―brain of the gut‖ or the
• body‘s ―second brain.‖ The ENS is an immense system—almost as many

neurons exist in the ENS as in the spinal cord.

Action Potentials

• Neurons function through the generation and propagation of electrochemical
• signals known as action potentials (APs). An AP is created
• of sodium and potassium ions through the membrane of neurons.
• Resting Potential. 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 (mV), which means that the inside of the cell has a
• negative charge compared to its surroundings.
• Threshold Potential. If a stimulus permits enough positive ions to enter a
• region of the cell to cause it to reach -55 mV, that region of the cell will
• open its voltage-gated sodium channels and allow sodium ions to diffuse
• into the cell. -55 mV is the threshold potential for neurons as this is the
• ―trigger‖ voltage that they must reach to cross the threshold into forming an
• action potential.
• Depolarization. Sodium carries a positive charge that causes the cell to
• become depolarized (positively charged) compared to its normal negative
• charge. The voltage for depolarization of all neurons is +30 mV. The
• depolarization of the cell is the AP that is transmitted by the neuron as a
• nerve signal. 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.
• Repolarization. After the depolarization voltage of +30 mV is reached,
• voltage-gated potassium ion channels open, allowing positive potassium ions
• to diffuse out of the cell. 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: chemical synapses and electrical
• synapses.
• Chemical synapses. At the end of a neuron‘s axon is an enlarged region of
• the axon known as the axon terminal. 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. The NT molecules cross the synaptic cleft and bind to
• receptor molecules on the cell, forming a synapse with the neuron. These
• receptor molecules open ion channels that may either stimulate the receptor
• cell to form a new action potential or may inhibit the cell from forming an
• action potential when stimulated by another neuron.
• Electrical synapses. Electrical synapses are formed when 2 neurons are
• connected by small holes called gap junctions. The gap junctions allow
• electric current to pass from one neuron to the other, so that an AP in one
• cell is passed directly on to the other cell through the synapse.
• Myelination
• The axons of many neurons are covered by a coating of insulation known as
• myelin to increase the speed of nerve conduction throughout the body.
• Myelin is formed by 2 types of glial cells: Schwann cells in the PNS and
• oligodendrocytes in the CNS. In both cases, the glial cells wrap their plasma
• membrane around the axon many times to form a thick covering of lipids.
• The development of these myelin sheaths is known as myelination.
• Myelination speeds up the movement of APs in the axon
• number of APs that must form for a signal to reach the end of an axon. The
• myelination process begins speeding up nerve conduction in fetal
• development and continues into early adulthood. Myelinated axons appear
• white due to the presence of lipids and form the white matter of the inner
• brain and outer spinal cord. White matter is specialized for carrying
• information quickly through the brain and spinal cord. The gray matter of
• the brain and spinal cord are the unmyelinated integration centers where
• information is processed.
• Reflexes
• Reflexes are fast, involuntary responses to stimuli. The most well known
• reflex is the patellar reflex, which is checked when a physicians taps on a
• patient‘s knee during a physical examination. Reflexes are integrated in the
• gray matter of the spinal cord or in the brain stem. Reflexes allow the body
• to respond to stimuli very quickly by sending responses to effectors before
• the nerve signals reach the conscious parts of the brain. This explains why
• people will often pull their hands away from a hot object before they realize
• they are in pain.
• Functions of the Cranial Nerves
• Each of the 12 cranial nerves has a specific function within the nervous
• system.
• The olfactory nerve (I) carries scent information to the brain from the
• olfactory epithelium in the roof of the nasal cavity.
• The optic nerve (II) carries visual information from the eyes to the brain.
• Oculomotor, trochlear, and abducens nerves (III, IV, and VI) all work
• together to allow the brain to control the movement and focus of the eyes.
• The trigeminal nerve (V) carries sensations from the face and innervates
• the muscles of mastication.
• The facial nerve (VII) innervates the muscles of the face to make facial
• expressions and carries taste information from the anterior 2/3 of the
• tongue.
• The vestibulocochlear nerve (VIII) conducts auditory and balance
• information from the ears to the brain.
• The glossopharyngeal nerve (IX) carries taste information from the posterior
• 1/3 of the tongue and assists in swallowing.
• The vagus nerve (X), sometimes called the wandering nerve due to the fact
• that it innervates many different areas, ―wanders‖ through the head, neck,
• and torso. It carries information about the condition of the vital organs to the
• brain, delivers motor signals to control speech and delivers parasympathetic
• signals to many organs.
• The accessory nerve (XI) controls the movements of the shoulders and
• neck.
• The hypoglossal nerve (XII) moves the tongue for speech and swallowing.