Nervous system Flashcards

Question

What are action potentials?

Answer 1

Large alterations in membrane potential, e.g. a change of 100mV. This change is an ‘all or nothing’ response. Action potentials are very rapid and may repeat at frequencies of several hundred per second. The ability to generate action potential is excitability, possessed by neurons, muscles cells and some others. Action potential signal over long distances.

Answer 2

Ligand-gated channels and mechanically gated channels often serve as the initial stimulus for an action potential. Voltage-gated channels give a membrane the ability to undergo action potentials by allowing the rapid depolarisation and repolarisation. Many voltage-gated ion channels vary by the ion and how they behave as the membrane voltage changes. Similarity- Na+ and K+ have charged amino acid residues allowing a change in shape in response to membrane potential. Difference- Na+ are much faster to respond and have an ‘inactivation gate’.

Answer 3

Resting membrane potential close to K+ equilibrium as normal K+ channels are leaky. Action potential begins when a stimulus binds to a specific ion channel, allowing Na+ to enter. Other Na+ channels stimulated to open by the depolarisation- positive feedback. Na+ channels become inactivated and K+ channels now open with a delayed effect halting depolarisation. K+ fluxes out of the cell rapidly repolarising the membrane. The return of resting membrane potential closes Na+ channels, but the sluggish K+ results in hyperpolarisation. As K+ channels close, resting membrane potential returns.

Answer 4

Not all membrane depolarisations in excitable cells trigger the positive feedback relationship leading to an action potential. The initial stimulus and the current through the Na+ channels must be sufficient to elevate the membrane potential beyond the threshold potential. Stimuli that are just strong enough to depolarise the membrane to this level are threshold stimuli. The threshold of most excitable membranes is 15mV less negative than the resting membrane potential. Weak polarisations- subthreshold potentials, caused by subthreshold stimuli.

Answer 5

Refractory periods limit the number of action potentials an excitable membrane can produce in each period. They also contribute to the separation of these action potentials so that electrical signals pass down the axon. Two types of refractory periods that cells undergo following an action potential: Absolute and Relative. A second stimulus will never produce a second action potential if one is already happening, that region of the membrane is in its absolute refractory period, occurring when the voltage-gated Na+ channels are already open or have proceeded to the inactivated state during the first action potential.

Answer 6

After the absolute refractory period, an interval allows a second action potential to be produced, if the strength is much greater than usual- this is the relative refractory period. Some voltage-gated Na+ channels will have returned to a resting state; the magnitude of the action potential is temporarily reduced and some K+ channels that repolarised the membrane are still open. During the relative refractory period, a new stimulus can depolarise the membrane above the threshold potential, if the stimulus is large in magnitude or outlasts the relative refractory period.

Answer 7

New action potentials produce local currents of its own that depolarise the region adjacent to it, producing another action potential at the next site, causing action potential propagation along the length of the membrane. There is a sequential opening and closing of Na+ and K+ channels. The action potential doesn’t move but ‘sets off’ a new action potential in the region of the axon just ahead of it. The end of the membrane is virtually identical in form to the initial one, so they don’t decrease in magnitude with distance.

Answer 8

Action potentials in neurons are unidirectional (can only go forward down the axon, as the space behind is in its refractory period). If the membrane through which the action potential travels is not refractory, excitable membranes can conduct action potentials in either direction. In most neurons, action potentials are initiated at one end of the cell and propagate toward the other end. The larger the fibre diameter, the faster the action potential propagates, larger fibres offer less internal resistance to local current.

Answer 9

Myelin is an insulator that makes it harder for charge to flow between intracellular and extracellular fluid compartments. As there’s less leakage of charge across the myelin, a local current can spread farther along an axon. Action potentials occur only at the nodes of Ranvier, where the myelin coating is interrupted and the concentration of voltage-gated Na+ channels is high.

Answer 10

Action potentials jump from one node to the next as they propagate along a myelinated fibre; an action potential is regenerated at each node. Saltatory conduction is faster than propagation in non-myelinated fibres of the same axon diameter: less charge leakage, membrane pumps need to restore fewer ions.

Answer 11

Amplitude varies with size of the initiating event. Can be summed. Has no threshold. Has no refractory period. Is conducted decrementally- amplitude decreases with distance. Duration varies with initiating conditions. Can be a depolarisation or a hyperpolarisation. Initiated by receptor, synapse, or randomly. Mechanism depends on ligand-gated channels or other changes.

Answer 12

All-or-none. Cannot be summed. Has a threshold (15mV) depolarised relative to the resting potential. Has a refractory period. Depolarisation is amplified to a constant value at each point along the membrane. Duration is constant under constant conditions. Only depolarisation. Initiated by graded potential. Mechanism depends on voltage-gated channels.

