Neuron Imp Gen Reflex arc notes
M1. NEURONS
Functions
- dendrite: conducts message towards the cell body
- axon: conducts message away from the cell body
- synapse: message jumps from one neuron to another
- myelin sheath: insulates the nerve fibre electrically
- node of Ranvier: speeds up transmission of nerve impulses
- cell body: conducts normal functions (respiration, etc)
Functions
- dendrite: conducts message towards the cell body
- axon: conducts message away from the cell body
- synapse: message jumps from one neuron to another
- myelin sheath: insulates the nerve fibre electrically
- node of Ranvier: speeds up transmission of nerve impulses
- cell body: conducts normal functions (respiration, etc)
M2. SENSORY, MOTOR AND INTERNEURONS
a) Motor neuron - connected to an effector (muscle, fibre or gland)
- short dendrite, long axon
- signal causes effector to react (eg. muscle to contract)
- cell body is inside CNS
- message is travelling away from the brain
b) Sensory neuron - starts with a sensory receptor (pressure, heat, light etc)
- message travels towards CNS
- cell body is outside CNS in Ganglia
- long dendrite, short axon
c) Association (inter) neuron
- smaller than a & b above
- entirely within CNS
- both long and short axons and dendrites
- conveys messages between system parts in CNS B. IMPULSE GENERATION
1. Nerve transmission
- the nerve impulse is electrical in nature.
Normal (no impulse), at rest.
a) Motor neuron - connected to an effector (muscle, fibre or gland)
- short dendrite, long axon
- signal causes effector to react (eg. muscle to contract)
- cell body is inside CNS
- message is travelling away from the brain
b) Sensory neuron - starts with a sensory receptor (pressure, heat, light etc)
- message travels towards CNS
- cell body is outside CNS in Ganglia
- long dendrite, short axon
c) Association (inter) neuron
- smaller than a & b above
- entirely within CNS
- both long and short axons and dendrites
- conveys messages between system parts in CNS B. IMPULSE GENERATION
1. Nerve transmission
- the nerve impulse is electrical in nature.
Normal (no impulse), at rest.
Action (an impulse) depolarization
Two phases
a) Na+ pours inside the axon due to sodium gates in the fibre membrane opening
Two phases
a) Na+ pours inside the axon due to sodium gates in the fibre membrane opening
b) K+ pours outside the axon due to pottasium gates opening
Many impulses can pass down a nerve because only a small fraction of the Na+ and K+ actually move.
1 - Resting potential (sod.-pot. pump)
2 - Action potential, Na+ gates opening, Na+ pouring in
3 - Action potential, K+ gates opening, K+ pouring out
4 - Resting potential, fibre is ready to conduct again (after rec. period)
After a time however, a nerve can lose its ability to make an action potential because too much Na+ and K+ have traded places. The nerve needs a resting period, and the Na+ and K+ are actively pumped back across the membrane.
The impulse moves down the fibre, because reversal at one point of the fibre (an action pot.) stimulates the sodium gates to open at the very next point. The gates that have just opened and closed cannot be restimulated for a very brief period of time, (Recovery period ) so the impulse moves in one direction only.
2 - Action potential, Na+ gates opening, Na+ pouring in
3 - Action potential, K+ gates opening, K+ pouring out
4 - Resting potential, fibre is ready to conduct again (after rec. period)
After a time however, a nerve can lose its ability to make an action potential because too much Na+ and K+ have traded places. The nerve needs a resting period, and the Na+ and K+ are actively pumped back across the membrane.
The impulse moves down the fibre, because reversal at one point of the fibre (an action pot.) stimulates the sodium gates to open at the very next point. The gates that have just opened and closed cannot be restimulated for a very brief period of time, (Recovery period ) so the impulse moves in one direction only.
- an action potential can only be initiated at the dendrite end of a nerve fibre (receptor or synapse) Thus the impulse can only travel away from the receptor towards the axon. It never goes in the opposite direction ( although it can be made to do so artificially)
ALL OR NONE
- action potentials, once stimulated, are all the same (-60 to +40 mv). It is either there, or not.
