Week 3: Nervous System Flashcards
What is the function of the nervous system?
To receive sensory input, process and integrate this sensory input, and activate an appropriate motor response.
What are the 2 major divisions of the nervous system?
Central and peripheral.
What components make up the central nervous system?
Brain and spinal cord.
What is the function of the CNS?
- Processes incoming sensory information.
- Source of thoughts, emotions and memories.
- Source of most signals that stimulate muscles to contract and glands to contract.
What components make up the peripheral nervous system?
All nervous tissue outside of the CNS:
* Nerves that attach to the spinal cord - spinal, cranial, peripheral, ganglia, enteric plexuses, sensory receptors.
* Special sense organs: ears, eyes, tongue, olfactory nerves.
What is a nerve?
A bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels, that lies outside the brain and spinal cord.
How many pairs of cranial nerves emerge from the brain?
12 pairs.
How many pairs of spinal nerves emerge from the spinal cord?
31 pairs.
What is a ganglia?
Ganglia are small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord.
What are ganglia closely associated with?
cranial and spinal nerves.
What are enteric plexuses, and what is their function?
Extensive networks of neurons located in the walls of the organs of the gastrointestinal tract.
The neurons of the enteric plexuses help regulate the digestive system.
What are sensory receptors, and what are examples of them?
A structure of the nervous system that monitors changes in the external or internal environment.
Examples are touch receptors in the skin, photoreceptors in the eye, olfactory receptors in the nose.
What are the functional divisions of the PNS?
- Somatic nervous system
- Autonomic nervous system
- Enteric nervous system
What neurons make up the somatic nervous system?
- Motor neurons that conduct impulses to skeletal muscle only.
- Sensory neurons that convey information from somatic receptors in the head, limbs, skin, and special sense receptors.
Is the somatic nervous system voluntary or involuntary?
The actions of the motor neurons of the SNS are voluntary.
What neurons make up the autonomic nervous system?
The ANS consists of:
* sensory neurons that convey information from autonomic sensory receptors located primarily in visceral organs.
* Motor neurons that conduct nerve impulses to smooth muscle, cardiac muscle, and glands.
Are the actions of the autonomic nervous system voluntary or involuntary?
The ANS is involuntary.
What are the 2 branches of the autonomic nervous system?
- Sympathetic division
- Parasympathetic division
What is the function of the sympathetic division of the autonomic nervous system?
The sympathetic division supports ‘fight-or-flight’ responses, such as increasing heart rate and blood pressure, pupil dilation, slows digestion.
What is the function of the parasympathetic division of the autonomic nervous system?
The parasympathetic division of the autonomic nervous system supports ‘rest-and-digest’ responses, such as lowering heart rate, increasing digestion rate, increasing salivation.
What neurons make up the enteric nervous system?
Neurons in enterix plexuses that extend most of the length of the GI tract. They operate independently of the ANS and CNS.
* Sensory neurons monitor chemical changes within the GI tract as well as the stretching of its walls.
* Motor neurons govern contractions of GI tract smooth muscle, secretions of GI tract organs, and activities of GI tract endocrine cells.
Is the enteric nervous system voluntary or involuntary?
Involuntary
What are the 3 basic functions of the nervous system?
- Sensory function
- Integrative function
- Motor function
What is the sensory function of the nervous system?
Sensory receptors detect internal or external stimuli.
What is the integrative function of the nervous system?
The activity of integration: processing sensory information by analysing it and making decisions for appropriate responses.
What is the motor function of the nervous system?
The activation of effectors (muscles - contraction, and glands - secretion) to provide an appropriate motor response to sensory information.
What are the 2 types of cells that make up nervous tissue?
- Neurons
- Neuroglia
What is a neuron?
A nerve cell that posesses electrical excitability - the ability to respond to a stimulus and convert it into an action potential.
What is a stimulus?
Any change in the environmnentthat is strong enough to initiate an action potential.
What is an action potential?
An electrical signal that propagates along the surface of the membrane of a neuron.
What are the 3 main body parts of a neuron?
- Cell body
- Dendrites
- An axon
What is the neuron cell body composed of?
A neucleus surrounded by cytoplasm that includes typical cell organelles.
What is a nerve fibre, and what are the 2 kindes that a neuron has?
Any neuronal process that emerges from the cell body of a neuron.
The 2 kinds are:
* Multiple dendrites
* A single axon
What is a dendrite?
A neuronal process that carries electrical signals towards the cell body.
The receiving, or input portions of a neuron.
What is an axon?
The single, long neuronal process that propagates a nerve impulse towards the axon terminals.
What is the axon hillock?
The location of the join between the axon and the cell body. The site where action potentials are triggered.
What is axoplasm?
The cytoplasm of an axon.
What is a synapse?
The site of communication between 2 neurons or between a neuron and an effector cell.
