Nervous Control Flashcards
What is the central nervous system made up of ?
The brain and the spinal cord
What is the peripheral nervous system made up of ?
made up of pairs of nerves originating from the brain or the spinal cord.
what is the motor nervous system divided into ?
the voluntary nervous system
the autonomic nervous system
What is the autonomic nervous system divided into ?
the sympathetic nervous system
the parasympathetic nervous system
what is the role of the sympathetic nervous system ?
stimulates effectors
speeds up activity
preparation for stressful activities ‘fight or flight’
increase heart rate
dilate pupils
inhibit activity of intestines and stomach
What is the role of the parasympathetic nervous system ?
inhibits effectors slows down activity
Controls activities at rest
conserves energy and replenishes the bodies reserves
slow heart rate
constrict pupils
stimulate activity of stomach
What 3 neurones does a REFLEX ARC involve ?
Sensory neurone
Intermediate neurone
Motor neurone
What are the main stages of a spinal reflex arc when touching a hot object ?
Stimulus is the heat from the hot object
Receptors detect temperature change in the skin and generate a nerve impulse.
Sensory neurone passes the nerve impulse to the spinal cord.
Intermediate neurone links the sensory neurone to the motor neurone in the spinal cord.
Motor neurone carries the nerve impulse from the spinal cord to a muscle in the upper arm.
The muscle in the upper arm which is stimulated to contract.
What is the effector in a simple reflex arc when touching a hot object ?
the muscle in the upper arm which is stimulated to contract.
What is the response in a simple reflex arc when pulling the hand away from the hot object ?
pulling the hand away from the hot object.
What is the role of reflex actions ?
Avoiding damage to tissues and prevents injury
Used to escape from predators
Describe the importance of REFLEX ARCS
protect the body from harmful stimuli
Make survival more likely
What is resting potential ?
In a resting axon (one that is not transmitting impulses), the inside of the axon always has a negative electrical potential compared to outside the axon.
Describe the structure of a neurone.
They have :
a cell body : contains a nucleus with large amounts of Rough ER associated with the production of neurotransmitters.
Dendrons and dendrites :This means they can connect to many other neurones and receive impulses from them, forming a network for easy communication
An axon : a single long fibre that carries nerve impulses away from the cell body
A myelin sheath and Nodes of Ranvier - allows nerve impulses to travel by saltatory conduction which speed up the rate of transmission.
What is the numerical value for the resting potential of the axon membrane ?
-70mv
Explain how resting potential is maintained in a neurone.
Sodium-potassium pumps are present in the membranes of neurones
These pumps use ATP to actively transport 3 sodium ions out of the axon for every 2 potassium ions that they actively transport in.
This means that there is a larger concentration of positive ions outside the axon than there are inside the axon
The movement of ions via the sodium-potassium pumps establishes an electrochemical gradient.
The cell-surface membrane of neurones has selective protein channels that allow sodium and potassium ions to move back across the membrane by facilitated diffusion.
The protein channels are less permeable to sodium ions than potassium ions
This means that potassium ions can diffuse back down their concentration gradient, out of the axon, at a faster rate than sodium ions.
What is the defintion of an Action Potential ?
The change that occurs in the electrical charge across the membrane of an axon when it is stimulated and a nerve impulse passes.
also known as a ‘wave of depolarisation along the axon membrane’
What causes the production of an action potential ?
a stimulus
Explain how an action potential is produced when a stimulus arrives at a resting neurone.
Sodium ion channels in the axon membrane open
Sodium ions pass into the axon down the electrochemical gradient.
This reduces the potential difference across the axon membrane as the inside of the axon becomes less negative (depolarisation)
Depolarisation triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation (positive feedback)
If the potential difference reaches around -55mV (known as the threshold potential), many more channels open and many more sodium ions enter causing the inside of the axon to reach a potential of around +40mV
An action potential is generated
The depolarisation of the membrane at the site of the first action potential causes sodium ions to diffuse to along the axon, depolarising the membrane in the next section of the axon and causing sodium ion voltage-gated channel proteins to open there. (conduction)
What happens during repolarisation ?
Voltage gated sodium ion channels close
Voltage gated potassium ion channels in the axon membrane open.
This allows the diffusion of potassium ions out of the axon, down their concentration gradient
This returns the potential difference to normal resting potential (-70mv)
What is Hyperpolarisation ?
