9.2 The Mammalian Nervous System Flashcards
Name the 2 main divisions of the nervous system.
Central nervous system (CNS)
Peripheral nervous system (PNS - all neurons that are not part of the CNS)
Describe the central nervous system.
Comprised of brain and spinal cord
Specialised system of nerve cells processes stimuli and propagates impulses
Name the 2 main divisions of the PNS.
Voluntary (under conscious control)
Autonomic (not under conscious control)
Name the 2 main divisions of the autonomic nervous system.
Sympathetic
Parasympathetic
They act antagonistically to regulate response of effectors e.g., heart rate
Describe the sympathetic nervous system.
Usually stimulates effectors (coordinates fight-or-flight response)
Neurotransmitter noradrenaline
Ganglia are located near CNS
Describe the parasympathetic nervous system.
Usually inhibits effectors (coordinates rest/digest response)
Neurotransmitter acetylcholine
Ganglia located far from CNS
Describe the structure of the spinal cord.
Cylindrical bundle of nerves fibres runs from brain stem to lower back - surrounded by spinal vertebrae
Consists of nerve tissue
Grey matter - H-shaped region contains neurons
White matter - myelinated axons
What is the function of the cerebellum?
Controls execution of movement
Possible role in cognition
What is the function of the medulla oblongata?
Controls a range of autonomous functions including breathing and heart rate
What is the function of the cerebrum?
Uppermost part of the brain that is organised into lobes which control voluntary functions
What is the function of the hypothalamus?
Includes anterior pituitary gland
Involved in thermo and osmoregulation
What is resting potential?
Potential difference across neuron membrane when not stimulates - about -70mV in humans
How is resting potential established?
- Membrane is more permeable to K+ than Na+
- Sodium-potassium pump actively transports 3Na+ out of cell and 2K+ into cell
Establishes electrochemical gradient
Name the stages in generating an action potential.
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Return to resting potential
What happens during depolarisation?
- Stimulus —> facilitated diffusion of Na+ into cell down electrochemical gradient
- P.D across membrane becomes more positive
- If membrane reaches threshold potential (-50mV) voltage gated Na+ channels open
Significant influx of Na+ ions reverses P.D to +40mV
What happens during repolarisation?
- Voltage gated Na+ channels close and voltage gated K+ channels open
- Facilitated diffusion of K+ ions out of cell down their electrochemical gradient
- P.D across membrane becomes more negative
What happens during hyperpolarisation?
- Overshoot when K+ ions diffuse out of= P.D becomes more negative than resting potential
- Refractory period: no stimulus is large enough to raise membrane potential to threshold
- Voltage gated K+ channels close and sodium-potassium pump re-establishes resting potential
Explain the importance of the refractory period.
No action potential can be generated in hyperpolarised sections of membrane:
- Ensures unidirectional impulse
- Ensures discrete impulses
- Limits frequency of impulse transmission
How is an action potential propagated along an unmyelinated neuron?
- Stimulus leads to influx of Na+ ions - first section of membrane depolarised
- Local electrical currents cause sodium voltage-gated channels further along membrane to open - meanwhile the section behind begins to depolarise
- Sequential wave of depolarisation
Describe the structure of a motor neuron.
Cell body: contains organelles & high proportion of RER
Dendrons: branch into dendrites which carry impulses towards cell body
Axon: long, unbranched fibre carries nerve impulse away from cell body
Describe the additional features of a myelinated motor neuron.
Schwann cells: wrap around axon many times
Myelin sheath: made from myelin-rich membranes of Schwann cells
Nodes of Ranvier: very short gaps between neighbouring Schwann cells where there is no myelin sheath
Explain why myelinated axons conduct impulses faster than unmyelinated axons.
Saltatory conduction: impulse jumps from one node of ranvier to another - depolarisation cannot occur where myelin sheath acts as electrical insulator
So impulse does not travel along whole axon length
Describe the structure of a synapse.
