Download The Nervous System

The Nervous System
Function
• the major controlling, regulatory, and
communicating system in the body.
• the center of all mental activity including thought,
learning, and memory.
• together with the endocrine system, the nervous
system is responsible for regulating and
maintaining homeostasis.
• Through its receptors, the nervous system keeps us
in touch with our environment, both external and
internal.
• The activities of
the nervous system
can be grouped
together as three
general,
overlapping
functions:
– Sensory
– Integrative
– Motor
Sensory:
– millions of sensory receptors detect
changes, called stimuli, which occur
inside and outside the body.
– They monitor such things as
temperature, light, and sound from
the external environment.
– Inside the body (the internal
environment), receptors detect
variations in pressure, pH, carbon
dioxide concentration, and the levels
of various electrolytes.
– All of this information is called
sensory input.
Integrative:
– Sensory input is converted into
electrical signals called nerve
impulses that are transmitted to the
brain.
– There the signals are brought
together to create sensations, to
produce thoughts, or to add to
memory.
– Decisions are made each moment
based on the sensory input. This is
integration.
Motor:
– Based on the sensory input and
integration, the nervous system
responds by sending signals to
muscles, causing them to contract,
or to glands, causing them to
produce secretions.
– muscles and glands are called
effectors because they cause an
effect in response to directions from
the nervous system. This is the
motor output or response
Structure of the Nervous System
The Nervous system has two major divisions
1. The Central Nervous System (CNS)
– consist of the Brain and the Spinal Cord.
– The average adult human brain weighs 1.3 to 1.4
kg .The brain contains about 100 billion nerve
cells,called Neurons and trillons of "support cells"
called glia.
– The spinal cord is about 43 cm long in adult
women and 45 cm long in adult men and weighs
about 35-40 grams.
2. The Peripheral Nervous System (PNS)
– consists of the neurons NOT Included in the
Brain and Spinal Cord.
– Is divided into two divisions:
• Somatic nerves ( voluntary)
• Autonomic nerves (involuntary)
• The somatic nerves
– Controls the skeletal muscles, bones and skin
– Some neurons collect information from the
Body and transmit it TOWARD the
CNS. These are called AFFERENT
NEURONS.
– Other neurons transmit information AWAY
from the CNS. These are called EFFERENT
NEURONS.
• The autonomic nerves
– Control the internal organs of the body
– Regulatory system that works with the
endocrine system to maintain homeostasis
– Consist of motor neurons that operate without
conscious control.
– Organized into the sympathetic nervous system
and the parasympathetic nervous system
– Autonomic regulation involves constant
interplay of balance between sympathetic and
parasympathetic
• The sympathetic
nervous system
– Prepares the body
for stress: increases
heart rate, increases
the release of
glucose, dilates the
pupils, increases
blood flow to the
skin, causes release
of epinephrine
The parasympathetic
nervous system
– Restores normal
balance: decreases
heart rate, stores
glucose, constricts
pupils, decreases
blood flow to the
skin.
Activity of autonomic system is regulated by
neurons in brain and spinal cord
– brainstem contains centers necessary for control
of heart rate, blood pressure, and body
temperature
– control of brainstem regions exerted by
hypothalamus
– hypothalamus, in turn, influenced by limbic
system structures
– emotional responses accompanied by extensive
changes in autonomic function
The Neuron
The Neuron
The Neuron
• is the functional unit of the nervous system.
• Humans have about 100 billion neurons in
their brain alone!
• While variable in size and shape, all
neurons have three parts:
– Dendrites
– The cell body
– The axon
Dendrites
– Bring information to the cell body (incoming)
– Rough Surface (dendritic spines)
– Usually many dendrites per cell ( can be up to a
thousand)
– No myelin sheath
– Branch near the cell body
• The Cell Body (Soma)
– contains the nucleus,
mitochondria and other
organelles typical of
eukaryotic cells.
– Is the metabolic control
center of the neuron and
its manufacturing and
recycling center: it is in
the cell body that
neuronal proteins are
synthesized
The Axon:
–
–
–
–
–
Take information away from the cell body
Smooth Surface
Generally only 1 axon per cell
Can have myelin
Branch further from the cell body
Types of Neurons
• Three types of neurons:
– Sensory neurons
– Interneurons
– Motor neurons
Sensory Neurons
– typically have a long dendrite and
short axon
– carry messages from sensory
receptors to the CNS.
– The cell bodies of the sensory
neurons leading to the spinal cord
are located in clusters, called
ganglia, next to the spinal cord.
– The axons usually terminate at
interneurons.
Interneurons
-Interneurons are found only in the
central nervous system where they
connect neuron to neuron.
-They are stimulated by signals reaching
them from sensory neurons, other
interneurons or both.
-are also called association neurons.
-It is estimated that the human brain contains
100 billion (1011) interneurons averaging
1000 synapses on each or some 1014
connections
Motor Neurons
– Typically have a long
axon and short dendrites
– Transmit messages from
the central nervous
system to the muscles (or
to glands).