This is when the potential difference across this section of axon membrane briefly becomes more negative than the normal resting potential
What is the Refractory Period ?
Once an action potential has been generated there is a period when sodium ions cannot move into the axon because the voltage gated sodium ion channels are closed. (unresponsive)
During this time it is impossible to generate a new action potential.
(a period of recovery)
What is the importance of the Refractory Period ?
Ensures that an action potential is propagated in one direction only
Produces discrete impulses
It limits the frequency of impulses that can pass along an axon in a given time.
What is the ALL-OR-NOTHING LAW ?
An impulse (action potential) is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential
How can an organism perceive the size of a stimulus ?
the larger the stimulus, the more nerve impulses are generated in a given time.
In unmyelinated Neurones the speed of conduction is very slow. Why is this ?
This is because depolarisation must occur along the whole membrane of the axon
What is a myelin sheath made from ?
Schwann cells
What does the myelin sheath act as ?
an electrical insulator
How does a myelin sheath increase the speed at which action potentials can travel ?
In sections of the axon that are surrounded by a myelin sheath, depolarisation cannot occur, as the myelin sheath stops the diffusion of sodium ions and potassium ions
Action potentials can only occur at the nodes of Ranvier
The presence of Schwann cells means the action potentials ‘jump’ from one node to the next, this is known as saltatory conduction.
What are nodes of Ranvier ?
Small uninsulated sections of the axon
What is SALTATORY CONDUCTION ?
Jumping of action potentials across the nodes of Ranvier.
What is the speed of conductance like if the axon diameter is thicker ?
The speed of conductance is faster. This is due to less resistance of flow so ions can be pushed into the next section faster.
greater surface area over which the diffusion of ions can occur
This increases the rate of diffusion of sodium ions and potassium ions through protein channels, which in turn increases the rate at which depolarisation and action potentials can occur
How does temperature affect the speed of conduction along an axon ?
Colder conditions can slow down the conduction of nerve impulses.
The colder temperatures mean there is less kinetic energy available for the facilitated diffusion of potassium and sodium ions during an action potential.
Some animals, such as mammals, maintain very stable body temperatures. Temperature does not usually affect the speed of nerve impulses in these animals
What is the name for synapses which use acetylcholine as a neurotransmitter ?
Cholinergic synapses.
Describe the sequence of events following a nerve impulse arriving at a cholinergic synapse
The arrival of an action potential at the presynaptic membrane causes depolarisation of the membrane
This stimulates voltage-gated calcium ion channel proteins to open
Calcium ions diffuse down an electrochemical gradient from the tissue fluid surrounding the synapse (high concentration of calcium ions) into the cytoplasm of the presynaptic neurone (low concentration of calcium ions)
This stimulates ACh-containing vesicles to fuse with the presynaptic membrane, releasing ACh molecules into the synaptic cleft
The ACh molecules diffuse across the synaptic cleft and temporarily bind to receptor sites on sodium ion channels in the postsynaptic membrane
This causes the sodium ion channels to open allowing sodium ions to diffuse down an electrochemical gradient into the the cytoplasm of the postsynaptic neurone.
The influx of sodium ions generates a new action potential in the post synaptic neurone.
Acetylcholine is broken down into choline and Acetate by the enzyme Acetylcholineesterase The choline is absorbed back into the presynaptic membrane and reacts with acetyl coenzyme A to form ACh, which is then packaged into presynaptic vesicles ready to be used when another action potential arrives.
Why must Acetylcholine be broken down by acetylcholineesterase in the synaptic cleft ?
To prevent the sodium ion channels staying permanently open and to stop permanent depolarisation of the postsynaptic membrane, the ACh molecules are broken down and recycled.
Why can synapses only pass impulses in one direction ?
(They are Unidirectional)
Vesicles containing neurotransmitter only found in the presynaptic neurone.
Post synaptic neurone only has receptors for the neurotransmitter.
When an impulse arrives at a synapse it does not always cause impulses to be generated in the next neurone. What are reasons for this ?
Only a small amount of acetylcholine is released into the synaptic cleft
A small number of the gated ion channels are opened in the axon membrane
An insufficient number of sodium ions pass through the membrane
The threshold potential is not reached
The small amount of acetylcholine attached to receptors is broken down rapidly by acetylcholinesterase