Lecture 4

Nodes of Ranvier:
- The axon is the only part of a neuron that is myelinated. There is no myelin on the cell body or dendrites. Myelination stops just before the axon terminals.
- Nodes of Ranvier are regions of a myelinated axon that are unmyelinated, meaning myelination is discontinuous along the axon.
- Contain voltage-gated $Na^+$ channels. Voltage-gated $Na^+$ channels are not found along the axon in the myelinated regions.
- Voltage-gated $Na^+$ channels are also found clustered at the axon hillock, which is unmyelinated.
Nodes of Ranvier in the nervous system:
- In the peripheral nervous system (PNS), a single Schwann cell myelinates one segment of the axon.
- In the central nervous system (CNS), a single oligodendrocyte myelinates several axons and several regions within a given axon.

Saltatory Conduction:
- Propagation of action potentials along a myelinated axon, where action potentials "jump" from one node of Ranvier to the next.
- It involves electrotonic conduction at the nodes of Ranvier.
- Myelin prevents depolarization of the neighboring tissue at every point along the axon.
- The action potential is generated at the first node of Ranvier and then passes to the second node of Ranvier, where depolarization of the neighboring tissue occurs.
- The action potential can pass undiminished in size from node to node.
- Depolarization of the neighboring tissue and opening of voltage-gated sodium channels occur only at the nodes of Ranvier.
Classification of Afferent Fiber Type:
- Two factors determine the speed at which an axon propagates an action potential from point A to point B:
  - Size of the axon: Thicker axons propagate action potentials faster.
  - Myelination: Myelinated axons propagate action potentials faster than unmyelinated axons.
Speed of Propagation:
- By the time the absolute refractory period of any neuron (myelinated or unmyelinated) has been completed, the action potential has traveled far enough down the axon that there will not be depolarization of the neighboring tissues upstream.
- The action potential will always travel down the axon to the synaptic terminals or axon terminals.

Synaptic Transmission:
- The process by which one neuron communicates with other neurons or effectors (e.g., a muscle cell) at a synapse.
Stretch reflex:
- Afferent neuron:
  - Myelinated – action potential travels by saltatory and electrotonic conduction to the spinal cord.
  - The afferent neuron activates the efferent neuron (to the quadriceps) and the inhibitory interneuron (inhibits efferent neuron to hamstrings) via chemical synaptic transmission.
Synaptic Transmission can be either chemical or electrical.

Electrical Synapse:
- Physical connection between two cells that are very close together, allowing the passage of ions and small molecules.
- Connexin:
  - A protein channel connecting the two cells; each connexin is made up of 6 connexin subunits.
  - Can be open or closed.
- Bidirectional.
- Fast communication between two cells.

Chemical Synapses:
- Involve a presynaptic cell and a postsynaptic cell with no physical connection.
- Do not have bidirectional transmission – transmission is only from the presynaptic cell to the postsynaptic cell.
Chemical Synapses:
- Have a definitive gap called the synaptic cleft or the synaptic gap between the presynaptic cell and the postsynaptic cell.
- Since there is no physical connection, the presynaptic cell excites or inhibits the postsynaptic cell by releasing a neurotransmitter.
- The neurotransmitter is stored in the presynaptic terminal in synaptic vesicles.
- Neurotransmitter is released from the vesicles and enters the synaptic gap, then binds to receptors on the postsynaptic cell.
- Binding of neurotransmitter to receptors on the postsynaptic cell opens ion channels on the postsynaptic membrane, resulting in depolarization or hyperpolarization.
- Depolarization or hyperpolarization of the postsynaptic cell depends on which neurotransmitter is in the synaptic vesicles.
- Neurotransmitters can be inhibitory or excitatory.