Nervous System AnatomyNervous 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 effector cells in the
There are 3 basic classes of neurons: afferent neurons, efferent neurons, and interneurons.
- 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. 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 the cauda equina (due to its resemblance to a horse’s tail) that continues inferiorly to the sacrum 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 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 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
- 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 of lumbar 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.
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.
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.
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.
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.
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.
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.
- 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.
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.
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 by reducing the 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 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.