5.1.3 Neuronal communication Flashcards
(39 cards)
Why is communication needed?
-so that organisms can respond to changes in their environment to increase chances of survival
-can be in response to internal or external environment
Homeostasis
-coordinating the functions of organs in order to maintain a relatively constant internal environment
Cell signalling
-occurs through one cell releasing a chemical which has an effect on a target cell
-cells can transfer signals locally i.e neurotransmitters across synapses or across large distances i.e secretion of hormones
What is the nervous system?
-made up of the brain and spinal cord(CNS) and the neurones that connect CNS to the rest of the body(PNS)
-responsible for detecting stimuli and coordinating a response–} faster and more targeted than hormonal communication
Neurones
-specialised network of nerve cells that can transmit electrical impulses rapidly around the body so the organism can respond to internal/external changes
What are the 3 main types of neurones?
-sensory neurones= transmit nerve impulses from receptors to CNS
-relay neurones= transmit nerve impulses from sensory to motor
-motor neurones= transmit nerve impulses from CNS to effectors
Describe the general structure of a neuron
-cell body= contains nucleus, has some ER and mitochondria within cytoplasm(involved in production of neurotransmitters)
-dendrites=transmit electrical impulses towards cell body
-axons=transmit impulses away from cell body
-axon terminal=end of an axon where neuro transmitters are released
-myelin sheath=layers of lipid rich plasma membrane produced by Schwann cells, act as an insulating layer to speed up electrical impulse, can also provide nutritional support(phospholipids)
-Nodes of Ranvier=gaps in the myelin sheath that speeds up transmission because electrical impulses must jump from one node to the next
Structure of a sensory neuron
-one long dendron to carry nerve impulses from receptor cells to cell body
-one short axon that carries nerve impulses from cell body to CNS
-cell body in the middle
-dendrites are not connected to cell body
Structure of a motor neuron
-many short dendrites that carry nerve impulses from CNS to cell body
-one long axon to carry nerve impulses from cell body to effector cells
-dendrites are connected to cell body
-cell body at the top
Structure of a relay neuron
-many short dendrites that carry nerve impulses from sensory neurons to cell body
-many short axons to carry nerve impulses from cell body to motor neurones
-no myelin sheath
-cell body in the middle
Describe the pathway of nervous communication
-stimulus is detected by a receptor
-electrical impulse passes from receptor to CNS via sensory neurone
-impulse is passed from sensory to motor neurone via the relay neurone in the CNS
-the motor neurone passes along electrical impulse from CNS to effectors(muscles/glands)
-effectors coordinate a response
What are sensory receptors?
-specialised cells that detect changes in the environment
-convert the stimulus they detect into a nerve impulse–} info is passed through the nervous system where a response is coordinated
What are the 2 main features of a sensory receptor?
-it is specific to a single type of stimulus
-act as a transducer= convert a stimulus into a electrical impulse called a generator potential
4 types of sensory receptors
-thermoreceptors= respond to heat in the end bulbs of Krause in the tongue
-chemoreceptors= respond to chemicals in the olfactory receptor of the nose
-photoreceptors= respond to light the cone cells of the eye
-mechanoreceptors= respond to pressure in the Pacinian corpuscle in the skin
What is the resting potential?
-the potential difference* across a neurone’s membrane when it is at rest
-happens because the inside of the cell is more negatively charged than the outside–} results in a voltage/PD across the membrane, therefore the membrane is polarised
*the difference in charge between the inside and outside of around -70mv
What is the generator potential?
-the change in potential difference due to a stimulus
-occurs when a stimulus is detected and the cell membrane becomes more permeable–} allows more ions to move in and out of cell, which alters PD
What is an action potential?
-if the generator potential is big enough/ threshold of positive charge inside the axon compared to our tide the axon is reached, it will trigger formation of an action potential
-energy of the stimulus temporarily reverses the charges on the axon membrane–} PD across membrane increases to +40mv
What are Pacinian corpuscles?
-specific sensory receptors that detect mechanical pressure
-located joints and deep within the skin
Describe the structure of a Pacinian corpuscle?
-the end of the sensory neurone is found in the centre surrounded by connective tissue/lamellae (each layer separated by a layer of gel)
-sodium ion channels within the membrane (responsible for transporting sodium ions across the membrane)
-neurone ending has a stretch-mediated sodium channel–} permeability changes when they change shape
Explain the steps of how a Pacinian corpuscle converts mechanical pressure into a nervous impulse
-in its resting state, the stretch-mediated sodium channels in the sensory receptor’s membrane are too narrow to allow sodium ions to pass through them(at resting potential)
-when pressure is applied to the Pacinian corpuscle it changes shape. This causes the membrane surrounding its neurone to stretch
-when the membrane stretches, the sodium ion channels present widen–} sodium ions can diffuse into the neurone
-influx of positive sodium ions changes the PD of the membrane= becomes depolarised–} this results in a generator potential
-in turn, the generator potential creates an action potential (a nerve impulse) that passes along the sensory neurones.
-action potential will then be transmitted along neurones to the CNS
Why does the resting potential occur?
-occurs as a result of the movement of sodium and potassium ions across the axon membrane(electrochemical gradient)
-must be transported via sodium-potassium pumps + ion channels because the phospholipid bilayer prevents these ions from diffusing across the membrane
Describe the events that result in the creation of a resting potential
1) sodium ions are actively transported out of the neurone and potassium ions are actively transported into the neurone by the potassium-sodium pump–} for every 3 sodium ions pumped out, 2 potassium ions are pumped in
2) therefore, there are more sodium ions outside of the membrane than inside the axon cytoplasm in comparison to potassium(has more inside cytoplasm)
3) sodium ions can diffuse back into the axon down its electrochemical gradient, whereas potassium ions move out of the axon via facilitated diffusion through the potassium ion channels
4) however when the cell is at rest, most of the gated sodium ion channels are closed, preventing the sodium ions to move back into axon whereas many potassium ion channels are open, so lots of potassium ions can diffuse out of the axon
5) therefore, outside of the cell is more positively charged than inside the cell, meaning the resting potential inside the cell is -70mv
Describe the sequence of events that leads to an action potential
1) the neurone is at resting potential, not transmitting any impulse. Some K+ ion channels are open but sodium voltage-gated ion channels are closed
2) STIMULUS: energy of stimulus triggers some sodium voltage-gated ion channels to open, making membrane more permeable to sodium. Sodium ions can diffuse into the neurone down their electrochemical gradient–} makes the inside of the neurone less negative
3) DEPOLARISATION: if the potential difference reaches the threshold(around -55mv), more voltage-gated sodium ion channels open and sodium ions to diffuse into neurone(positive feedback)
4) REPOLARISATION: at a PD of around +40 mv, the voltage-gated sodium ion channels close and potassium ion channels open. Sodium ions can no longer enter the neurone, but potassium ions can diffuse out of membrane down electrochemical gradient–} reduces charge inside neurone(negative feedback)
5) HYPERPOLARISATION: potassium ion channels are slow to close so there’s a slight overshoot, where too many potassium ions diffuse out of neurone. PD becomes more negative than the resting potential
6) RESTING POTENTIAL: voltage-gated potassium channels now close. Sodium potassium pump causes sodium ions to move out of the cell + potassium ions to move in–} resting potential is maintained
What is the refractory period?
a short period of time after an action potential where the neurone can’t be excited again–} ion channels are recovering and they can’t be made to open
-important because it makes sure that the action potentials are unidirectional and prevents them moving backwards along the axon
-also ensures that action potentials don’t overlap but instead pass along as discrete impulses
