Transcript action potential

Muscle excitation
•The excitation of muscles depend on the arrival of a
stimulus to the muscle cell membrane to generate an
excitation (action potential ) in the muscle membrane.
•Some types of muscle also have the potential to be self
excitable such as smooth and cardiac muscle ( cells of
the conducting system )
•synapse :
•Point of contact
•is a site where information is transmitted from one cell to another. The
information can be transmitted :electrically
(electrical synapse)
Chemically
(chemical synapse).
•for example the synapse between nerve fiber and muscle fiber is the site of
transmission of the neural order to the muscle to contract )
•Electrical synapses allow current to flow from one excitable cell to the
next via low resistance pathways between the cells called gap
junctions.
•chemical synapses: there is a gap between the presynaptic cell membrane
and the postsynaptic cell membrane, known as the synaptic cleft.
Information is transmitted across the synaptic cleft via a neurotransmitter .
Electrical synapse
general sequence of events that occurs at
chemical synapses:
•.
.
change in membrane potential
The change in membrane potential on the postsynaptic cell
membrane can be either excitatory or inhibitory, depending
on the nature of the neurotransmitter released from the
presynaptic nerve terminal:
Excitatory:
ACh, norepinephrine ,epinephrine,
dopamine, glutamate,
Inhibitory
glycine GABA) and
and serotonin .
depolarization of the postsynaptic cell
hyperpolarization of the postsynaptic cell
•A motor unit comprises a single motor neuron and the muscle fibers it
innervates.
•Motor units vary considerably in size:
•A single motorneuron may activate a few muscle fibers or thousands of muscle fibers.
•Predictably, small motor units are involved in fine motor activities (e.g., facial
expressions), and large motor units are involved in gross muscular activities (e.g.,
quadriceps muscles used in running (
Muscles EXCITATION
• Through
Motor End Plate
=
neuromuscular
junction.
Ach esterase
mitochondria
An action potential in the motor neuron produces an
action potential in the muscle fibers it innervates by
the following sequence of events
• Action potentials.
• presynaptic terminal is depolarized.
• voltage-gated Ca channels.
• Ca influx.
• release of the neurotransmitter.
• ACh binds to the α subunits of the nicotinic receptor.
• Open ligand-gated ion channel.
• permeability of the motor end plate to both Na +and K +increases.
• both Na +and K +flow down their respective electrochemical
gradients
• drive the motor end plate to its equilibrium potential
• end plate potential (EPP)
 )The EPP is not an action potential ,but it is simply a depolarization
of the specialized motor end plate adjacent muscle fibers, which are
depolarized to threshold and fire action potentials
 Although the motor end plate itself cannot fire action
potentials, it depolarizes sufficiently to initiate the
process in the neighboring "regular" muscle cells
 Ach degradation
Then NTs undergo degradation
Several agents interfere with normal activity at the
neuromuscular junction,
Myasthenia Gravis
DESCRIPTION OF CASE .An 18-year-old college woman comes to the student
health service complaining of progressive weakness. She reports that occasionally
her eyelids "droop" and that she tires easily, even when completing ordinary daily
tasks such as brushing her hair. She has fallen several times while climbing a flight
of stairs. These symptoms improve with rest. The physician orders blood studies,
which reveal elevated levels of antibodies to ACh receptors. Nerve stimulation
studies show decreased responsiveness of skeletal muscle upon repeated
stimulation of motoneurons. The woman is diagnosed with myasthenia gravis and
is treated with the drug pyridostigmine. After treatment, she reports a return of
muscle strength .
EXPLANATION OF CASE ??
INTEGRATION OF SYNAPTIC INFORMATION
Spatial Summation
two or more presynaptic inputs arrive at a postsynaptic cell simultaneously.
•If both inputs are excitatory, they will combine to produce greater depolarization
than either input would produce separately.
• If one input is excitatory and the other is inhibitory, they will cancel each other
out
Temporal Summation
Temporal summation occurs when two presynaptic inputs arrive at the
postsynaptic cell in rapid succession. Because the inputs overlap in time, they
summate .
Facilitation, augmentation ,posttetanic potentiation:
are phenomena that may occur at synapses. In each
instance, repeated stimulation causes the response of the
postsynaptic cell to be greater than expected. The common
underlying mechanism is believed to be an increased
release of neurotransmitter into the synapse, possibly
caused by accumulation of Ca +in the presynaptic terminal
Synaptic fatigue
may occur where repeated stimulation produces a smaller
than expected response in the postsynaptic cell, possibly
resulting from the depletion of neurotransmitter stores from
the presynaptic terminal.
Excitation of cardiac
muscle
Excitation of cardiac muscle
Excitation of smooth
muscle
Excitation of smooth muscle
smooth muscle can be stimulated to contract by
multiple types of signals:
1. nervous signals
2. hormonal stimulation
3. stretch of the muscle, and in several other ways .
Membrane Potentials and Action Potentials in Smooth Muscle
In the normal resting state, -50 to -60 millivolts
The action potentials of visceral smooth muscle occur in forms:
action potentials with plateaus
spike potentials
slow wave rythm
Action potential with a
plateau, recorded from a
smooth muscle fiber of the
uterus.
Typical smooth muscle action
potential (spike potential)
elicited by an external stimulus
The importance of the plateau is that it can
account for the prolonged contraction that
occurs in some types of smooth muscle,
such as the ureter, the uterus under some
conditions, and certain types of vascular
smooth muscle .
•Some smooth muscle is self-excitatory.
•often associated with a basic slow wave rhythm of the membrane potential
•The cause of the slow wave rhythm is unknown
•The importance of the slow waves is that,
when they are strong enough, they can initiate
action potentials
•Therefore, the slow waves are called pacemaker waves