B15- Nervous Coordination and Response Flashcards
What is the nervous system?
rapid communication between nerve cells via electrical impulses and neurotransmitters. The response is short-lived. E.g. reflex action
What is the structure of a mammalian motor neurone?
Cell body - usual organelles with a large amounts of RER. Production of proteins and neurotransmitters.
Dendrons - extension which divide into dendrites, carry impulses towards the cell body
Axon - single long extension that carries the impulse away from the cell body. It can be metres at length and less than a micrometer in diameter.
What is the axon?
The axon is covered by myelin sheath which insulates the axon called myelinated neurones.
What are the Schwann cells?
Schwann cells - surround the axon with their myelin-rich membranes. They protect and insulate. They help removing debris by phagocytosis and play part in nerve regeneration.
What are the Nodes of Ranvier?
Axon - single long extension that carries the impulse away from the cell body. It can be metres at length and less than a micrometre in diameter.
What is a nerve impulse?
A nerve impulse can be defined as a self propagating wave of electrical activity.
How is the rest potential achieved?
Resting potential is the electrical charge difference across a neuron’s membrane when it’s not transmitting a signal. In humans, it’s typically around -65 mV (ranging from -50 to -90 mV).
Maintaining this potential requires constant energy expenditure. The neuron’s interior is more negatively charged than its exterior due to an uneven distribution of ions across the cell membrane.
The unequal distribution of ions?
The sodium-potassium pump actively transports 3 Na+ ions out and 2 K+ ions into the cell. The cell membrane is significantly more permeable to K+ ions. During resting potential:
- Some ion channels remain open, allowing facilitated diffusion of Na+ and K+
- A small amount of Na+ ions diffuse back into the axon
- K+ ions freely diffuse out through permanently open channels
This ion distribution maintains a resting potential of the neurone .
How is an action potential achieved?
An action potential occurs when a stimulus (e.g., chemical, mechanical, thermal, electrical, or light) triggers a change in the neuron’s membrane potential. This causes:
- Rapid depolarization of the membrane
- Potential difference shifting from negative to positive (reaching about +40 mV)
- This sudden voltage change propagates along the neuron as the action potential
What is the process of the action potential?
- Depolarization occurs when channel proteins change shape at the stimulus point. During resting potential, voltage-gated Na+ channels are closed and most K+ channels are closed.
- A stimulus causes voltage-gated Na+ channels to open, allowing Na+ influx. This triggers more Na+ channels to open, further increasing Na+ influx.
- If the stimulus reaches threshold, an action potential occurs. Na+ channels close, K+ channels open, allowing K+ efflux. More K+ efflux leads to more K+ channels opening. Repolarization of axon begins.
- K+ channels close slowly, causing a brief hyperpolarization where the membrane potential becomes more negative than the resting potential.
- The Na+/K+ pump then restores ion balance, returning the neuron to its resting state.
Action Potential Propagation
An action potential initiates a chain reaction along the axon, causing a nerve impulse to travel. This process involves:
- Localized depolarization of axon segments
- Stimulation of adjacent regions
- Continuous impulse propagation
How do unmyelinated axon transmission work?
In unmyelinated neurons, the action potential generates a small current flow. This local current acts as a stimulus for the next membrane segment, enabling the impulse to travel along the axon.
How do myelinated neurones work?
In myelinated neurons, the myelin sheath insulates the axon, concentrating ion flow at the nodes of Ranvier. This results in saltatory conduction, where the action potential effectively “jumps” from node to node, allowing for faster and more efficient signal transmission.
What is the all or nothing principle of action potentials?
Threshold Value:
- Below this stimulus level, no action potential occurs
- Any stimulus above threshold triggers an action potential
What is the refractory period?
A brief period when voltage-gated sodium channels are closed, preventing further action potentials. Ensuring one way prorogation of nerve impulses prevent continuous firing?
Why is there a refractory period?
- Unidirectional: Impulses travel only from active to resting regions
- Discrete: Action potentials are separated, preventing signal overlap
- Limiting: Caps the frequency of action potentials, protecting against sensory overload
What is the structure of a synapse?
- Synaptic cleft: Narrow gap (20-30 nm) between neurons
- Neurotransmitters: Chemical messengers for neuronal communication
- Presynaptic neuron: Releases neurotransmitters into the cleft
- Postsynaptic neuron: Contains receptors for neurotransmitters
What are the two types of synapse?
- Excitatory synapses: Trigger new action potentials in the postsynaptic neuron
- Can become fatigued with continuous stimulation due to neurotransmitter depletion
- Inhibitory synapses: Prevent or reduce the likelihood of action potentials in the postsynaptic neutron
What is the neurotransmitter?
Acetylcholine: Prevalent in vertebrate central nervous systems and neuromuscular junctions
How does a cholinergic synapse function?
1)Action potential causes Ca2+ channels to open, influx of Ca2+ ions.
2)Ca2+ ions cause synaptic vesicles to migrate to and fuse with the presynaptic membrane
- Acetylcholine molecules diffuse across synaptic cleft (Na+ channels are closed.)
- When the acetylcholine molecules bind to the receptor site on the Na+ channels, the channels open and Na+ions diffuse into the postsynaptic cell.
- Influx of Na+ ions generate a new action potential in the membrane of the postsynaptic neurone (remember resting potential Na+/K+ pump -> voltage-gated Na+ channels -> depolarisation -> voltage-gated K+ channels - > repolarisation)
- Acetylcholinesterase hydrolyses acetylcholine into ethanoic acid and choline which diffuse back into the presynaptic neurone (recycling)
- ATP from mitochondria is used to recombine choline and ethanoic acid into acetylcholine.
What do inhibitor synapses do?
Neurotransmitter binds to Cl channels. When Cl- channels open, Cl- ions move into the postsynaptic neurone. This causes K+ channel to open and K+ ions to diffuse out of the postsynaptic neurone. Axon membrane is hyperpolarised (-80mV), it is more difficult to excite it.
Effects of Drugs on the Nervous System
- Impact:
- Drugs significantly affect brain and nervous system functioning
- Prescription drugs: Potentially beneficial for neurological disorders
- Recreational drugs: Possibly damaging or fatal
- Synaptic Transmission Effects:
- Many drugs specifically target synaptic transmission
- Modes of action include:
- Stimulating neurotransmitter release
- Providing neurotransmitter synthesis materials
- Mimicking neurotransmitters at receptors
- Preventing neurotransmitter reuptake