What are synaptic end bulbs?
Bulb-shaped structures at the tips of some axon terminals that contain synaptic vesicles which store neurotransmitters.
What is a neurotransmitter?
A molecule released from a synaptic vesicle that excites or inhibits another neuron, muscle fibre, or gland cell.
What are the 2 types of fast axonal transport movement directions?
- Anterograde - moves organelles and synaptic vesicles from the cell body to the axon terminals.
- Retrograde - moves membrane vesicles and other cellular materials from axon terminals to the cell body for degrading or recycling.
What are sensory neurons?
Sensory (afferent) neurons convey action potentials from sensory receptors into the CNS.
Sensory neurons either contain sensory receptors at their distal ends or are located just after sensory receptors that are separate cells.
What are motor neurons?
Motor (efferent) neurons convey action potentials away from the CNS to effectors (muscles and glands) in the PNS.
What are interneurons?
Interneurons (association neurons) are located in the CNS inbetween sensory and motor neurons.
They process incoming sensory information from sensory neurons and activate the appropriate motor neurons.
What are neuroglia?
Neuroglia, or glia, are several types of neural cells that support the function of neurons.
What is mylination?
The surrounding of an axon by a multilayered lipid and protein covering called a myelin sheath.
What is the function of myelination?
Electrical insulation of the axon and increased speed of nerve impulse conduction.
What are graded potentials?
Graded potentials are subthreshold changes in membrane potential that occur mainly in the dendrites and cell body of a neuron and diminish as they travel away from their point of origin due to decremental conduction.
What is summation in relation to graded potentials?
Summation is the process where 2 graded potentials add together. If they are both hyperpolarising or depolarising, the net result is a larger graded potential. If the 2 graded potentials are equal but opposite, then they will cancel out.
What are the 2 types of summation?
- Temporal summation
- Spatial summation
What is temporal summation?
Multiple, rapid and successive EPSPs from a single presynaptic neuron reaching a single postsynaptic neuron.
What is spatial summation?
2 different presynaptic neurons transmitting a single EPSP each onto the same postsynaptic neuron at approximately the same time.
What is hyperpolarisation?
When the resting membrane potential shifts more negative than the base resting potential.
What is depolarisation?
Movement of the action potential towards a value of 0 mV.
What is the term for a graded potential that makes the membrane more polarised?
hyperpolarising graded potential.
What is the term for a graded potential that makes the membrane less polarised?
Depolarising graded potential.
When does a graded potential occur?
When a stimulus causes mechanically gated or ligand-gated channels to open or close in an excitable cell’s plasma membrane.
What is a ligand?
A chemical messenger.
What is a ligand gated channel?
Transmembrane ion channels which are opened when a ligand binds to them to allow ions to pass through the membrane.
What is a mechanically gated channel?
A transmembrane ion channel which opens in response to mechanical forces such as vibration, stretch, or sound waves.
Where do graded potentials mainly occur in the anatomy of a neuron?
In the dendrites and cell body of a neuron.
What does it mean to say that these electrical signals are graded?
It means that they vary in amplitude depending on the strength of the stimulus.
What does it mean to describe the flow of current as localised?
The current spreads to adjacent regioins along the plasma membrane in either direction from the stimulus source for a short distance and then gradually dies out.
What is decremental conduction?
The mode of graded potential travel by which they die out as they spread along the membrane.
What is a postsynaptic potential?
A graded potential that occurs in the dendrites or cell body in response to a neurotransmitter.
What are action potentials?
Electrial signals that are brief, large alterations in membrane potential and are generated along the axon of a neuron.
What causes an action potential to occur, and at what part of the neuron do they begin?
They begin at the axon hillock, and occur when it is depolarised to -55 mV.
What value is the resting membrane potential?
-70 mV
What is threshold potential?
The threshold at which the voltage-gated sodium channels open. -55mV.
What occurs when the membrane is depolarised to -55 mV?
- There is a large jump in depolarisation which reverses the polarity of the membrane to positive.
- This is followed by a period of repolarisation, where the membrane potential returns towards the resting state.
- There is a period of hyperpolarisation where the membrane potential becomes more negative than the resting state.
- The membrane potential returns to the resting state.
What are voltage gated channels and where are they found?
Voltage gated channels are transmembrane ion channels that open and close in response to changes in membrane potential.
They are only found on axon membranes of a neuron.
Which fluid compartment contains the higher concentration of sodium ions?
Extracellular fluid contains the higher concentration of sodium ions, as such they constantly want to diffuse into the cell.
Which fluid compartment contains the higher concentration of potassium ions?
Intracellular fluid contains the higher concentration of potassium ions, as such they constantly want to diffuse out of the cell.
What are the 2 gates on a voltage-gated sodium channel?
- Activation gate: opens in response to change in membrane potential.