Answer 13

Certain chemical messengers elicit complex responses that can’t be described as ESPSs or IPSPs. Modulation is used for these complex responses and the messengers that cause them are neuromodulators. Certain neuromodulators are co-released with the neurotransmitter. Many messengers used by the immune system serve as neuromodulators. Receptors for neurotransmitters influence ion channels that directly affect excitation or inhibition of the post-synaptic cell. Receptors for neuromodulators more often bring about changes in metabolic processes in neurons via G-proteins coupled to second-messenger systems. Neurotransmitters are involved in rapid communication. Neuromodulators are associated with slower events (learning, development states).

Answer 14

Acetylcholine (ACh)- neurons that release it are cholinergic neurons. It’s synthesised from chlorine and acetyl coenzyme A. It’s concentration at the postsynaptic membrane decreases due to the action of enzyme acetylcholinesterase, which destroys ACh, releasing choline and acetate. The two types of ACh receptors are Nicotinic Acetylcholine Receptors and Muscarinic Acetylcholine Receptors. Biogenic Amines- small, charged molecules that are synthesised from amino acids and contain an amino group. These include: Catecholamines (dopamine, norepinephrine, epinephrine), Serotonin and Histamine.

Answer 15

Amino Acid Neurotransmitters- the most prevalent neurotransmitter in the CNS. The most common excitatory amino acid is glutamate. GABA is the major inhibitory neurotransmitter in the brain, it’s a modified form of glutamate. Neuropeptides- composed of two or more amino acids linked together by peptide bonds. Many function as hormones or paracrine substances. They’re derived from larger precursor proteins. Neurons that release one or more of the peptide neurotransmitters are called peptidergic. Often, neuropeptides are co-secreted with another neurotransmitter and act as neuromodulators.

Answer 16

Gases- nitric oxide, carbon monoxide and hydrogen sulphide are emitted by neurons as signals. They’re produced by enzymes in axon terminals and diffuse from their sites of origin in one cell into the intracellular fluid of other neurons, where they bind to activate proteins. Purines- ATP and adenosine act as neuromodulators. ATP is co-released with one or more neurotransmitters in response to Ca2+ influx into the terminal. Adenosine is derived from ATP via enzyme activity in the extracellular compartment.

Answer 17

A synapse is an anatomically specialised junction between two neurons, either chemical or electrical. Activity at synapses can increase or decrease the likelihood that the post-synaptic neuron will fire action potentials by producing a brief graded potential. Excitatory synapse- membrane potential of post-synaptic neuron is brought closer to threshold (depolarised) Inhibitory synapse- membrane potential of post-synaptic is either driven farther from threshold (hyperpolarised) or stabilised at resting potential

Answer 18

In an electrical synapse, the electrical activity of the presynaptic neuron affects the electrical activity of the postsynaptic neuron. Pre and post synaptic cells are connected by gap junctions. In chemical synapses, neurotransmitters transmit the signal. Pre-synaptic neurons release neurotransmitters from their axon terminals.

Answer 19

Neurotransmitters are stored in vesicles within lipid bilayer membranes; vesicles are docked on the presynaptic membrane in release regions called ‘active zones’. Receptors can be ionotropic (ion channels) or metabotropic (G protein/second messenger signalling). To terminate the signal in a chemical synapse the neurotransmitter must be removed. Diffusion of the transmitter from the cleft. Degradation of the transmitter by enzymes. Reuptake into the pre-synaptic cells for reuse.

Answer 20

Excitatory chemical synapses generate an excitatory postsynaptic potential (EPSP)- a graded potential. EPSPs serve to bring the membrane potential closer to threshold for generating an action potential. Inhibitory chemical synapses generate an inhibitory postsynaptic potential (IPSP) which make the cell membrane potential more negative, making it harder to generate an action potential.

Answer 21

If synapses become active simultaneously the stimulus can be summed. Temporal summation- one axon is stimulated again before the first EPSP has died down, so second synaptic potential adds to the first, resulting in greater depolarisation. Spatial summation- two axons are stimulated resulting in input from two different neurons, which results in summation and greater depolarisation.

Answer 22

The neurons involved in controlling skeletal muscle can be thought of as being organised in a hierarchical fashion, with each level having a certain function. General intention at the highest level of the hierarchy is generated for a consciously planned movement. Information is then relayed to middle level structures that specify posture and movements needed to carry out the action. As neurons in the middle level receive input from the command neurons, they also receive afferent information from receptors in the muscles, tendons, joints, skin, etc (local level).

Answer 23

Voluntary movements- movement accompanied by a conscious awareness of what we are doing and why we’re doing it; our attention is directed toward the action or its purpose. Involuntary movements describe actions that are unconscious and automatic, described by ‘reflex’.

