Nervous System Flashcards
State the components of a nervous response.
- Receptors
- Effectors
Nervous system or hormones transfer information from receptors to effectors
Name the two main divisions of the nervous system.
- Central nervous system (CNS)
- Peripheral nervous system (PNS)
What is the central nervous system (CNS)?
Brain and spinal cord.
What is the peripheral nervous system (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 neurones enter the spinal cord via the dorsal root
What is the ventral root?
- One of two roots that emerges from the spinal cord
Motor neurones 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.
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
neurones.
Sensory neurone
* Relay neurone
* Motor neurone
State the function of a sensory neurone.
Carries nerve impulses from the
receptors to the CNS via the dorsal root. .
State the function of a motor neurone
Carries nerve impulses from the CNS to
the effectors via the ventral root.
Describe the structure of a motor
neurone.
Short dendrites carry impulses from
CNS to cell body
Cell body found at one end of neurone dendrites
cell body at the end of axon
Long axon carries impulses from cell body to effectors
State the function of a relay neurone.
Located in the spinal cord .
Links the sensory neurone to the
motor neurone
What is the function of dendrites?
Short, branched extensions of the cell body
* Receive nerve impulses from other
neurones
Describe the cell body.
The region of the neurone 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 and contains synaptic vesicles filled with neurotransmitters.
Describe the additional features of a myelinated neurone.
Schwann cells: wrap around axon; involved in electrical insulation, phagocytosis, 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
neurone membrane when not stimulated (-70 mV).
How is resting potential established?
Membrane more permeable to K* than Na*
Sodium-potassium pump actively transports 3Na* out of cell and 2K* into 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
- Hyperpolarisation
- 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 repolarisation?
- Membrane potential reaches +40 mV
- 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 repolarises
What happens during hyperpolarisation?
- ‘Overshoot’ when K* ions diffuse out
- Potential difference becomes more negative than resting
potential - Membrane hyperpolarises, preventing another impulse
occuring
What is the refractory period?
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 does not 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 neurones or from neurones to effectors
Summation of sub-threshold impulses
New impulses can be initiated in several different neurones for multiple simultaneous responses
Describe the structure of a synapse.
Presynaptic neurone 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 neurone has complementary receptors to neurotransmitter (ligand-gated Na* channels)
What is the synaptic cleft?
A small gap between neurones across which a nerve impulse is transmitted via
neurotransmitters.
Describe synaptic transmission in the presynaptic neurone.
- Wave of depolarisation travels down presynaptic neurone, causing
voltage-gated Ca2+ channels to 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 neurone.
- 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 neurone
- 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 the postsynaptic
neurone - Results in continuous stimulation of the neurone
Where 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
