- outline the roles of sensory receptors in mammals in converting different forms of energy into nerve impulses
Sensory receptors – specialised cells that can detect changes in our surroundings. They are energy transducers that convert one form of energy to another. A transducer is adapted to detect changes in a particular form of energy.
Stimulus – a change in energy levels in the environment. Whatever the stimulus the sensory receptors convert the energy into a form of electrical energy called nerve impulse.
- describe, with the aid of diagrams, the structure and functions of sensory and motor neurones
There is a number of different neurones, including:
- Sensory neurone – carry action potentials from a sensory receptor to the central nervous system.
- Relay neurone – connect sensory and motor neurones.
- Motor neurone – carry action potentials from the central nervous system to an effector, e.g. muscle or gland.
- describe and explain how the resting potential is established and maintained
Resting potential– the potential difference or voltage across the neurone cell membrane while the neurone is at rest. It is about -60mV inside the cell compared with the outside. The membrane is said to be polarised.
- The sodium-potassium pumps in the membrane of the axon continually pump 3 Na+ ions out the cell for every 2 K+ ions moved into the cell against a concentration gradient, using energy provided by a large number of mitochondria found in the neurone.
- Due to the concentration gradients, Na+ ions tend to leak back into the cell, and K+ ions tend to leak out.
- K+ ions leaks out at a faster rate (1.5 times) than the rate of Na+ ions leaking back in, meaning that there are more positive ions (cations) outside the cell than inside the cell, and more negative ions (anions) in the cytoplasm.
- This all results in a potential difference of -60mV between the inside and outside of the cell. The difference is called the resting potential. The axon membrane is said to be polarised when it is in this state.
- describe and explain how an action potential is generated
Action potential– the depolarisation of the cell membrane so that the inside is more positive than the outside, with a potential difference across the membrane of +40mV.
- A stimulus causes sodium ion channels in the axon membrane to open, allowing Na+ ions to move into the cell, down an electro-chemical gradient (more positive to more negative).
- The movement of Na+ ions increases the potential difference resulting in voltage-dependent gates opening, which allow even more Na+ ions in. The large number of positive ions cause the potential difference to rise to +40mV.
- The sodium ion channels close and the potassium ions channels open, allowing K+ ions to move out of the cell, to try to restore the balance of charges either side of the membrane – this is repolarisation.
- So many K+ ions leave the cell causing the potential difference to drop below the –60mV (called hyperpolarisation), which causes the potassium ions channels to close.
- The membrane’s sodium-potassium pumps will quickly restore the potential difference to the normal resting potential figure of -60mV.
Refractory period– the short time when the membrane is hyperpolarised. In this time it is impossible to stimulate the cell membrane to reach another action potential.
Threshold potential– The potential difference across the membrane of about –55mV. If the depolarisation of the membrane does not reach the threshold potential then no action potential is created.
- describe and explain how an action potential is transmitted in a myelinated neurone, with reference to the roles of voltage-gated sodium ion and potassium ion channels
- When an action potential occurs, the sodium ion channels open at a particular point along the neurone, allowing Na+ ions to diffuse across the membrane from high concentration outside the neurone to low concentration inside the neurone.
- The movement of Na+ ions into the neurone upsets the balance of ionic concentrations created by the sodium-potassium pumps.
- The concentration of sodium ions inside the neurone rises at the point where the sodium ion channels are open, causing the Na+ ions to diffuse sideways, away from this region of increased concentration.
- This is called a local current – the movements of ions along the neurone. The flow of ions is caused by an increase in concentration at one point, which causes diffusion away from the region of higher concentration.
The Myelin Sheath:
The myelin sheath is an insulating layer of fatty material, made of Schwann cells, which is impermeable to Na+ and K+ ions. Therefore, the ionic movements that create an action potential cannot occur at the parts of axon with the myelin sheath. Instead, the action potentials only occur between the Schwann cells. These gaps are called nodes of Ranvier. The action potentials appear to jump from one node to the next. This is called saltatory conduction. This speeds up the transition of the action potential. Myelinated neurones conduct action potentials more quickly than non-myelinated neurones.
Saltatory conduction – ‘jumping conduction’ refers to the way that the action potential appears to jump from one node of Ranvier to the next.
