Chapter 10
The Nervous System
Introduction
• Types of neural tissue:
• 1. Neurons – react to changes around them & send
impulses
• 2. Neuroglia – support tissue with a variety of
functions
• Functions of nervous system:
• 1. Sensory – use sensory neurons to gather info.
inside & outside the body
• 2. Motor – use motor neurons to help the body react
to stimuli
• 3. Integrative – integrate signals from sensory & motor
neurons to produce thought, memory, etc.
Divisions of the Nervous System
• Central Nervous System (CNS) –
consists of the brain & spinal cord
• Peripheral Nervous System (PNS) –
consist of nerves that connect the CNS to
other body parts
Structure of a Neuron
• Dendrites – pick up
impulses
• Cell body –
contains cell parts
• Axon – sends
impulses
• Schwann cells – wrap
around the axon
• Myelin – lipid covering
formed by Schwann cells; speeds
rate of impulse
• Axon terminals – end of axon
Structure of A Neuron
• Axon hillock – slight
elevation where
axon originates
• Node of Ranvier –
gap in myelin
Structure of A Neuron
• Neurofibrils – network of fine threads
that extend into the axon; for
support
• Nissl bodies – consist of
rough ER
Neurilemmal sheath – formed by
the cytoplasm & nucleus of the
Schwann cell that remain on the
outside
Direction of Impulse
• Impulse always
travels from dendrites,
through cell body, &
down axon
• Axon synapses w/next
neuron or an effector
(muscle or gland)
Structural Classification of
Neurons
• Bipolar – has 2 processes from the cell body, 1 at each
end; in sense organs
• Unipolar – has 1
process from c.b. that
divides into 2; in PNS
• Multipolar – have many
processes from c.b; in CNS
and motor neurons.
Functional Classification of
Neurons
• Sensory (afferent) – unipolar & carry
impulses from body parts to brain or s.c.
• Interneurons (association neurons) –
multipolar & in CNS; form links b/t other
neurons
• Motor (efferent) – multipolar & carry
impulses from brain or s.c. to muscle or
gland
Types of Neuroglia
• Support tissue w/a variety of functions:
1. Astrocytes –star-shaped;
found b/t neurons & b.v.;
support, transport &
communication
b/t nerves & b.v.
•
Transport glucose to
Neuron and store glycogen
• Separate neurons from
each other.
Types of Neuroglia
2.Microglia – small w/few processes; found
throughout CNS; support & phagocytosis
of harmful substances
Types of Neuroglia
3. Oligodendrocytes – resemble astrocytes
but w/fewer processes; form myelin sheath
in CNS
Types of Neuroglia
4. Ependyma – columnar & cuboidal shaped
cells; form inner lining
of brain & s.c.;
provide a layer
for diffusion to
occur
Types of Neuroglia
Cell suicide
• Microglia can destroy cells
that are old &/or damaged
• A – healthy neuron
• B – neuron being
destroyed & DNA
breaking apart
• C – microglia removing
debris
Nerve Impulse Cartoon
• Impulse Animation
Resting Potential
• A resting neuron is one not sending an impulse & is in resting
potential
• The cell membrane of this neuron is polarized b/c of an un=
distribution of ions on either side
• Outside the neuron – greater concentration of Na+ ions
• Inside the neuron – greater concentration of K+ ions & negatively
charged proteins
Resting Potential
• K+ leak out of K+ channels at a slow rate leaving behind
negatively charged proteins
• This makes the charge on the inside of the membrane
negative
• The voltage meter (next pg.) shows a charge of -70 mv &
refers to the charge of a neuron in resting potential
Resting Potential
Movement of Ions
• Ions follow the laws of diffusion (movement
from high to low concentrations) when moving
thru membranes
• Ions enter & leave the membrane thru channels
or gates that
are specific for
that ion
Ion Channels
• 3 types
– Passive- always open
– Ligand gated- opened by a chemical
compound. (neurotransmitter)
– Voltage gated- opened in response to a
change in electric potential.
Resting Potential
The charge outside the cell is positive b/c:
1. the high concentration of Na+ ions
2. the movement of K+ ions to the outside
Resting Potential Animation
• Resting Potential Animation
Sodium Potassium Pump
• Membrane protein used for the active
transport of Na+ and K+ across
membrane.
• Requires ATP
• Removes 3 Na+ ions and accepts 2 K+ for
every ATP molecule used.
• Maintains resting potential.
Action Potential
• An abrupt change in the electrical potential across the cell
membrane that occurs after a stimulus (a.k.a. nerve impulse):
1. Resting neuron stimulated (remember – a resting neuron is
polarized)
2. Na+ channels open & Na+ move into membrane; charge inside cell
becomes + (+30mv) & neuron is depolarized
3. Na+ channels close & K+ channels open; K+ move out & charge
reverts back to negative
(-70mv); cell is repolarized
Resting Potential → Action
Potential
A)Resting
potential
(polarized)
B)Action potential
A.P. in the
1st region
stimulates
adjacent
region (depolarized)
C)1st region
repolarized
Action Potential Animation
• Action Potential Animation
Graphing Action Potential
After repolarization
a brief period of
delay occurs when
Na+ gates cannot
temporarily open;
called refractory
period
Graphing Action
Potential
Hyperpolarization
when the cell
becomes more
negative than
-70mv; depends on
which ions are
allowed to enter the
cell, + or – ions (i.e.
