Inside potentials Pt1
Transmissive Segment Overview
The transmissive segment is the last of the four functional regions in a neuron where transmission occurs.
This segment is essential for sending impulses either to the next neuron or to an effector, such as a muscle cell.
Arrival of Action Potential
Action Potential Arrival:
Arrows indicate the arrival of the action potential at the synaptic knobs after being conducted down the axon.
The arrival triggers a change in the electric potential difference across the membrane.
Voltage Change and Calcium Channels:
This change in voltage leads to the opening of voltage-gated calcium channels.
Calcium ions (Ca²⁺) are more concentrated outside the cell than inside due to previous actions of calcium pumps that have expelled Ca²⁺ from the cell, creating a concentration gradient.
Calcium Ion Influx
When voltage-gated calcium channels open, calcium ions flow into the synaptic knob along their concentration gradient.
Inside the knob, calcium ions bind to synaptic vesicles, causing them to release neurotransmitters into the synaptic cleft.
Gradient Potentials in Brain Cells
Ion Movement and Gradient Potentials:
The opening of ion channels leads to the movement of ions across the cellular membrane, resulting in changes in electric potential.
Direction of movement can either render the inside of the cell less negative (depolarization) or more negative (hyperpolarization).
Example of Ion Movement:
Positive sodium ions (Na⁺) entering the cell will make the inside less negative, facilitating depolarization.
Intensity of Graded Potentials:
Graded potentials vary in intensity based on the degree of stimulation, i.e., more neurotransmitters binding to chemically gated channels leading to greater ion exchange.
Local Potentials
Local Potentials:
These are generated in the dendrites and travel short distances to the initial segment of the axon.
They result from ion movement via chemically gated channels and are only effective over short distances.
Action Potentials and Their Propagation
Action Potential Generation:
Action potentials occur only when the threshold is reached; they propagate down the axon.
The process involves voltage-gated channels where depolarization is caused by Na⁺ influx and repolarization is due to K⁺ (potassium) efflux.
The action potential reaches a maximum of approximately +30 millivolts.
Conduction:
Unlike local potentials, action potentials propagate over long distances, potentially several feet in length.
Factors Influencing Action Potential Velocity
Velocity of action potential transmission is influenced by:
Axon Diameter:
A larger diameter axon allows for faster impulse transmission due to reduced resistance to ion flow.
Analogy: It is quicker to push water through a wider hose than a narrow one.
Myelination:
Myelinated axons transmit impulses much faster due to saltatory conduction, where the action potential jumps between the nodes of Ranvier (neurofibrillar nodes).
Each node houses voltage-gated sodium channels, amplifying the electrical signal as it travels.
Myelinated Axon Structure
Schwann Cells:
These are the specific neuroglial cells responsible for myelination in the peripheral nervous system.
They cover segments of the axon (approximately one millimeter each) but do not overlap. Spaces between adjacent Schwann cells are known as nodes of Ranvier.
Mechanism of Saltatory Conduction:
As the action potential travels along myelinated axons, it moves beneath the myelin sheath and electrical current becomes weaker due to energy loss.
However, it remains strong enough to reach the next node of Ranvier, ensuring that it still reaches the threshold for action potential generation.
Conclusion
Understanding the mechanisms of the transmissive segment is crucial for grasping how neuronal communication occurs, including the roles of ion gradients, action potentials, and the structural components that facilitate rapid signal transmission in the nervous system.