Nerve cells and nerve impulses Flashcards
Describe the nervous system
is the communication network & control centre of the body.
is also involved in maintaining a constant environment inside the body, along with endocrine system
The nervous system can be divided into two main parts:
- Central Nervous System (CNS)
- Peripheral Nervous System (PNS)
Nerve cells (neurons) consist of:
a cell body – the cytoplasm contains nucleus and other organelles
dendrites - short extensions from the cell body. Carry nerve impulses towards the cell body.
an axon (nerve fibre) - single, long extension of the cytoplasm. Carries nerve impulses away from the cell body.
What forms the myelin sheath ?
special cells called Schwann cells, which wrap around the axon. In the brain and spinal cord, the myelin sheath is produced
What are the intervals along the axon are gaps in the myelin sheath are called ?
Nodes of ranvier
The outermost coil of the Schwann cell forms a structured called ?
neurilemma which helps in the repair of injured fibres
Roles of myelin sheath:
Protects nerve fibre
Insulates axon
Speeds up transmission of nerve impulses
TYPES OF NEURONS (FUNCTIONAL)
Sensory neurons carry messages from receptors to CNS
Interneurons are located in the CNS and are the link between the sensory and motor neurons
Motor neurons carry messages from the CNS to the effectors
Explain the reflex action when a finger touch object
- Sensory neurone conduct signals from receptors to the CNS
- Interneurons send messages from sensory neutrons to motor neurons
- Motor neurons conduct signals from the CNS to effectors such as muscles and glands
Describe features of multipolar neurons
Have one axon & multiple dendrites extending from the cell body.
Includes most of the interneurons and motor neurons
Describe features of bipolar neurones
Have one axon & one dendrite extending from the cell body
Axon and dendrite have many branches at their ends.
Occur in the eye, ear and nose, where they take impulses from the receptor cells to other neurons.
Explain the features of unipolar neurons
Have just one extension from the cell body, an axon
The cell body is to one side of the axon
Mostly sensory neurons
What are nerve impulses
A nerve impulse is an electrochemical change that travels along a nerve fibre.
It involves change in electrical voltage
Nerve impulses are transmitted very quickly – so body can respond rapidly to any changes
What is membrane potential?
Membrane potential is the potential difference created by the difference in the ion concentration on either side of the cell membrane
What is resting membrane potential ?
The membrane potential of unstimulated nerve cells, known as the resting membrane potential is about –70 mV.
The resting membrane potential is -70mV because:
- The sodium potassium pump actively transports 3 Na+ ion out of the cell and 2 K+ into the cell
- The permeability of cell membrane is not equal
- Highly permeable to K+ and Cl-
- Slightly permeable to Na+
- Impermeable to large negative ions
What happened if a stimulus is strong enough to reach threshold (-55mV)?
Action potential occurs as a sequence of 3 events:
Depolarization
Repolarization
Reestablishment of resting potential
The rapid depolarisation and repolarisation of the membrane is called?
Action potential
What is depolarisation ?
When threshold is exceeded, voltage-sensitive Na+ channels open
Na+ move into the cytoplasm of the axon
What is repolarization ?
At +30mV, Na+ channels close
Voltage-gated K+ channels open
K+ move out of the cell
What is the reestablishment of resting potential ?
K+ channels close slowly, excess K+ leave neuron hyperpolarization
Sodium-potassium pump restores membrane potential
What is an action potential ?
An all or none response
What is the size of nerve impulse that travels along a fibre
always the same
Does a weak stimulus, provided it exceeds the threshold , produces the same action potential as a strong one?
Yes
Describe stronger stimulus
stimulate more receptors or more nerve fibres depolarized
trigger a greater frequency of impulses in sensory neurons
What is a refractory period?
During an action potential, and for a very brief time afterwards, that part of the nerve fibre cannot be stimulated to respond again
What is the importance of refractory period ?
The refractory period ensures the nerve impulse is propagated in one direction
Explain the transmission of the nerve impulse
The speed at which an impulse travels depends on whether:
1. the nerve fibre is myelinated (faster) or unmyelinated (slower)
- diameter of the fibre – the greater the diameter, the faster the speed of transmission
Explain the continuous propagation along unmyelinated fibres
Depolarisation of one area of the membrane causes a local current flow between neighbouring areas on the membrane
This current flow causes depolarisation immediately adjacent to the site of the original stimulus.
