5.1.3: Neuronal communication Flashcards
Why do multicellular organisms require a communication system?
- Distance: cells monitoring change far away from those responding to it.
- Specialisation: cells are specialised to do a particular job (i.e. either detect or respond to stimuli)
- Seasonality: need to produce a coordinated response e.g. detect increased day length to flower at right time.
Homeostasis
the maintenance of a constant internal environment despite external changes; communication between cells allows multicellular organisms to achieve this.
Nerve impulse
Action potentials propagated along the axon of a neurone.
Nerve impulses can be described as “__-or-______”
All-or-nothing i.e. action potentials are always the same size
How is a response to a stimulus coordinated?
Coordination relies on communication between cells known as cell signalling, either between adjacent cells or cells that are far apart.
Action potential
The reversal and restoration of the electrical potential across the plasma membrane as an electrical impulse passes along the axon.
Membrane potential
voltage inside the axon compared to that outside the axon.
Sequence of events in an action potential
1) Resting potential
2) Depolarisation
3) Repolarisation
4) Hyperpolarisation
5) Return to resting potential
Describe resting potential
- Na⁺ ions pumped out of neurone and K⁺ pumped in by sodium-potassium pump
- K⁺ diffuse out more than Na⁺ diffuse in
- An electrochemical gradient of sodium-potassium ions is built up across the membrane
- The inside of the axon has a negative electrical charge compared to the outside
- Membrane potential is -70mV
Describe depolarisation
- Voltage-gated Na⁺ channels open when voltage reaches threshold potential (-50mV)
- Na⁺ rapidly diffuse into the neurone down their electrochemical gradient
- The inside of the neurone becomes more +vely charged w.r.t. the outside
Describe repolarisation
- Na⁺ channels close and voltage-gated K⁺ channels open
- The inside of the neurone become less +vely charged
- Membrane potential becomes -ve
Describe hyperpolarisation
- K⁺ ions diffuse into the neurone down their electrochemical gradient
- The electrical potential across the plasma membrane becomes more -ve than resting potential
- A new action potential cannot be generated immediately
Describe the return to resting potential
- Resting potential is re-established by the sodium-potassium pump and outward diffusion of K+ ions
- The potential difference across the membrane returns to -70mV
Propagation of an action potential along a non-myelinated neurone
1) At resting potential, outside of the neurone is more positive w.r.t. the inside. Membrane potential is -70mV.
2) A stimulus causes a sudden influx of Na⁺ ions (known as the action potential). The membrane is depolarised.
3) The influx of Na⁺ ions causes the formation of a localised electrical current, and thus the opening of voltage-gated Na⁺ channels a little further along the axon. The resulting influx of Na⁺ ions causes depolarisation.
4) Behind this new region of depolarisation, voltage gated Na⁺ channels close and voltage gated K⁺ channels open. K⁺ ions leave the neurone.
5) The action potential is propagated in the same way further along the neurone. The outward movement of K⁺ ions leads to depolarisation of the area of the axon behind the action potential.
6) Following repolarisation, the axon membrane returns to resting potential, allowing a new action potential to be generated.
Propagation of an action potential along a myelinated neurone
- In a myelinated neurone, there are gaps between each section of myelin sheath. These are known as the nodes of Ranvier.
- An action potential forms a localised circuit as it is propagated along a neurone.
- In a myelinated neurone, the localised circuit is longer than in a non-myelinated neurone, because this circuit must stretch between the adjacent nodes of Ranvier.
- This means that the action potential effectively ‘jumps’ from node to node along the neurone in a process known as saltatory conduction.
- Saltatory conduction is faster than the continuous movement of an action potential along a non-myelinated neurone (where the lack of myelin sheath and nodes of Ranvier means the action potential cannot ‘jump’).
Dendron
Carries an impulse towards the cell body; structurally the same as an axon.
Axon
Carries an impulse away from the cell body; structurally the same as a dendron.
Role of the axons/dendrons
Transmit nerve impulses away from/towards the cell body.
Role of the cell body
Contains the endoplasmic reticulum and mitochondria necessary for production of neurotransmitters.
Sensory neurones transmit impulses…
from sensory receptor cells to CNS
Relay neurones transmit impulses…
between neurones within the CNS
Motor neurones transmit impulses…
from the CNS to an effector
An effector can be a…
muscle or a gland
Sensory neurone myelination
Axons have myelin sheath
Relay neurone myelination
No myelin sheath
Motor neurone myelination
Axons have myelin sheath
Speed of impulse transmission (Sensory neurone)
Up to 100m/s
Speed of impulse transmission (Relay neurone)
Approx. 1 m/s
Speed of impulse transmission (Motor neurone)
Up to 100 m/s
Position of the cell body (Sensory neurone)
Outside the CNS
Position of the cell body (Motor neurone)
Inside the CNS
What is meant by myelination?