Cl- ions)
Threshold – the minimum amt. of stimulus
required to cause an action potential
Impulse Conduction
• Saltatory conduction – impulse jumps from 1 node of
Ranvier to another; why?
• Myelin covering – mostly lipids which prevent flow of ions
• channels - are located at nodes of Ranvier for ions to
diffuse in & out
• Myelinated axons (white matter) - conduct impulses
faster than unmyelinated axons (gray matter)
Saltatory Conduction Animation
• Animation
The Synapse
• Junction b/t 2 neurons
• Presynaptic neuron –
occurs before the syapse
• Postsynaptic neuron –
occurs after the synapse
• Synaptic knob –
enlargement of axon
terminal
• Synaptic vesicles – store
ntm
• Synaptic cleft – space
b/t neurons
Actual Synapse
Events at the Synapse
• Action potential travels down presynaptic neuron &
arrives at synapse
• Synaptic knob becomes more
permeable to Ca+ & they
diffuse inward
• This causes vesicles to
release ntm
• Ntm causes A.P. to enter
postsynaptic neuron
• A.P. continues to travel down
postsynaptic neuron
The Synapse
Types of Neurotransmitters
• The nervous system produces approx. 30
different types of ntm
• Some open ion channels, others close them
• Monoamines:
Neuropeptides:
- epinephrine
- endorphins
- norepinephrine
- enkephalins
- dopamine
- substance P
- serotonin
Acetylcholine (ACh)
Effects of Ntms
• Epinephrine & norepinephrine – hormones when
released in blood, but ntm in the n.s.; stimulate
autonomic n.s.; incr. HR, resp. rate, etc.; “fight-or-flight”
response
• Dopamine – excitatory or inhibitory; create a sense of
well-being; insufficient levels associated with Parkinson’s
disease
• Serotonin – inhibitory; insufficient levels associated
with insomnia
• Endorphins & enkephalins – generally inhibitory &
influence mood; released under stress to reduce
pain (blocks substance P)
• Substance P – excitatory; helps in perception of pain
• ACh – stimulates muscles to contract
Synaptic Potentials
• Ion channels that respond to ntm are called
chemically gated channels (as opposed to
those that are voltage-gated & are involved in
sending A.P.)
• Changes in chem. gated channels create local
changes called synaptic potentials (a small,
temporary change in the potential charge of a
neuron)
• They allow one neuron to influence another
The Synapse
Synaptic Potentials
• 2 types:
1. Excitatory postsynaptic potential (EPSP) – occurs
when the neuron is depolarized (or becomes less
negative), but the charge is subthreshold (<+30mv).
A true A.P. won’t occur, but will be more likely to occur
if the neuron receives more subthreshold stimuli
Synaptic Potentials
2.
Inhibitory postsynaptic potential (IPSP) occurs
when the neuron is hyperpolarized (or becomes more
negative than -70mv). An A.P. will be less likely to
occur.
• The type of ntm secreted will decide the effect that
occurs.
Effects of Ntm on Synaptic
Potentials
• If a ntm opens Na+ channels & Na+
diffuse in, the membrane is depolarized
(EPSP)
• If a ntm opens K+ channels & K+ diffuse
out, the membrane is hyperpolarized
(IPSP)
• A neuron can receive EPSP’s & IPSP’s
simultaneously; the neuron responds to
the algebraic sum of the + and - charges
Synaptic Potential vs.
Action Potential
• 2 differences:
1. P.S.P. are graded (depends on amt. of ntm)
& their effect adds up (called summation)
whereas A.P. are all-or-none
2. P.S.P. decr. in intensity w/incr. distance from
synapse
• Facilitation – when a neuron receives
subthreshold stimuli & gets closer to sending an
A.P.
Convergence vs. Divergence
• Convergence – impulses from
2 or more fibers converge on
a single neuron (summation
will occur)
• Divergence – when outgoing
impulses are divided onto several
branches of an axon
Convergence vs. Divergence
Importance of Ions
• Ca+ are needed for the release of ntm
• Ca+ are also needed to close Na+
channels
• Insufficient Ca+ levels result in channels
remaining open & impulses repeatedly
transmitted; results in tetany
• May occur in pregnancy (as fetus uses
maternal Ca+), when diet lacks Ca+ or Vit
D during dehydration
Importance of Ions
• An incr. in extracellular K+ causes neuron
to be less negative; threshold is reached
sooner & neurons are very excitable; may
result in convulsions
• A decr. in extracellular K+ causes neuron
to be more neg.; does not allow an A.P. to
occur & muscles may become paralyzed
Resting Potential
Action Potential
Saltatory Conduction
EPSP
IPSP
Convergence vs. Divergence