Nerves Flashcards
What two parts is the human nervous system composed of?
-the central nervous system (CNS) composed of the brain and spinal cord
the peripheral nervous system (PNS)
consisting of the nerves and ganglia on the outside of the CNS.
In mammals, what do responses to many external and internal stimuli involve?
the reception of information and its transfer from a receptor to an effector via the nervous system (or as hormones via the blood in the endocrine system).
What is a stimulus?
A detectable change in the external or internal environment of an organism
What are examples of stimuli?
-Pressure on skin
-Chemicals in food
-light turning on
What does a receptor do?
Detects stimulus (one form of energy) and converts this to electrical
energy to send the information as an electrical impulse to the central nervous system via neurones
What are examples of receptors?
-Photoreceptor
-Thermoreceptor
-Mechanoreceptor
-Osmoreceptor/Chemoreceptor
What does an effector do?
Receives electrical impulses from the CNS and brings about a response
What are examples of effectors?
-Muscles/glands
What are neurones?
Highly specialised cells that generate and transmit nerve impulses
In mammals what are the three functional types of neurones?
-Sensory
-Motor
-Relay
What do sensory neurones do?
carry impulses from the receptors to the CNS
What are motor neurons?
Carry impulses from the CNS to the effector organs (muscles/glands)
What are relay neurones? (connector or association)
-found within the spinal cord receive impulses from sensory neurons or other intermediate neurons and relay them to motor neurons or other intermediate neurons
Compare the structure of the three main types of neurones (based on basic drawings)
sensory- cell body in middle of neuron whereas motor relay cell body at the start of neuron
What are components of a neurone?
-Axon
-Dendrites
-Cell body/centron
-Synaptic end bulb
-Axon ending
What is the function is an axon?
extension of the cytoplasm that transmit impulses away from the cell body to the axon endings
What are dendrites and what are their function?
Many thin extensions of cytoplasm that receive impulses from other nerve cells and transmit impulses towards the cell body
What is the function of the cell body/centron?
Contains the nucleus and groups of ribosomes in the cytoplasm (Nissl granules) that synthesise neurotransmitters
What is the function of the synaptic end bulb?
Swelling at end of axon in which neurotransmitter is stored
What is the function of the axon ending?
Secretes neurotransmitter by exocytosis into synaptic cleft
What components are only found in the neurones of the peripheral nervous system?
-Schwann cells
-Myelin Sheath
-Nodes of Ranvier
What is the function of Schwann cells?
Surround peripheral neurones in vertebrates and grow around the axon to form a multi-layered myelin sheath
How is myelin sheath formed and what is its function?
-Formed as Schwann cells grow around the axon in peripheral neurones.
-Provide electrical insulation to speed up nerve impulse transmission
What is the function of the nodes of ranvier?
Intervals in the myelin sheath between adjacent Schwann cells.
What is the spinal cord?
long, thin, tubular bundle of nervous tissue and support cells that extends from the brain
What is the spinal cord protected by?
the spinal column.
What originates form the spinal cord?
Most of the peripheral nerves (nerves which connect the CNS to the other organs of the body)
What is the function of the spinal cord?
-the transmission of neural signals between the brain and the rest of the body
-it also contains neural circuits that can independently control numerous reflexes.
What is the spinal cord made up of?
of a central area of grey matter which mainly contains nerve cell bodies.
What is grey matter surrounded by?
white matter which consists of nerve fibres surrounded by myelin sheath (axons).
What is the spinal cord surrounded by?
membranes called meninges.
Where do sensory fibres from the peripheral nervous system enter the spinal cord?
-on the dorsal root
-and the cell bodies of the sensory fibres are found in the dorsal root ganglia
Where do motor fibres leave? (spinal cord)
via the ventral roots
What is the simplest type of a response?
A reflex arc which is an inborn response to a stimulus and is rapid automatic and beneficial
why does reflex arc not involve the brain?
-The reaction is involuntary
What are examples of reflexes?
-blink reflex
-Pupil reflex
-knee-jerk reflex
what type of cell is a neurone and what does this mean?
excitable cell, which means it can change its resting potential (the potential difference across the membrane when no nervous impulse is being conducted).
How can the potential difference across membranes be measured?
in experiments involving inserting microelectrodes into axons and measuring the changes in electrical potential which can be read on a cathode ray oscilloscope
why are neurones from giant squid use to test the transmission of impulses?
because they are such a large neurones
What happens when the two microelectrodes are both outside the neurone? (measuring membrane PD)
no difference in electrical potential is recorded
What happens when the two microelectrodes are place on either side of the axon of and the difference between the outside and inside is the axon is measured? (measuring membrane PD)
This is known as potential difference
how do neurones transmit electrical impulses along the cell membrane? (nerve transmission)
by changing the potential difference across the axon membrane of the neurone
What is resting potential?
when no impulse is being sent, the inside of the cell has a negative electrical charge compared to the outside
what is the resting potential maintained via?
The movement of sodium and potassium ions
Why do we say that the membrane is polarised at the resting potential?
large protein anions and organic phosphates (e.g. ATP4-) remain in the cytoplasm thus producing a negative potential difference across the membrane at around -70mv relative of the axon
when is action potential created?
-When the nerve impulse is initiated
how is the action potential generated? (depolarisation)
The energy of a stimulus causes some sodium ion gated channels to open.
-The voltage gated potassium ion channels close.
