The Nervous System
Functions of the Nervous System
• 1. Communication and coordination system in
the body
• 2. Seat of intellect and reasoning
• The goal is to monitor changes in the
environment inside and outside the body,
interpret the changes, and initiate a response
in an effort to maintain homeostasis.
• Electrochemical messages called Nerve
impulses race through your body every
moment, traveling along special routes, or
Nerves, at high speeds.
• This is how communication happens
Divisions of the Nervous system
• Two main groups
• Central Nervous System (CNS) which includes
the brain and spinal cord
• Peripheral Nervous System (PNS) which includes
the nerves and sensory receptors
• The PNS is divided into the Autonomic Nervous
System (ANS) which controls unconscious
activities and the Somatic Nervous System (SNS)
which oversees conscious activities
Nervous tissue
• The functional cells of nervous tissue are
called neurons, which receive support from
nearby neuroglial cells (connective part)
• Each neuron consists of a cell body and
branches. The cell body contains the nucleus
and most of the cytoplasm, and the branches
include many dendrites which carry impulses
toward the cell body, and a single axon which
carries impulses away.
Nervous tissue
Nervous tissue
• In many neurons, the axon is covered with
numerous neuroglial cells known as Schwann
cells, which provide a white-colored
protective sheath that is mostly fat.
• This fat layer is called the myelin sheath and it
insulates and protects the axon (some axons
are nearly one meter – about 3 feet – long).
Types of neurons
• Sensory (afferent) neurons carry nerve
impulses from body parts into the brain or
spinal cord. They have receptors at the end of
their dendrites
• Motor (efferent) neurons carry nerve
impulses out of the brain or spinal cord to
effectors
• Interneurons (mixed nerves) lie within the
brain or spinal cord and link other neurons
Cell membrane potential
• The surface of a cell membrane is usually
electrically charged, or polarized, with respect
to the outside
• This polarization is due to an unequal
distribution of positive and negative ions
between sides of the membrane
• Important in the conduction of muscle and
nerve impulses
• Because of the active transport of sodium and
potassium ions, cells throughout the body
have a relatively greater concentration of
sodium ions (Na+) outside and a relatively
greater concentration of potassium ions (K+)
inside
• The cytoplasm of these cells has many large
negatively charged particles that cannot
diffuse across the cell membranes.
• Sodium and potassium ions follow the laws of
diffusion and show mvmt from high to low
concentration as permeability permits
• The difference in electrical charge between two
regions is called a potential difference and in a
resting nerve cell this is called resting potential
• When permeability changes in the region of cell
membrane that is being stimulated, channels
open and allow sodium to diffuse inward
• This mvmt is aided by the negative electrical
condition on the inside of the membrane, which
attracts the positively charged sodium ions
• Now the membrane loses its negative charge and
becomes depolarized
• At almost the same time, membrane channels
open and allow potassium ions to pass through
and diffuse outward so the inside again becomes
negatively charged and repolarized
• This rapid sequence of depolarization and
repolarization takes about one-thousandth of
a second and is called Action Potential
• www.youtube.com/watch?v=yQ-wQsEK21E
• A wave of action potentials moves down the
fiber to the end. This delivers a nerve impulse
The Synapse
• Neurons have the ability to conduct nerve
impulses very quickly, but how does one cell
communicate with another cell?
• Adjacent neurons communicate by releasing
chemicals across tiny gaps that separate them,
called synapses (synaptic cleft)
• The chemicals, known as neurotransmitters,
are released by a neuron when a nerve
impulse reaches its distal end
• The neurotransmitters then diffuse across the
synaptic cleft to contact the adjacent cell
• Contact with the next neuron may stimulate it
to trigger a nerve impulse, or it may inhibit it
(pain meds)
The Reflex Act
• The simplest type of nervous response is the
reflex act, which is unconscious and
involuntary <examples>
• Every reflex act is preceded by a stimulus (any
change in the environment)
• Special structures called receptors pick up
these stimuli
• Afferent neurons take the message to the
interneuron
• The interneuron interprets the info and
decides the action
• The efferent neuron takes the message to the
responding organ (effectors)
• Reaction to the stimulus is called the response
and if there is mvmt the muscles are the
effectors
The Reflex Arc
• A simple reflex is one in which there is only a
sensory nerve and a motor nerve involved
• Classic example is the knee-jerk reflex
• Also the withdrawal reflex
Central Nervous System
• The “central station” for incoming and
outgoing nerve impulses
• Includes the brain and spinal cord
• Both are protected by bones (cranium and
vertebral column) and a thick set of
membranes called the meninges (located
between the soft nervous tissue and the hard
bones, and is several layers thick)
Meninges
• Outer layer is tough, white fibrous connective
tissue and cantains many nerves and bv - dura
mater
• As it continues down the spinal cord it does not
attach directly to the vertebrae but is separated
by an epidural space (filled with adipose and
connective tissue to pad around the spinal cord)
• Middle layer is thin weblike arachnoid (lacks
nerves and bv)
• Inner layer is thin and plastered to the nervous
tissue pia mater (contains many nerves and bv)
• Between middle and inner is subarachnoid
space that is filled with a slightly yellowish
fluid called cerebrospinal fluid (CSF)
• CSF is also found in the four ventricles of the
brain (small spaces “little bellies” within the
brain’s center)
The Spinal Cord
• The spinal cord extends from the medulla of
the brain (at the foramen magnum) down the
back to L1 and L2 (about 18 inches) where it
“splits”
• It is divided into 31 segments and each
segment includes a pair of spinal roots, which
form spinal nerves as they leave the spinal
cord (so there are 31 pairs of spinal nerves)
• It consists of both white and gray matter –
gray is in the center of the cord, white is on
the outside (opposite of the brain)
• The function of the spinal cord is a reflex
center and two-way conduction pathway
(ascending and descending tracts)
The Brain
• The brain receives sensory information,
interprets and integrates this info, and
controls muscle and glandular responses
• Its nerve impulse activity also provides your
memory, thoughts, dreams, and personality
• It receives a large blood supply to fuel its
constant activity (10s, 30s, few mins, 4-8mins)
• Located in the cranial cavity and weighs 3 lbs
• The brain is composed of both gray and white
matter
• Gray matter is mainly neuron cell bodies and
dendrites (integrative centers)
• White matter is the axons covered with myelin
(carries nerve impulses)
• The brain includes four main parts: cerebrum,
cerebellum, diencephalon, and brain stem
The Cerebrum
• Consists of two large masses called cerebral
hemispheres (mirror images of each other)
• The hemispheres are connected by a deep
bridge of nerve fibers called the corpus
callosum
• The surface has many ridges, or convolutions
(gyri), separated by grooves. A shallow groove
is a sulcus, and deep groove is a fissure.
