NEURAL COMMUNICATION Flashcards
List the three different communication systems
Endocrine system
Neural system
Neuro-endocrine system
Define a reflex
An involuntary, unpremeditated, unlearned response
What is the function of afferent neurones (sensory neurones)?
The nerve fibers responsible for bringing sensory information from the outside world into the central nervous system
What is the function of interneurons?
The neurones connecting the afferent and efferent neurone in the central nervous system
What is the function of the efferent neurones (motor neurones)?
The nerve fibres responsible for carrying information from the central nervous system to the peripheral nervous system to initiate an action
What are the two categories of efferent neurones and which tissues do they target?
Somatic: muscle
Autonomic: visceral organs
What is the function of the neurone dendrites?
The point of the neurone that receives electrical signals from another neurone
What is the function of the neurone cell body (soma)
Contains the organelles
The site where the electrical signals are integrated
What is the function of the axon hillock?
The junction between the cell body (soma) and the axon
Where an action potential is initiated
What is the function of the axon?
Transfers integrated signals to the axon terminal
What are synapses?
The junctions between excitable cells
What is the function of the myelin sheath?
The myelin sheath acts as an ‘insulator’ for some axons to minimise the dissipation of the electrical signal that is travelling along the axon, speeding up action potential transmission
Which cells produce the myelin sheath?
Oligodentrocyte cells
What are the Nodes of Ranvier?
The periodic gaps between the myelin sheath where action potentials are triggered along a myelinated axon
Why is the axon hillock the point where action potentials are most likely to be stimulated?
The axon hillock has a high concentration of fast voltage gated Na+ channels and slow voltage gated K+ channels (which are required to trigger an action potential)
Which factors influence the membrane potential?
Sodium potassium ATPase pump
Negatively charged impermeable proteins within the cell membrane
Membrane permeability to particular ions
List the types of protein channels which allow ions to move in and out of the cell, influencing the membrane potential
Leaky channels
Voltage gated ion channels
Ligand gated ion channels
Mechanical gated ion channels
Which cells can alter membrane permeability?
Excitable cells (muscle cells and neurones)
In a pathological state, what can alter membrane permeability?
Composition of intra- and extracellular fluid
If the cell membrane was only permeable to potassium, what would the equilibrium potential be for potassium?
-90mV
If the cell membrane was only permeable to sodium, what would be the equilibrium potential for sodium?
+60mV
What are the lowest and highest membrane potential values that would be seen in a healthy mammalian cell?
Lowest: -90mV
Highest: +60
What can be used to predict the equilibrium potential for any ion?
The Nernst equation
What is the resting potential value of a healthy mammalian cells?
-70mV
What determines a cell’s resting membrane potential?
The relative permeability of the membrane to ALL ions
What can result in a change in a cell’s membrane potential?
A change is membrane permeability to any ion
What is membrane depolarisation?
When the membrane potential is more positively charged relative to the resting potential
What is membrane repolarisation?
When the membrane potential is returned back to the resting membrane potential
What is membrane hyperpolarisation?
When the membrane potential is more negatively charged relative to the resting potential
What is the average value for threshold potential?
-50mV
What is threshold potential?
The minimum membrane potential required to fire an action potential
What stimulates a graded potential?
Graded potentials are stimulated by external stimuli via sensory (afferent) neurones or by neurotransmitters
Why can graded potentials provide only short distance information transfer?
Graded potentials undergo decremental transfer where the electrical signal gets smaller and smaller as the graded potential moves away from the initial site of stimulation
What determines the size of the graded potential?
The size of the graded potential is directly related to the size of the stimulus
What stimulates an action potential?
Membrane depolarisation to reach threshold
Describe the positive feedback system which mediates action potentials
A stimulus increases the membrane Na+ permeability –> Increased Na+ movement into the cell –> Membrane is depolarised and reaches threshold –> Opening of fast Na+ voltage gated ion channels –> Increases membrane Na+ permeability
Describe the cellular changes that occur during an action potential
- A stimulus is applied causing depolarisation and the membrane reaches threshold, triggering the opening of fast voltage gated Na+ channels (which are very quickly inactivated at the peak of the action potential)
- Opening of slow voltage gated K+ channels, repolarising the membrane through the movement of K+ out of the cell
- When the membrane reached resting potential, the fast voltage gated Na+ channels return to their original conformation. The slow voltage gated K+ channels are very slow to close, so K+ continues to exit the cell, hyperpolarising the membrane
- The slow voltage gated K+ channel returns to its original conformation and the sodium potassium ATPase pump distributes the Na+ and K+ so the membrane returns to resting potential
How do local anesthetics work?
Through the inhibition of fast voltage gated Na+ channels, preventing the triggering of action potentials
How does stimulus size influence action potentials?
Action potentials are always the same size, so the stimulus size is related to the action potential frequency
Describe how a larger stimulus increases action potential frequency
A larger stimulus causes a larger graded potential, meaning the graded potential will be higher above threshold. The membrane will be brought back to threshold faster after each action potential, thus increasing the action potential frequency
What is the absolute refractory period?
The period of time after an action potential has been triggered where no further stimulation can depolarise the membrane to trigger another action potential
What is the relative refractory period?
The period of time where a larger stimulus is required to stimulate an action potential
Describe how action potentials are propagated over large distances along a non-myelinated axon
- An action potential is fired and the membrane is depolarised via the movement of Na+ through fast voltage gated sodium channels
- The infiltrating Na+ ions move to the areas of the membrane immediately adjacent to the point of action potential stimulation (this is due to the Na+ being attracted to negative charge further along the membrane)
- Depolarisation of the adjacent membrane occurs, bringing the membrane up to threshold and triggering another action potential
Explain why action potentials are propagated away from the initial point of action potential stimulation
Because that initial point of action potential stimulation will be within it’s absolute refractory period
Why is action potential propagation along a myelinated axon faster and more energy efficient than along a non-myelinated axon?