- Creation of a myelin sheath around an axon
- Sheath consists of many layers of plasma membrane, produced by Schwann cells, which grows around the axon
- Sheath acts as a layer of insulation and enables impulses to be transmitted at a faster speed
Autoimmune disease
a disease in which the immune system mistakenly attacks healthy body tissue.
Role of myelin
Forms an electrically insulating sheath around axons of certain neurones in the body, speeding up the transmission of nerve impulses.
Why might a damaged myelin sheath be a problem/lead to e.g. blindness?
Impulse received is too slow/does not reach from sensory receptor cells in eye to brain
Factors that affect the speed of nerve impulse transmission
- Myelination
- Axon diameter
- Temperature (after 40°C, proteins denatured)
Positive feedback
the enhancing or amplification of an effect by its own influence on the process which gives rise to it.
Example of positive feedback
- As soon as Na⁺ ions enter the axon, the potential difference across the membrane becomes less -ve
- This prompts the opening of more sodium channels, so more Na⁺ ions enter the axon
What type of sensory receptor is in the skin?
Mechanoreceptor
What is the name given to the receptor cells in the skin?
Pacinian corpuscles
How does a Pacinian corpuscle convert mechanical pressure into a nervous impulse?
- At resting potential, stretch mediated Na⁺ channels in neurone membrane are too narrow to let Na⁺ through
- When mechanical pressure applied to the Pacinian corpuscle, the lamellae are deformed and press on tip of neurone
- This deforms the neurone’s plasma membrane and widens the stretch-mediated Na⁺ channels
- Na⁺ diffuse into the neurone, depolarising it –> produces generator potential
- Threshold reached: action potential generated at first node of Ranvier
- Action potential propagated along sensory neurone to CNS
Transduction
Changing stimulus into a nerve impulse (called a generator potential).
What term describes specialised cells which convert one type of energy into an electrical nerve impulse?
Transducer
Transducer
Specialised cells which are capable of converting one type of energy into an electrical nerve impulse.
Process at a synapse
1) Action potential arrives at the end of presynaptic neurone.
2) Depolarisation of the membrane of the presynaptic neurone causes Ca2+ channels to open.
3) Ca2+ ions diffuse into the synaptic knob.
4) Vesicles containing neurotransmitter fuse with the plasma membrane and release neurotransmitter into the synaptic cleft by exocytosis.
5) Neurotransmitter diffuses across synaptic cleft.
6) Neurotransmitter binds with receptor protein on the post-synaptic membrane.
7) Binding of neurotransmitter results in Na+ channels opening and Na+ ions diffusing into the postsynaptic neurone.
8) This causes depolarisation of the postsynaptic membrane. If threshold potential reached, action potential generated and propagated along postsynaptic neurone.
9) Neurotransmitter is broken down by specific enzymes to prevent continuous synaptic transmission.
10) Neurotransmitter fragments diffuse back across the synaptic cleft to be reabsorbed, reassembled, and repackaged into vesicles in the synaptic knob.
11) Ca2+ ions pumped out of synaptic knob.
What is the neurotransmitter at a cholinergic synapse?
Acetylcholine (ACh)
What enzyme hydrolyses the acetylcholine neurotransmitter?
Acetylcholinesterase (AChE)
Where are cholinergic synapses found?
In the CNS of vertebrates
At neuromuscular junctions
What type of synapse is found at neuromuscular junctions?
Cholinergic synapse
What does acetylcholinesterase hydrolyse acetylcholine into?
Ethanoic acid and choline
Synapse
Junction between neurones or between a neurone and an effector.
Neurotransmitters can be either _________ or _________
excitatory or inhibitory
What is a cholinergic synapse?
A synapse which has acetylcholine as a neurotransmitter.
Example of an excitatory neurotransmitter?
Acetylcholine
Example of an inhibitory neurotransmitter?
Gamma aminobutyric acid
Why is there a high number of mitochondria in the synaptic knob?
Mitochondria needed as they are the site of respiration to release ATP. ATP needed for:
• Fusion and formation of vesicles
• Active pumping of Na⁺ and Ca²⁺ ions out of synaptic knob
• Synthesis of neurotransmitter
• Movement of vesicles along cytoskeleton
Why is endoplasmic reticulum needed in the synaptic knob?
Many vesicles to package neurotransmitters must be made.
