As the most
complex system, the nervous system serves as the body control center
and communications electrical-chemical wiring network. As a key
homeostatic regulatory and coordinating system, it detects, interprets,
and responds to changes in internal and external conditions. The
nervous system integrates countless bits of information and generates
appropriate reactions by sending electrochemical impulses through
nerves to effector organs such as muscles and glands. The brain
and spinal cord are the central nervous system (CNS); the connecting
nerve processes to effectors and receptors serve as the peripheral
nervous system (PNS). Special sense receptors provide for taste,
smell, sight, hearing, and balance. Nerves carry all messages exchanged
between the CNS and the rest of the body.
Back to top
neurons, brain, spinal cord
The neuron transmits
electric signals like an electric wire. The perikaryon (cell body)
is the neuron central part. Dendrites, short branches, extend from
the neuron. These input channels receive information from other
neurons or sensory cells (cells that receive information from the
environment). A long branch, the axon, extends from the neuron as
its output channel. The neuron sends messages along the axon to
other neurons or directly to muscles or glands.
be linked to each other in order to transmit signals. The connection
between two neurons is a synapse. When a nerve impulse (electrical
signal) travels across a neuron to the synapse, it causes the release
of neurotransmitters. These chemicals carry the nerve signal across
the synapse to another neuron.
are propagated (transmitted) along the entire length of an axon
in a process called continuous conduction. To transmit nerve impulses
faster, some axons are partially coated with myelin sheaths. These
sheaths are composed of cell membranes from Schwann cells, a type
of supporting cell outside the CNS. Nodes of Ranvier (short intervals
of exposed axon) occur between myelin sheaths. Impulses moving along
myelinated axons jump from node to node. This method of nerve impulse
transmission is saltatory conduction.
The brain has
billions of neurons that receive, analyze, and store information
about internal and external conditions. It is also the source of
conscious and unconscious thoughts, moods, and emotions. Four major
brain divisions govern its main functions: the cerebrum, the diencephalon,
the cerebellum, and the brain stem.
is the large rounded area that divides into left and right hemispheres
(halves) at a fissure (deep groove). The hemispheres communicate
with each other through the corpus callosum (bundle of fibers between
the hemispheres). Surprisingly, each hemisphere controls muscles
and glands on the opposite side of the body. Comprising 85 percent
of total brain weight, the cerebrum controls language, conscious
thought, hearing, somatosensory functions (sense of touch), memory,
personality development, and vision.
(unmyelinated nerve cell bodies) composes the cerebral cortex (outer
portion of the cerebrum). Beneath the cortex lies the white matter
(myelinated axons). During embryonic development, the cortex folds
upon itself to form gyri (folds) and sulci (shallow grooves) so
that more gray matter can reside within the skull cavity.
forms the central part of the brain. It consists of three bilaterally
symmetrical structures: the hypothalamus, thalamus, and epithalamus.
The hypothalamus 'master switchboard' resides in the brain stem
upper end. It controls many body activities that affect homeostasis
(maintenance of a stable internal environment in the body).
is the main neural control center (brain part that controls endocrine
glands). The pituitary gland lies just below the hypothalamus. The
pituitary gland is a small endocrine gland that secretes a variety
of hormones (organic chemicals that regulate the body's physiological
processes). When the hypothalamus detects certain body changes,
it releases regulating factors (chemicals that stimulate or inhibit
the pituitary gland). The pituitary gland then releases or blocks
various hormones. Because of this close association between the
nervous and endocrine systems, together they are called the neuroendocrine
also regulates visceral (organ-related) activities, food and fluid
intake, sleep and wake patterns, sex drive, emotional states, and
production of antidiuretic hormone (ADH) and oxytocin. The pituitary
gland produces both these hormones.
is a relay and preprocessing station for the many nerve impulses
that pass through it. Impulses carrying similar messages are grouped
in the thalamus, then relayed to the appropriate brain areas.
is the most dorsal (posterior) portion of the diencephalon. It contains
a vascular network involved in cerebrospinal fluid production. Extending
from the epithalamus posteriorly is the pineal body, or pineal gland.
Its function is not yet fully understood; it is thought to control
At the rear
of the brain is the cerebellum. The cerebellum is similar to the
cerebrum: each has hemispheres that control the opposite side of
the body and are covered by gray matter and surface folds. In the
cerebellum, the folds are called folia; in the cerebrum, sulci.