Due to the ‘insulation’ effect of the myelin sheath, instead of action potentials being triggered at the immediately adjacent membrane, the action potentials are triggered at the Nodes of Ranvier - a process known as saltatory conduction
List the two types of communication systems between excitable cells
Convergent system
Divergent system
What is a convergent system?
The activity of many excitable cells influences the activity of one excitable cell
What is a divergent system?
The activity of one excitable cell influences the activity of many excitable cells
List the two types of synapses
Electrical synapses/gap junctions
Chemical synapses
Describe electrical synapses/gap junctions and why they are less efficient than chemical synapses
Electrical synapses/gap junctions allow direct electrical current flow between the excitable cells, however, this current can flow bidirectionally, making these synapses less efficient
Which structures make up electrical synapses/gap junctions?
Six proteins subunits known as connexins
Describe how chemical synapses convert an electrical signal to another electrical signal via a chemical intermediate
- When an action potential reaches the axon terminal, this triggers the opening of voltage gated Ca2+ channels, allowing an influx of Ca2+ into the cell
- Ca2+ induced changes stimulate synaptic vesicles to move to the pre-synaptic membrane and release the neurotransmitters into the synaptic cleft
- The neurotransmitters binds to specific receptors on the post-synaptic membrane, triggering the opening of ligand-gated channels within the post-synaptic membrane which result in either an EPSP or IPSP graded potential
- Eventually, the post-synaptic membrane will reach threshold, triggering an action potential
List the different methods in which neurotransmitters can be removed from the synaptic cleft?
Diffusion away from the synaptic cleft
Re-uptake into the presynaptic cell
Destroyed by enzymes
Why are there methods of neurotransmitter removal from the synaptic cleft?
To prevent constant stimulation of the post-synaptic cell and an excessive firing of action potentials
What are excitatory post-synaptic potentials (EPSPs)?
Depolarisation of a postsynaptic cell, bringing the membrane potential of that cell closer to the threshold for triggering an action potential
- this is caused by the neurotransmitters opening ligand gated sodium channels
What are inhibitory post-synaptic potentials (IPSPs)?
Hyperpolarisation of the membrane, bringing the membrane potential further from threshold and thus further from triggering an action potential
- this can be caused by the neurotransmitter opening ligand gated potassium channels, closing ligand gated sodium channels or opening ligand gated chloride channels
Why would opening ligand gated chloride channels result in an inhibitory post-synaptic potential (IPSP)?
Chloride has an equilibrium potential of -70mV (the same as resting membrane potential), so the opening of ligand gated chloride channels would result in the stabilisation of the membrane
What is the function of temporal and spatial summation?
Within excitatory cells, temporal and spatial summation is the integration of information (graded potentials) over time and space in order to determine an overall response
i.e. this process is the foundation of decision making
What is the difference between temporal and spatial summation?
TEMPORAL SUMMATION: The summation of a single stimuli over a short period of time
SPATIAL SUMMATION: The summation of several spatially separated stimuli at the same time
List three pre-synaptic factors which can alter chemical synapse efficiency
Neurotransmitter availability
Enzyme availability
Calcium concentration
List five post-synaptic factors which can alter chemical synapse efficiency
EPSP or IPSP graded potential
Drugs (can manipulate synaptic function)
Disease (can manipulate synaptic function)
Post-synaptic receptor concentration
Neurotransmitter concentration in the synaptic cleft
What is the function of sensory receptors?
Converts and information from environmental stimuli into action potentials
Describe why sensory receptors are such efficient systems
Sensory receptors are very sensitive systems which can operate over a wide range of stimulus strengths
List the five different classifications of sensory receptors
Mechanoreceptors
Thermoreceptors
Nocieptors
Electromagnetic receptors
Chemoreceptors
What is adaptation?
Adaptation is a decrease in neural response due to a continued application of a stimulus to sensory receptors
What is complete adaptation?
When a stimulus is applied to a sensory receptor, action potentials are fired and very quickly and the membrane is repolarised until that stimulus is changed and another action potential is fired
What is incomplete adaptation?
When a stimulus is applied to a sensory receptor, an action potential is fired, the membrane begins to repolarise but remains slightly elevated above resting potential until that stimulus is changed, and another action potential is fired
What information is provided by slow adapting sensory reflexes?
Stimulus strength
What information is provided by fast adapting sensory reflexes?
Stimulus frequency
List mechanisms of adaptation
Energy filtering
Membrane effect
Describe how energy filtering acts as a mechanism of adaptation
Energy filtering is the neural process of filtering out redundant or irrelevant stimuli from all possible environmental stimuli reaching the brain
Describe how the membrane effect acts as a mechanism of adaptation
- When an action potential is triggered and voltage gated Na+ channels are opened, Ca2+ can also move through these channels
- Ca2+ can bind to and trigger the opening of ligand gated K+ channels
- This leads to less likelihood of an action potential being fired, reducing action potential frequency, allowing adaptation of the sensory receptors to occur
Describe a monosynaptic reflex arc
Only one synapse is involved
Faster reflex, brief response
Describe a spinal reflex arc
Reflexes where all of the reflex components are contained within the spinal cord
the brain can impose control over these reflexes, but doesn’t need to
Describe a polysynaptic reflex arc
More than one synapse involved
Involves interneurons
Slower reflex, prolonged response
What is the function of a sign inverting synapse?
Allows a coordinated response between different muscle groups within the body
