Chapter 11 Notes
UEQ: Describe the physiology of nerve conduction, including the generator potential, and the synapse.
LEQ: Why is the nervous system important?
LEQ: How does the CNS and PNS work together?
LEQ: How are reflex arcs different from a regular nerve transmission?
LEQ: What diseases are associated with this system?
Nervous System- Master controlling and communicating system of the body
Functions of the Nervous System –
A. Sensory input – collect input from the senses(Peripheral Nervous System) and send it to the
Central Nervous system
B. Integration – processes the collected input and decides on a response. Sends response to
Peripheral Nervous System
C. Motor output – carries out action that was delivered from the Central Nervous System
a. Example: You perceive thirst, and you see a glass of water. The brain process the thirst
and vision of the glass, and tells your arm to reach out and grab the water and drink it.
Two main parts of Nervous System - Central Nervous System (CNS) and Peripheral Nervous System(PNS)
CNS
Contains the Brain and spinal cord
Responsible for Integration
Dictates motor responses based on reflexes, current conditions, and past experiences
PNS
Responsible for Sensory input and motor output
Extension from CNS contains: Sensory (afferent) and Motor (efferent) neurons
Motor (efferent) neurons are split into:
Somatic(voluntary) and Autonomic (ANS) (involuntary)
The Autonomic Nervous System is further split into:
Sympathetic (stimulates) and Parasympathetic (inhibits)
Histology - Study of tissue (in this chapter - nervous tissue)
Nervous tissue is composed of two main types of cells
A. Neurons – excitable (sends the impulses)
Ratio- 1neuron:10neuroglia
B. Neuroglia – support the neurons – “nerve glue” also called glial cells
CNS has these cells:
Astrocytes – most versatile, anchor neurons to capillaries
Microglia – monitor neuron health
Ependymal cells – provide barrier from cerebral spinal fluid
Oligodendrocytes – make myelin sheaths by wrapping neurons
PNS has these cells:
Satellite cells – function like astrocytes
Schwann Cells – function like oligodendrocytes
Neurons
a. Long lasting – last a lifetime
b. Are amitotic – don’t divide into/make new cells
C. Have a high metabolic rate – use lots of ATP/Oxygen
D. Release neurotransmitters
Structure
a. Cell Body – aka perikaryon or soma – contains the usual cell organelles.
b. Processes – extensions from the cell body
a. In the CNS they are called Tracts
b. In the PNS they are called Nerves
c. Two types of processes
i. Dendrites – major receptive and input regions
ii. Axons – Come from the axon hillock, and there is only one per neuron.
1. Long axons are called a nerve fiber
2. Axon collaterals – small branches that extend off the axon
3. Axon terminals (branched end of axon) aka terminal branches have
knob-like ends sometimes referred to as synaptic knobs or boutons.
c. Myelin Sheath and Neurilemma
a. Myelin sheath – whitish fatty (protein-lipid) covering, occurring in segments along the
axon of many nerves.
i. Myelin serves as an insulator to the axon of the neuron. Impulses are
conducted faster on myelinated neurons as compared to unmyelinated ones.
ii. Myelin sheaths in the PNS are created by Schwann Cells. Schwann cells wrap
themselves around the axon like a jelly roll.
iii. The outer most layer of the Schwann Cell that is wrapped many times around
the axon is called the Neurilemma “neuron husk”
iv. The gaps between the Schwann Cells are called the Nodes of Ranvier. Axon
collaterals can extend from these gaps.
v. Myelin Sheaths in the CNS are made from Oligodendrocytes. There is no
Neurilemma in these, because the cytoplasm is evenly distributed, and not
squeezed to the outside layer.
Classification of Neurons (structure and function)
Structure:
Multipolar
Bipolar
Many processes extend from
Two processes extend from one
one cell body. Lots of dendrites cell body. One side is a
and one axon.
dendrite, and one side is the
axon.
Most abundant, especially in
Rare
the CNS.
Example: Muscle motor
Example: Sensor neurons of the
neurons
eye
Function:
Sensory
Afferent (send info to CNS)
Interneurons
Between sensory and motor
neurons.
Unipolar
One process extends from cell
body. (axon only)
Mostly located in the PNS
Example: sensory neurons of
the skin.
Motor
Efferent (receive info from CNS)
Membrane Potentials
Neurons are highly excitable/irritable
Electrical Impulse – action potential or nerve impulse
Voltage – Measure of potential energy by separated charge.
a. Measured in volts or millivolts.
b. Always measured between two points
a. Gives the potential difference or simply the potential.
b. An increase in the difference between 2 points = an increase in voltage.