- Inactivation gate: closes the channel shortly after activation gate has been opened.
What causes the depolarisation of the membrane at the axon hillock?
Graded potentials depolarising the membrane potential to -55 mV.
What occurs when the threshold potential of -55 mV is met?
Voltage-gated sodium channels rapidly open and sodium ions rush into the cell, depolarising the membrane potential to +30 mV.
What occurs when the membrane potential reaches its peak positive value?
Inactivation gates close the sodium channels. Simultaneously, voltage-gated potassium channels open, causing potassium ions to leave the cell. This causes repolarisation to begin.
What occurs when the membrane potential returns to the baseline following depolarisation?
The return to baseline membrane potential triggers the K+ channels to begin closing. They do this slowly
What causes the period of hyperpolarisation following the repolarisation of the cell?
As the voltage-gated K+ channels close slowly, this allows more K+ ions to leave the cell. This causes the membrane potential momentarily to keep dropping past -70 mV, creating a period of hyperpolarisation.
What are the 7 main steps in the generation of an action potential?
- Graded depolarisation brings an area of excitable membrane to threshold.
- Sodium channel activation occurs.
- Sodium ions enter the cell and depolarisation occurs.
- Sodium channels are inactivated.
- Voltage-gated potassium channels open and potassium moves out of the cell initiating repolarisation.
- Sodium channels regain their normal properties.
- A temporary hyperpolarisation occurs.
What are the 4 phases of an action potential activation?
- Resting state
- Depolarising phase
- Repolarising phase begins
- Repolarising phase continues
Explain what occurs in the resting state phase of action potential activation?
All voltage-gated sodium and potassium channels are closed.
The axon membrane potential is at -70mV. There is a small buildup of negative charges along the inside membrane surface and an equal number of positive charges along the outside membrane surface.
Explain what occurs in the depolarising phase of action potential activation?
When membrane potential reaches threshold (-55mV), the sodium channel activation gates open. As Na+ ions move through these channels into the neuron, a buildup of positive charges forms along the inside membrane surface and the membrane becomes depolarised.
Explain what occurs in the repolarisation phase begins phase of action potential activation?
When the membrane potential peaks (approx. +30mV), Na+ channel inactivation gates close and K+ channels open slowly. The membrane starts to become repolarised as some K+ ions leave the neuron and negative charges begin to build up along the inside membrane surface.
Explain what occurs in the repolarisation phase continues phase of action potential activation?
K+ outflow continues. As more potassium ions leave the neuron, more negative charges build up on the inside membrane surface. K+ outflow eventually restores resting membrane potential. Na+ channel activation gates close and inactivation gates open.
When is the return to resting state of membrane potential complete?
The return to resting state is complete when the K+ gates close.
What are the 2 refractory periods of an action potential, and when do they occur in the action potential timeline?
- Absolute refractory period: occurs during depolarisation and repolarisation phases.
- Relative refractory period: coincides with the hyperpolarisation phase.
What is the absolute refractory period, and what does it prevent?
The period of an action potential during which a second action potential cannot be produced in the same region of membrane.
The absolute refractory period prevents the immediate generation of a second action potential, which would cause overlapping action potentials.
What causes the absolute refractory period?
The voltage-gated sodium channels are either already open or inactivated and cannot be restimulated.
When does the absolute refractory period end?
It ends when voltage-gated Na+ channels begin to transition from inactivated to a resting/closed state.
What is the relative refractory period, and what does it allow?
It is the period immediately following the absolute refractory period, during which a second action potential can be generated if strongly stimulated.
This period ensures that action potentials only travel forward towards the axon terminal.
What causes the relative refractory period?
In this period inactivated Na+ channels are transitioning to resting state, meaning an increasing number of Na+ channels are capable of opening.
K+ is still leaving through voltage-gated K+ channels which close slowly.
What is propagation of an action potential?
The transmission of an action potential along an axon’s length.
How does depolarisation propagate along an axon?
The Na+ influx that initiates depolarisation moves both forwards and backwards along the axon as it initiates depolarisation at the current membrane.
The refractory phase of the previous membrane prevents this Na+ influx from initiating depolarisation, ensuring that only the Na+ that moves forward will trigger an action potential.
What is the name for conduction of an action potential along an axon?
Propagation
What is a myelinated axon?
An axon surrounded by a myelin sheath.
What is an unmyelinated axon?
An axon that lacks a myelin sheath.
What is myelin?
An insulative material made of lipids and proteins which wraps tightly in layers around the axon.
Explain myelinated axons.
Myelinated axons have myelin producing neuroglia (oligodendrocytes in the CNS and Schwann cells in the PNS) wrapped around them at regular intervals along their length, creating myelin segments. Each myelin segment is separated by small, uncovered regions of the axon called nodes of Ranvier.
