Nerves and Muscles 2 Flashcards
What is the average resting potential of a neurone?
- 65 mV
How do charged intracellular proteins contribute to resting potential?
Large negatively charged proteins cannot mass through the plasma membrane, and so keeping the potential negative.
How does the Na+/K+ pump contribute to resting potential?
The Sodium-potassium pump is an ATPase pump that pumps out 3 Na+ ions for every 2 K+ ion pumped into the cell.
How dos Sodium and Potassium ions contribute to the resting potential?
The plasma membrane is more permeable to K+ ions than Na+. As a result, K+ leaves the cell; more K+ leaves than Na+ enters. These ions move through ion channels.
What is the Potassium equilibrium? How is it achieved?
- 70 mV. K+ moves out of the cell down a concentration gradient but moves into the cell by an electrochemical gradient. An equilibrium between these gradients are achieved. The potassium equilibrium is close to resting potential, the discrepancy is most likely due to Sodium ions.
What is the Nernst Equation?
The equilibrium potential for any ion is worked out using this. It takes into account the temperature and the concentration of the ion both inside and outside the cell.
How does Na+ add to the positivity of the cell?
The membrane is only slightly permeable to Sodium and so the net inward diffusion of Na+ slightly adds to the positivity of the cell.
How is an action potential produced?
- A stimulus causes Na+ ions to enter the cell. this causes membrane potential to rise.
- If the potential reaches threshold potential, voltage gated ion channels are opened and an influx of sodium ions enters the cell.
- This causes an action potential spike. Na+ voltage gated ion channel close when equilibrium potential is reached.
- K+ voltage gated ion channels open and K+ rushes out of the cell, thereby reducing potential .
- The K+ ions continue to flow out fo the cell while Na+ channels close.
- Hyper-polarisation occurs as more K+ leaves even once resting potential is reached.
What is the ‘all or nothing’ principle?
An action potential occurs when the membrane depolarises to a certain threshold, if this threshold is not reached the action potential will not be triggered. The size of the action potential is the same regardless of the size of the stimulus.
What does a larger stimulus result in?
More frequent action potentials being sent.
What is the refractory period?
Refractory period is a period immediately following a stimulus during which further stimulation has no effect.
Absolute refractory period: Where no other action potentials can be sent (usually during depolarisation). This determines the direction the action potential is sent.
Relative refractory period: If there stimulus is large enough, you can generate another action potential (usually during hyper polarisation)
What is a local current?
The next section of the axon in which the action potential is sent, can be depolarised as sodium ions can diffuse across to the Nodes of Ranvier.
Local current flow following depolarisation results in depolarisation of the adjacent axonal membrane and where this reaches threshold, further action potentials are generated.
Action potentials cannot occur in regions behind as that region is in absolute refractory period.
What is sensory transduction?
The conversion of environmental or internal signals into electrochemical energy.
What is the muscle spindle?
A proprioceptor and mechanoreceptor. It is a modified muscle fibre enclosed in connective tissue that responds to stretch. They imitate reflexes which cause the muscle to contract to reduce the danger of over stretching. When a muscle is stretched passively the spindle is activated and so initiates a reflex. When the muscle contracts and shortens it is switched off.
What is the Golgi tendon organ?
The Golgi tendon rogan lies in the tendon. A small bundle of collagen fibres enclosed in a layered capsule with a terminal branches of a large diameter afferent fibre intertwined with collagen bundles. Stimulated when the associated muscle contracts or is stretched. Sets up reflex that cause the muscle to relax an avoid stimulation. It is activated when the muscle is actively stretched.
What is the receptor potential?
The transmembrane potential difference produced by activation of a sensory receptor. A receptor potential is often produced by sensory transduction.
What is the knee-jerk reflex?
A reflex initiated by the muscle spindle. When the knee is tapped, it causes a muscle contraction as if the muscle has been stretched.
There are no interneurones involved in this response. This reflex is a reflex of proprioception which helps maintain posture and balance, allowing to keep one’s balance with little effort or conscious thought.
What are electrical synapses?
Direct passages of current via ions flowing through gap junctions. These gap junctions formed by interlocking connexions - made up of connexin proteins.
These synapses occupe in parts of the CNS such as the retina and hypothalamus where fast burst firing occurs. Current flow is unidirectional but can occur in both directions in invertebrates.
What are chemical synapses?
These cells synapses use neurotransmitters to deliver the information from the presynaptic neurone to the postsynaptic neurone.
How is a neurotransmitter released?
- The action potential reaches the axon terminal of the presynaptic cell.
- The action potential triggers calcium entry causing release of neurotransmitter from vesicles which fuse with the synaptic membrane.
Vesicles are docked and primed before action potential arrives.
- Upon calcium entry, the vesicle fuses with the plasma membrane and the contents of the vesicles are related into the synaptic cleft.
What are three groups of neurotransmitters?
Amines, amino acids and betides
What is an ionotrophic receptor?
Ionotropic receptors are ion channels and comprise mostly 4 or 5 similar protein subunits arranged around a central pore that is normally closed to ion movements. When the transmitter (ligand) binds it causes a conformational change that briefly opens the pore and ions pass through to cause a rapid change in the resting potential of the underlying cytoplasm.
What is a metabotrophic receptor?
Metabotropic receptors are single, long protein molecules, mostly crossing the cell membrane 7 times (7 trans-membrane domains). There is no ion pore. When the ligand binds there is a conformational change in the molecule that causes the intracellular part to interact with a G-protein that then sets off a chain of intracellular events, that may include opening of ion channels. This is a slower response.
What is an IPSP?
Binding of GABA or glycine to their receptors causes an influx of Cl- ions giving rise to an inhibitory post-synaptic potential (IPSP) in postsynaptic cell. Makes it longer to reach threshold and so makes it harder to longer for an AP to be sent. IPSPs tend to hyperpolarise cell and make may initiation of an AP less likely i.e. inhibition