Each action potential generates another action potential just in front of it
Explain the saltatory conduction along myelinated fibres
Saltatory conduction along myelinated fibres
The nerve fibres are insulated from the extracellular fluid except at the nodes of Ranvier ions can readily cross the plasma membrane only at the nodes
As a result, depolarization can only occur at the nodes. This increases the distance that a given local current flows
Action potential jumps from one node of Ranvier to the next (saltatory conduction)
What is synapse ?
The junction between the branches of adjacent neurons is called the synapse.
Where does synapse occur ?
Most synapses occur between the end branches of an axon of one neuron and a dendrite or the cell body of another neuron.
What does the pre-synaptic neutron has ?
Pre-synaptic neuron has synaptic knob which fits into depression on post-synaptic neuron.
What is neuromascular junction?
Axon meets a skeletal muscle cell
Explain the synaptic transmission
- When action potential reaches axon terminals, voltage gated calcium channels open and triggers the influx of calcium ions
- This triggers vesicles containing neurotransmitters to fuse with the cell membrane, which releases neurotransmitters into the synaptic cleft
- Neurotransmitter diffuses across synaptic cleft and binds to receptor on the dendrite of post-synaptic membrane_ (ligand-gated) Na+ channels open, allowing Na+ to enter post-synaptic cell _ action potential generated if threshold value is reached
What breaks down the neurotransmitter ?
specific enzyme (eg. acetylcholinesterase). The components re-enter the axon terminal by endocytosis and are reformed into new neurotransmitters, ready to be used again
As synaptic transmission can only occurs in one direction , what are the direction ?
– from axon to dendrite or cell body
What are the examples of neurotransmitters ?
Examples of neurotransmitters: acetylcholine, adrenaline, dopamine and histamine.
What are the effect of chemicals on synaptic transmission ?
Stimulants such as caffeine and Benzedrine stimulate transmission at the synapse.
Other drugs, such as anaesthetics or hypnotics, depress the transmission.
Venom from certain species of snakes and spiders also affects the neuromuscular junction.
The chemicals can act in different ways, what are the ways ?
Some have similar structures to neurotransmitters
have the same effect as the neurotransmitter but are not removed so that the response is prolonged
Or block receptors preventing a response
prevent neurotransmitters being released.
increase the release of neurotransmitters.
Some prevent neurotransmitters being broken down and so prolong their effects.
Nerve agents (nerve gases) used during World War II contained organophosphates. Organophosphates are used in some insecticides. What are the effects ?
Cause build-up of acetylcholine at the neuromuscular junction.
All muscles in the body then try to contract and the loss of muscle control prevents breathing.
Generation of action potential: (max 10 marks)
· Neurons are normally polarised. Resting membrane potential is at –70mV.
· This imbalance is maintained by 3 sodium ions being actively pumped out and 2 potassium pumped into the axon by sodium-potassium pumps.
· If a stimulus is strong enough to reach threshold (-55mV), action potential will occur
· Voltage-gated sodium ion channels open
· Influx of sodium ions
· Membrane depolarises
· At +30mV, voltage gated sodium ion channels close, voltage gated potassium ion channels open
· Potassium ions leave the axon
· Membrane is repolarised
· Membrane is temporarily hyperpolarised as too many potassium ions exit the axon
· Sodium-potassium pumps restores the resting membrane potential
Propagation refers to the movement of a nerve impulse along a neuron: (max 5 marks)
· Depolarisation of one area of the membrane causes a local current flow between neighbouring areas
· and causes depolarisation in adjacent membrane
· During refractory period the membrane will not respond to stimulus
· Impulse transmission is unidirectional
· In myelinated axon, impulse move via saltatory conduction/ nerve impulse jumps from one node of Ranvier to the next
· In unmyelinated axon, impulses are propagated continuously throughout the axon
Transmission from one neuron to the next refers to synaptic transmission: (max 5 marks)
· When the nerve impulse arrives at the synaptic knob
· vesicles containing neurotransmitters will fuse with the membrane
· releasing the neurotransmitters in to the synaptic cleft
· Neurotransmitters will diffuse across synaptic cleft
· Binds to the receptors on the membrane of postsynaptic cell
· Membrane of the next neuron becomes permeable to Na+ and depolarisation occurs