– The axons connecting
your spinal cord to your
foot can be as much as 1
m long (although only a
few micrometers in
diameter).
The Nerve Impulse.
The Neuron at Rest
• The plasma membrane of neurons contains
many active Na-K-ATPase pumps.
• These pumps shuttle Na+ out of the neuron
and K+ into the neuron when ATP is
hydrolyzed.
• Three Na+ are pumped out of the neuron at
a time and two K+ ions are pumped in
• This creates a concentration gradient for
Na+. As Na+ accumulates on the outside of
the neuron, it tends to leak back in.
• Na+ must pass through proteins channels to
leak back through the hydrophobic plasma
membrane. These channels restrict the
amount of Na+ that can leak back in.
• This maintains a strong positive charge on
the outside of the neuron
• The K+ inside the neuron also tends to follow its
concentration gradient and leak out of the cell.
• The protein channels allow K+ to leak out of the
cell more easily.
• As a result of this movement in Na+ and K+ ions,
a net positive charge builds up outside the neuron
and a net negative charge builds up inside.
• This difference in charge between the
outside and the inside of the neuron is
called the Resting Potential.
• The resting potential in most neurons is
–70 mV.
• When the neuron is at rest, it is polarized
Initiation of the Action Potential
• A change in the environment ( pressure,
heat,sound, light) is detected by the receptor and
changes the shape of the channel proteins in part
of the neuron –usually the dendrites.
• The Na+ channels open completely and Na+ ions
flood into the neuron. The K+ channel close
completely at the same time and K+ ions can no
longer leak out of the neuron in that particular
area.
• The interior of the neuron in that area becomes
positive relative to the outside of the neuron.
• This depolarization causes the electrical potential to
change from –70 mV to + 40 mV
• The Na+ channels remain open for about 0.5
milliseconds then they close as the proteins enter an
inactive state.
• The total change between the resting state (-70 mV)
and the peak positive voltage ( +40mV) is the action
potential ( about 110 mV)
• The spike in voltage causes the K+ pumps to open
completely and K+ ions rush out of the neuron.
The inside becomes negative again. This is
repolarization.
• So many K+ ions get out that the charge goes
below the resting potential. While the neuron is in
this state it cannot react to additional stimuli.
• The Refractory period lasts from 0.5 to 2
milliseconds.
• During this time, the Na-K-ATPase pump
reestablishes the resting potential.
Transmission of the impulse
• The stimulus induces depolarization in a
very small part of the neuron, at the
dendrites.
• The sequence of depolarization and
repolarization generates a small electrical
current in this localized area.
• The current affects the nearby protein
channels for Na+ and causes them to open.
• When the adjacent channels open, Na+ions flood
into that area of the neuron and an action potential
occurs. This in turn will affect the areas next to it
and the impulse passes along the entire neuron.
• The electric current passes outward over the
membrane in all directions BUT the area to one
side is still in the refractory period and is not
sensitive to the current. Therefore the impulse
moves from the dendrites toward the axon.
Threshold stimulus
• Action potentials occur only when the membrane
in stimulated (depolarized) enough so that sodium
channels open completely.
• The minimum stimulus needed to achieve an
action potential is called the threshold stimulus.
• If the membrane potential reaches the threshold
potential (generally 5 - 15 mV less negative than
the resting potential), the voltage-regulated
sodium channels all open. Sodium ions rapidly
diffuse inward, & depolarization occurs.
All-or-None Law
• Action Potentials occur maximally or not at all.
• In other words, there's no such thing as a partial or
weak action potential. Either the threshold
potential is reached and an action potential occurs,
or it isn't reached and no action potential occurs.
• However, different neurons have different
densities of Na+ channels and therefore have
different APs
• The AP remains constant as it travels down
the neuron. Its amplitude is always the
same because it corresponds to wide open
Na+ channels.
• The frequency of the AP can change.
Conduction Velocity
• impulses typically travel along neurons at a speed
of anywhere from 1 to 120 meters per second
• the speed of conduction can be influenced by:
– The diameter of a fiber. Velocity increases as diameter
increases.
– Temperature. As temperature increases, the velocity
increases. Axons of birds and mammals can be very
small because of the high body temperature.
– the presence or absence of myelin.
• Neurons with myelin (or myelinated neurons)
conduct impulses much faster than those without
myelin.
• Because fat (myelin) acts as an insulator,
membrane coated with myelin will not conduct an
impulse.
• So, in a myelinated neuron, action potentials only
occur along the nodes and, therefore, impulses
'jump' over the areas of myelin - going from node
to node in a process called saltatory conduction
(the word saltatory means 'jumping')
Summary
• The Action Potential, or nerve impulse is an
electrochemical event involving the rapid
depolarization and repolarization of the
nerve cell membrane.
• The axon terminals of one neuron do not
touch the dendrites of other neurons. What
happens when the impulse reaches the axon
terminal?