Excitable tissues

2.  Define excitable tissues
 Name parts and functions of a neuron
 Describe the changes in ionic channels that
underlie action and membrane potentials
 Draw a diagram showing action potential
stages.
 Describe the mechanism of synaptic
transmission

3.  There are two excitable tissues in the body
 nerve and muscle.
 Excitability simply refers to an ability of a
tissue to receive stimuli and respond to them.
 These stimuli can be electrical, chemical,
mechanical, or thermal.

4.  Excitable tissues respond to various stimuli
by rapidly changing their resting membrane
potentials and generating nerve impulses
(action potentials).
 A nerve impulse is simply an electrical signal
that travels along an axon.
 Action potentials are propagated throughout
an excitable cell once started.

5.  Neurons are the basic building blocks of the
nervous system.
 We have billions of neurons , linked together
to form rapid control center of the body.
 Neurons communicate or relay information to
another cell by way of an electrical impulse.

6. Neurons are structurally divided into
 Unipolar
 Bipolar
 Multipolar

9.  Unipolar neurons are most common in
invertebrates
 Pseudo unipolar neurons resemble unipolar
and are responsible for the sense of touch,
pain and pressure
 Bipolar neurons usually found in sensory
organs i.e the eyes, nose and ears
 Multipolar neurons most found in vertebrates

10.  Functionally they are divided in to:
 Sensory (Afferent) neurons ─ conduct
impulses from the periphery to the CNS.
 Motor neurons (Efferent) ─ conduct impulses
from the CNS to the periphery.
 Interneurons ─ serve as connectors. They
conduct nerve impulses from sensory area to
motor area ─ exclusively found within the
CNS.

12.  Neurons are in different shapes and sizes,
but have the same parts as the typical spinal
motor neuron.
 Have 5 to 7 processes called dendrites or
receptors that extend from the cell body and
branch out like trees.

14.  The dendrites conduct stimuli information to
the nerve cell body.
 Assists in nerve impulse formation.
 Multiple local potential changes generated by
synaptic connections are integrated in the
dendrites.

15.  Cell body is often located at the dendritic
zone of the axon
 Primary site for maintaining the life of the
nerve cell.

16.  An axon originates from a thickened area of
the cell body.
 It is estimated to be up to 1.6 km long and
only 13 mm in diameter.
 Axons transmit propagated impulses(action
potential) to the nerve endings.
 The junction site between the nerve cell body
and the axon is called Axon Hillock

17.  The Hilllock is where processing of voltage
changes or generation of potentials takes
place.
 The first portion of the axon is called the
initial segment.

19.  The axon divides into terminal branches, each
ending in a number of synaptic knobs.
 The knobs are also called terminal buttons.
 They contain granules or vesicles in which the
synaptic transmitters secreted by the nerves
are stored.
 Synaptic knobs serve as a secretory
component that releases neurotransmitters in
response to action potential.

20.  Axons of many neurons are myelinated, they
are wrapped up to 100 times by a sheath of
myelin which is a protein lipid complex
produced by Schwann cells.
 Most neurons in the CNS are myelinated, but
the cells that form the myelin are
oligodendrogliocytes not Schwann cells.

 oligodendrogliocytes send off multiple
processes that form myelin on many
neighboring axons.

22.  The myelin is then compacted when the
extracellular portions of a membrane protein
called protein zero (P0).
 Various mutations in the gene for P0 cause
peripheral neuropathies e.g Multiple Sclerosis

23.  Three major types of Na+ channels
 I. Na+-leak channels
 II. Voltage-gated Na+ channels
 III. Ligand-gated (chemical-gated) Na+
channels

24.  There are four major K+ channels
 I. K+-leak channels
 II. Voltage-gated K+ channels
 III. Ligand-gated K+ channels
 IV. G-protein-gated K+ channels

25.  There are three major Ca2+ channels
 I. Voltage-gated Ca2+ channels
 II. Ligand-gated Ca2+ channels
 III. G-protein-gated Ca2+ channels

26.  A membrane potential is a voltage difference
across the cell plasma membrane.
 At rest it is called resting membrane potential
(RMP).
 Voltage sensitive Na+ and K+ gates in the
plasma membrane of a nerve cell are nearly
closed at RMP.
 Resting membrane potential is caused mainly
by diffusion of potassium and sodium
through a leak channel called K+N+ channel.

27.  The leak channel is 100 times more
permeable to potassium than it is to sodium.
 Potassium and sodium diffusion give a
membrane potential of about –86 millivolts.
 An additional –4 millivolts is contributed by
Na+-K+ pump, which moves 3Na+ ions to
the ECF and 2K+ ions to the ICF, giving a net
resting membrane potential of –90 millivolts.

28.  This combination of activity contributes to
the formation of electro- positivity outside
the cell and electro-negativity inside the cell
membrane of a neuron.
 The negativity inside the cell is created by
non-diffusible proteins within the ICF that
cannot travel through the membrane.