• If a threshold voltage is reached, the permeability of the membrane to sodium ions increases, and they rush into the axon depolarising the membrane.
• The negative charge of -70Mv inside the axon rapidly becomes a positive charge of +40.
• The is membrane is said to be depolaried.
what happens after depolarisation occurs?
The membrane is repolarised
how does repolarisation occur?
-The voltage-gated sodium ion channels close
-The voltage-gated potassium ion
channels open
This causes K+ to rapidly diffuse out of the axon, reducing the potential difference across the membrane.
• An overshoot causes the membrane to become hyperpolarised.
what happens during the refractory period?
-the concentrations of K+ and Na+ are restored to that of the resting potential
-(about 1ms) this portion of the axon cannot transmit another action potential ensuring that transmission is in one direction only.
what happens if the intensity of a stimulus is below the threshold (approximately -55mv)?
no action potential is initiated
What happens if the intensity of the stimulus is below threshold and when it exceeds this threshold and what does it ensure? (-55mV)
If the intensity of a stimulus is below the threshold (approximately -55mV) then no action potential is initiated, but if the intensity of the stimulus exceeds this threshold then an action potential is initiated. This ensures that low level stimuli are filtered out. The action potential when initiated is always the same size (+40mV).
what is the all or nothing law state?
‘An action potential is either initiated or it is not and it is always the same size’
-The size of the impulse is independent of the size of the stimulus
-The speed of conduction of the stimulus is not altered by the intensity of the stimulus
-A large stimulus will simply produce a greater frequency of impulses
what factors affect the speed of conduction of an impulse??
-temperature
-diameter of axon
-myelination
how does temperature affect the speed of conduction of an impulse?
-lons move faster at higher temperatures as they have more kinetic energy.
-Warm-blooded animals (mammals / birds) transmit nervous impulses more quickly and have faster responses.
how does the diameter of axon affect the speed of conduction of an impulse?
The larger the diameter, the greater its volume in relation to the area of the membrane.
More Na can flow along the axon so impulses travel faster.
how does myelination affect the speed of conduction of an impulse?
electrical insulation of the axon speeds up the rate of transmission as the action potential jumps from node to node in a process called saltatory propagation.
-impulses in unmyelinated neurones have a maximum speed of around 1 m/s, in myelinated neurones they travel at 100 m/s.
What is Cnidaria?
A phylum containing aquatic animals such as hydra, jellyfish and sea anemone.
What is a nerve net?
The simplest type of nervous system
In a nerve net what do the sense receptors do? (in cnidaria)
they respond to a limited number of stimuli and there is only a small number of effectors
what does the nerve system of cnidaria consist of?
simple nerve cells with short extensions joint to each other and branching in a number of different directions
what is the nervous system type in a hydra?
nerve net
Is a myelin sheath present in the nervous system of hydra?
absent
what is a conduction speed of the nervous system of hydra?
slow (approx 5m/s-1)
Do hydra have the ability to regenerate neurones?
yes
what is the nervous system type in a human?
CNS
Do human neurones have a myelin sheath?
present
what is the conduction speed of human nervous system?
Fast (up to 120 m/s)
Does the human nervous system have the ability to regenerate neurons?
no
What are neurones separated by?
gaps called synapses
what are the two types of synapses?
Electrical and chemical
what is an electrical synapse?
a tiny gap (approx. 3m) that is small enough for an electrical impulse to be transmitted directly across. an electrical impulse.
what is a chemical synapse?
-involves a gap of around 20mm
-Branches of axons lie close to dendrites and the impulse is transmitted across the synaptic cleft chemically as a neurotransmitter before being converted back into an electrical impulse.
What are some neurotransmitters?
Acetylcholine then e.g. Dopamine, Serotonin, noradrenalin, GABA, endorphins, Glycine
How do organophosphates affect the transmission of impulses?
-Organophosphorus insecticides are acetylcholinesterase inhibitors, which cause repeated firing of the post-synaptic neurone.
-This results in repeated, uncontrolled contractions of the muscle.
how do psychoactive drugs affect the transmission of impulses?
They act on the CNS and brain function resulting in temporary changes and perception, mood, consciousness and behaviour
What do Excitatory (agonist) drugs do?
stimulate the nervous system- increase number of action potentials
what are excitatory drugs possible methods of action?
-Mimic the action of natural NT - have similar shape and bind to post-synaptic neurone - agonist
-Prevent breakdown of neurotransmitter by inhibiting Ach esterase
-Block reuptake of natural NT
what are examples of excitatory drugs?
-Nicotine
-organ phosphates
insecticides, herbicides and nerve gases
-cocaine and ecstasy
what do inhibitor (antagonist) drugs do?
-inhibit the nervous system- decreased the number of action potentials
what are possible method of action of inhibitor drugs?
-Blocking receptors in post-synaptic membrane preventing neurotransmitters binding to them - antagonist
-Blocking calcium ion uptake
-Inhibit exocytosis of neurotransmitters
what are examples of inhibitor drugs?
-Alcohol
-Benzodiazepines e.g. diazepam (valium)
how can SSRI’s treatment disorders?
-inhibit the reuptake process, leaving the serotonin active for longer within the cleft and the post synaptic receptors will continue to be stimulated
-this has the effect of alleviating the persons mood or reducing anxiety and depression
-Many anti depressants are marketed as SSRI’s
