3 - Excitable Tissues Flashcards
What are excitable tissues? And what are the major types?
- Tissues that utilise electrical signals that travel along cells and can be readily transferred from cell to cell
- Neurones and Muscle (skeletal, cardiac and smooth)
How do you record the membrane potential in a single cell?
Use a voltmeter with microelectrodes. Place one extracellularly and one intracellularly, then measure the potential difference
What is resting membrane potential?
Membrane potential of an excitable cell at rest. Membrane is polarised
Define polarised
2 sides are different charges
What happens during depolarisation
Membrane potential decreases in magnitude from RMP (inside of the cell becomes less negative)
What happens during repolarisation
- Restoration of the difference in charge
- MP increases in magnitude back towards RMP
- Inside of the cell becomes more negative
What happens during hyperpolarisation?
- MP increases in magnitude from RMP
- Membrane is more polarised (more negative)
What happens during hyperpolarisation?
- MP increases in magnitude from RMP
- Membrane is more polarised (more negative)
What are graded potentials? And what are their key characteristics?
- Occur when an excitable tissue is subjected to an excitatory/inhibitory stimulus
- Small changes in MP (1-30mv)
- Transient (lasting 10’s of ms)
- Proportional to size of stimulus
- Produce local not global effects (only effects part around the stimulus, not the whole cell)
Explain depolarising graded potentials
- Produced by an excitatory stimulus applied to the cell
- Causes a transient depolarisation of the membrane
Describe hyperpolarising graded potentials
- Inhibitory stimulus applied to the cell
- Causes a transient increase in the membrane potential
- More negative than the RMP
What are the general features of an action potential?
- Large, fast complex changes in MP by large excitatory stimulus
- Affects the whole cell (once initiated, travels over the whole cell)
- To get an AP, depolarising GP must be large enough to reach threshold (varies from cell to cell)
Describe the phases of action potentials
- **Depolarising phase: **period between threshold and the peak. Inside of the cell is positive compared to the outside
- Repolarising phase: Period between peak and RMP
- Hyperpolarising phase: Becomes more negative before returning to RMP
What are the typical membrane values at threshold and peak of action potential?
- Threshold: -65mV
- Peak: +30mV
Key differences between graded and action potentials
- AP always the same size, unlike GP (all or none principle. if threshold is reached, whole sequence occurs)
- APs are quicker (AP = few ms)
- Action potential propagation. Movement of AP. GP is localised.
What is conduction velocity?
- Speed of AP propagation is rapid
- Vary b/n cells (0.5-130 m/sec)
What is frequency encoding?
- APs encode info by frequency of APs
- Expressed in Hz
- Freq in excitable tissues = no. AP per second
Describe an experiment for frequency coding
- Measuring MP of a neurone in response to skin indentation by blunt probe
- As the probe is advanced, series of AP in neurone that last as long as skin deformation and have a consistent frequency
- Increase the force applied to the probe, higher freq of AP happens
- Therefore, higher stimulus intensity is encoded by a higher freq of AP
What are the normal intracellular and extracellular ion concentrations for major anions and cations in mammals
What is the functional significance of ion selective channels, resting channels and gated ion channels?
Bc ions are dissolved in aqueous ECF and ICF, can’t move through hydrophobic core of lipid bilayer so they move through these channels instead.
Explain voltage-gated ion channels and an example
- State of the gate is determined by MP
- Has a molecular sensor that measures MP
- Opens/closes depending on value
- E.g: VGNa+ and K+ channels in AP
Explain ligand-gated ion channels and an example
- Gated by binding of chemicals to a receptor closely associated with the channel
- Interaction between ligand/chemical and receptor enables high specificity in controlling ion channel opening
- E.g: acute sense of smell
- Important in a wide variety of physiological systems and are implicated in a number of diseases and actions of therapeutic drugs
Explain stretch-gated ion channels and an example
- Regulated by degree of stretch (mechanical deformation) exerted on membrane where they’re embedded
- Stetch produces a conformational shape change, opens the gate so ions pass
- Involved in initiating GP associated with sensory stimuli
Explain resting channels
- Ungated
- Open most of the time, not affected by stimuli
- Important for RMP