M4. MYELIN SHEATH
In myelinated cells, the action potential cannot change under the myelin, so they "jump" from one node of Ranvier to another. This way they travel approx. 200 X faster.
- Myelinated nerve cells are covered by a myelin sheath
- has white appearance
- interrupted by gaps at intervals: nodes of Ranvier.
Non myelinated fibers: Impulse goes point to point (slower)
Myelinated fibers: Impulse goes node to node. (faster)
( Demo using tennis ball)
M5. THE SYNAPSE - Junctions between nerve cells
ALL OR NONE
- action potentials, once stimulated, are all the same (-60 to +40 mv). It is either there, or not.
M4. MYELIN SHEATH
In myelinated cells, the action potential cannot change under the myelin, so they "jump" from one node of Ranvier to another. This way they travel approx. 200 X faster.
- Myelinated nerve cells are covered by a myelin sheath
- has white appearance
- interrupted by gaps at intervals: nodes of Ranvier.
Non myelinated fibers: Impulse goes point to point (slower)
Myelinated fibers: Impulse goes node to node. (faster)
( Demo using tennis ball)
M5. THE SYNAPSE - Junctions between nerve cells
M6. SYNAPTIC TRANSMISSION
As the impulse moves into the synaptic ending, it stimulates the synapse vesicles to move to the pre synaptic membrane of the axon. (This involves Ca+2 ions). These fuse to the membrane which then empty their contents (neurotransmitter substance) into the synaptic cleft. These diffuse across the cleft to the receptor sites in the post synaptic membrane of the dendrite. These can either:
i) increase action potentials in the dendrite
ii) decrease action potentials in the dendrite
iii) initiate an action potential in the dendrite (open Na+ gates)
- The impulse can only go one way across the gap because only the axon has the vesicles and the dendrite the receptors.
Neurotransmitters
- Different nerve cells can have different neurotransmitters.
Some examples are: Acytlcholine
Noradrennalin
Seratonin
M7. NEUROTRANSMITTER BREAKDOWN
- Enzymes exist in the synaptic cleft that break apart the neurotransmitter. This clears out the gap, so that a new impulse can be rapidly transmitted. It also allows reuse of the neurotransmitters as the parts diffuse back into the axon to be rebuilt into new neurotransmitter substance.
Example: Acytlcholinesterase (destroys Acytlcholine into choline and acetic acid)
M8. REFLEX ARC
As the impulse moves into the synaptic ending, it stimulates the synapse vesicles to move to the pre synaptic membrane of the axon. (This involves Ca+2 ions). These fuse to the membrane which then empty their contents (neurotransmitter substance) into the synaptic cleft. These diffuse across the cleft to the receptor sites in the post synaptic membrane of the dendrite. These can either:
i) increase action potentials in the dendrite
ii) decrease action potentials in the dendrite
iii) initiate an action potential in the dendrite (open Na+ gates)
- The impulse can only go one way across the gap because only the axon has the vesicles and the dendrite the receptors.
Neurotransmitters
- Different nerve cells can have different neurotransmitters.
Some examples are: Acytlcholine
Noradrennalin
Seratonin
M7. NEUROTRANSMITTER BREAKDOWN
- Enzymes exist in the synaptic cleft that break apart the neurotransmitter. This clears out the gap, so that a new impulse can be rapidly transmitted. It also allows reuse of the neurotransmitters as the parts diffuse back into the axon to be rebuilt into new neurotransmitter substance.
Example: Acytlcholinesterase (destroys Acytlcholine into choline and acetic acid)
M8. REFLEX ARC
- A reflex action (eye blink, hand jerking away from a hot object) in which a stimulus causes a response, without the brain being invloved in making a decision.
- The inter neuron bypasses the brain
- Another neuron also goes to the brain, but by the time the impulse reaches the brain, the motor neuron has already contracted the muscle.
- The inter neuron bypasses the brain
- Another neuron also goes to the brain, but by the time the impulse reaches the brain, the motor neuron has already contracted the muscle.