The nervous system Flashcards
State the components of a nervous response.
- Receptors.
- Effectors.
- Nervous system or hormones transfer info from receptors to effectors.
What is the CNS?
Brain and spinal cord.
What is the PNS?
Pairs of nerves that originate from the CNS and carry nerve impulses into and out of the CNS.
What is the dorsal root?
- One of two roots that emerges from the spinal cord.
- Travels to the dorsal root ganglion.
- Sensory neurons enter the spinal cord via the dorsal root.
What is the ventral root?
- One of two roots that emerges from the spinal cord.
- Motor neurons leave the spinal cord via the ventral root.
What is a reflex?
A rapid, automatic response to a sensory stimulus by the body. It serves as a protective mechanism.
Outline a simple reflex arc.
Stimulus - receptor - sensory neuron - relay neuron (in CNS) - motor neuron - effector - response.
How does a reaction differ from a reflex?
- Reaction is voluntary and coordinated by the brain.
- Reflex is non-voluntary and does not involve the brain.
What is a nerve net?
- Simplest form of nervous system found in Cnidarians.
- Consists of interconnected nerve cells with short extensions allowing a response to a limited number of stimuli.
State the three types of functional neurons.
- Sensory
- Relay
- Motor
State the function of a sensory neuron.
Carries nerve impulses from the receptors to the CNS via the dorsal root.
State the function of a motor neuron.
Carries nerve impulses from the CNS to the effectors via the ventral root.
Describe the structure of a motor neuron.
- Short dendrites carry impulses from CNS to cell body.
- Cell body found at one end of neuron.
- Long axon carries impulses from cell body to effectors.
State the function of a relay neuron.
- Located in the spinal cord.
- Links the sensory neuron to the motor neuron.
What is the function of dendrites?
- Short, branched extensions of the cell body.
- Receive nerve impulses from other neurons.
Describe the cell body.
The region of the neuron that contains the organelles, notably the nucleus and the rough endoplasmic reticulum.
What is the function of the axon?
A long fibre that conducts nerve impulses away from the cell body.
What are axon terminals?
Branched endings of an axon that approach the muscle fibre.
What is a synaptic end bulb?
The end of an axon that is bulbous shaped contains synaptic vesicles filled with neurotransmitters.
Describe the additional features of a myelinated neuron.
- Schwann cells: wrap around axon, involved in electrical insulation, phagocytosis and nerve regeneration.
- Myelin sheath: made from myelin-rich membranes of Schwann cells.
- Nodes of Ranvier: small gaps between neighbouring Schwann cells where there is no myelin sheath.
Define action potential.
The temporary change in electrical potential across the membrane of an axon in response to the transmission of a nerve impulse.
What is resting potential?
Potential difference (voltage) across a neuron membrane when not stimulated (-70 mV).
How is resting potential established?
- Membrane more permeable to K+ than Na+.
- Sodium-potassium pump actively transports 3 Na+ out of the cell and 2K+ into the cell.
- Organic phosphates and large protein anions remain in cytoplasm.
- Establishes electrochemical gradient: cell contents more negative than extracellular environment.
Name the stages of an action potential.
- Depolarisation
- Repolarisation
- Hyperpolarization
- Return to resting potential
What happens during depolarisation?
- Stimulus causes a change in the voltage across an axon membrane, opening voltage-gated Na+ channels.
- Na+ diffuse into the axon.
- Potential difference across membrane becomes more positive, membrane depolarises.
What happens during repolarization?
- Membrane potential reaches +40mV.
- Voltage-gated Na+ channels close and voltage-gated K+ channels open.
- Facilitated diffusion of K+ ions out of cell down their electrochemical gradient.
- Potential difference across membrane becomes more negative, membrane repolarizes.
What happens during hyperpolarization?
- ‘Overshoot’ when K+ ions diffuse out.
- Potential difference becomes more negative than resting potential.
- Membrane hyperpolarizes, preventing another impulse occuring.
What is the refractory period?
The time period after an action potential during which further action potentials are prevented.
Why is the refractory period important?
Ensures that action potentials can only be propagated in one direction.
Describe the ‘all or nothing’ law.
Principle that states that all stimuli above a certain threshold value will generate the same size of action potential, regardless of the strength of the stimulus.
State the factors affecting the speed of conduction of a nervous impulse.
- Temperature
- Axon diameter
- Myelin sheath
Explain why myelinated axons conduct impulses faster than unmyelinated axons.
Saltatory propagation: impulse ‘jumps’ from one node of Ranvier to another (depolarisation cannot occur where myelin sheath acts as electrical insulator). Therefore, impulse doesn’t travel along whole axon length.
How does temperature affect the speed of conduction of nervous impulses?
The higher the temperature, the faster the speed of conduction.
How does axon diameter affect the speed of conduction of nervous impulses.
The larger the diameter, the faster the speed of conduction.
What is a synapse?
The junction between two nerve cells or a nerve cell and an effector.
What is the function of synapses?
- Electrical impulse cannot cross junction.
- Neurotransmitters send impulses between neurons of from neurons to effectors.
- Summation of sub-threshold impulses.
- New impulses can be initiated in several different neurons for multiple simultaneous responses.
Describe the structure of a synapse.
- Presynaptic neuron ends in synaptic knob.
- Synaptic knob contains a high concentration of mitochondria, endoplasmic reticulum and vesicles of neurotransmitter.
- Synaptic cleft, 20-30 nm gap.
- Postsynaptic neuron has complementary receptors to neurotransmitter (lingang-gated Na+ channels).
What is the synaptic cleft?
A small gap between neurons across which a nerve impulse is transmitted via neurotransmitters.
Describe synaptic transmission in the presynaptic neuron.
- Wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca2+ channels open.
- Ca2+ cause vesicles of acetylcholine to move towards and fuse with presynaptic membrane.
- Exocytosis of neurotransmitter (e.g. acetylcholine) into synaptic cleft.
How do neurotransmitters cross the synaptic cleft?
Via simple diffusion.
Describe synaptic transmission in the postsynaptic neuron.
- Acetylcholine diffuses across synaptic cleft and binds to specific receptors on postsynaptic membrane.
- Ligand-gated Na+ channels open.
- If influx of Na+ ions raises membrane to threshold potential, action potential is generated.
How is the merging of impulses prevented during synaptic transmission?
- Active transport of Ca2+ out of synaptic knob.
- Role of cholinesterase.
- Reabsorption of neurotransmitters.
Describe the role of cholinesterase in synaptic transmission.
- Hydrolyses acetylcholine in the postsynaptic neuron.
- Products diffuse back across the cleft.
What is the effect of organophosphates on the transmission of impulses?
- Act as cholinesterase inhibitors, preventing the hydrolysis of acetylcholine in postsynaptic neuron.
What are organophosphates commonly found?
Component of insecticides.
How do psychoactive drugs (e.g. amphetamine) affect the transmission of impulses?
- Excitatory drugs stimulate the release of neurotransmitters such as noradrenaline.