The vermis (central constricted area) connects the hemispheres.
The cerebellum controls balance, posture, and coordination.
The brain stem
connects the cerebrum and cerebellum to the spinal cord. Its superior
portion, the midbrain, is the center for visual and auditory reflexes;
examples of these include blinking and adjusting the ear to sound
volume. The middle section, the pons, bridges the cerebellum hemispheres
and higher brain centers with the spinal cord. Below the pons lies
the medulla oblongata; it contains the control centers for swallowing,
breathing, digestion, and heartbeat.
formation extends throughout the midbrain. This network of nerves
has widespread connections in the brain and is essential for consciousness,
awareness, and sleep. It also filters sensory input, which allows
a person to ignore repetitive noises such as traffic, yet awaken
instantly to a baby's cry.
The spinal cord
is a continuation of the brain stem. It is long, cylindrical, and
passes through a tunnel in the vertebrae called the vertebral canal.
The spinal cord has many spinal segments, which are spinal cord
regions from which pairs (one per segment) of spinal nerves arise.
Like the cerebrum and cerebellum, the spinal cord has gray and white
matter, although here the white matter is on the outside. The spinal
cord carries messages between the CNS and the rest of the body,
and mediates numerous spinal reflexes such as the knee-jerk reflex.
connective tissue layers, protect the brain and spinal cord. The
outermost dura layer forms partitions in the skull that prevents
excessive brain movement. The arachnoid middle layer forms a loose
covering beneath the dura. The innermost pia layer clings to the
brain and spinal cord; it contains many tiny blood vessels that
supply these organs.
substance, cerebrospinal fluid, surrounds the brain and spinal cord.
The brain floats within the cerebrospinal fluid, which prevents
against crushing under its own weight and cushions against shocks
from walking, jumping, and running.
Back to top
somatic (voluntary) nervous system, autonomic (involuntary) nervous
nervous system includes sensory receptors, sensory neurons, and
motor neurons. Sensory receptors are activated by a stimulus (change
in the internal or external environment). The stimulus is converted
to an electronic signal and transmitted to a sensory neuron. Sensory
neurons connect sensory receptors to the CNS. The CNS processes
the signal, and transmits a message back to an effector organ (an
organ that responds to a nerve impulse from the CNS) through a motor
The PNS has
two parts: the somatic nervous system and the autonomic nervous
system. The somatic nervous system, or voluntary nervous system,
enables humans to react consciously to environmental changes. It
includes 31 pairs of spinal nerves and 12 pairs of cranial nerves.
This system controls movements of skeletal (voluntary) muscles.
of spinal nerves emerge from various segments of the spinal cord.
Each spinal nerve has a dorsal root and a ventral root. The dorsal
root contains afferent (sensory) fibers that transmit information
to the spinal cord from the sensory receptors. The ventral root
contains efferent (motor) fibers that carry messages from the spinal
cord to the effectors. Cell bodies of the efferent fibers reside
in the spinal cord gray matter. These roots become nerves that innervate
(transmit nerve impulses to) muscles and organs throughout the body.
of cranial nerves transmit from special sensory receptors information
on the senses of balance, smell, sight, taste, and hearing. Cranial
nerves also carry information from general sensory receptors in
the body, mostly from the head region. This information is processed
in the CNS; the resulting orders travel back through the cranial
nerves to the skeletal muscles that control movements in the face
and throat, such as for smiling and swallowing. In addition, some
cranial nerves contain somatic and autonomic motor fibers.
nervous system (autonomic nervous system) maintains homeostasis.
As its name implies, this system works automatically and without
voluntary input. Its parts include receptors within viscera (internal
organs), the afferent nerves that relay the information to the CNS,
and the efferent nerves that relay the action back to the effectors.
The effectors in this system are smooth muscle, cardiac muscle and
glands, all structures that function without conscious control.
An example of autonomic control is movement of food through the
digestive tract during sleep.
portion of the autonomic system is divided into sympathetic and
parasympathetic systems. The sympathetic nerves mobilize energy
for the 'Fight or Flight' reaction during stress, causing increased
blood pressure, breathing rate, and bloodflow to muscles. Conversely,
the parasympathetic nerves have a calming effect; they slow the
heartbeat and breathing rate, and promote digestion and elimination.
