Download 1. nervous system

1. NERVOUS SYSTEM
a memory (the smell of a meal), or it could be
discarded as not important (the feeling of a breeze).
FUNCTION
Control of muscles and glands
After being stimulated through the nervous system,
most skeletal muscles contract, causing some type
of body movement. Some of this activity is done as
a reflex (to maintain standing position) while others
are done voluntarily (running to catch a pray).
Smooth muscles contract in response to either
hormonal action (smooth muscles) or in response to
direct nervous impulses (muscles in the wall of blood
vessels). The activity of many secretory glands
(sweat, salivary, digestive) is also controlled by the
nervous system.
The major functions of the nervous system can be
summarized as follows (Figure 1-1).
FUNCTIONS OF THE NERVOUS
SYSTEM
Homeostasis
Most of the activities of the nervous system are
aimed at maintaining homeostatic conditions. This
requires the synchronization of the trillions of cells
that an animal has. The nervous system coordinates
this.
Mental activity
Thinking, storing and recalling memories, generation
of emotional responses, the state of awareness or
consciousness are all taking place within the brain.
Figure 1-1. Role of the nervous system
Sensory input.
Specialized cells located throughout the organism
detect multiple signals from both, the internal and
the external environment and they are sent for
processing at different levels of the nervous system.
Some inputs result in sensations that are
consciously recognized or that they become aware
of, such as images that are seen or food that is
tested. Other inputs are dealt within an unconscious
level, such as maintaining blood pressure within
normal range or moving food through the digestive
tract and secreting the proper digestive enzymes.
Integration
Within the central nervous system (brain and spinal
cord) the sensory inputs are processed and an
outcome is decided upon. The outcome could be an
active response (moving away); it could be stored as
V BS 121 Physiology I
1
STRUCTURE AND DIVISIONS OF
THE NERVOUS SYSTEM
The nervous system is divided into two main
components, (Fig. 1-2) the central nervous system
(CNS) and the peripheral nervous system (PNS).
The CNS is made up of the brain, which is located in
the skull, and the spinal cord that is located within
the vertebral canal.
The peripheral nervous system is made up of all the
components located outside the CNS. These include
ganglia, which are groups of neuron's cell bodies
located outside the CNS; plexuses, which are large
networks of axons and neuron’s cell bodies located
outside the CNS; sensory receptors, which are
either specialized cells connected to afferent
Class of 2016
PERIPHERAL N. SYSTEM
NERVOUS SYSTEM
A Sensory (Afferent)
B Somatic (Efferent)
Figure 1-2.
Components and divisions of the
nervous system
neurons or the nerve ending of efferent neurons;
and nerves, which are made of axons and their
respective sheaths.
The PNS is divided into a set of components that
bring information into the CNS, which is the sensory
or afferent division. The set of components
sending instructions, in the form of action potentials,
to the different parts of the body is called the motor
or efferent division.
C Autonomic (Efferent)
The sensory division is made of neurons which have
their bodies located in the dorsal root ganglion; they
are the receptor or they connect to the specific
receptor to bring the information in the form of an
action potential to the CNS (Fig. 1-3A).
The motor or efferent division is further divided into
the somatic nervous system and the autonomic
nervous system (ANS). The somatic nervous
system specifically connects the CNS with skeletal
muscles (Fig. 1-3B). It is characterized by its cell
bodies, which have all of its neurons, located within
the CNS, specifically in the spinal cord. Their axons
leave the spinal cord through the ventral root of a
V BS 121 Physiology I
2
Figure 1-3. Components and divisions of the
peripheral nervous system
Class of 2016
given spinal nerve and connect directly with a given
skeletal muscle through a synapse.
All the activity of the ANS takes place in an
involuntary or subconscious manner. The ANS takes
care of the ongoing functioning of the organism such
as respiration, digestion, cardiac function, etc.
that functionally act as a single axon. The branch
that reaches the periphery has receptor like
dendrites, which generate action potentials that are
transmitted through the second branch to the CNS.
The body of the monopolar neurons resides in
dorsal root ganglia and is a part of the sensory or
afferent division of the peripheral nervous system.
