Nervous System: CNS and PNS

• The nervous system uses electrical and chemical signals for rapid communication and control in the body.
• It allows organisms to gather information, process data, and generate responses.

Central Nervous System (CNS)

• Includes the brain and spinal cord.
• Processes sensory data and makes conscious or unconscious decisions.

Peripheral Nervous System (PNS)

• Includes all neural elements outside the CNS.
• Has sensory (afferent) and motor (efferent) divisions.
• The sensory division transmits information from receptors to the CNS.
• The motor division carries signals from the CNS to effector organs.

Somatic Motor Division

• Controls skeletal muscles voluntarily.

Autonomic Motor Division

• Controls cardiac and smooth muscles, and glands involuntarily.
• Has sympathetic and parasympathetic divisions.
• The sympathetic division prepares the body for stress.
• The parasympathetic division maintains routine functions and conserves energy.

Sensory Neurons

• Carry sensory information from receptors to the CNS.
• Have dendrites and cell bodies in the periphery and axon terminals in the CNS.

Motor Neurons

• Convey motor commands from the CNS to muscles/glands.
• Have cell bodies and axon segments in the CNS and axon terminals in the periphery.

Interneurons

• Facilitate communication within the CNS.
• Connect sensory and motor pathways.
• Located entirely within the CNS.

Reflex Arc

• Neural pathway mediating a reflex action: sensory receptor, sensory neuron, interneuron, motor neuron, effector.
• A sensory receptor detects a stimulus.
• Sensory neurons transmit information to the CNS.
• Interneurons process information within the CNS.
• Motor neurons send a response signal from the CNS to the effector.
• The effector (muscle or gland) executes the response.

Withdrawal Reflex Example

• Sensory receptors send a signal to the spinal cord via sensory neurons.
• Interneurons process the stimulus and activate motor neurons.
• Motor neurons stimulate muscles, causing movement.

Neuron Properties: Excitability

• Ability to respond to stimuli and convert them into nerve impulses.

Neuron Properties: Conductivity

• Ability to transmit an action potential along the neuron.

Neuron Properties: Secretion

• Release of neurotransmitters to pass the signal to the next cell.

Neuron Properties: Longevity and Amitotic Nature

• Neurons can last a lifetime but do not typically undergo mitosis, making neural damage often irreversible.

Neuron Structure: Dendrites

• Receive signals from other neurons.
• Convey information towards the cell body.

Neuron Structure: Soma (Cell Body)

• Contains the nucleus and cytoplasm with organelles.
• Synthesizes neurotransmitters and proteins essential for neuron function.

Neuron Structure: Axon

• Conducts electrical impulses away from the cell body.
• Axon Hillock: The trigger zone where action potentials are initiated.

Neuron Structure: Myelin Sheath

• Insulating layer that increases impulse transmission speed.
• Envelops the axon.

Neuron Structure: Axon Terminals (Synaptic Knobs)

• Contain neurotransmitters.
• Release neurotransmitters into the synapse to communicate with neighboring cells.

Synapses: Chemical Synapses

• Neurons communicate via neurotransmitters across synaptic clefts.
• The presynaptic neuron releases neurotransmitters that bind to receptors on the postsynaptic cell.

Synapses: Electrical Synapses

• Ions flow directly between cells through gap junctions.
• Enables rapid and bidirectional communication.
• They are less common than chemical synapses due to limited functional versatility.

Nerves: Cranial Nerves

• Associated with sensory and motor functions of the head and neck.
• Optic nerve example: It is responsible for vision.

Nerves: Spinal Nerves

• Connect the spinal cord with motor and sensory structures.
• Sciatic nerve example: It controls leg movements and relays sensory information from the lower limbs.

Membrane Potential: Electrochemical Gradient

• Combines chemical (solute concentration differences) and electrical (charge differences) gradients across a membrane.

Membrane Potential: Resting Membrane Potential

• It is typically around -70 mV in neurons.
• It indicates that the inside of the cell is more negative compared to the outside.

Membrane Potential: Dynamic Equilibrium

• Dynamic Equilibrium: It is maintained by ion pumps and channels managing ion flow across the membrane.

Establishing Membrane Potential

• Potassium is more concentrated inside cells.
• Sodium, chloride, and calcium are more concentrated outside cells.

Ion Channels

• Pores in the membrane allow ions to pass.
• Each is specific to an ion type.