- Presynaptic neuron ends in synaptic knob - contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter
- Synaptic cleft
Postsynaptic neuron: has complementary receptors to neurotransmitter
Explain the role of acetylcholine.
Causes muscle contraction at motor end plate
Causes excitation at preganglionic neurons
Causes inhibition at postganglionic neurons
What happens to acetylcholine from the synaptic cleft?
- Hydrolysis into acetyl and choline by acetylcholinesterase
- Acetyl & choline diffuse back into presynaptic membrane
- ATP is used to reform acetylcholine from storage in vesicles
What happens in the presynaptic neuron when an action potential is transmitted between neurons?
- Wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca2+ channels to open
- Vesicles move towards & fuse with presynaptic membrane
- Exocytosis of neurotransmitter into synaptic cleft
How do neurotransmitters cross the synaptic cleft?
Via simple diffusion
What happens in the postsynaptic neuron when an action potential is transmitted between neurons?
- Neurotransmitter binds to specific receptor on postsynaptic membrane
- Ligand-gated Na+ channels open
- If influx of Na+ ions raises membrane to threshold potential, action potential is generated
What happens in an inhibitory synapse?
- Neurotransmitter binds to an opens Cl- channels on postsynaptic membrane & triggers K+ channels to open
- Cl- moves in & K+ moves out via facilitated diffusion
- P.D becomes more negative: hyperpolarisation so no action potential is generated
How does nicotine work?
Absorbed in lungs - travels to brain via bloodstream
Similar shape to acetylcholine = bids to cholinergic receptors
Stimulant effect causes symptoms of sharpness & relaxation
How does lidocaine work?
Absorbed through skin or injected
Blocks voltage gated Na+ ion channel = prevents postsynaptic neuron from depolarising
Prevents transmission of pain signals so acts as anaesthetic
How does cobra venom work?
Blocks acetylcholine receptors on diaphragm = muscles cannot contract during inhalation
Disrupts normal breathing
Name the 2 types of photoreceptor cells located in the retina.
Cone cells
Rod cells
Where are rod and cone cells located in the retina?
Rod: distributed evenly around periphery but NOT in central fovea
Cone: mainly central fovea
No photoreceptors at blind spot where ganglion axon fibres form optic nerve
Explain why rod cells do not generate action potentials in the dark.
- Na+ enters outer segment of red cell via on-specific cation channels - active transport of Na+ out of inner segment = rod cell is slightly depolarised
- Action potential = voltage gated Ca2+ channels open - triggers exocytosis of glutamate
- Glutamate acts as inhibitory neurotransmitter to hyperpolarisation bipolar neuron
Explain how rod cells generate an action potential in the light.
- Rhodopsin pigment bleaches when it absorbs light & breaks down into opsin + retinal
- Opsin closes cation channels via hydrolysis - active transport of Na+ out of inner segment continues
- Rod cell becomes hyperpolarised - no glutamate is release, so no inhibitor signal
- Bipolar neuron depolarises
Describe the pigments in rod and cone cells.
Rod: rhodopsin absorbs all wavelengths of light
Cone: 3 types of iodopsin which absorbs red, blue or green wavelengths of light
Describe the visual acuity of rod and cone cells.
Rod: many told cells synapse with 1 bipolar neuron - low resolution
Cone: 1 cone cell synapse 1 bipolar neuron so there is no retinal convergence = high resolution
Describe the light sensitivity of rod and cone cells.
Rod: very sensitive due to spatial summation of subthreshold impulses = vision in low-light conditions
Cone: less sensitive = vision in bright light
Describe how light causes a change in the release of glutamate from rod cells.
Rhodopsin is involved
Retinal and opsin is formed from rhodopsin
Na+ channels are closed
Na+ is moving out of the cell
Stops glutamate/neurotransmitter release
Explain the role of mitochondria in the functioning of rod cells.
Produces ATP
Needed for active transport of ions