Current – flow of electrical charge from one point to another.
Resistance – hindrance to charge flow
The relationship between voltage, current and resistance is called Ohm’s Law
Current(I)= Voltage(V)/resistance(R)
Membrane Ion Channels
a. Leakage or non-gated channels - always open
b. Chemically gated/ligand gated – open with appropriate chemical
c. Voltage gated – open/close with changes in membrane potential.
Electrochemical gradient – concentration gradient (high concentration to low concentration)and
electrical gradient (positive and negative charges attract each other)
Resting Membrane Potential – usually -70mv. This means that it is more negative on the inside
compared to the outside.
Depolarization – (undo polarization) make charges more equal on the inside compared with the
outside ie. -70mv to -65mv voltages move towards zero.
Hyperpolarization – make the voltage move farther from zero ie. -70mv to -75mv.
Graded Potentials – short distance transmission of impulse. The transmission is reduced because the
current dissipates due to leakage channels.
Action Potentials – long distance transmission that is only used by neurons and muscle cells
a. Brief reversal of membrane potential with a total amplitude (change in voltage) of about
100mv (-70mv to +30mv).
b. Unlike graded potentials, action potentials do not decrease in strength with distance.
c. Aka nerve impulse
Propagation of an action potential
1. Resting state
a. All gated K+ and Na+ channels are closed (only the leakage channels are open,
maintaining resting potential)
2. Depolarizing phase
a. Na+ channels open. Cell interior becomes progressively less negative.
b. Once threshold is reached (usually around -50mv to -55mv)the depolarization continues
due to positive feedback. As more Na+ channels open, the change in charge opens even
more Na+ channels.
c. Results in the sharply upwards spike of the action potential.
3. Repolarizing phase: Na+ channels start inactivating and K+ channels open
a. No more influx of Na+. At this time the slower reacting K+ channels open and K+ rushes
out of the cell traveling down its electrochemical gradient. This changes the charge
inside the cell back towards the resting level. (re-polarization)
4. Hyperpolarization – some K+ channels remain open and Na+ channels reset.
a. The over shoot of K+ leaving the cell results in a slightly more negative interior than
resting levels.
b. Even though the cell has restored the electrical charges, the ions are not back to their
original places. This occurs with the Na+/K+ pump.
Propagation of an Action Potential – movement of the potential down the length of an axon.
a. Because the steps listed above for creating an action potential, the result is a wave like
response of depolarizing, repolarizing, and hyperpolarizing. Action potentials travel away
from their point of origin.
b. Impulse requires that it meet threshold levels before the axon can “fire”.
a. Threshold is typically met when the membrane has been depolarized by 15 to 20mv.
c. All or none phenomenon. Like electricity – the light is either on or off.
d. Refractory periods a. When Na+ channels are open, any addition stimuli would not affect it, no matter
how strong. They cannot be re-opened until they have been reset to their original
resting state. Absolute refractory period
b. After the absolute refractory period, the relative refractory period is when most
Na+ channels are back to resting, and the K+ channels are open. (repolarization)
Very large impulses can re-open Na+ channels and send another Action Potential.
Items that affect conduction speed(velocity)
a. Axon diameter – larger axons conduct AP’s faster.
b. Degree of myelination – more myelin = faster propagation of impulse
a. Unmyelinated axons exhibit continuous conduction
b. Myelinated axons exhibit salutatory conduction (saltare=to leap) Action potentials
are triggered at the nodes, and the electrical signal jumps from node to node. This
results in 30x faster conduction than continuous conduction.
Synapse – syn “to clasp or join” a junction that mediates information transfer from one neuron to the
next or from a neuron to an effector cell
Axodendritic synapses – junction between an axon and dendrites
Axosomatic synapses – junction between the end of an axon and the body of another neuron.
Rare are dendrodendritic and dendrosomatic.
Presynaptic neuron – the neuron sending the signal to the synapse
Post synaptic neuron – the neuron accepting the signal from the synapse
Two types
Electrical Synapses
Chemical Synapses
Less common
Designed to release and accept chemical
neurotransmitters.
Function like gap junctions. Allows ions to travel Axon terminal releases neurotransmitter from
between cells through the “connexons”.
vesicles into the synaptic cleft. The
Transmission is very rapid, and can be uni or
neurotransmitter is picked up in the receptor
bidirectional.
region on the adjacent cell’s dendrite or cell
body.
