Download Lecture 12 - Taft College

12-The Nervous System
Taft College Human Physiology
Introduction To The Nervous System
• The nervous system is a ‘wired’ system with
discrete pathways (nerves) and local actions.
– The effects of nervous stimulation are usually
immediate and short lived.
• E.g. muscle movement
• The nervous system in conjunction with the
endocrine system is responsible for coordination
of all of the other human body systems.
Introduction To The Nervous System
• The endocrine system is specialized to
control activities that require duration
(time) not speed.
• Hormones are secreted into the
bloodstream and have wide ranging
effects on target cells that contain
hormone receptors.
• E.g. growth patterns, reproduction cycles,
metabolism, water balance
Functions of the Nervous System
1.
Sensory: via afferent neurons
–
Millions of sensory (afferent) receptors monitor changes both inside and
outside the body.
The changes are called stimuli and the information gathered sensory
input.
–
•
Examples:
–
–
2.
Internal: stretching of stomach, pH changes in blood.
External: Smells, sights, sounds, pressure, pain.
Integration: via interneurons
–
3.
It analyzes sensory inputs, stores some information, and makes
decisions about what should be done based on the information.
Motor: via efferent neurons
–
–
It may cause a response by activating effector (target )organs:
causing muscles to contract and glands to secrete.
This response would be called motor output.
3 Types of Neurons
•
•
•
Afferent (sensory)
Efferent (motor)
Interneurons (“linker”)
Functions of the Nervous System
4.
•
•
•
•
•
Conceptual thought
Capacity to record, store, and relate information
received and use it at a later date.
Like on Exam 2 coming up shortly!!!
A high level of self awareness.
The human brain represents the peak of
development of animal brains.
This trend has enabled such adaptive capacities
as learning, planning, speech, and language.
Classification of the Nervous System
• It is very important that you understand
which divisions of the nervous system are
anatomical structures (i.e. a structure you
would actually see during the course of a
dissection or operation) and which
nervous system terms are based on
function that is, how it works.
Anatomical Classification of the N.S.
Nervous System
Central Nervous System
1. Brain
2. Spinal Cord
Peripheral Nervous System
1. Nerves
2. Ganglia
3. Sensory Receptors
Anatomical Classification of the N.S.
• From an anatomical perspective the nervous
system has 2 major divisions:
• 1. The Central Nervous System = CNS.
• 2. The Peripheral Nervous System =PNS
•
(drawing on board)
• The Central Nervous system consists of the
brain and the spinal cord and nothing else!
• Any other nervous system structure that
connects to the brain or spinal cord would be a
part of the Peripheral Nervous system.
Anatomical Classification of the N.S.
• The 3 major components of the peripheral
nervous system are:
• 1. Nerves
• 2. Ganglia
• 3. Sensory receptors
3 Major Components of the Peripheral N.S.
1. Nerves – A nerve is a bundle of neurons
= nerve cells.
• Nerves can be motor, sensory or mixed.
• 2 important categories of nerves that are
a part of the PNS:
–
–
Cranial nerves (12 pr.) that branch to and from the
brain.
» You need to know these for exam 2. See the
detail in the course outline.
» For each nerve, know number, name, whether it is
sensory or motor or both (mixed), and it’s function.
Spinal nerves (31 pr.) that exit the spinal cord
bilaterally from between each vertebrae.
Cranial nerves
#
I
II
III
IV
V
For each cranial nerve you must know the Roman numeral, name,
whether it is sensory (S) or motor (M) or both (B) (mixed), and
function.
Name
Olfactory
Optic
Oculomotor
Trochlear
Trigeminal
S,M, or B
S
S
M
M
B
VI Abducent
VII Facial
M
B
VIII Vestibulocochlear
IX Glossopharyngeal
S
B
X
Vagus
B
XI
Accessory
M
XII Hypoglossal
M
Function
smell
vision
eye muscles
eye muscles
S- face & scalp
M- muscles of mastication
eye muscles
S- tongue & taste
M- facial muscles
hearing & equilibrium
S- tongue & taste
General sensation of pharynx
M- pharyngeal muscles(swallowing)
S- visceral sensation
M- visceral movement
swallowing, head & shoulder
movement
tongue- speech & swallowing
3 Major Components of the Peripheral N.S.
1.
2.
