15.Nervous Coordination and Muscles Flashcards
Describe the structure of a myelinated motor neurone.
- Cell body which contains usual organelles a nucleus and large amounts of RER (this is associated with the production of proteins and neurotransmitters).
- dendrons are extensions of the cell body which subdivide into branched fibres which carry nerve impulses towards the cell body.
- axon is a single fibre that carriers nerve impulses away from the cell body.
- myelinating schwann cells which surround the axon providing electrical insulation (also carry out phagocytosis and play a part in cell regeneration).
- myelin sheath covers the axon and is made from the membranes of schwann cells (which is rich in the lipid myelin).
- nodes of Ranvier gaps between myelin sheath (schwann cells).
Function of sensory neurone
Transmits nerve impulses from a receptor to a relay neurone in the spinal chord.
Function of relay neurone
Transmits nerve impulses from sensory to relay neurone
Function of motor neurone
Transmits nerve impulse from relay neurone, out the spinal chord, to an effector (gland/muscles).
An effector can be a
Muscle or a gland.
Neurones are adapted to carry electrochemical charges called (1). Each neurone comprises a cell body that contains a (2) and large amounts of (3), which is used in the production of proteins and neurotransmitters. Extending from the cell body is a single long fibre called an axon and smaller branched fibres called (4). Axons are surrounded by (5) cells, which protect and provide (6) because their membranes are rich in a lipid known as (7). There are three main types of neurone. Those that carry nerve impulses to an effector are called (8) neurones. Those that carry nerve from a receptor are called (9) neurones and those that link the other two types of neurones are called (10) neurones.
(1) electrical impulses
(2) nucleus
(3) rough endoplasmic reticulum
(4) dendrons
(5) schwann
(6) electrical insulation
(7) myelin
(8) motor
(9) sensory
(10) relay
Define a nerve impulse
- a self-propagating wave of electrical activity that travels along the axon membrane
- it is a temporary reversal of potential difference across the axon membrane (from resting potential to the action potential)
How is the movement of ions across the axon membrane controlled
- The phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions from diffusion across it
- Protein channels span the phospholipid bilayer which have ion channels. These ion channels can be opened or closed so that sodium ions or potassium ions can move through them via facilitated diffusion.
- Sodium-potassium ion pump. A form of active transport using carrier proteins to move Na+ ions out the axon and K+ ions in.
What is a resting potential
Potential difference (difference of change between in and outside the axon) caused by the movement of positive ions in and out the axon. In this condition the axon is said to be polarised.
How is a resting potential established
- Sodium ions are actively transported out and potassium ions in by the sodium-potassium ion pumps. -This caused 3 Na+ ions to move out while 2 K+ ions move in.
- This causes an electrochemical gradient as the movement of Na+ ions out is greater than the K+ ions in.
What is an action potential
A travelling wave of depolarisation.
What is depolarisation
Temporary reversal of charges on the cell surface membrane of a neurone which takes place when a nerve impulse passes.
What causes depolarisation
- voltage-gated Na+ channels open
- allowing the movement of sodium ions into the axon
- via facilitated diffusion down an electrochemical gradient
Describe the events of depolarisation, hyperpolarisation and repolarisation
- At resting potential some K+ ion voltage-gated channels are open but Na+ ion voltage-gated channels are closed
- The energy of the stimulus causes some Na+ voltage-gated channels to open and therefore Na+ ions diffuse into the axon through these channels down the electrochemical gradient
- As Na+ ions diffuse into the axon, they diffuse further down within the axon causing more Na+ ion channels to open causing an influx of Na+ ions.
- Once the action potential of around +40mV gas been established the Na+ ion channels shut and some K+ ion voltage-gated channels open.
- This causes K+ ions to diffuse out of the axon down the electrochemical gradient. This starts the repolarisation of the axon.
- The outward diffusion of these K+ ions causes a temporary overshoot of the electrical gradient. The inside of the axon is more negative than usual relative to the outside ( =hyperpolarisation ).
- The K+ ion voltage gated proteins close. This activates the Na+ K+ ion pump to pump Na+ ions out and K+ ions in. The resting potential is reestablished, this is known as repolarisation
Name the chain of events that occur before, during and after a nerve impulse passes
Depolarisation > Hyperpolarisation > Repolarisation
What mechanism is resting potential maintained by
Acitve transport of K+ and Na+ ions (pump)
What mechanism does an action potential rely on
Facilitated diffusion
What is saltory conduction
Describes the movement of an action potential in a myelinated axon as it skips from node to node.
Name the factors that affect the speed at which an action potential travels
- myelin sheath
- diameter of axon
- temperature
How does the myelin sheath affect action potential speed
- myelin is an electrical insulator
- stops an action potential from forming
- action potentials therefore only occur at breaks in this myelin (Node of Ranvier)
- the action potential jumps between Nodes of Ranvier and is faster (saltory conduction)
How does the axon diameter affect action potential speed
- larger diameter means faster conductance
- as there is less leakage of ions if it is a bigger axon
How does the temperature affect action potential speed
- Temperature increases diffusion rate of ions (due to a increase in kinetic energy) meaning faster nerve impulse.
- Respiration is controlled by enzymes and is necessary for active transport. If temperature gets to high these enzymes and plasma membrane proteins are denatured so impulses fail to be conducted at all (active transport needed in repolarisation for Na+ K+ pump )
What is the all or nothing principle
There is a certain level of stimulus needed to trigger an action potential, this is called the threshold value.
How do organisms perceive the size of a stimulus
- frequency of impulses, larger stimulus = more impulses generated in a given time
- by having different neurones with different threshold values
What is the refractory period
- once an action potential in an axon has been created, there is a period of time when inward movement of sodium ions is prevented
- as the voltage-gated sodium ion channels are closed
- action potential can no longer be generated
Name 3 purposes the refractory period serves
- It ensures that action potentials are propagated in one direction only. As action potentials cannot be propagated in a region that is refectory, they can only move in one direction.
- It produces discrete impulses. It means a new action potential cannot be formed immediately behind another one so action potentials are separated from one and other
- Limits the number of action potentials that can be passes along in a given time thus limiting the strength of a stimulus that can be detected
Define a synapse
A junction between neurones across which neurotransmitters diffuse across
What is the function of a synapse
- transmits information (not electrical impulses) from one neurone to another
- by means of chemicals know as neurotransmitters
- allows a single stimulus to create a number of simultaneous responses
- or allows impulses reacting from different stimuli to contribute to a single response
Describe the structure of a synapse
- synaptic gap, the gap between neurones
- presynaptic knob, the swelling which releases the neurotransmitter. This possesses large amounts of mitochondria and endoplasmic reticulum (required to manufacture the neurotransmitter). Has large amounts of Ca 2+ ion protein channels on cell surface membrane.
- synaptic vesicles, where the neurotransmitter is stored in the presynaptic knob
- post synaptic knob, where neurotransmitter diffuses to. Has Na+ ion protein channels on cell surface with receptors that are complementary to the neurotransmitter.
Describe unidirectionality of a synapse
- neurotransmitter is only present in the presynaptic knob
- receptors are only present on the post synaptic knob
- means one of the features of a synapse is they can only pass information in one direction