Information transfer across a chemical synapse:
I.
Action potential arrives at the axon terminal.
II.
Voltage-gated Ca2+ channels open along with the Na+ channels and Ca2+ also enters the axon
terminal
III.
Ca2+ entry causes neurotransmitter-containing vesicles to release their contents by exocytosis.
IV.
Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the post
synaptic membrane.
V.
Binding of the neurotransmitter opens ion channels, resulting in graded potentials.
VI.
Neurotransmitter effects are terminated.
a. Neurotransmitter is reclaimed by astrocytes of the pre-synaptic terminal
b. Neurotransmitter is broken down by enzymes.
c. Neurotransmitter diffuses away from the synaptic cleft.
Excitatory Synapses and EPSP’s
a. EPSP – excitatory post synaptic potentials – help trigger an AP distally to allow it to send
information the post cell. Usually occurs in the cell body, or dendrites, and allows the AP to
spread to the axon. Moves membrane potential toward threshold. Na+ and K+ move through
the same channel.
Inhibitory Synapses and IPSP’s
a. IPSP - Inhibitory post synaptic potentials- Moves membrane potential away from threshold for
generation of AP. Opens only K+ or Cl- channels causes hyperpolarization. Harder to generate
an AP.
Temporal summation – rapid fire of EPSPs in succession. Each one is fired before the previous can
dissipate. These add up and cause threshold to be reached, and an AP is achieved.
Spatial summation – post synaptic neuron receives EPSPs from multiple neurons at the same time.
These small movements towards threshold add up, and threshold is reached and an AP is generated.
Neurotransmitters and receptors
Neurotransmitter
Function
Acetylcholine
Excitatory in CNS
Excitatory or inhibitory
in PNS
Norepinephrine
(biogenic amine)
Excitatory or inhibitory
depending on receptor
location
CNS – cerebral cortex,
hippocampus and
brainstem
PNS – neuromuscular
junctions
CNS – brain stem,
cerebral cortex
PNS – main
neurotransmitter of
ganglionic neurons in
sympathetic nervous
system.
Info
Prolonged effects lead
to tetanic muscle
spasm.
Alzheimer’s patients
experience lower than
normal levels in certain
areas of the brain.
“feeling good”
neurotransmitter. A
common target for
abusive drugs.
Dopamine (biogenic
amine)
Excitatory or inhibitory
depending on receptor
CNS – midbrain,
hypothalamus,
principle
neurotransmitter of
extrapyramidal system
PNS – some
sympathetic ganglia
CNS – brain stem,
hypothalamus, limbic
system, cerebellum,
pineal gland and spinal
cord.
Serotonin (biogenic
amine)
Mainly inhibitory
Histamine (biogenic
amine)
Excitatory or inhibitory
depending on
receptors
CNS - hypothalamus
GABA (amino acid)
Generally inhibitory
Glutamate (amino acid)
Generally excitatory
CNS – cerebral cortex,
hypothalamus,
purkinjie cells of
cerebellum, spinal
cord, granule cells of
olfactory bulb, retina
CNS – spinal cord and
brain
Glycine (amino acid)
Generally inhibitory
CNS – spinal cord, brain
stem and retina
Endorphins (peptides)
Generally inhibitory
CNS – brain and spinal
cord
A “feeling good”
neurotransmitter.
Target of illicit drugs.
Deficient in Parkinson’s
Disease, Increased in
Schizophrenia
May play a role in
sleep, appetite,
nausea, migraines, and
regulates mood.
Too much causes
anxiety. Some illicit
drugs target this
neurotransmitter.
Involved in
wakefulness, appetite
control, learning and
memory. Also a
paracrine from the
stomach causing more
acid to be released,
and mediates
inflammation and
vasodilation.
Principle inhibitory
neurotransmitter of
the brain. Inhibitory
affects augmented by
alcohol and other
drugs.
Major excitatory
neurotransmitter in the
brain. Important in
learning and memory.
Excessive release
results in cell death
(extra release caused
by lack of oxygen)
Principle inhibitory
neurotransmitter of
the spinal cord.
Natural opiates, inhibit
pain. Some
prescription and illicit
drugs mimic this.