Nerves
Ganglia – aggregations of nerve cell bodies.
•
E.g. Dorsal root ganglia.
3.
Sensory receptors (sense organs)
– Serve to give the body information about the
immediate environment, both internal and external.
– They include the special sense organs involved in
your sense of taste, touch, sight, hearing, or smell.
Functional Classification of the NS
Nervous System
CNS Integration
Afferent Division
(sensory division)
Efferent Division
(motor division)
Involuntary or Voluntary Control
Autonomic
(involuntary)
Sympathetic
“Stress”
Afferent: → brain
Efferent: brain →
Somatic
(voluntary)
Parasympathetic
“Calm”
•
Afferent Division
•
•
The afferent division of the nervous system is responsible for
carrying information toward the brain.
The afferent division is also called the sensory division as it picks
up information about the environment and takes that information
to the brain (CNS).
•
Efferent Division
•
The efferent division of the nervous system is responsible for
carrying information out and away from the brain.
The efferent division is also called the motor division
The efferent division is further divided into 2 parts based on whether
the information coming from the brain is under voluntary or
involuntary control.
•
•
1.
•
•
2.
•
•
Somatic Nervous System = Somatic Division
The somatic portion of the efferent division is under voluntary control and
is composed of somatic motor neurons.
It is the part of the nervous system that serves to innervate skeletal
muscle tissue.
Autonomic Nervous System (ANS)
You do not have control over the information that passes through the ANS
and is sometimes referred to as the automatic or involuntary division.
The specific tissues that are innervated by visceral motor nerves of the
autonomic nervous system are:
– E.g. Smooth muscle, cardiac muscle, and glands.
2 Divisions of the Autonomic
Nervous System
• The 2 divisions of the autonomic nervous
system are:
• a. The Parasympathetic division.
• b. The Sympathetic division.
2 Divisions of the ANS
• Parasympathetic
Division
– The parasympathetic
division attempts to
conserve body resources.
Restore after exertion.
– It is primarily dominant
under calm conditions –
‘rest and digest’.
– SLUDD = salivation,
lacrimation, urination,
digestion, defecation.
– The 3 Decreases: heart
rate, diameter of airway,
diameter of pupils.
2 Divisions of the ANS
•
Sympathetic Division
– The sympathetic division
dominates under conditions
of stress.
– It serves to prepare for the
quick utilization of body
resources.
– The sympathetic division is said
to be the division that prepares
you for “fight or flight”.
– Supports vigorous physical
activity and ATP production.
– E situations: exercise,
excitement, emergency,
embarrassment. Aided by
hormone (E and NE) release
from adrenal medulla.
– Dilates eyes, increases heart
rate and contractions, increase
BP, dilate airway, increase
blood to skeletal and cardiac
muscle, digestion inhibited.
Electrical Properties of Cells
• If you were to stick an electrode into any cell of the body
and compare it to the outside of the cell you would be
able to measure an electrical potential.
ECF = extracellular
ECF
Volt meter
- 70 mV
fluid
ICF = intracellular
ICF
fluid
• This electrical potential is produced by differences in
concentration of ions (ion imbalance) on either side of
the cell membrane. Your book calls it a membrane
potential.
• The resting potential is determined mainly
by 2 cations: Na+ and K+.
If the positive and negative ions are in balance on each side of the
membrane, the voltage is zero (0).
– Since such small amounts of electricity are involved the voltage is
measured in millivolts = 1/1000th of a volt.
•
At rest there are more positive ions outside the cell than inside so the
resting potential of the cell is given as negative voltage ( average -70
mV, range –40 to –90 mV).
Voltage mV
•
+ 40
0
-70
zero
Resting
Potential
By convention the voltage
inside the cell (intracellular
fluid ICF) is compared to the
voltage out the cell
(extracellular fluid ECF)
therefore, negative ( − )
voltage.
Mechanisms of Establishment of
Membrane Potentials
•
There are 2 basic processes that may lead to an
unequal distribution of charges (ionic imbalance) on
either side of the membrane.
1. Differential permeability of membranes to
specific ions moving by diffusion.
2. Active transport (Na+/K+ pump)
(Na+ pumped out & K+ pumped in)
Mechanisms of Establishment of Membrane
Potentials
•
1. Differential Permeability of Membrane.
There is leakage of ions through protein channels in the membrane that
are always open. Since they are always open they are called leakage or
nongated ion channels.