Role of synapses in the CNS
- Ensure impulses are unidirectional (neurotransmitter receptors only on one side)
- Allows impulses from one neurone to be transmitted to many/Allows impulses from many neurones to be transmitted to one
- Enables control by summation
Two types of summation
- Spatial
* Temporal
Spatial summation
- Several presynaptic neurones connect to 1 postsynaptic neurone
- Each synaptic knob releases neurotransmitter; the cumulative effect triggers an action potential in the postsynaptic neurone
Temporal summation
- Single presynaptic neurone releases neurotransmitter several times in quick succession
- High frequency of neurotransmitter results in action potential in postsynaptic neurone
Central Nervous System
- Brain and spinal cord
* Relay neurones
Peripheral Nervous System
- Everything apart form the CNS
* Sensory and motor neurones
Somatic nervous system
- Conscious control
- Input from sense organs
- Output to skeletal muscles
Autonomic nervous system
- Subconscious control
- Input from internal receptors
- Output to smooth muscles and glands
The mammalian nervous system can be divided into
the CNS and PNS
The PNS can be divided into
• the somatic nervous system
and
• the autonomic nervous system
The autonomic nervous system can be divided into
• the sympathetic motor system
and
• the parasympathetic motor system
Sympathetic motor system
• Fight or flight responses
⟶Outcome increases activity
• Neurotransmitter: noradrenaline
Parasympathetic motor system
• Relaxing response
⟶ Outcome decreases activity
• Neurotransmitter: acetylcholine
The somatic nervous system is stimulatory/inhibitory?
Stimulatory
The autonomic nervous system can be…
stimulatory or inhibitory
What type of muscle does the sympathetic motor system increase blood flow to?
Skeletal muscle
What type of muscle does the parasympathetic motor system increase blood flow to?
Smooth muscle
How do drugs target synapses (stimulants)
• Stimulate the nervous system by creating more postsynaptic action potentials
⟶ mimic the shape of neurotransmitter
⟶ Stimulate release of more neurotransmitter
⟶ Inhibit enzyme from breaking down neurotransmitter
How do drugs target synapses (depressants)
• Inhibit the nervous system by allowing fewer postsynaptic action potentials to be generated
⟶ Block receptors so neurotransmitters can’t bind, preventing action potentials
⟶ Help inhibitory neurotransmitters to bind
Pituitary gland function
Stores and releases, regulates hormones produced by the hypothalamus
Cerebrum function
Coordinates some voluntary and some involuntary responses (memory, thinking, speech, sight, hearing)
Hypothalamus function
- Regulates temperature, water balance, glucose concentration
- Controls behaviour: feeding, sleeping, aggression
- Produces hormones
Cerebellum function
Controls unconscious functions e.g. posture, balance, non-voluntary movement
Medulla oblongata function
• Used in autonomic control e.g. heart rate, breathing rate (ventilation), swallowing, peristalsis, coughing, bladder control
Injury to the medulla oblongata is usually
fatal
Reflex path
Receptor ⟶ Sensory neurone ⟶ Relay neurone ⟶ Motor neurone ⟶ Effector
Why are reflexes vital for survival?
- Fast
- Innate
- Allows brain to focus on complex responses
Why is the knee-jerk reflex important?
For balance and posture
⟶ detect stretching of tendon, contract muscles accordingly
Reflex
response to stimulus without conscious thought.
Blink reflex process
1) Cornea is touched (stimulus)
2) Action potential along sensory neurone
3) Relay neurone in lower brain stem
4) Motor neurone to eyelid
5) Blinking response
3 types of muscles
- Skeletal
- Smooth (involuntary)
- Cardiac
Skeletal muscle: striated or non-striated
Striated
Skeletal muscle cellular configuration
Parallel cylindrical cells
Speed of contraction (skeletal muscle)
Fast
Skeletal muscle voluntary or involuntary
Voluntary
Skeletal muscle: cells uni/multinucleate
multinucleate
Cardiac muscle: cells uni/multinucleate
Uninuclear
Cardiac muscle: describe striations
Specialised and not parallel
Cardiac muscle cell config
Branched and interlocking cells which are connected
Smooth muscle uni/multinucleate
Uninuclear
Smooth muscle: cell shape
Spindle shaped
Smooth muscle: striated or non-striated
Non striated
Smooth muscle: contraction speed
Slow
Cardiac muscle: contraction speed
Fast
Muscle fibres are what
Large multinucleate cells (skeletal muscle)
Sarcolemma =
plasma membrane (muscle fibre)
sarcoplasm =
cytoplasm (muscle fibre)
sarcoplasmic reticulum =
endoplasmic reticulum (muscle fibre)
Sarcolemma is folded to form a system of…
transverse (t) tubules
Thick filaments made of
Myosin
Thick filaments diameter
15nm
Thin filaments made of
actin
Thin filaments diameter
7nm
Motor unit
The combination of the motor neurone and all the muscle fibres it innervates
How to vary the strength of muscle contraction
1) Motor units
2) Wave summation
How do motor units vary the strength of muscle contraction?
- All muscle cells in a motor unit contract simultaneously
- Stronger force required means greater number of motor units will be stimulated
- One motor neurone can synapse up to 200 muscle fibres (like spatial summation)
How does wave summation vary the strength of muscle contraction?
• An increase in the frequency with which a muscle is stimulated increases the strength of contraction
