Nervous System Flashcards
Nerve Cells
Transmit information via electric impulse
Structure of a Neuron
The cell body or ‘soma’ hold the nucleus and other cell organells of the neurons.
Emitting outwards from the cell are the dendrites, and play the role of accepting information from upstream neurons and passing that information on.
The other end of the soma is the axon hillock. This is where action potentials are generated. It decides whether or not the given neuron will transmit a message to the next neuron in line.
The multiple signals from the dendrites sums up at the axon hillock and if the signal is strong enough then the neuron will depolarize and pass the message on.
The axon hillock leads to the axon, which is like an electrical wire where the signal either goes to the next neuron or to a target cell such as a muscle or the cells of a gland.
Axons are sheath by fatty membranes called myelin sheaths, which prevents signal moss and increase conduction speed.
The myelin sheath are produced by different cells depending on whether you consider this central or peripheral nervous system.
At the end of the axon are the nerve terminals which are sometimes called the synaptic terminus.
The synaptic terminus passes information alone either to the dendrites, or other target organs. The connection is not direct and relies on the diffusion of neurotransmitters.
Myelin sheets and Nervous system in the CNS
Produced by olidogendrocytes. Oligo means many, and this is because the space in the central nervous system is at a premium.
- In the nervous system the dendrites and axon terminal make up a synapse in the CNS
Myelin Sheets in the PNS
Produced by Schwann cells.
Wraps around just one axon axon. Many myelinated cells are requires to sheath the whole axon.
It leads to gaps in the myelin sheath called nodes of Ranvier, which speeds up action potential conduction.
Rather than depolarization occurring smoothly across the entire axon, depolarization occurs at one gap which causes a change in potential in the next gap, leading to depolarization at the gap, and then the next and so on.
The result is that the signal jumps from one node to the next, skipping the entire myelinated section in between.
If you compare the conduction speed between a myelinated neuron and an unmyelinated neuron the conduction speed is much faster in the myelinated neuron.
When demyelination occurs it leads to disorders such as sclerosis.
Three parts of the synapse
The junction between two neurons is calls a synapse, which has three parts
- Presynaptic neuron
- Postsynaptic neuron
- Synaptic Cleft
Some synaptic gaps imply that action potential cannot directly pass from one neuron to the next. Instead a change in the mode of communication is required.
How Signals travels from one neuron to the next
Thus, chemical messengers known as neurotransmitters are used to carry the signal.
These neurotransmitters are stores in pouches called vesicles in the synaptic terminus of the presynaptic neuron.
When the action potential reaches the nerve terminal, it triggers the opening of voltage-gated calcium channels.
The influx of calcium, triggers the vesicles to fuse to the cell membrane, dumping neurotransmitters into the synaptic cleft.
How Action Potential is experiences (K/Na+ Pumps)
In order for a neuron to experience an action potential, which in a sense, a wave of charge, their neuron itself must be charged even at rest.
To facilitate this charge, sodium and potassium cations slowly leak across the membrane of the cell. Potassium (K) leaks OUT of the cell and sodium (Na) leaks INTO the cell.
Both sodium and potassium leak positive charge, but more potassium leaks out than sodium brings in.
The cell is slowly leaking positive charge, explaining the negative resting potential, which is around -70 mV.
If these leaks continued unabaided then the cell would eventually lose all charge, so a Na/K pumps continues to move these ions in the oppiste direction of their leak so that the leaking and the resting potential can continue.
The sodium/potassium pump moves more cations out of the cell that it brings into the cell, maintaining a negative membrane potential
Action Potential
- In order for a neuron to experience an action potential, which in a sense, a wave of charge, their neuron itself must be charged even at rest. To facilitate this charge, sodium and potassium cations slowly leak across the membrane of the cell.
- Potassium (K) leaks OUT of the cell and sodium (Na) leaks INTO the cell.
- Both sodium and potassium leak positive charge, but more potassium leaks out than sodium brings in. The cell is slowly leaking positive charge, explaining the negative resting potential, which is around -70 mV. If these leaks continued unabaided then the cell would eventually lose all charge, so a Na/K pumps continues to move these ions in the opposite direction of their leak so that the leaking and the resting potential can continue.
- When an action potential reaches the axon terminal it triggers the opening of the voltage gated calcium channels.
Depolarization
Depolarization:
Occurs when the membrane potential rises above the resting potential. The potential is heading away from -70 mV and closer to 0. it the membrane potential depolarizes above a threshold value (-55 mV to -40mV) then an action potential will occur.
Activates voltage-gated sodium channels and the slow leak of sodium suddenly turns into a sodium surge. As a result the membrane potential rapidly increases.
Even though its called depolarization because the potential heads towards zero, the action potential is so powerful that the potential actually overshoots and ends up at +35 mV. At this point sodium channels are deactivated.
As the sodium channels close voltage-gates potassium channels are activated, and now, the slow leak of potassium out of the cell turns into a surge.
Very few potassium ions actually move, but for a moment, with those potassium channels wide open, they have the potential to move. (same as sodium channels when they are open).
Hyperpolarization
Hyperpolarization:
Occurs when the membrane potential goes below the resting potential and prevents generation of an action potential.
The potential for a positive charge to escape the cell drives the potential in the negative direction, past the resting potential which is why it is called hyperpolarization.
During this hyperpolarization phase, that segment of the axon cannot fire another action potential which is known as the refractory period .
The refractory period benefits the neurons by preventing backflow of the action potential
Organization of the Nervous System
- Central Nervous System (CNS)
- Organizes and directs our nervous system.
- Send information to the PNS
- Peripheral Nervous System (PNS)
- Receive information from the CNS and sen it towards the rest of our body
- Provide sensory feedback to the CNS
Central Nervous System
- Includes the brain and spinal cord
- Made up of Neurons
- Sensory neurons
- Motor neurons
- Interneurons
- Cell bodies and dendrites of the neuron come together to make the gray matter
- Myelinated axons of neurons come together to form the white matter
Sensory (Afferent) Neurons
- Bring information into the CNS
- Afferent neurons are Affected by sensations such as temperature, taste or sound
Motor (Efferent) Neurons
Take information from the CNS to the rest of the body
Efferent neurons cause an effect like flexing your muscles
Interneurons
- Interconnect the efferent and afferent neurons