Two neurons complete the connection between the
CNS and the effector organ. The first
neuron is located within the CNS and
its axon leaves through the ventral root
of a spinal nerve and synapses with
another neuron, the second motor
neuron whose body is located in an
autonomic ganglion and the axon
synapses with the effector organ (Fig.
1-3C).
The ANS in turn is divided into two
large divisions, the sympathetic and
parasympathetic divisions and an
entirely separate system called the
enteric nervous system. You have
dealt with the ANS earlier and will deal
with the enteric nervous system next
semester. Now we will concentrate in
Figure 1-4. Structure of a neuron
the central nervous system.
The basic unit of the nervous system
is the neuron. The generic structure of
a neuron is presented in figure 1-4.
There are three basic types of neurons
(Fig, 1-5). The multipolar neurons are
characterized for having a highly
branched dendritic terminal and a
single axon capable of interacting with
a very large number of other neurons.
These are found principally in the
central nervous system (CNS). The
bipolar neurons have one dendritic
process and one axon. They are part
of a sequence of neurons that convey
action potentials to the CNS. The
monopolar neurons consist of a
single process leaving the cell body, Figure 1-5. Types of neurons
which branches into two components
V BS 121 Physiology I
3
Class of 2016
CENTRAL NERVOUS SYSTEM
GLIAL CELLS OF THE CNS
The central nervous system is made up of the brain
and the spinal cord. The CNS is characterized by
being covered or constrained by a multilayer
protective membrane of connective tissue called the
meninges. The CNS is supported by a variety of
cells, called glial cells, which perform very specific
functions to protect, or to enhance its functioning
(Fig. 1-6). The supportive activities are of various
types such as making available oxygen and
nutrients. They also provide a physical support to
the neurons to maintain them in place and in some
cases to insulate them. The last known role of these
supportive cells is to scavenge any debris of dead
neurones and deal with inflammatory processes.
Figure 1-6.
Supportive cells of the CNS and
their respective roles
Astrocytes, ependymal and microglia cells contribute
to the protection of the CNS by establishing a blood
microglia are the smallest and in the CNS play a
barrier. This barrier forms the choroid plexus which
similar role to that of macrophages in circulation.
secretes cerebrospinal fluid and cleans inflamed
They participate in immune protection of the CNS
tissue through phagocytic
action,
respectively.
Astrocytes in particular
ANATOMY OF THE BRAIN
provide physical support to
neurones in the CNS and
form the physical barrier
making the blood-brain
barrier. They also provide the
conduit for the transfer of
nutrients from circulation to
the neurones and contribute
to removing debris through
phagocytosis. Ependymal
cells are a type of epithelial
cells lining the ventricles of
the brain which are filled with
CSF. The ependymal cells
are responsible for the
movement or circulation of
the CSF and for their
production. Ependymal cells
located in the choroid plexus
are specifically designed to
produce CSF. Of all the Figure 1-7 Structures of the brain
supportive or glial cells the
V BS 121 Physiology I
4
Class of 2016
preventing
invasion
by
microorganisms.
Oligodendrocytes enhance the function of the CNS
by providing myelin sheaths to axon bundles.
Brain
The brain is divided into the telencephalon or
cerebrum, diencephalon, cerebellum, brainstem and
reticular formation (Figs. 1-7; 8).
The telencephalon or cerebrum is the largest
component of the brain. It is divided into the cortex
(grey matter, because it is made up of unmyelinated
neurons) and the cerebral medulla (white matter,
because it is made up of myelinated neurons). The
cortex is responsible for memory, awareness,
perception, language, consciousness and thought.
The medulla is made up of nervous tracts
connecting different areas of the brain and the rest
of the CNS.
parietal lobe is mostly in charge of sensory
information, except for special senses such as
vision, which is processed in the occipital lobe, as
well as smell, and hearing that are processed in the
temporal lobe.
