Topic 8 - Grey Matter Flashcards
What is the nervous system divided into?
The central nervous system and the peripheral nervous system
What does the central nervous system consist of?
- The brain
- The spinal cord
What does the peripheral nervous system consist of?
- Sensory nerves which carry sensory information from the receptors to the central nervous system
- Motor nerves carrying the motor commands from the CNS to the effectors
What is the peripheral nervous system subdivided into?
- The autonomic nervous system
- The somatic nervous system
What are the characteristics of the autonomic nervous system?
- It is involuntary
- It stimulates smooth-muscle, cardiac muscle and glands
What are the characteristics of the somatic nervous system?
- Voluntary
- Stimulates skeletal muscle
What is the autonomic nervous system subdivided into?
- The sympathetic nervous system
- The parasympathetic nervous system
What is the function of the sympathetic nervous system?
It prepares the body for fight or flight response (e.g. Speeds up heart rate)
What is the function of the parasympathetic nervous system?
Prepared body for rest and digest
What is the difference between a neurone and a nerve?
A neurone is a single cell whereas the nerve is a more complex structure containing a bundle of axons of many neurones surrounded by protective covering.
What is the cell body of a neurone?
It contains the nucleus and cell organelles within the cytoplasm.
What are the two types of thin extensions from the cell body of a neurone and describe their function?
- Dendrites - conduct impulses towards the cell body (very thin)
- The axon - A single long process, transmits impulses away from the cell body
List the three types of neurones
- Motor neurones
- Sensory neurones
- Relay neurones
What are motor neurones and what is the function?
These conduct impulses from the CNS to the effector (muscles or glands). The cell body is always situated within the central nervous system and the axon extends out. The axons of some motor neurones can be extremely long such as those that run the full length of the leg.
What is the function of sensory neurones?
These carry impulses from sensory cells to the CNS.
What is the function of relay neurones?
These are found mostly within the CNS. They can have a large number of connections with other nerve cells. Relay neurones are also known as connector neurones and as interneurones.
What are motor neurones also known as?
Motor neurones are also known as effector neurones.
What is the myelin sheath?
Fatty material which forms a layer round the axons in many
mammal neurones. It acts as an insulator.
What is the importance of the myelin sheath in ensuring that nerve impulses pass quickly along the axon?
Myelin sheath only allows the electric currents which are set up by an action potential to flow at the gaps where no myelin is present. Nerve impulses therefore travel along myelinated axons in a series of jumps. This allows them to transmit impulses much faster than non-myelinated neurones.
What is the myelin sheath made up of?
Schwann cells
What are Schwann cells?
A cell that produces myelin, the fatty material that surrounds many axons in mammalian nerve cells. Each Schwann cell wraps round and round the axon forming a layer of myelin.
What are simple nerve pathways known as?
Reflex arcs
Define reflexes
Rapid, in voluntary responses to stimuli
Describes a reflex arc including all the different receptors and neurones that are used
1) Stimulus- a change in the environment (usually outside body)
2) Receptor - Detects stimulus + generates an impulse
3) Sensory neurone - Carries impulse to the CNS along a Sensory Pathway.
4) Sensory neurone forms a synapse with a relay neurone.
5) Relay neurone - Carries impulse through the CNS. Connects Sensory neurone to motor neurone . Relay neurone forms a synapse with a motor neurone.
6) Motor neurone - Carries impulse away from CNS
7) Effector - Muscle or gland which produce a response
8) Response - Movement or releases hormone
What are the advantages of a reflex arc?
- Fast
- Reflex arcs do not require conscious thought
- They are protective responses to threat
- Minimum number of neurons to minimise the number of synapses
What are synapses?
A small gap between two neurones. A nerve impulse travels along the presynaptic neurone. When it arrives at the synapse, it causes small amounts of a chemical neurotransmitter to be released. The neurotransmitter diffuses across the synapse and produces a nerve impulse in the postsynaptic neurone. Synapses of the slowest parts of the nervous system as they rely on diffusion not electrical impulses.
What controls the size of the pupil?
The muscles of the iris.
What are the two types of muscles in the iris?
- Circular muscles
- Radial muscles
How do the muscles of the iris respond to light?
