Neuronal communication Flashcards
Homeostasis -
Function of organs must be coordinated, in order to maintain a relatively constant internal environment, seen in large multicellular animals, with different organs having different functions in the body.
Cell signalling -
Nervous and hormonal systems coordinate the activities of whole organisms. the communication is required on a cellular level. Occurs through one cell releasing a chemical which has an effect on another cell (target cell) Can be done locally or across large distances.
Cell signalling - Short distances
For example between neurons at synapses, signal used from neurotransmitters.
Cell signalling - longer distances
Transfer signals across large distances, from the use of hormones. E.G, the cells of the pituitary gland secrete ADH which acts on the kidneys to maintain the water balance in the body.
Coordination in plants -
Plants do not have a nervous system, but still respond to their external and internal influences, E.G plant stems grow towards a light source, to maximise the rate of photosynthesis, this achieved from plant hormones.
Neurons -
NS is made up of billions of specialised nerve cells called neurons, which transmit electrical impulses rapidly around the body, so the organism can rapidly respond to changes to its internal and external environment, several within a mammal that are different, work together to carry out information detected by sensory receptors to the effector, which in turn carries out the appropriate response.
Structure of a mammalian neuron - Cell body
Cell body - contains the nucleus surrounded by the cytoplasm and contain large amounts of mitochondria and ER. Involved in the production of neurotransmitters.
What are neurotransmitters?
Chemicals which are used to pass signals from one neuron to the next.
Structure of a mammalian neuron - Dendrons
Short extensions which come from the cell body. these divide into smaller and smaller branches known as dendrites, responsible for transmitting electrical impulses towards the cell body.
Structure of a mammalian neuron - Axon
Axons - They are singular elongated, nerve fibres that can be very long, for example those that transmit impulses from the tips of the toes and fingers to the spinal cord, the fibre is cylindrical in its shape and is surrounded by a plasma membrane
Types of neuron - Sensory neurons
Sensory neurons transmit impulses from a receptor cells to a relay neuron, motor neuron or the brain, have one dendron which carries the impulse to the cell body and one axon which carries the impulse away from the cell body.
Types of neuron - Relay neuron
Relay neurons transmit impulses between neurons, for example between sensory and motor neurons, have short axons and dendrons.
Types of neuron - Motor neuron
Motor neurons transmit impulses from a relay or sensory neuron to an effector, such as a muscle or gland, have one long axon and many short dendrites.
How does the nervous response of electrical impulse usually follow in a pathway.
Receptor-sensory neuron-relay neuron-motor neuron-effector cell.
The use of myelinated neurons -
The axons on some neurons are covered in a myelin sheath made up of layers of plasma membrane, these are produced from special cells called Schwann cells and these membranes produced grow around the axon many times, each time causes a double layer of a phospholipid bilayer to be laid down. Acts as an insulating layer and allows the electrical impulse to be conducted at much faster speeds compared to unmyelinated neurons.
Nodes of Ranvier -
Between each adjacent Schwann cell there is a small gap known as a node of Ranvier, this creates small gaps in the myelin sheath, the myelin sheath acts as electrical insulating layer, in these myelinated neurons the electrical impulse jumps from one node to the next node as it travels along the neuron. This allows the impulse to be transmitted much further. In non-myelinated neurons the impulse does not jump, it transmits continuously along the nerve fibre so its much slower.
Features of sensory receptors -
Specific to a single type of stimulus and are transducer.
What is a transducer?
Convert stimulus into a nerve impulse (electrical energy)
Example of a sensory receptor (photoreceptors)
Stimulus - light
Receptor - cone cell detect differences in light wavelengths
Sense organ - eye
The role of a transducer -
The receptor converts the stimulus into a nervous impulse, which is a generator potential. E.G a rod cell (in the eye) responds to lights and produces a generator potential.
Pacinian corpuscle - (as a sensory receptor)
These are specific sensory receptors, that detect mechanical pressure and are found deep inside the skin and are most common in the fingers and the soles of the feet, found in joints to enable you to know which joints change direction.
Pacinian corpuscle anatomy -
Within the membrane of the neuron there are sodium ion channels. These are responsible for transporting sodium ion channels across the membrane.
Stretch-mediated sodium channel -
Found in the ending neuron in a Pacinian corpuscle, consist of special type of sodium channel, when these channels change shape, their permeability to sodium ions also changes.
How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse (as a transducer)? Step 1 - Resting state
Normal state is known as the resting state, the specific sodium channel inside the Pacinian corpuscle are too narrow and not stretched to allow sodium ions through, the neuron of the Pacinian corpsucle is at resting state. (resting potential)
How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse (as a transducer)? Step 2 -
When mechanical pressure is applied to the Pacinian corpuscle, the corpuscle changes shape , causing the membrane around it to stretch.
How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse (as a transducer)? Step 3 -
When the membrane stretches, the sodium ion channel widens, so sodium ions can now diffuses into the neuron.