Answer 24

Local control systems are the relay points for instructions to the motor neurons from centres higher in the motor control hierarchy. They play a major role in adjusting motor unit activity to unexpected obstacles to movement and to painful stimuli in the surrounding environment. To do this: Carry out adjustments based on information from sensory receptors. Also, by transmitting information via afferent fibres to higher levels of the hierarchy.

Answer 25

Stretch receptors embedded within muscles monitor muscle length and the rate of change in muscle length. Receptors consist of peripheral endings of afferent nerve fibres wrapped around modified muscle fibres enclosed within a connective tissue capsule, collectively called a muscle spindle. Intrafusal fibres- modified muscle fibres. Extrafusal- fibres that form the bulk of the muscle.

Answer 26

Tension depends on muscle length, load on the muscle and fatigue. Golgi tendon organs are the endings of afferent nerve fibres that wrap around collagen bundles in the tendons near their junction with the muscle. These sensors discharge action potentials upon stretching or contraction.

Answer 27

Afferent fibres from the muscle spindle enter the CNS and divide: Excitatory synapse directly onto motor neurons in extensor muscle. Inhibitory synapse with interneuron that inhibit motor neurons in flexor muscle. Reciprocal innervation. Excitatory synapse directly onto motor neurons of other synergistic extensor muscles. Information to higher centres.

Answer 28

Stand on a pin. Pain detected by nociceptor and signal sent to CNS. Ipsilateral extensor muscle in thigh relaxes due to inhibitory synapse. Ipsilateral flexor muscle contracts due to excitatory synapse. Contralateral effects: Contraction of extensor muscle. Relaxation of flexor muscle. Result: stand up on other leg. Signal sent to brain.

Answer 29

A recording of brain electrical activity. EEG patterns are due to graded potential, summed postsynaptic potentials in the many brain neurons that underlie the recording electrodes. The alpha rhythm is associated with decreased levels of attention. When alpha rhythms are generated, subjects are often relaxed and happy. When people are attentive to external stimulus or are thinking hard about something, the alpha rhythm is replaced by the beta rhythm.

Answer 30

Sensory receptors that receive stimuli from the external or internal environment. Neural pathways that conduct information from receptors to the brain/spinal cord. The parts of the brain that deal with this information (sensory information). Sensation- aware of the information. Perception- aware and understand the meaning.

Answer 31

After the CNS has modified/processed the sensory information: Transduction of stimulus to graded potentials (receptor potentials). Action potentials in afferent neurons. Pattern of action potentials is the code providing info about stimulus. Primary sensory areas of CNS receive this input. Further communication with other brain regions/spinal cord.

Answer 32

Sensory receptors are specialised cells that generate graded potentials called receptor potentials in response to a stimulus. 5 major divisions of these sensory receptors based on stimuli that they respond to: Mechanoreceptors- pressure/stretch. Thermoreceptors- cold/warmth. Photoreceptors- different ranges of light. Chemoreceptors- binding of chemical to the receptor (smell, taste, pH, O2). Nociceptors- painful stimuli (heat, tissue damage). Sensory receptors are the ends of afferent neurons and can convert stimuli into graded potentials. Stimuli open ion channels directly or indirectly.

Answer 33

Transduction in sensory neurons involve opening or closing ion channels, directly by the stimulus, or through a second messenger system. Ion channels present at the distal tip of a single axon or are on a specialised receptor cell. An ion flux results in a change in membrane potential which results in a local graded potential (receptor potential). Current flows to a region where there are membrane voltage gated channels, usually at the first node of Ranvier. A larger stimulus results in a larger graded potential which results in more frequent action potentials.

Answer 34

Coding is the conversion of a stimulus into a signal that conveys the relevant information. The area of the body that when stimulated, leads to activity in a particular afferent neuron: receptive field. Modality- a term for a type of stimulus (heat, cold, sound, pressure). Modalities can be sub-divided (salty, sweet, bitter, sour of Taste). A given receptor is particularly sensitive to one stimulus modality the ‘adequate stimulus’. This is optimal for the signal transduction mechanism and ion channels.

Answer 35

As the strength of a stimulus increases, receptors on adjacent branches of an afferent neuron are activated, resulting in a summation of their currents. Stimulus location is coded by the site of a stimulated receptor and potentials travelling along unique pathways to a specific part of the CNS. In vision, hearing and smell, the stimulus location is interpreted as outside the body, rather than at the site of receptor stimulation. A neuron responds vigorously when stimulus is in the middle of the receptive filed (more receptors activated=more action potentials). The precision (acuity) for locating the stimulus depends on convergence. Greater convergence=less acuity.

Answer 36

It enables the localisation of a stimulus site for some sensory systems. Information form afferent neurons whose receptors are at the edge of a stimulus is strongly inhibited compared to information from the stimulus’ centre. It enhances the contrast between the centre and periphery of a stimulated region.

Nervous system Flashcards

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