The size of the pupil is controlled by the autonomic nervous system. The radial muscles are controlled by the sympathetic reflex which means when they contract and the circular muscles relax that the pupil dilates allowing increased light to be taken in. The circular muscles are controlled by the parasympathetic reflex and they contract while the radial muscles relax to decrease the amount of light coming into the eye.
What are photoreceptors?
A receptor cell which is stimulated by light and enables us to detects changes in our environment. In the human eye photoreceptors are contained in the retina. There are two distinct types, rods and cones.
What happens when high light levels strike the photoreceptors in the retina regarding the pupil size? Describe how the pupil size changes because of this.
High light levels striking the photoreceptors in the retina cause nerve impulses to pass along the optic nerve to a number of different sites within the CNS, including a group of coordinating cells in the midbrain. Impulses from the cells are sent along parasympathetic motor neurones to the circular muscles of the iris, causing them to contract. At the same time, the radial muscles relax. This constricts the pupil, reducing the amount of light entering the eye.
What is the resting potential?
The difference in electrical charge across the membrane of a resting neurone. When the inside of the axon is more negative than outside; the membrane is said to be polarised. The potential difference across the membrane is about –70 mV and this is known as the resting potential. This is due to the difference in permeability of the cell surface membrane to different ions which results in their uneven distribution.
Describe how a resting potential is formed and maintained.
1) The action of Na+/K+ pumps creates concentration gradients across the membrane. The pumps generate a high concentration of K+ (potassium ions) inside the cell and a high concentration of an Na+ (sodium ions)outside the cell.
2) Protein channels allow K+ ions to cross the membrane but not the Na+. K+ leaves the cell by facilitated diffusion, moving from a high concentration inside the cell to a low concentration through the protein channels outside the cell.
3) As the K+ leaves, the inside of the cell becomes more negatively charged due to the loss of positively charged ions relative to the outside.
4) The more potassium ions that the diffuse out of the cell, the larger the potential difference across the membrane. The increased negative charge created inside the cell as a consequence attracts potassium ions back across the membrane into the cell down an electrical gradient.
5) When the potential difference across the membrane is around -70mV, the electrical gradient exactly balances the chemical gradient. There is no net movement of potassium ions and hence a steady state exists maintaining the potential difference and -70mV.
Why is the axon resting potential - 70mV?
- The concentration gradient generated by the Na+/K+ pump
- The electrical gradient due to the difference in charge for the two sides of the membrane resulting from K+ diffusion
What is the electrochemical equilibrium regarding the resting potential of an axon?
When the potential difference across the membrane of the axon is around -70 mV, the electrical gradient exactly balances the chemical gradient. There is no net movement of K+ and hence a steady state exists.
The sodium potassium pumps of the axon pump against the concentration gradient of the two ions. How is the energy supplied?
By the hydrolysis of ATP
Define depolarisation
When the potential difference across the neurone is reversed, making the inside of the axon more positive than the negative outside.
Define repolarisation
The return of the resting potential of -70mV.
Define action potential
A large change in the voltage across the membrane.
What are the three stages of an action potential?
1) depolarisation
2) Repolarisation
3) hyperpolarisation and the restoring of the resting potential
What occurs before the depolarisation phase of an action potential?
A stimulus
Explain the function of a stimulus for an action potential
A stimulus excites the neurone cell membrane. This causes sodium ion channels to open and therefore the membrane becomes more permeable to sodium. Sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neuron less negative.
Describe what happens during the depolarisation phase of an action potential.
As the sodium continues to move by facilitated diffusion more voltage dependent sodium channels open causing more sodium ions to diffuse into the neurone. Depolarisation increases at a threshold of around -55 mV. There is a higher concentration of sodium ions outside the axon, so sodium ions flow rapidly inwards through the voltage-dependent Na+ channels, causing a buildup of positive charge inside. The potential difference across the membrane reaches +40mV.
Describe what happens during the repolarisation phase of an action potential.
At a potential difference of around +30/40mV voltage dependent sodium channels close and the voltage dependent potassium ion channels open. As a result, the potassium ions move out of the axon down the electrochemical gradient. As potassium ions flow out of the cell, the inside of the cell once again becomes more negative than the outside.
Describe what happens during the hyperpolarisation and restoring of the resting potential phase of an action potential.
Voltage dependent potassium ion channels are slow to close, so there is a slight overshoot with too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential. Therefore the resting potential is re-established by the closing of the voltage-dependent potassium ion channels and potassium ions diffuse into the axon, through K+ channels.