Explain unmyelinated axons.
Multiple axons are wrapped by a neuroglia. This neuroglia does not produce myelin. There are no segments or nodes of ranvier in unmyelinated axons.
Where are action potentials generated on myelinated axons?
At the nodes of ranvier.
What effect does the lack of myelination have on propagation?
The lack of insulation (myelination) allows sodium ions leak away from the axon. This means that the sites of action potentials must be close together to allow propagation to occur.
This also means that the propagation speed is much slower than in myelinated axons.
What is the process of conduction called in an unmyelinated axon?
Continuous conduction.
What effect does myelination have on propagation?
The action potential at the first node of ranvier generates ionic currents in the cytosol and interstitial fluid of the myelin sheath, which depolarises the next node, and so on.
This means that the distance between the nodes can be much greater than that between action potential sites in unmyelinated axons, and thus the speed of propagation is greatly increased.
What is the process of propagation in myelinated axons called?
Saltatory conduction.
What are 3 factors that affect the speed of propagation?
- Amount of myelination: More myelination causes faster propagation.
- Axon diameter: larger diameter axons propagate faster.
- Temperature: Axons propagate at lower speeds when cooled.
How does the sensory system differentiate between stimuli of varied intensities?
Light intensity stimuli generate a low frequency of action potentials.
The stronger the stimuli, the higher the frequency of action potentials.
Does stimulus intensity affect the voltage or duration of an action potential?
No, action potential amplitude and duration do not vary in size for any reason.
Name the points on the picture.
- Presynaptic neuron
- Ca2+
- Synaptic end bulb
- Synaptic cleft
- Neurotransmitter
- Ligand-gated channel closed
- Postsynaptic potential
- Nerve impulse
- Ligand-gated channel open
- Voltage-gated Ca2+ channel
What is a synapse and what is it’s function?
A synapse is a specialised junction or connection between a nerve cell and another cell (effector cell, endocrine cell etc.).
The primary function of synapses is to enable the modulation and control of processes at these junctions.
In a chemical synapse, what is a presynaptic axon terminal?
The end of the first neuron, where the action potential arrives.
In a chemical synapse, what is a postsynaptic second cell?
The other nerve, or an effector cell.
In a chemical synapse, what is a presynaptic terminal?
The region at the end of the first neuron, where the action potential triggers a synaptic transmission.
In a chemical synapse, what is a postsynaptic membrane?
The mebrane of the second cell that faces the presynaptic terminal.
In a chemical synapse, what is an action potential/nerve impulse?
It initiates the events at the synapse when it reaches the axon terminal.
What is exocytosis?
Exocytosis (in a synapse) is the process by which a synaptic vesicle releases its contents into the synaptic cleft.
In a chemical synapse, what is a synaptic transmission?
A sequence of events initiated by the action potential, leading to the release of neurotransmitters.
What are the 6 events in the sequence of synaptic transmission?
- Arrival of action potential: AP arrival at the axon terminal triggers the opening of voltage-gated Ca2+ channels.
- Calcium influx: Ca2+ enters the presynaptic terminal.
- Vesicle movement and fusion: Ca2+ stimulates vesicles to release neurotransmitters into the synaptic cleft.
- Neurotransmitters: Diffuse across the synaptic cleft and bind to postsynaptic membrane receptors.
- Postsynaptic response: Neurotransmitter binding triggers a response on the postsynaptic membrane.
-
Termination of transmission: to prevent continuous stimulation
- Ca2+ is actively removed from the presynaptic terminal.
- Enzymes on the postsynaptic membrane break down neurotransmitters to prevent further stimulation.
What are the 5 events in the sequence of synaptic transmission?
- Voltage-gated Ca2+ channels open and Ca2+ flows into the synaptic end bulbs.
- Vesicles filled with neurotransmitters migrate towards the pre-synaptic membrane.
- Neurotransmitters are released into the synaptic cleft via exocytosis.
- Neurotransmitters diffuse across the synaptic cleft and bind to ligand-gated ion channels on the post-synaptic membrane.
- Permeability of the post-synaptic membrane is altered and a post-synaptic action potential occurs.
What does EPSP and IPSP stand for?
- Excitatory postsynaptic potential
- Inhibitory postsynaptic potential
What is an Excitatory Postsynaptic Potential (EPSP)?
A depolarising postsynaptic potential, caused by a neurotransmitter, that makes the postsynaptic cell more excitable.
What is an Inhibitory Postsynaptic Potential (IPSP)?
A hyperpolarising postsynaptic potential, caused by a neurotransmitter.
What affects the grading of an EPSP or IPSP?
The distance it has to travel to the trigger point.
What causes an EPSP or IPSP?
Neurotransmitter binding to specific receptors on the postsynaptic membrane opens ligand-gated ion channels, allowing ion influx.
