Nerves Flashcards
What are the main divisions of the nervous system?
Central Nervous System (CNS): Consists of the brain and spinal cord; processes information and coordinates responses.
Peripheral Nervous System (PNS): Comprises all neurons outside the CNS; transmits signals between the CNS and the rest of the body.
How is the Peripheral Nervous System (PNS) subdivided?
Sensory Division: Carries sensory information from receptors to the CNS.
Motor Division: Transmits commands from the CNS to effectors (muscles and glands).
Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion) by controlling smooth muscle, cardiac muscle, and glands.
Sympathetic Division: Prepares the body for ‘fight or flight’ responses.
Parasympathetic Division: Promotes ‘rest and digest’ activities.
What are neurons, and what are their main components?
Neurons are specialized nerve cells that transmit electrical signals throughout the nervous system. Their main components are:
Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.
Dendrites: Branch-like structures that receive messages from other neurons and conduct impulses toward the cell body.
Axon: A long, slender projection that transmits impulses away from the cell body to other neurons or effectors.
Myelin Sheath: A fatty layer insulating the axon, increasing the speed of impulse transmission.
Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated, facilitating rapid conduction.
Axon Terminals (Synaptic Boutons): Endings of the axon that release neurotransmitters to communicate with other neurons or effectors.
What is the resting membrane potential, and how is it maintained?
The resting membrane potential is the electrical potential difference across the neuronal membrane when the neuron is not transmitting an impulse, typically around -70 mV. It is maintained by:
Sodium-Potassium Pump: Actively transports 3 Na⁺ ions out and 2 K⁺ ions into the neuron, creating a concentration gradient.
Selective Membrane Permeability: The membrane is more permeable to K⁺ ions, allowing them to diffuse out, contributing to a negative charge inside.
What are the different types of neurons and their functions?
Sensory Neurons: Transmit information from sensory receptors to the CNS.
Motor Neurons: Carry commands from the CNS to muscles or glands (effectors).
Interneurons (Relay Neurons): Connect sensory and motor neurons within the CNS, processing information.
What is an action potential, and what are its phases?
An action potential is a rapid, temporary change in the membrane potential that travels along the neuron. Phases include:
Depolarization: Voltage-gated Na⁺ channels open, allowing Na⁺ to enter the neuron, making the inside more positive.
Repolarization: Na⁺ channels close, and voltage-gated K⁺ channels open, allowing K⁺ to exit, restoring a negative interior.
Hyperpolarization (Undershoot): K⁺ channels remain open slightly longer, causing the membrane potential to become more negative than the resting potential.
How is an action potential propagated along a myelinated neuron?
In myelinated neurons, action potentials propagate via saltatory conduction, where the impulse jumps from one Node of Ranvier to the next, significantly increasing conduction velocity.
What is a synapse, and what are the steps of synaptic transmission?
A synapse is a junction between two neurons or a neuron and an effector cell. Steps of synaptic transmission include:
Arrival of Action Potential: Depolarizes the presynaptic membrane.
Opening of Voltage-Gated Ca²⁺ Channels: Ca²⁺ enters the presynaptic neuron.
Neurotransmitter Release: Ca²⁺ influx causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.
Neurotransmitter Binding: Neurotransmitters bind to specific receptors on the ligand gated ion channels on the postsynaptic membrane.
Generation of Postsynaptic Potential: Binding opens ion channels, leading to depolarization (excitatory) or hyperpolarization (inhibitory) of the postsynaptic neuron.
Termination of Signal: Neurotransmitters are degraded by enzymes or reabsorbed into the presynaptic neuron.
What are neurotransmitters, and can you provide examples?
Neurotransmitters are chemical messengers that transmit signals across synapses. Examples include:
Acetylcholine (ACh): Involved in muscle contraction and autonomic nervous system functions.
What is a reflex arc?
A reflex arc is the neural pathway that controls an involuntary response to a stimulus, bypassing the brain to allow for fast, automatic actions.
What is the functional significance of a reflex?
Reflexes are:
Rapid and protect the body from harm (e.g., pulling away from heat).
Involuntary, not under conscious control.
Useful for maintaining homeostasis and postural stability.
What are the key components of a reflex arc?
Stimulus – e.g., heat from a flame.
Receptor – detects stimulus (e.g., thermoreceptor in the skin).
Sensory neuron – carries impulse to the CNS.
Relay neuron (interneuron) – in the spinal cord; processes signal.
Motor neuron – carries impulse to the effector.
Effector – usually a muscle or gland that responds (e.g., contracting a muscle to pull away).
Response – the action taken (e.g., withdrawal of hand).
In which part of the CNS does processing in the reflex arc usually occur?
Usually in the spinal cord, not the brain — this allows the response to be fast and automatic.
What is the difference between a spinal reflex and a cranial reflex?
Spinal reflexes are processed in the spinal cord (e.g., withdrawal reflex).
Cranial reflexes are processed in the brainstem (e.g., blinking or pupil reflex).
How is the reflex arc different from conscious neural pathways?
Reflex arcs are faster, involuntary, and involve fewer synapses.
They often bypass higher brain centers, though the brain may be made aware of the stimulus afterward.
Why are reflexes described as innate?
Reflexes are present at birth, not learned — they are hardwired into the nervous system.
What is the role of the relay neuron in a reflex arc?
It links the sensory neuron to the motor neuron and allows for integration within the spinal cord, sometimes enabling modulation of the reflex.
Why do reflex arcs often involve only three neurons?
To minimize delay:
Fewer synapses = faster transmission.
This supports the protective function of reflexes.
How does the body become aware of a reflex action if the brain is initially bypassed?
While the initial motor response is rapid and spinal, sensory input is also sent to the brain afterward, so you become aware of what happened just after the action occurs.
Describe the withdrawal reflex when touching a hot object.
Heat detected by thermoreceptors in skin.
Impulse travels along sensory neuron to spinal cord.
Relay neuron processes and connects to motor neuron.
Motor neuron carries impulse to biceps (effector).
Biceps contract, pulling hand away from the heat.
What is a nerve net?
A nerve net is the simplest type of nervous system, made of a network of nerve cells (neurons) with no brain or central coordination.
Which organism is commonly used to study nerve nets in the Eduqas spec?
Hydra, a small freshwater cnidarian, is the model organism used to study primitive nerve nets.
What are the characteristics of Hydra’s nerve net?
Diffuse network of unmyelinated neurons.
No central nervous system or brain.
Neurons connect in a mesh-like arrangement.
Can respond to stimuli from any direction.
Coordinates simple movements like contracting and feeding.
How do nerve nets differ from more advanced nervous systems?
Nerve nets, like those found in Hydra, are simple and uncentralised, whereas advanced systems (e.g., in humans) are complex and centralised. Key differences include:
Structure: Nerve nets are diffuse with no brain or spinal cord, while advanced systems have a central nervous system.
Neuron type: Neurons in nerve nets are unmyelinated, leading to slow conduction; advanced systems often have myelinated neurons for faster signal transmission.
Impulse direction: Nerve nets transmit impulses in both directions; advanced systems are unidirectional.
Coordination: Nerve nets can only coordinate basic responses; advanced systems allow complex processing and integration.
Synapses: Nerve nets have few, often electrical synapses; advanced systems use chemical synapses extensively.
Speed: Nerve nets conduct impulses slowly; advanced systems are much faster.
