Neurons, Synapses, and Signaling Notes

Neurons, Synapses and Signaling

What Does the Nervous System Do?

The nervous system has three primary functions:

1. Receiving Information: Nerves receive information from sense organs and transmit it to the brain.
2. Interpreting Information: The brain interprets the sensory information and generates a response.
3. Transmitting Orders: Nerves transport the response order to the locomotor system (muscles).
4. Muscles Move: The muscles execute the response.

Sensory Input, Integration, and Motor Output

• Sensory Input: This involves the gathering of information via sensory organs.
• Integration: The processing and interpretation of sensory input within the brain.
• Motor Output: The response or action generated as a result of integration, leading to muscle movement.

Example organisms for sensory input, integration and motor output include cone snails.

Central Nervous System (CNS) and Peripheral Nervous System (PNS)

The nervous system is divided into two main parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord.
• Peripheral Nervous System (PNS): Includes:
  • Sensory pathways with sensory neurons registering internal and external stimuli
  • Motor pathways
    • Somatic nervous system (voluntary control)
    • Autonomic nervous system (involuntary control):
      • Sympathetic nervous system ("fight or flight" response)
      • Parasympathetic nervous system ("rest and repose")

Afferent and Efferent Neurons

• Afferent neurons: Carry information toward the CNS.
• Efferent neurons: Carry information away from the CNS.

Neuron Structure

A neuron consists of the following parts:

• Dendrites: Receive signals.
• Cell Body: Contains the nucleus.
• Axon Hillock: Where the axon originates from the cell body.
• Axon: Transmits signals.
• Synapse: The junction between two neurons, or a neuron and an effector cell.
• Synaptic Terminals: Located at the end of the axon; release neurotransmitters.
• Neurotransmitter: Chemical messenger that transmits signals across a synapse.
• Presynaptic Cell: The neuron sending the signal.
• Postsynaptic Cell: The neuron receiving the signal.

Types of Neurons

• Sensory Neurons: Afferent neurons that carry impulses to the CNS.
• Motor Neurons: Efferent neurons that carry impulses from the CNS to muscles, glands, or organs.
• Interneurons: Connect sensory and motor neurons within the CNS.

Physiology of a Neuron

Key concepts include:

• Resting potential
• Action potential
• Depolarization
• Repolarization
• Refractory period

Membrane Potential

• Membrane Potential: The charge difference between the outside and inside of a neuron.

Resting Potential

• Resting Potential: The neuron's polarity at rest, where the outside is positive and the inside is negative. The neuron is considered polarized.

• Resting Potential in Vertebrates: $-70 \, mV$

Maintaining Resting Potential

• Maintained by the sodium-potassium pump.

Action Potential

• Action Potential: The reversal of polarity in a neuron where the outside becomes negative and the inside becomes positive. The neuron is depolarized.

Threshold Voltage

• Threshold Voltage: The voltage required to trigger an action potential.

Nerve Signal Transmission

1. Resting State: Sodium ($Na^+$) and potassium ($K^+$) channels are closed.
2. Depolarization: Some sodium channels open, causing depolarization.
3. Rising Phase: More sodium channels open, and more sodium ions enter the cell.
4. Falling Phase: Sodium channels close, and potassium channels open, causing repolarization.
5. Undershoot: Brief hyperpolarization before returning to the resting state.

Conduction of an Action Potential

• The action potential propagates along the axon.

Refractory Period

• Refractory Period: The time during which a neuron is incapable of conducting another impulse.

• Importance: Ensures that the action potential travels in one direction along the axon and prevents signal overlap.

Key Terms

• Gated Channels: Channels through which ions flow.
  • Ligand-gated channels: Open when a ligand attaches to them.
  • Voltage-gated channels: Open in response to changes in membrane potential.
• Graded Potential: Small, continuous changes to the resting potential.
• Action Potential: A transient, significant change in the resting potential.
• Depolarization: Makes the membrane potential less negative.
• Hyperpolarization: Makes the membrane potential more negative.

Evolutionary Adaptations of Axon Structure

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

Signal Transmission at the Axon Terminal

• Understand the structure and function of a synapse.

Synapse

• Presynaptic Membrane: The membrane of the axon terminal sending the signal.
• Synaptic Cleft: The gap between the presynaptic and postsynaptic membranes.
• Postsynaptic Membrane: The membrane of the receiving cell (neuron or effector).

Events at a Synapse

1. Action potential arrives at the presynaptic terminal.
2. Voltage-gated calcium channels open, and $Ca^{2+}$ enters the presynaptic terminal.
3. $Ca^{2+}$ influx triggers the fusion of synaptic vesicles with the presynaptic membrane.
4. Neurotransmitters are released into the synaptic cleft and bind to ligand-gated ion channels on the postsynaptic membrane.

Types of Synapses

• Electric Synapses: Direct cytoplasmic connections via gap junctions; common in invertebrates.
• Chemical Synapses: Electric signal converted to a chemical signal (neurotransmitter release); common in vertebrates.

Neurotransmitters

Examples:

• Acetylcholine
• Dopamine
• Serotonin
• Norepinephrine

Neurotransmitters and Drug Addiction

• Cocaine: Binds to proteins that break down dopamine, causing dopamine accumulation and pleasure pathway activation.
• Opiates (Codeine, Morphine, Heroin): Bind to endorphin receptors, producing well-being feelings.
• Nicotine: Binds to acetylcholine receptors, giving pleasure feelings.

Termination of Neurotransmission

Two Mechanisms:

• Enzymatic Breakdown: Neurotransmitter is broken down by enzymes in the synaptic cleft.
• Reuptake: Neurotransmitter is transported back into the presynaptic neuron via transport channels.