Tachykinins (peptides)
Excitatory
CNS – midbrain,
hypothalamus, cerebral
cortex
PNS – some sensory
neurons of the dorsal
root ganglia, enteric
neurons
CNS – hypothalamus,
hippocampus, cerebral
cortex
pancreas
CNS – throughout
Small intestine
Somatostatin
(peptides)
Generally inhibitory
Cholecystokinin
(peptides)
Generally excitatory
ATP (purines)
Excitatory or inhibitory
depending on receptor
Adenosine (purines)
Generally inhibitory
Nitric Oxide (gasses
and lipids)
Excitatory
CNS – brain and spinal
cord
PNS – adrenal gland,
nerves to penis
Carbon monoxide
(gasses and lipids)
Excitatory
Endocannabinoids
(gasses and lipids)
Inhibitory
Brain and some
neuromuscular and
neuroglandular
synapses
Throughout CNS
CNS- induces Ca2+
wave propagation in
astrocytes
PNS – dorsal root
ganglion neurons
Throughout CNS
Mediates pain
transmission in PNS.
CNS – involved with
respiratory and
cardiovascular controls
and in mood.
Often released with
GABA, inhibits HGH
release.
Involved in anxiety,
pain, memory. Inhibits
appetite.
ATP released by
sensory neurons or
damaged cells, invokes
a pain response
May be involved with
sleep/wake cycle, and
terminating seizures.
Dilates arterioles,
increasing blood flow.
Can increase stroke
damage. Male
impotence drugs
(Viagra) target Nitric
Oxide action.
Stimulates synthesis of
cyclic GMP
Involved in memory,
appetite control,
nausea and vomiting ,
neuronal development,
receptors also found
on immune cells
Neurotransmitter receptors
Channel linked receptors – ligand gated ion channels. These are always located directly across from the
site of neurotransmitter release. Channels open instantly when neurotransmitters attach. Fast and
short lived.
G-protein linked receptors - more complex, slower to respond, and stay open longer. Muscarinic ACH
receptors and those that bind to the biogenic amines and neuropeptides. Activated G-proteins typically
work by controlling the production of second messengers such as cyclic AMP, cyclic GMP and
diacylglycerol or Ca2+. These second messengers act as go-betweens to regulate ion channels or
activate kinase enzymes that initiate a cascade of enzymatic reactions in target cells.
Neural Integration – Neurons must function in groups. How they all work together
Neuronal Pools – functional groups of neurons that integrate incoming information received from
receptors or different neuronal pools and then forward the processed information to other destinations.
Circuits – patterns of synaptic connections in neuronal pools
a. Diverging circuits – amplifying circuits – one neuron stimulates multiple neurons, which in turn
continue to stimulate more neurons. Common in sensory and motor systems. One impulse from
brain can fire a hundred or more motor neurons, and in turn thousands of muscle fibers.
b. Converging Circuits – multiple neurons are sending the impulse to one neuron. Also common in
sensory and motor pathways.
c. Reverberating or oscillating circuits – some of the neurons in the pool send the impulse through
the circuit again. Example -Breathing
d. Parallel after discharge circuits – a neuron stimulates multiple neurons, then later, the signal
narrows back down to one neuron.
Reflexes – rapid autonomic responses to stimuli. The same response is given for a certain stimulus.
Reflex arcs – pathway for reflexes that contain: receptor (detect the changes in internal/external
environment), sensory neuron, CNS integration center, motor neuron, and effector(muscle or glands).
Instant reflexes use serial processing – touching stove
More complex responses use parallel processing - stepping on a thorn
Developmental aspects of Neurons:
Nervous system originates from the dorsal neural tube and neural crest from the surface of the
ectoderm. The Neural tube becomes the CNS. Neurons grow from the CNS (neuroblasts) and eventually
become amitotic. The distal end of the neuron has a neural cone to help it find the way to the
destination innervation site.(second month of development) We originally produce more neurons than
we need and about 2/3 of them die/apoptosis before we are born. Those that remain, are the ones we
have for life.
Some diseases that affect the nervous system –
Multiple Sclerosis – autoimmune disease that affects the myelin sheaths. The myelin sheaths of the CNS
are gradually destroyed and replaced by hardened lesions called scleroses.
Neuroblastoma – a malignant tumor in children where the some cells retain a neuroblast type structure.
Neuroapathy – degenerative disease of nerves.
Rabies – (madness) – vector borne virus that after entry to the body travels via nerves to the CNS where
it causes brain inflammation. This swelling causes delirium and death. Treatment is only effective
before symptoms appear.
Shingles – comes from the herpes zoster virus which causes chicken pox. Once infected, the virus
remains in the body in the cell bodies of the sensory ganglia. The immune system keeps it in check, but
if it is weakened, the virus makes a reappearance with painful, scaly blisters/rash.