Download Chapter 12- Intro to NS

Fundamentals of Nervous
System and Tissue
Chapter 12
Anatomy 32
Simplyfied schema of the nervous system
Basic Division of the Nervous System
The nervous system is designed to gather
information from the environment (sensory
input), process it (integration), and stimulate
the body to respond appropriately (motor
output). All of this occurs through the
interaction of the central nervous system
(CNS) and the peripheral nervous system
(PNS)
A. Central Nervous System (CNS)- the brain
and spinal cord make up this branch of the
nervous system, both are protected by
bone.
B. Peripheral Nervous System (PNS)- all the
nerves that exit the spinal cord and
innervate the body make up the PNS. This
branch has two subdivisions
1. Autonomic NS- this branch sends
impulses to control involuntary actions
2. Somatic NS- this branch sends
impulses to control voluntary
actions
C. When a stimulus is detected by the NS the impulse is sent to the brain and
is called afferent signal. Only the sensory organs are able to send this type
of impulse. The brain receives the information and processes it; this is
called integration. When it sends out a respond, the impulse sent to the
body (ex. muscles) is called an efferent signal. This is possible only
through motor neurons.
D. Sensory input and motor output is divided according to the region
they intervate:
1. Somatic motor: efferent impulses that reach skeletal
muscle (voluntary movement).
2. Branchial motor: The branch that innervates
pharyngeal muscles (ex. those use to swallow) is called
3. Visceral motor: efferent impulses that reach the
internal organs and regulates involuntary muscle
contractions (makes up the autonomic NS). Thisi ncludes the
heart and smooth muscle.
4. Somatic sensory: afferent impulses that originate
from non-visceral areas such as skin and muscle.
Proprioceptive senses sends sensory information
from muscle, ligaments, tendons, and indicate body
position. Special somatic senses are localized
receptors such as those of the ear and eyes.
5. Visceral sensory: afferent impulses generated by
internal organs, a specialized visceral sensory organ
would be the taste buds.
II. Nervous TissueNervous tissue develops from the embryonic neural tube and neuro crest.
Two types of cells form: neurons and glial cells (supporting cells)
A. The Neuron- these types of cells are excitable and can send an impulse
(electrical signal). Neurons have three major parts: cell body, dendrites,
axon. These cells live for many years, do not under mitosis, and are highly
dependant on oxygen due to a high metabolic rate.
1. The cell body (soma)- contains a nucleus, cytoplasm,
mitochondria, and a large number of rough ER (chromatiphilic
bodies). They are usually found within the CNS and in the PNS
they are called ganglia (ganglion)
2. Neuron processes- extensions of the cell body, may be termed
dendrites or axons. Dendrites are receptive ends that allow signals to
travel towards the cell body. Axons extend out of one side of the cell
body called the axon hillock, it may be several feet in length and
carry afferent signals. Collateral axons result when axons branch. At
the end of an axon there are multiple branches called the terminal
branches that end in knobs called terminal axons. The length of the
axon is surrounded by schawnn cells that together make the myelin
sheath. In between each schawnn cell is a gap called the node of
Ranvier.
3. Synapses- the connection that allows one neuron to communicate with
the next. If the axon makes a synapse with a dendrite it’s called
axodendritic synapse. If an axon connects with a cell body it’s called
axosomatic synapse. If a synapse exists between two axons it’s called
axoaxonic synapses. The cell conducting the impulse towards the
synapse is called presynaptic neuron and the one that receives the
impulse and sends it away from the synapse is called postsynaptic
neuron.
Synaptic vesiclesmembrane bound sacs
containing
neurotransmitter
Neurotransmitter- chemical
substance that allows an
impulse to “jump” from
neuron to neuron.
Neurotransmitter vary
depending on the type of
signal that’s sent
Synaptic cleft- the gap that
separates the plasma
membranes of the two
neurons
4. Impulse-an electrical signal is
sent down the plasma
membrane of a neuron. When
an impulse reaches the synaptic
end of a neuron the following
occurs:
a. The impulse signals the
release of synaptic vesicles to
fuse with the cell membrane with
the presynaptic membrane at
presynaptic density.
b. Neurotransmitters that
have been exocytosed enter the
synaptic cleft and bind to the
postsynaptic membrane at the
postsynaptic density.
c. The postsynaptic
membrane changes from its
polarized state (inside negative,
outside positive) to being
depolarized (charges switch
sides).
d. The depolarization
continues down the neuron until
the impulse reaches the
next synaptic site.
5. Types of potentials- the type of potential is defined by the activity
stimulated
a. Resting Potential- the neuron is not sending any type of
impulse. The arrangement of ions makes the inside relatively more
negative than the outside.
b. Action Potential- also known as an impulse, the cells
depolarizes to the extent that the inside is more positive than the
outside. The impulse travels down the axons and as it
leaves the axon
returns to its resting potential.
