Nerves Flashcards
What is a nerve net?
Made of only one type of nerve cell with short extensions joined to each other and branching in a number of different directions.
Differences between a Hydra and a Human (in terms of CNS/nerve net, types of nerve cell present, length of cell processes, presence on myelination, conduction speed, direction of impulse, number of stimuli that can be detected, number of effectors):
Hydra: Nerve net, one type of nerve cell, short, no myelination, slow, both directions, limited, small number of effectors.
Human: Central Nervous System, three types of nerve cell (sensory, motor, relay), long, yes (both myelinated and non myelinated), fast, one way, various stimuli, skeletal muscles/glands.
Difference between a sensory, motor and relay neurone (in relation to length of axons, location of cell body and function)?
Sensory: Short, Dorsal root ganglion, transmit impulses from sense organs towards the CNS.
Motor: Long, grey matter, transmit impulses from CNS to an effector.
Relay: Short, grey matter, connect the sensory and motor neurones.
What is the Central Nervous System?
Comprised of the brain and the spinal cord. These are both surrounded and protected by a tough protective membrane called meninges.
What is the Peripheral Nervous System?
Comprised of the somatic nervous system-nerves that originate in the brain or spinal cord. Includes sensory neurones (impulses from receptors to CNS) and motor neurones (CNS to effectors). It has unconscious control of internal organs.
What is a Sensory neurone?
Carries nerve impulses from receptor cells (in sense organs) to the coordinator. The coordinator is the central nervous system.
What is a Relay neurone?
Also known as connector/intermediate neurones. Thee lie in the CNS. They receive impulses from the sensory neurones and send impulses to motor neurones.
What is a Motor neurone?
These carry impulses from the coordinator (CNS) to the effector organs. The effector brings about a response. If the effector is a muscle the response will be contraction. If the effector is a gland, the response will be secretion of hormones.
What are Dendrites?
Thin extensions which carry the impulses towards the cell body.
What is an Axon?
Long cytoplasmic extension, which transmits impulses away from the cell body.
What are Schwann cells?
Cells which surround neurones and insulate them.
What is the Myelin Sheath?
Schwann cells grow around the axons to create this multi-layered fatty sheath. Acts as an electrical insulator that speeds up transmission along the axon- cause saltatory conduction.
What are the Nodes of Ranvier?
Areas along the axon where the myelin sheath is missing.
What is the cell body?
Part of the neurone which contains the nucleus, RER, numerous mitochondria and other cell organelles.
What is the function of a reflex arc?
Protection.
What is a reflex (action)?
Rapid, involuntary response to a stimulus (doesn’t involve brain).
What is a Reflex Arc?
The pathway travelled by the nerve impulses during a reflex action.
What is a Spinal Reflex?
Most reflexes are spinal reflexes. Information may be transmitted to the brain, but it is the spinal cord that is responsible for the integration of sensory information and a response transmitted to motor neurones.
What is a cranial reflex?
Have pathways through cranial nerves and the brainstem.
What is the process of a reflex arc?
A stimulus (change in environment) is picked up/detected by receptor. Receptors convert stimulus to an electrochemical signal. The sensory neurone (usually myelinated) transmits the impulse to the spinal cord. The sensory neurones enter the spinal cord via the dorsal root. The sensory neurone synapses with a relay neurone (usually unmyelinated) in the grey matter of the spinal cord. Release of a neurotransmitter generates an action potential in the relay neurone. This is repeated at the synapse between the relay and motor neurone (usually myelinated). The motor neurone transmits a nerve impulse from the spinal cord to an effector (muscle/gland) to bring about a response.
Label a reflex arc/spinal cord:
Answer on page 91 of blue booklet.
What is a Synapse?
Small gap between 2 neurones. A neurotransmitter carries the message across the gap.
What happens at Resting potential in an action potential?
The sodium potassium pump is open and requires ATP to pump 3Na+ out of the neurone for every 2K+ in. Resting potential is at -70mv (more negative inside the axon) and is when the neurone is not firing. The potassium channel is open and so K+ diffuses out of the neurone, down the conc gradient. There are proteins in the axoplasm, keepting it negative.
What happens at depolarisation of an action potential?
Na+ voltage gated channels open and Na+ enters the axon by facilitated diffusion-causes membrane potential to move towards 0mv, but not all Na+ channels open at this time. If depolarisation reaches threshold limit (-50mv), more Na+ voltage gated channel proteins open= causes Na+ to enter-cell more positive-potential difference reaches peak of +40mv=action potential.
What happens in repolarisation in an action potential?
Voltage gated K+ channel open and voltage gated Na+ channels close and membrane becomes more permeable to K+, which diffuse out of the axon, down the conc gradient. This removal of positive charge from inside the axon starts to repolarise the membrane. Due to so many K+ leaving axon, inside of membrane briefly becomes more negative than at normal resting potential. This overshoot= hyperpolarisation. Voltage gated K+ channels close and the Na+/K+ pump restarts and restores resting potential. Membrane is polarised again.
What is Absolute Refractory Period?
Ensures each wave of action potential travels as a discrete impulse in 1 direction.
What is the refractory period?
During the refractory period after an action potential, a second action potential cannot be initiated.
The refractory period is a result of a temporary inactivation of the Na+ channels while the resting potential is restored.
It ensures the active potentials are separated from eachother.
How does the diameter of the axon affect the speed of conduction of the nerve impulse?
The greater the diameter of the axon the greater the speed of conductance: due to less leakage of ions from the axons.
How does temperate affect the speed of conduction of the nerve impulse?
At a higher temperature, there are faster nerve impulses. This is due to an increase in speed of diffusion but also because the energy for active transport comes from respiration, which is controlled by enzymes. There is an optimum temperature, as, if proteins become too hot they denature.
What is the process of a Cholinergic Synapse?
- Impulse travels along the axon of a presynaptic neurone.
- Impulse/depolarisation arrives at synaptic knob.
- Permeability of presynaptic membrane to calcium ions increases.
- Calcium ions diffuse into presynaptic knob.
- This causes synaptic vesicles to migrate towards the presynaptic membrane.
- Synaptic vesicles contain neurotransmitters (acetylcholine).
- Synaptic vesicles fuse with the presynaptic membrane.
- Synaptic vessels release neurotransmitters by exocytosis.
- Neurotransmitters diffuse across synaptic cleft.
- Neurotransmitters binds to receptors on the post synaptic membrane.
- Post synaptic membrane becomes depolarised/creates action potential/causes influx of sodium ions.
- Neurotransmitter is broken down by an enzyme called acetylcholinesterase.
- Products of the breakdown are reabsorbed in to the pre-synaptic neurone and recycled.
What is Summation?
Low frequency action potentials often release insufficient amounts of neurotransmitter to exceed the threshold in the post-synaptic neurone. Summation allows action potentials to be generated. This enables a build up of a neurotransmitter in the synapse.
What is inhibition?
The slowing down or stopping of a biological reaction, often by an enzyme, by a molecule called an inhibitor
What are Organophosphates?
Are excitatory because they inhibit acetylcholinesterase, so acetylcholine isn’t broken down and continues to stimulate receptors.
What is the relative refractory period?
Another action potential can be generated here, but only by a greater stimulus current than that originally needed.
What is the ‘All or nothing principle’?
If the stimulus is below threshold, no action potential takes place and if the stimulus is above threshold, a whole action potential takes place.