This example of intimate interaction with the endocrine system is
one of many that explain why the two systems are called the neuroendocrine
between sensory and motor neurons can be seen in a reflex (rapid
motor response to a stimulus). Reflexes are quick because they involve
few neurons. Reflexes are either somatic (resulting in contraction
of skeletal muscle) or autonomic (activation of smooth and cardiac
muscle). All reflex arcs have five basic elements: a receptor, sensory
neuron, integration center (CNS), motor neuron, and effector.
are somatic reflexes mediated by the spinal cord. These can involve
higher brain centers. In a spinal reflex, the message is simultaneously
sent to the spinal cord and brain. The reflex triggers the response
without waiting for brain analysis. If a finger touches something
hot, the finger jerks away from the danger. The burning sensation
becomes an impulse in the sensory neurons. These neurons synapse
in the spinal cord with motor neurons that cause the burned finger
to pull away. This spinal reflex is a flexor, or withdrawal reflex.
reflex occurs when a muscle or its tendon is struck. The jolt causes
the muscle to contract and inhibits antagonist muscle contraction.
A familiar example is the patellar reflex, or knee-jerk reflex,
that occurs when the patellar tendon is struck. The impulse travels
via afferent neurons to the spinal cord where the message is interpreted.
Two messages are sent back, one causing the quadriceps muscles to
contract and the other inhibiting the antagonist hamstring muscles
from contracting. The contraction of the quadriceps and inhibition
of hamstrings cause the lower leg to kick, or knee-jerk.
Back to top
The sense organs
are highly specialized structures that receive information from
the environment. These organs contain special sense receptors ranging
from complex structures, such as eyes and ears, to small localized
clusters of receptors, such as taste buds and olfactory epithelium
(receptors for smell).
Smell and taste
are chemical senses, which contain chemoreceptors that respond to
chemicals in solution. Food chemicals dissolved in saliva stimulate
taste receptors in taste buds. The nasal membranes produce fluids
that dissolve chemicals in air. These chemicals stimulate smell
receptors in olfactory epithelium. The chemical senses complement
each other and respond to many of the same stimuli.
which include rods and cones, in back of the eye respond to light
energy. Rods provide dim-light, black-and-white vision, and are
the source of peripheral vision. Cones operate in bright light and
provide color vision. Cones are most concentrated at the back center
of each eye. Rods are more numerous than cones, and surround the
cones. Information from the rods and cones travels via the optic
nerve into the brain for interpretation.
The ear has
two specialized functions: sound wave detection and interpretation
of the head position in space. Sound waves enter the outer ear through
the external auditory canal (ear canal) and strike the tympanic
membrane (eardrum). Vibration of the eardrum moves three ossicles
(small bones) inside the middle ear, which in turn stimulate the
organ of Corti (hearing receptor in the inner ear). Impulses travel
from the organ of Corti through the vestibulocochlear nerve to be
interpreted by the brain.
The ear also
contains equilibrium (sense of balance) receptors. The vestibular
apparatus, a group of equilibrium receptors in the inner ear, sense
movement in space. Maculae receptors in the vestibule monitor static
equilibrium (head position with respect to gravity when the body
is still). Cristae receptors in the semicircular canals monitor
dynamic equilibrium (movement). Impulses from the vestibular apparatus
travel along the vestibulocochlear nerve to appropriate brain areas.
These centers start responses that fix the eyes on objects and stimulate
muscles to maintain balance.
respond to mechanical energy forces: touch, pressure, stretching,
and movement. Ranging in complexity from free nerve endings beneath
the skin to more complex tactile receptors at the bases of hair,
mechanoreceptors change shape when pushed or pulled.
of skin receptors sense different environmental stimuli. Free nerve
endings sense pain. Specialized receptors such as Merkel's discs
and Meissner's corpuscles sense touch. Pacinian corpuscles sense
deep pressure. Naked nerve endings are thought to be responsible
for sensing temperature.
of sensory receptors provide the brain information on the body.
Interoreceptors in body organs inform the CNS about internal conditions
such as hunger and pain. Proprioceptors in joints, tendons, and
muscles detect changes in position of skeletal muscles and bones.
This information allows humans to be aware the positions of their
trunk and limbs without having to see them.
Back to top