29.  An action potential is a change in electrical
potential.
 This change occurs between the inside and
outside of a nerve or muscle fiber when it is
stimulated serving to transmit nerve signals.
 Action potentials are useful in cell-to-cell
communication for example neurons
communicate with one another.

30.  Action potentials obey the all or none law.
 Once elicited at any point on the membrane
of a normal fiber, the depolarization process
travels over the entire membrane if
conditions are right, or it does not travel at
all if conditions are not right.
 The action potential fails to occur if the
stimulus is sub threshold in magnitude and
once elicited it occurs with constant
amplitude and form regardless of the
strength of the stimulus.

31.  All cells have slightly excess of positive
charges outside and negative charges inside
of the membrane.
 Most of the membrane potential is due to the
passive diffusion of Na+ and K+ ions down
their concentration gradients.

34.  Resting stage ─ is the resting membrane
potential before the action potential begins.
It’s also known as a polarized stage with -90
to -70 mV negative membrane.
 Depolarization stage ─ is when there is rapid
inflow of Na+ ions into the inside, creating
positivity inside. There is increased
membrane permeability to Na+ ions.

35.  Repolarization stage ─ is initiated when Na+
channels begin to close and K+ channels
open.
 The membrane potential begins to recover
back toward the resting membrane state so,
K+ ions will diffuse rapidly to the exterior
and the inside regains the negativity.

36.  Nerve signals are transmitted by the action
potentials, which are rapid changes in the
membrane potential that spread rapidly along
nerve fiber membrane.

37.  Each action potential begins with a sudden
change from a normal negative resting
membrane potential to a positive active
potential then ends with an almost equally
rapid change back to a negative potential.

38.  This change causes the voltage to change to
a less negative state and when the ICF voltage
reaches a threshold of about -55 mV, the
Na+ gates open completely and increase the
inward flow of Na+ ions through Na+
channels this quickly changes the voltage
from a resting level of -90 mV to about +35
mV.

39.  The rapid shift from a negative to a positive
state is called depolarization. Immediately a
state of depolarization is attained at +35 mV,
the Na+ gates close and the depolarization
process stops.

40.  The depolarization state of +35 mV causes
the K+ gates to open and allow K+ ions to
flow from the ICF to ECF.

41.  The rapid flow of K+ reverses the membrane
potential from +35 mV to about -90 mV and
is called repolarization.
 This all process happens in a millisecond.
 At the conclusion of each repolarization
event, the Na+/K+ pump move Na+ and K+
ions back to their main storage areas and
reset the membrane back to RMP.

44.  Synapse is a junction between two cells in
which one must be a neuron.
 It is the site of transmission from one neuron
to the next.
 There are two modes of synaptic
transmission:
 (1) Chemical synapses
 (2) Electrical synapses

45.  Almost all the synapses used for signal
transmission in the CNS of humans are
chemical synapses.
 One neuron will transmit impulse to another
neuron or to a muscle or a gland cell by
releasing chemicals called neurotransmitters.

46.  There are 3 types of chemical synapses:
 1. Neuroneuronal junction (presynaptic &
postsynaptic neurons)
 2. Neuromuscular junction
 3. Neuroglandualr junction

48. Electrical synapses
 Faster signal transmission and only
excitatory.
 Can synchronize the activity of postsynaptic
neurons.

49. Chemical synapses
 Are slower signal transmission, which can be
either excitatory or inhibitory.
 The signal can be modified as it passes from
one neuron to next.

50.  The action potential or nerve impulse will
travel in the axon and when it reaches the
axon terminal, a set of events will be
triggered which will release a certain amount
of neurotransmitter (NT) e.g acetylcholine
(Ach).
 The NT then accumulates in a synapse and
generates a postsynaptic voltage potential in
the next cell of a nerve pathway sequence.

51.  If the postsynaptic potential is positive, then
it is called an Excitatory Postsynaptic
Potential (EPSP) and If negative, then it is
called an Inhibitory Postsynaptic Potential
(IPSP).
 EPSP’s stimulate further nerve impulses,
whereas, IPSP’s inhibit nerve impulses.

52.  If the axon is myelinated due to a Schwann
cell, nerve impulse forms only in the Nodes of
Ranvier & skips over the insulating myelin
sheath from node to node.

53.  As long as a stimulus is strong enough to
cause depolarization to threshold, voltage-
gated Na & K channels open, and an AP
occurs.

 A refractory period is a brief time after an AP
begins when a muscle fiber or neuron cannot
generate another Ap. It is unresponsive to a
stimulus no matter how strong.

54.  A refractory period is the time interval
between the opening of Na+ activation gate
and a time when a Na+ channel cannot be
stimulated.
 There are two forms of refractory period:
 Absolute Refractory Period- when another AP
can not be generated regardless of the
strength of the stimulus. ARP begins at the
start of the upward stroke & extends into the
downward stroke.

55. Relative Refractory Period
The RRP begins when the ARP ends. A new AP
can occur in an excitable fiber if the stimulus is
strong enough.