What is the refractory period?
Following an action potential, the neurone cell can’t be excited again straight away. This is because the ion channels are recovering and they can’t be made to open.
Explain how the impulse passed along an axon (wave of depolarisation -> non-myelinated)?
1) At resting potential there is a positive charge on the outside of the membrane and negative charge on the inside.
2) When stimulated, voltage-dependent sodium ion channels open, and sodium ions flow into the axon, depolarising the membrane. Localised electric currents are generated in the membrane. Sodium ions move to the adjacent polarised (resting) region causing a change in the electrical charge (potential difference) across this part of the membrane.
3) The change in the potential difference in the membrane adjacent to the first action potential initiates a second action potential. At the site of the first action potential the voltage dependent sodium channels close and voltage dependent potassium ion channels open. Potassium ions leave the axon, repolarising membrane. The membrane becomes hyperpolarised.
4) The third action potential is initiated by the second. In this way local electric currents cause the nerve impulse to move along the axon. At the site of the first action potential, potassium ions diffused back into the axon, restoring the resting potential.
What is all or nothing ?
An expression used to describe an action potential. When an action potential is produced in a nerve cell, it is always the same size. It does not matter how big the initial stimulus, the action potential will always involve the same change in potential difference across the cell surface membrane. Because of this, the only way that
information about the strength of a stimulus can be carried is by varying the number of nerve impulses in a given time.
How long is the refractory period?
About 5 milliseconds
Are impulses different sizes?
The same size action potential is generated by stimuli of different sizes and so impulses are the same size. A stimulus must be above a threshold to generate an impulse. The all or nothing law states that the size of a stimulus, assuming it is above the threshold has no effect on the size of the action potential.
What are the different mechanisms that are used to communicate the intensity of a stimulus?
- The frequency of impulses
- The number of neurones in a nerve that are conducting impulses
What are the characteristics of the strongest stimuli?
High firing frequency + Many neurones used
What are the two main factors that determine the speed of the conduction of impulses?
- The diameter of the axon
- Presence or absence of a myelin sheath
How does the diameter of the axon impact the speed of conduction?
Wider the diameter = faster the impulse travels
How does the presence of a myelin sheath increase the speed of the conduction of impulses?
Axons that are myelinated have a faster impulses.
- Myelin sheath acts as an electrical insulator, which prevents any flow of ions across the membrane.
- Myelin sheath is made of Schwann cells and between Schwann cells are tiny patches of bare membrane called the nodes of ranvier where the sodium ion channels are concentrated. Depolarisation occurs here. As ions flow across the membrane at one node during depolarisation, a circuit is set up which reduces the potential difference of the membrane at the next node, triggering an action potential. This is much faster than a wave of depolarisation along the whole membrane and means a myelinated axon has a higher impulse velocity than an unmyelinated axon. This is known as saltatory conduction which is very fast.
What is saltatory conduction?
The way in which a nerve impulse travels along a myelinated axon in a series of jumps. Myelin is fatty material which forms a layer round the axons in many mammal neurones. It acts as an insulator and only allows the electric currents which are set up by an action potential to flow at the gaps where no myelin is present. This allows myelinated axons to transmit impulses much faster than non-myelinated ones.
What is the synaptic cleft?
The gap between a synaptic knob and post-synaptic membrane at a synapse. When a nerve impulse arrives at synapse, it causes small amounts of a chemical neurotransmitter to be
released. The neurotransmitter diffuses across the synaptic cleft and produces a nerve impulse in the second neurone.
What the presynaptic cell?
A cell or membrane which comes before a synapse is described as presynaptic.
What is the postsynaptic cell?
A cell or membrane which comes after a synapse is described as postsynaptic.
What are synaptic vessels?
A small membrane-surrounded sac in the presynaptic neurone containing neurotransmitter.
What are neurotransmitters?
Chemical that is responsible for carrying a nerve impulse across a synapse.
Explain how the synapse transmits an impulse?
1) An action potential arrives at the presynaptic membrane.
2) The membrane depolarises. Calcium ion channels open. Calcium ions enter the neurone.
3) Calcium ions cause synaptic vessels containing neurotransmitter to fuse with the presynaptic membrane.