How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse (as a transducer)? Step 4 -
The influx of positive sodium ion channels change the potential of the membrane and it becomes depolarised, resulting in a generator potential.
How does the Pacinian corpuscle convert mechanical pressure into a nervous impulse (as a transducer)? Step 5 -
The generator potential creates an action potential (nerve impulse) that can pass along the sensory neuron. The action potential will then be transmitted along the neurons until it reaches the CNS.
Resting potential -
When a neuron is not transmitting an impulse, the potential difference across the membrane is known as the resting potential. In this state the outside of the membrane is more positively charged than the inside of the axon. Said to be polarised.
Sodium and potassium ions across the axon membrane -
Prevented by the phospholipid bilayer prevents the ions diffusing across the membrane and therefore have to be transported by via channel protein. Some are gated - must be opened to allow specific ions to pass through. Other channels remain open all the time allowing sodium and potassium ions to simply diffuse through.
Events of a resting potential - movement of sodium and potassium ions -
Sodium ions - are actively transported out of the axon.
Potassium ions - are actively transported into the axon by a specific intrinsic protein. The sodium/potassium pump. Movement is not equal. For every 3 sodium ions pumped out there are 2 potassium ions pumped in.
What does the sodium/potassium pump show?
As a result there are more sodium ions on the outside of the membrane than inside the axon cytoplasm, whereas there are more potassium ions inside the axon cytoplasm compared to the outside the axon. Therefore the sodium ions diffuse back into the axons as a result of it moving down the electrochemical gradient. whereas the potassium ions move out of the axon.
What does having most of the sodium ion channels closed do?
Prevents the movement of sodium ions, whereas most of the potassium ion channels are opened, therefore there are more positively charged ions outside the axon than inside the cell, this creates the resting potential across the membrane of -70mV, which is negative relative to the outside of the cell.
Action potential -
When a stimulus is detected by a sensory receptor, the energy of the stimulus temporarily reserves the charges on the axon membrane, this results potential difference across the membrane to rapidly to change and becomes positively charged at approximately +40mV. This is known as depolarisation.
Depolarisation -
A change in potential difference from negative to positive.
Repolarisation -
As the impulse passes repolarisation occurs a change from positive to negative, so it returns to resting potential.
How does an action potential occur?
An action potential occurs occurs when the protein channels change shape, this results in opening or closing (voltage gated ions channels)
Action potential - 1, neuron at resting potential
- Not transmitting a impulse
- Some potassium ion channels are open
- Sodium voltage - gated ion channels are closed
Action potential - 2, triggered stimulus
- Energy of the stimulus triggers some sodium voltage ions to open.
- Membrane becomes more permeable to sodium ions, they then diffuse into the membrane down a electrochemical gradient, this makes the neuron less negative (positive), the charge causes more sodium ion channels to open allowing more sodium ions to diffuse into the axon, example of positive feedback.
Positive feedback - 3
The change in charge has called more sodium ions to open, allowing more sodium ions to diffuse into the axon
The potential difference finally reaches +40mV - 4
The voltage gated sodium ion channels close and voltage gated ion channels and voltage gated potassium ion channels open, the membrane is more permeable to potassium ions but Sodium ions can no longer enter.
Movement of potassium ions - 5
Potassium ions diffuse out of the axon down their electrochemical gradient, reduces the charge resulting in the axon to be more negative than the outside. (Beginning of repolarisation)
Hyperpolarisation - 6
Initially, lot’s of potassium diffuse out of the axon, the axon becomes more negative to the outside, this is it’s normal resting state. This forms hyperpolarisation. The voltage gated potassium ion channels now close. The sodium potassium pump allow it to return to it’s resting potential where sodium ions move out of the cell an potassium ions to move inside the cell. returning to it’s resting potential and is now repolarised.
What is it called after the action potential where it cannot form again?
Refractory period, where voltage gated sodium ion channels remain closed. It prevents the propagation of an action potential backwards as well as forward. It makes the action potential unidirectional, non-overlapping and occur as discrete impulses.
Saltatory conduction - (myelinated axons)
These axons transfer the electrical impulses at much higher rates than non-myelinated, as the depolarisation of the axon membrane can only occur at the nodes of Ranvier where no myelin is present. Here only the sodium ions can pass through the protein channels in the membranes.
The speed of an action potential can also be influenced by two other factors other than the neuron being myelinated -
Axon diameter - the bigger the faster, less resistance of flow of ions in the cytoplasm compared with a smaller axon.
Temperature - higher temperature the faster as the ions diffuse at higher temperatures occurs up to 40 degrees as higher temperatures would denature the sodium-potassium pump.
All or nothing principle -
Nerve impulses are said to be all or nothing. A certain level of stimulus (threshold) will always trigger a response and if reached will result in an action potential. No matter how large the stimulus it will always trigger the same sized action potential. If the threshold value is not reached no action potential.
Synapses -
The junction between two neurons, impulses are transmitted across the synapse using chemicals called neurotransmitters.