The diencephalon is located between the brainstem
and the cerebrum. It contains the thalamus, which
synapses all sensory information (except olfactory)
before sending information to the different parts of
the cortex, thus it is considered the sensory relay of
the brain. Emotions such as rage or fear are
processed in the thalamus thus influencing and
integrating the appropriate sensory information and
determining mood.
The hypothalamus is the lowest component of the
diencephalon. It contains a variety of nuclei and
STRUCTURES OF THE BRAIN AND THEIR FUNCTION
Figure 1-8 Role of the different areas of the brain
The brain also contains two hemispheres and
several lobes, each of which has a specific function.
The frontal lobe is mostly concerned with
motivation, aggression, mood, voluntary motor
activity and some of the senses of smell. The
V BS 121 Physiology I
5
nervous tracts. Some of these are involved in
responding to olfactory stimulus but the majority are
involved in controlling and regulating the endocrine
system in conjunction with the hypophysis. Through
these nuclei, the animal controls its body
temperature, thirst, hunger, sex drive, blood volume,
Class of 2016
renal function and many productive functions such
as growth and milk production. The epithalamus,
which includes the pineal gland influences, several
biological rhythms including those related to
reproduction in seasonal breeders. The epithalamus
also has the habenular nucleus that participates in
innate response to odours.
spinal cord carries out a significant amount of
regulatory activity through reflexes. It extends from
The cerebellum interacts with the brainstem and
other components of the CNS. The most complex
cells in the cerebellum are Purkinje cells. These are
capable of receiving around 200,000 synapses.
The cerebellum is responsible for eye movement,
posture, locomotion and fine motor coordination.
Complex movements are learned in collaboration
with the frontal lobe of the cerebral cortex.
The brainstem is made up by the midbrain, pons
and medulla oblongata. The midbrain controls the
movement of the head, body and eyes in response
to sound, texture or sight. The pons serves as a
relay of information between the cerebrum and the
cerebellum. Furthermore, integrated within this area
are some of the components of the sleep and
respiratory center of the medulla oblongata. Within
the medulla, different nuclei control different
aspects, such as the conscious control of skeletal
muscles that permit the control of balance. In the
medulla, the nerve fibres cross from one hemisphere
of the brain to the opposite side of the PNS.
The reticular formation is made of several nuclei
distributed throughout the brainstem. These nuclei
regulate or control several cyclic activities such as
wake-sleep.
The brain gives rise to 12 pairs of cranial nerves;
two connect to the cerebrum, nine to the brainstem
and one to the spinal cord. The function of the
cranial nerves can be sensory (afferent), somatic
motor (efferent) or parasympathetic (efferent).
Spinal cord
The spinal cord plays the fundamental role of linking
the brain to the peripheral nervous system. The
V BS 121 Physiology I
6
Figure 1-9. Synaptic connection
the foramen magnum where it joins the brain to the
second lumbar vertebra.
The names of the sections of the spinal cord
correspond to the names of the vertebra from which
the corresponding nerves either leave or enter.
Therefore there is a cervical, thoracic, lumbar, and
sacral segment of the spinal cord. A total of 31 pairs
of spinal nerves connect the spinal cord with
different organs and tissues of the body. There are 8
cervical, 12 thoracic, 5 lumbar, and 6 sacral which
include the coccygeal nerves.
PERIPHERAL NERVOUS SYSTEM
Sensory division of the peripheral
nervous system
All signals conveying information from the internal or
external environment are generated by a stimulus
which can be light, touch, heat, vibration, chemical,
etc. The stimulus is sensed or detected by a
receptor and, if it is of enough strength, it will be
converted to an action potential and sent to the CNS
where it can reach different levels (spinal cord, mid
brain, cerebrum). Once the action potential reaches
Class of 2016
its destination it is processed to determine if and
what type of response is warranted —if it can be
stored as a memory or if it is simply to be ignored.
The transmission of an action potential from one cell
to another is carried out by a synaptic connection
(Fig. 1-9).