4) Neurotransmitter is released into the synaptic cleft.
5) Neurotransmitter binds with receptors of the post synaptic membrane. Cation channels open. Sodium ions flow through the channels.
6) The membrane depolarises and initiate an action potential.
7) When released from the receptor the neurotransmitter will be taken up across the presynaptic membrane (hole or after being broken down), or it can diffuse away and be broken down.
What are the three stages leading to the nerve impulse passing along the postsynaptic neurone?
- Neurotransmitter release
- Stimulation of the postsynaptic membrane
- Inactivation of the neurotransmitter
Why is the calcium important in the transmission of an impulse across the synapse?
When the presynaptic membrane is depolarised by an action potential, channels in the membrane open and increase the permeability of the membrane to calcium ions. These calcium ions are in greater concentration outside the cell, so they diffuse across the membrane and into the cytoplasm. The increased calcium concentration causes synaptic vessels containing acetylcholine to fuse with the presynaptic membrane and release their contents into the synaptic cleft by exocytosis.
Explain how the shape of cation channels changes to allow the flow of sodium ions into the postsynaptic membrane.
Embedded in the postsynaptic membrane are specific receptor proteins that have a binding site with a complimentary shape to part of the acetylcholine molecule. The acetylcholine molecule binds to the receptor, changing the shape of the protein, opening cation channels and making the membrane permeable to sodium ions.
What does the extent of the depolarisation of the postsynaptic membrane depend on?
The amount of the acetylcholine reaching the postsynaptic membrane. This will depend on the frequency of impulses reaching the presynaptic membrane. The number of functioning receptors in the postsynaptic membrane will also influence the degree of depolarisation.
What can happen to the neurotransmitter once it has been used?
- Some neurotransmitters are actively taken up by the presynaptic membrane and the molecules are used again.
- Some neurotransmitters rapidly diffuse away from the synaptic cleft or are taken up by other cells of the nervous system.
- Some neurotransmitters are broken down, for example in the case of the acetylcholine a specific enzyme (acetylcholinesterase) at the postsynaptic membrane breaks it down so that it can no longer bind to receptors.
What are the two roles of synapses?
- Control of nerve pathways, allowing flexibility of response
- Integration of information from different neurones, allowing a coordinated response.
What are the two main factors that affect the likelihood that the postsynaptic membrane will depolarise?
- The type of synapse
- The number of impulses received
What are the two types of synapses?
- Excitatory
- Inhibitory
What are excitatory synapses?
Neurotransmitters released from an excitatory synapse make the postsynaptic membrane more permeable to sodium ions, increasing the likelihood that an action potential will be triggered in the postsynaptic neurone. Several excitatory impulses added together produce sufficient depolarisation to produce an action
potential in the postsynaptic cell.
What is summation?
When several impulses added together produce an action potential in the postsynaptic cell.
What are the two types of summation?
- Spatial
- Temporal
What is spatial summation?
Here the impulses are from different synapses, usually from different neurones.
What is temporal summation?
When several impulses arrive along a single neurone.
What are inhibitory synapses?
A synapse which will make it less likely that an action potential will occur in the postsynaptic nerve cell.
How do inhibitory synapses work?
Neurotransmitters from these synapses open channels for chloride ions and potassium ions. Chloride ions move into the cell carrying a negative charge and potassium ions will move out of the cell carrying a positive charge. Hyperpolarisation occurs as it inside becomes more negative than usual and this makes depolarisation harder.
What are the two types of photoreceptors?
- Rods
- Cones
What are rods?
A receptor cell, found in the retina of the eye, which is sensitive to light. Rod cells contain a light-sensitive pigment called rhodopsin. When light falls on rhodopsin, it breaks down into retinal and opsin. This reaction leads to the generation of an action potential in a neurone in the optic nerve. Rod cells only give black and white vision but are able to work in dim light conditions.
What are cones?
A receptor cell, found in the retina of the eye, which is sensitive to light. Cone cells contain pigments which are sensitive to light of different wavelengths. Because of this, cones allow colour vision.
What are bipolar neurone cells?
A type of nerve cell which has a cell body with two long, thin branches. One of these is a dendrite which brings impulses to the cell body. The other is an axon which transmits impulses away from the cell body to other neurones. Bipolar cells in the retina of the eye are important in vision.
