Download Cell Week4

Human Cell Biology and
Physiology
Timothy Billington PhD
LET’S MUSCLE in on WEEK 4 (27 Oct)
MUSCLE CELLS (fibres)
1. Enormous compared with the cells we have seen so far
fibres in the thigh are 0.1 mm wide and up to 30 cm long!
2. Multinucleate. (>100 nuclei) with the nuclei lying very close to the cell membrane.
Genes in these nuclei control production of structural and catalytic (enzyme) proteins required for
muscle contraction.
The more copies of these genes, the faster those proteins can be produced
3. Membrane of a muscle fibre has a Transmembrane Electrical Potential (Voltage) due to the uneven
distribution of +ve and –ve charges across the membrane.
A sudden change in transmembrane potential is what initiates a contraction.
4. Inside each muscle cell there are many hundreds of Myofibrils, running the length of the cell.
Each myofibril is packed with Myofilaments, made of Actin and Myosin proteins
Muscle cells
----- short section of 10 cells lying side-by-side
nucleus
PART of an INDIVIDUAL MUSCLE CELL
Myofilaments, made of Actin
and Myosin linear proteins
SARCOMERE
ACTIN
MYOSIN
(thin filament)
(thick filaments)
}
ACTIN
(thin filament)
In a contraction:
The Myosin filament stays still and draws the Actin filaments in from both sides
Myofibrils are each composed of 10,000 Sarcomeres, arranged end-to-end
SARCOMERE
}
Sarcomeres are the smallest functional units of the
muscle cell.
They are about 2 micrometres long
In a contraction:
The myosin filament stays still and draws the actin filaments in from both sides
Contraction of skeletal muscle fibres
Myofibrils are able to actively shorten and are responsible for muscle fibre contraction.
Actin filaments slide over the fixed Myosin filaments.
Mechanism:
At each end of the muscle cell, the myofibrils are firmly anchored to the inner surface of the sarcolemma (cell
membrane)
In turn, the outer surface of the sarcolemma is attached to the collagen fibres of the tendon.
Result?
When the myofibrils contract, the ENTIRE cell shortens and pulls on the tendon
Muscle moves a joint
SLIDING FILAMENTS and MUSCLE CONTRACTION
SARCOMERE
Actin
Myosin
At rest
Contracted
Notice how the Myosin filaments stay still and the Actin filaments move in from either side
We now know which parts of the muscle cell contract.
Want to know: what event initiates a contraction?
Answer: An electrical impulse delivered by a motor nerve
So how does this happen?
The MOTOR CORTEX of the brain sends an impulse which travels the length of the
nerve pathway.
From brain to spine to leg muscle for example.
leg
Motor Cortex
nerve pathway
via the spine
Neuro-Muscular Junction
This is the point where the nerve interacts with an individual muscle cell
LEG MUSCLE cell
NERVE
NMJ
At the NMJ
Nerve releases a neurotransmitter called Acetyl Choline (ACh)
ACh arrives at the Sarcolemma and causes the release of Calcium ions ( Ca ++)
Ca++, in the presence of ATP, then cause the Sarcomere to contract
Contraction will then continue while there is sufficient ATP available and while there are still
impulses arriving at the NMJ
Impulse via a motor nerve
Nerve end
NMJ
sarcolemma
ACh ACh ACh
ATP
ATP
Calcium ions released
Sarcomere contraction
MUSCLE CELL TERMINOLOGY
Sarcolemma = muscle cell membrane
Sarcoplasm = muscle cell cytoplasm
Sarcoplasmic reticulum = rough endoplasmic reticulum
Sarcomere = smallest functional unit of a muscle cell
Sarcoplasmic reticulum is like a lacy network which surrounds each myofibril for its entire length
In the sarcoplasm , in between the myofibrils and the sarcolemma, are hundreds of mitochondria
Question:
Why is it necessary to have hundreds of mitochondria per muscle cell?
We now move on to our new cellular topic
CELLS in the NERVOUS SYSTEM
This system includes all the neural tissue in the body
Functional unit? a cell called a Neuron [NEW-RON]
What functions do Neurons have?
1. Communication
2. Information processing
3. Control
In the nervous system there are also cells called Neuroglia, which far out-number the neurons
Newro-glee-ah
Functions of neuroglial cells
1. support the neurons to survive and function optimally
2. separate & protect neurons
3. are a supportive framework for neural tissue
4. regulate the ionic composition of interstitial fluid which surrounds
the neurons
Let’s see what a typical NEURON looks like
dendrites
nucleus
axon
axon terminals
sheath
cell body
Cell body contains a prominent Nucleus
Cytoplasm surrounding the nucleus is known as the PERIKARYON
In the perikaryon are organelles responsible for energy generation and protein synthesis.
mitochondria
rough endoplasmic reticulum
Perikaryon also contains organelles which synthesise Neurotransmitters, pivotal for cell-to-cell communication
Grey matter of the brain is composed of neurons which have large clusters of RER and free ribosomes
clusters are so dense that they appear as grey regions
Now for some new definitions
DENDRITES:
Slender, sensitive, highly branched processes extending from the cell body.