• The most important functional part of the
cerebrum is the cerebral cortex, which is an
outer fringe of gray matter (contains nearly
75% of all the neuron cell bodies in the
nervous system)
• The cerebral cortex is divided into functional
zones known as lobes
The Lobes
• Frontal lobe – controls voluntary muscles (R to
L and L to R), emotions, personality, morality,
intellect, speech (Broca’s area – you know the
word but can’t say it)
• Parietal lobe – receives and interprets nerve
impulses from the sensory receptors for pain,
touch, heat, and cold. Also helps in
determining distances, sizes, and shapes
• Occipital Lobe – visual area, controlling
eyesight
• Temporal Lobe – auditory area (upper) and
olfactory (smell) area is anterior
Hemisphere Dominance
FYI
• 90% of the population is left brain dominant –
others are right or both equal
• Deep within each cerebral hemisphere are
several masses of gray matter called basal
ganglia. Their neuron cell bodies serve as relay
stations for motor impulses originating in the
cerebral cortex and passing into the brain stem
and spinal cord. They send impulses that inhibit
motor functions using dopamine.
Diencephalon
• Located beneath the cerebrum and anterior to
the cerebellum
• Means “double brain” and it contains 2 impt
areas
• 1. Thalamus – inner chamber, a relay station
that redirects nerve impulses to and from the
cerebrum
• 2. Hypothalamus – located below thalamus,
regulates involuntary activites (list of 7)
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1. heart rate and BP
2. body temp
3. water and electrolyte balance
4. control of hunger and body weight
5. control of mvmts and gland secretions
6. production of substances that stimulate the
pituitary gland to secrete hormones
• 7. sleep and wakefulness
Brain Stem
• A bundle of nervous tissue that connects the
cerebrum to the spinal cord
• Parts include the midbrain, pons, and medulla
oblongata
• Midbrain serves as a reflex center
• Pons relays impulses to and from the medulla
oblongata and cerebrum and regulates the
rate and depth of breathing
• Medulla oblongata connects the brain and
spinal cord and controls vital visceral activities
– Heart rate, contract and dilate certain blood
vessels, regulates breathing
Cerebellum
• Located below and posterior to the cerebrum
• Coordinates muscle responses, manages
equilibrium, and maintains muscle tone
The Peripheral Nervous System
• Consists of nerves that branch out from the
CNS and connect it to other body parts
– Cranial nerves, from the brain
– Spinal nerves, from the spine
Also includes ganglia and sensory receptors
• 12 pairs of cranial nerves come from the
underside of the brain
• Most are mixed nerves, but some are only
sensory (smell and vision)
• <see list in handout>
• 31 pairs of spinal nerves originate from the
spinal cord
• They are mixed nerves that provide two-way
communication between the spinal cord and
the neck, trunk, and limbs
• 8 pairs cervical, 12 pairs thoracic, 5 pairs
lumbar, 5 pairs sacral, 1 pair coccygeal
• A network of spinal nerves combine to form a
plexus except in the thoracic region
• Ganglia are clusters of neuron cell bodies that are
outside the brain and spinal cord, appearance is a
swelling or a knot
• These are centers where nerve impulses are
passed from one neuron to another across a
synapse
• Sensory receptors are nervous structures that
respond to temperature, pressure, touch, or pain
• They are embedded in the skin, but also within
the walls of some visceral organs
The Autonomic Nervous System
• The portion of the PNS that functions without
conscious effort
• Controls visceral functions to maintain
homeostasis
• Further divided
– The sympathetic system consists primarily of 2
cords, beginning at the base of the brain and
proceeds down both sides of the spinal column
and has nerves to all the vital internal organs
– This is referred to as the “fight or “flight” system
– When the danger passes, the parasympathetic
system will help restore the balance to the body
by the vagus and pelvic nerves
– Both systems are strongly influenced by emotions
– This leaves us with high levels of “stress
hormones”