• c. Graded Potential- stimulus received by the dendrites or cell body
determine whether an impulse will travel down the axon. A graded
potential is a localized depolarization that travels towards the axon
hillock and initiates the impulse.
• d. Synaptic potential- the potential changes at the synapse
determine if the postsynaptic neuron sends an impulse or not. An
excitatory synapse changes the potential so the postsynaptic neuron
can send an impulse. An inhibitory synapse stops the travel of an
impulse.
6. Classification- neurons are classified based on the location of the cell body in
relationship to the dendrites and axon. Different types of neurons carry out
different types of functions; neurons are also classified by function.
a. Multipolar neuron- about 99% of neurons are multipolar- meaning that
the cell body has multiple extension/process
b. Bipolar neuron- these are specialized sensory neurons found in the eye,
ear, and nose. There are only two extensions/process on opposite sides of
the cell body.
c. Unipolar neuron-also known as sensory neurons. A short extension off
the cell body connect the two process that leads to the axons and dendrites.
d. Sensory neuron (afferent)- These neurons have receptors in the PNS and
send signals towards the CNS. The receptors are cells that capture a stimuli
and transfer a signal onto the cendrite of the sensory neuron. The dendrite
then connects tot eh cell body that sends an axon into the CNS (spinal
cord). The bodies of sensory neurons are in ganglia outside the CNS.
peripheral process going to receptors in the body (axon and dendrites).
e. Motor neuron (efferent)- these are multipolar neurons and have cell bodies
in CNS and connect in the body with effector cells (muscles/glands).
f. Interneuron (association number)- multipolar neurons in the CNS that
“link” sensory and motor neurons, relays messages to and from the
brain and process information.
Neurons vary based on length of the axon
and on branching of dendrites. The more complex
the brandhing, the greater the synaptic input.
B. Supporting Cells- these cells are responsible for insulating the
neurons and preventing signal interference.
1. Supporting cell of Central Nervous system- Neurolia (glial
cells) are smaller than neurons and are more numerous (in the
brain) and have mitotic ability.
a. astrocytes- star shaped, control ionic environment (prevents
impulse interference), recapture and recycle neurotransmitters.
b. microglia- small, least abundant, act as phagocytes
c. epedymal- cells that make a simple epithelium lining the
central cavity of spinal cord and brain and help to circulate the
cerebrospinal fluid.
d. oligodendrocytes- produce myelin sheaths of the CNS
neurons.
2. Supporting cells of Peripheral Nervous system- the glial cells
of the PNS are :
a. satellite cells- surround cell bodies in the ganglion
b. Schwann cells- wrap around axons to create the myelin
sheath that assist in speeding up the rate at which the impulse
travels.
3. Myelin sheaths- these sheaths form an insulating layer to
prevent leakage of electrical current. The membrane tightly wraps
around the axon and the nucleus and cytoplasm together are called
neurilemma. The impulses travel by jumping from one node of
Ranvier to the other. Unmyelinated axons move signals slowly.
III. Nerves- these are
bundles of axons
(nerve fibers) that
are myelinated or
unmyelinated. Each
axon is surrounded
by an endoneurium,
groups of nerves are
bundles into nerve
fascicles surrounded
by perineurium and
the whole nerve is
surrounded by
epineurium. *Know
the difference
between neuron,
nerve fiber, and
nerve.
IV. Basic Neural Organization of the Nervous System
A. Reflex Arcs-Reflexes are involuntary unlearned
actions that stimulate the somatic muscles or viscera to
react quickly in response to a stimulus. There are five
steps to a reflex:
1. The receptor in the PNS perceives a stimulus and
sends a signal to the CNS (spinal cord) through
the sensory neuron (afferent).
2. The impulse reaches the integration center in the
CNS which may be simple called monosynaptic
(no interneuron synapse betwee motor and
sensory neurons), or very complex called
polysynaptic (one or more interneuron synapses).
3. The motor neuron is stimulated to send an
efferent impulse that reaches the effector.
4. The effector may be skeletal muscle (contracts), a
gland (secretes),
or visceral muscle (contraction).
B. Simplified design of the nervous system- The NS has areas called white
matter which contain multiple myelinated axons, the areas of gray matter
contain unmyelinated neuron parts such as cell bodies, dendrites, and
unmyelinated axons. The NS is arranged so that motor neuron run out of the
CNS on the ventral side and sensory neurons enter on the dorsal side. The cell
bodies of sensory neuron are outside the CNS in ganglion while the motor and
interneuron cell bodies are in the CNS gray matter. Interneurons process information received by sensory neurons and transmit signals to and from the brain.
V. Disorders of the nervous systemA. Multiple sclerosis- this is a disease in which
small areas in the brain and spinal cord
containing myelinated neurons are destroyed. It
may be an autoimmune disease that causes
repeating periods of disability and recovery.
During disability the neurons are destroyed but
during recovery the immune system may stop
attacking the nervous system and the nerves
regenerate. MS affects more women than men
and it is influenced by genetic and
environmental factors.