- interpret graphs of the voltage changes taking place during the generation and transmission of an action potential
- The neurone is at resting state (-60mV) and the membrane is said to be polarised.
- A stimulus causes sodium ion channels to open and Na+ ions move into the cell causing the membrane potential to increase to the threshold potential (-55mV).
- The membrane becomes depolarised and the cell becomes positively charged inside compared to outside.
- The membrane potential reaches +40mV. The sodium ion channels close and the potassium ion channels open and K+ ions diffuse out of the cell.
- The membrane becomes repolarised as the potential difference goes back to negative inside compared with outside.
- The membrane becomes hyperpolarised as the potential difference overshoots slightly from too many K+ ions move out the cell.
- The original potential difference is restored so that the cell returns to its resting state.
- outline the significance of the frequency of impulse transmission
When a stimulus is at a higher intensity the sensory receptor will produce more generator potentials. This will cause more frequent action potentials in the sensory neurone. When these arrive at the synapse they will cause more vesicles to be released. Therefore this creates a higher frequency of action potentials in the postsynaptic neurone. Our brain can determine the intensity of the stimulus from the frequency of signals arriving. A higher frequency of signals means a more intense stimulus.
Summation – refers to the way that several small potential changes can combine to produce one larger change in potential difference across the membrane.
Low-level signals can be amplified by a process called summation:
- Temporal summation
- If a low-level stimulus is persistent it will generate several action potentials in the presynaptic neurone releasing many vesicles which will produce an action potential in the postsynaptic neurone.
- Several impulses arriving at the same neurone via the same synapse.
- Spatial summation
- Several presynaptic neurones each release small numbers of vesicles into one postsynaptic neurone.
- Several impulses arriving at the same neurone via several synapses.
- compare and contrast the structure of myelinated and non-myelinated neurones
- describe, with the aid of diagrams, the structure of cholinergic synapse
Synapse – the junction between two or more neurones, where neurones can communicate with, or signal to, another neurone.
Cholinergic synapse – those that use acetylcholine as their transmitter substance.
Neurotransmitter – this is the transmitter substance which is a chemical released by the presynaptic neurone, that diffuses across the synaptic cleft (the gaps between two neurones) to transmit a signal to the postsynaptic neurone.
- outline the role of neurotransmitters in the transmission of action potentials
- An action potential arrives at the presynaptic neurone. The voltage-gated calcium ion channels open and Ca+ ions diffuse into the synaptic knob.
- The Ca+ ions causes the vesicles containing acetylcholine to move to and fuse with the presynaptic membrane. Acetylcholine is released by exocytosis.
- Acetylcholine diffuse across the synaptic cleft and they bind to the receptor sites on the sodium ion channels in the postsynaptic neurone, causing the sodium ion channels to open. Na+ ions diffuse across the membrane and move into the postsynaptic neurone and an action potential is created.
- Acetylcholinesterase, found in the synaptic cleft, hydrolyses acetylcholine to ethanoic acid and choline so that they can be recycled and to stop the action potential in the postsynaptic neurone. Ethanoic acid and choline re-enter the presynaptic knob by diffusion and are recombined to acetylcholine using ATP from respiration in the mitochondria.
- outline the roles of synapses in the nervous system
The main role of synapses is to connect two neurones together so that a signal can be passed from one to the other. Other functions include:
- Several presynaptic neurones converging to one postsynaptic neurone – allows several signals from different parts of the nervous system to create the same response.
- One presynaptic neurone diverging to several postsynaptic neurone – allows one signal to be transmitted to several parts of the nervous system to create different responses. This is useful in a reflex arc – one postsynaptic neurone elicits the response while another informs the brain.
- Ensures signals are transmitted in one direction – only the presynaptic neurone contains vesicles of acetylcholine.
- Acclimatisation – after repeated stimulation a synapse may run out of vesicles containing the transmitter substance. The synapse is said to be fatigued, which explains why we soon get used to a smell or a background noise. It may also help to avoid overstimulation of an effector, which could damage it.
- Creation of specific pathways – thought to be the basis of conscious thought and memory.
- Allows temporal and spatial summation.