Active in intercellular communication
AXON: Long cytoplasmic process capable of propagating electrical impulses [ Action Potentials ]
INPUT
OUTPUT
axon
Conducts electrical impulses away from cell body
Transmits information to other nerve cells, or to muscles
Makes electrical contact with other cells at junctions called SYNAPSES
Some single axons are over a metre long, as for example the sciatic nerve axon which runs
from the lumbar region of the spine to the big toe
NEURON – NEURON COMMUNICATION
A
The axon terminals of Cell A interact with
dendrites of Cell B at a SYNAPSE
B
SO, WHAT DOES a SYNAPSE LOOK LIKE?
SYNAPSE:
Junction between 2 nerve cells or between a nerve cell and a muscle cell.
A minute gap across which impulses jump by the diffusion of a neurotransmitter ACh
Impulse (Action Potential)
Synaptic Gap
Axon
ACh Receptor
Axon terminal
Dendrite
SIGH- NAPS
Neurotransmitter molecules (ACh)
SEQUENCE OF EVENTS AT A CHOLINERGIC SYNAPSE
[A cholinergic synapse is one where the neurotransmitter is Acetyl Choline (ACh)]
1. Action Potential arrives, from a cell body, along an axon and then depolarizes the Axon Terminal
membrane (pre-synaptic membrane)
2. Extracellular Calcium ions now enter the Axon Terminal and ACh molecules begin to leak into the synaptic
gap and diffuse across it, towards the post-synaptic membrane
SEQUENCE OF EVENTS AT A CHOLINERGIC SYNAPSE
[A cholinergic synapse is one where the neurotransmitter is Acetyl Choline (ACh)]
1. 3. ACh molecules bind to specific receptors on the post-synaptic membrane and this causes a depolarizing
of that membrane. Na rushes in and another Action Potential is generated and ? Propagated
2. 4. ACh is then removed by Acetyl Choline Esterase, an enzyme which occupies the synaptic gap. Ach is
degraded to Acetate and Choline. Choline is reabsorbed into the axon terminal. Acetate diffuses away into
the bloodstream nearby.
A synapse between a neuron and a muscle cell is a Neuromuscular Junction (NMJ)
Each axon terminal contains mitochondria ( guess why?) and thousands of vesicles filled with
neurotransmitter molecules ( Acetyl choline, ACh)
The membrane of an axon terminal is the pre-synaptic membrane
The membrane of a dendrite is the post-synaptic membrane
MYELIN SHEATH:
Composed of lipid-rich flat cells which wrap themselves around Axons
Electrically insulate axons
Causes such wrapped axons to appear white. (White matter in the brain)
Cross-section
Sheath
Axon (end-on)
Un-myelinated axons appear grey
TRANSMEMBRANE POTENTIAL:
As you have learned, ALL cells at rest have an electrical potential difference between inside and
outside.
Remember: the inside of a cell is negatively charged
Neurons are no exception.
All neural activities begin with a sudden change in Resting Transmembrane Potential
If that change is BIG enough, we get an ACTION POTENTIAL being initiated
Action Potential is a self-sustaining voltage change that propagates along a whole axon
membrane, from Cell Body all the way to the nearest Synapse
ACTION POTENTIAL:
Here is a plot of membrane voltage vs time
Depolarizing
Repolarizing
Hyperpolarizing
1 millisecond
Depolarization: Caused by Na channel gates opening which lets Na+ ions rush into the cytoplasm from the
outside
Transmembrane voltage increases from -70 to +30 m V
At peak voltage (+30 m V), Na channel gates now snap shut and…………….
Repolarization:
begins. K channel gates now open and K leaves the cytoplasm and goes to the outside.
Transmembrane voltage begins to drop and goes negative
Hyperpolarization: K channel gates now close. Na and K channels return to their resting state ( -70 m V)
Question: What is the magnitude of the voltage change when depolarisation of
the membrane takes place ?
Question: At peak voltage, the cytoplasm of the neuron is full of Na+ ions. The Na+
channel gates now snap shut. How do the Na+ ions then
leave the cell so that the resting state returns?
Question: When the transmembrane voltage has returned to the resting state,
what is the status of K+ and Na+ ions in the cell and in the extracellular
fluid?
END WK4