A couple of conditions have to be fulfilled for a
stimulus to trigger an action potential. The stimulus
has to be appropriate for the receptor to be
stimulated (a thermo receptor does not respond to
pressure, it only responds to certain temperatures or
changes in temperature). Secondly, it has to be
strong enough to reach a threshold, which causes
the depolarization. There are a variety of receptors
stimulus is not “felt”. If the stimulus is strong enough
it will generate a depolarization, which reaches the
threshold and triggers an action potential. If the
graded potential is not strong enough to trigger an
action potential, it will spread over the plasma
membrane with decreasing strength and decay
overtime until it disappears. It is possible, however,
that a local or graded potential is followed by a
second stimulus before the first potential has
decayed. This causes the summation of the
stimulus and may result in a local depolarization that
will trigger an action potential.
The organism is able to disregard any stimulus that
is continuous but does not present a threat to the
CLASSIFICATION OF RECEPTOR TYPES
Receptor type
Mechanoreceptors
Meissner corpuscle
Hair follicle receptor
Pacinian corpuscle
Merkel disk
Ruffini end organ
Free nerve ending
Nociceptors
Sensation
Touch, proprioception, pressure
Trigger
Compression
Stroking
Stroking
Vibration, proprioception
Pressure, texture
Skin stretch
Itch
Pain
Irritation
Free nerve ending
Thermoreceptors
Temperature
Temperature
Smell, taste
Binding of molecule
Sight
Light
Free nerve ending, cold, warm
Chemoreceptors
Special
Photoreceptors
Special
Figure 1-10. Receptor types, the sensations that they identify with and their triggering stimulus
capable of detecting different types of stimulus (Fig
1-10).
A single stimulus initially causes a graded potential
or local potential (Fig. 1-11). A graded potential
ranges from very weak to very strong. A very weak
graded potential is unable to reach the threshold, in
which case the signal is not transmitted and the
V BS 121 Physiology I
7
wellbeing of the animal. Therefore, stimuli that
initially may cause awareness of their existence,
over time disappear as such. An example is a dog’s
collar. The first time you put it on the animal it is
uncomfortable and the dog tries to remove it. After a
period of time the animal does not feel it any longer.
The same happens with rings on fingers or with
Class of 2016
sounds that eventually become “white
sound”. In summary the organism
adapts to these stimuli.
As a
protection mechanism, however, the
organism does not adapt to any
noxious stimuli.
Specifically pain
receptors keep sending action
potentials as long as the noxious
stimulus persists, making the animal
continuously aware of a problem
regardless of the duration.
GRADED POTENTIALS
Intensity. The intensity of a given
stimulus is determined by the
frequency code of stimulation and the
population code of receptors in the
area (Fig. 1-12). The frequency is Figure 1-11. Events taking place during a graded potential and
determined by the number of action its consequences
potentials taking place in a given unit
of time and it is quantified in Hertz (Hz). One Hz is
that more than one is stimulated, thus the stimulus is
one stimulus per second. Therefore if a receptor
perceived in a stronger manner. The lips are very
sensitive because they have many receptors but the
arms have less receptors, thus, they are more
difficult to stimulate.
STIMULUS STRENGTH OR
INTENSITY
Sensitivity. The sensitivity of a tissue to stimuli
depends on the number of receptors in the area
(population code) and the size covered by each
receptor. Based on these the ability of the tissues to
discriminate the number and location of stimuli
varies. You can touch your back with a two-point
compass separated by 3 or 4 cm but you feel only
one contact point. On the other hand you can touch
your lips at a distance of 2 mm and you are able to
recognize that there are two points of contact.
Figure 1-12. Factors determining stimulus
strength
fires at a rate of 4 Hz it is being stimulated four times
stronger than a receptor firing at a rate of 1 Hz.
The population code refers to the number of
receptors in a given volume or area of tissue. The
more receptors there are, the larger the possibility
V BS 121 Physiology I
8
Integration or summation of graded
potential. If the axons of several presynaptic
neurons deliver their action potential to a single
postsynaptic neuron, this is called spatial
summation. If an axon fires repeatedly (faster than
the time required for the graded potential to
disappear) this will trigger temporal summation. In
both cases it is possible that, as a result of the
summation, the graded potential reaches a threshold
capable of generating an action potential in the
postsynaptic neuron.
Class of 2016