• The major cations involved here are sodium (Na+) and potassium (K+).
The cell membrane has more K+ than Na+ leakage channels and is
therefore more permeable to K+ than Na+ (permeability of K+ 50-100x
>Na+).
• Therefore, Na+ ions are found primarily outside the cell.
• K+ ions are found primarily inside the cell but tend to leak out due to
diffusion through the “leaky” membrane. It is due to the leakyness of the
K+ ions that the inside of the cell is less positive and said to be slightly
negative.
•
Positive outside
Higher Na+
++++++++++
-------------Higher K+
Cell
Less positive or
negative inside cell
Mechanisms of Establishment of Membrane
Potentials
• K+ ions can’t leak out forever. There must be
something that serves to replace them.
• 2. Active Transport, the Na+/K+/ATPase
pump.
Through active transport there
is a K+ pump/ Na+ pump.
• The Na+/K+ pump serves to move K+ ions
into the cell and to pump Na+ ions out of the
cell. It requires ATP to provide energy for the
pump. For every 3 Na+ ions moved out, 2 K+
ions are moved in.
• Since K+ diffuses out of the cell easily, the net
effect of the pump is to remove Na+ from the
cell.
• Although many different
ions are found in the ECF
and ICF, the resting
potential is determined
mainly by 2 cations: K+
and Na+.
• The lipid bilayer acts as
an effective insulator.
•
•
•
•
Cell membrane is more permeable to
K+ than to Na+
Permeability K+ > Na+
Na+ found primarily outside of cell
K+ found primarily inside cell but tend
to leak out due to permeability which
makes the inside of the membrane
slightly negative.
Mechanisms of Establishment of
Membrane Potentials
–
–
–
The combination of the passive forces of diffusion through a
semipermeable membrane (leaky channels) and the active
force of active transport Na+/K+ pump), there is an unequal
distribution of ions leading to a membrane potential.
This type of membrane potential is called a resting potential as
the nerve cell is at rest.
It remains this way as long as there is no stimulation.
The membrane is said to be polarized.
++++++++++++++++++++
____________________
____________________
++++++++++++++++++++
•
Resting nerve cell or fiber, with polarized membrane.
•
Let’s compare a resting nerve fiber with a stimulated nerve fiber.
Nerve Impulse & Action Potential
• What happens when membrane is
stimulated (say during nerve excitation)?
• Step 1: becomes highly permeable to Na+
– Na+ rushes in due to the concentration
gradient & membrane is depolarized =
action potential
+++++++++++++++++
+++_________________
____________________
++++++++++++++++++++
Stimulated nerve fiber. Shows (+) ion attracted to consecutive (−) charges
and moves along fiber = impulse
Nerve Impulse & Action Potential
• Step 2: membrane regains original
permeability
– K+ renews outward movement and resting
potential is restored repolarization (in that
area of the membrane.
−++++++++++++++
_____+______________
____________________
++++++++++++++++++++
Resting potential is restored but (+) charge will depolarize adjacent part of
membrane and action potential will take place along entire fiber step-by-step
= nerve impulse.
Movement of the nerve impulse along the nerve= Propagation
Action potential can be graphed as follows:
An action potential or impulse consists of depolarization and repolarization.
It is a sequence of events that decrease and eventually reverse the membrane potential
(depolarization) and then restore it to the resting state (repolarization).
Na+
In
Step 1
K+
out
Step 2
Resting Potential = the state of a neuron
when it is not conducting an impulse.
Action Potential = the state of a neuron
when it is conducting an impulse.
Action potential consists of
depolarization followed by repolarization.
Effects of Chemicals and Drugs
on Nerve Cell Membranes
• DDT:
One of the reasons the pesticide DDT is so
dangerous is that it increases the nerve cell membrane’s
permeability to Na+ ions.
• This causes spontaneous action potentials to occur all
of the time.
This seriously disrupts nerve cell transmission of
information. This is how it kills insects!
In humans, too much DDT effects the diaphragm and
results in respiratory arrest.
• Local Anesthetics: Lidocaine and Novacaine have the
opposite effect of DDT. They serve to and decrease
the permeability of the membrane to Na+ and prevent
action potentials.
This serves then to “numb” the localized area.