Ion Channels: Leakage Channels

• Always open.
• Allow ions to leak across the membrane following concentration gradients.

Ion Channels: Gated Channels

• Have gates that open or close.
• Do so in response to stimuli.

Gated Channels: Chemically Gated

• Open/close when a chemical binds to the receptor.

Gated Channels: Mechanically Gated

• Open/close in response to mechanical deformation.

Gated Channels: Voltage-Gated

• Open/close in response to changes in membrane potential.
• Sodium channels have open, closed, and inactivated states.

Important Channels: Sodium-Potassium Pump (Na+/K+ ATPase)

• Pumps sodium out and potassium in against gradients, using ATP.
• Maintains resting membrane potential.
• Makes the outside of the cell more positive.

Important Channels: Sodium Leak Channels

• Allow sodium to move into the cell.
• Enhances resting membrane potential.

Functional Segments of Neurons: Receptive Segment

• Location: Dendrites and cell body
• Receives incoming signals.
• Channels: Chemically gated Na+, K+, and Cl- channels; sometimes mechanically gated.

Functional Segments of Neurons: Initial Segment

• Location: Axon hillock
• Integrates incoming signals to initiate action potential.
• Channels: Voltage-gated Na+ and K+ channels.

Functional Segments of Neurons: Conductive Segment

• Location: Axon
• Conducts action potentials.
• Channels: Voltage-gated Na+ and K+ channels.

Functional Segments of Neurons: Transmissive Segment

• Location: Axon terminals
• Transmits signals to the next cell.
• Channels: Voltage-gated Ca2+ channels and Ca2+ pumps.

Types of Electrical Potentials: Resting Membrane Potential

• Potential difference across the membrane of a resting neuron.

Types of Electrical Potentials: Action Potential

• Rapid rise and fall in voltage across a cellular membrane.
• Triggered when the initial segment reaches a threshold.
• Causes a temporary reversal of membrane potential, followed by return to resting potential.

Dynamics and Distribution

• Concentration gradients are maintained by active transport mechanisms.
• Leak channels and pumps ensure a constant negative charge inside the neuron at rest compared to the outside.

Sodium and Potassium Dynamics: At Rest

• Potassium ions are concentrated inside; sodium ions are concentrated outside.
• Sodium-potassium pumps maintain this gradient.

Sodium and Potassium Dynamics: During an Action Potential

• Voltage-gated sodium channels open, sodium rushes in, reversing membrane potential.
• Potassium channels open, potassium exits, restoring the negative interior.

Sodium and Potassium Dynamics: After Action Potential

• Sodium channels become inactivated until membrane potential returns to rest.
• Sodium/potassium pumps restore original ion distribution.

Synaptic Transmission: Synaptic Transmission Overview

• An action potential reaching the axon terminal prompts neurotransmitter release.
• Neurotransmitters are released into the synaptic cleft.
• Neurotransmitters bind to receptors on the postsynaptic membrane.
• The binding results in excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs).

Synaptic Transmission: EPSPs

• Occur when neurotransmitter binding opens Na⁺ channels.
• Allows positive ions to flow in and depolarize the postsynaptic membrane.

Synaptic Transmission: IPSPs

• Occur when neurotransmitter binding opens K⁺ or Cl⁻ channels.
• Potassium flows out, chloride flows in, making the inside of the cell more negative and hyperpolarized..

Types of Summation: Spatial Summation

• Multiple synapses at different positions on the postsynaptic neuron fire simultaneously.

Types of Summation: Temporal Summation

• A single synapse fires rapidly.
• Causes overlapping EPSPs that increase the likelihood of reaching the threshold.

Synapses: Excitatory Cholinergic Synapse

• Neurotransmitter: Acetylcholine
• Action: Results in EPSPs by opening Na⁺ channels.

Synapses: Inhibitory GABAergic Synapse

• Neurotransmitter: Gamma-aminobutyric acid (GABA)
• Action: Results in IPSPs by opening Cl⁻ channels.

Synapses: Excitatory Adrenergic Synapse

• Neurotransmitter: Norepinephrine
• Mechanism: Utilizes a second messenger system to open ion channels.

Signal Propagation: Continuous Conduction

• Occurs in unmyelinated axons where every segment of the membrane has to be depolarized sequentially.

Signal Propagation: Saltatory Conduction

• Occurs in myelinated axons.
• The action potential jumps from one Node of Ranvier to the next, speeding up transmission.

Refractory Periods

• A period during which no new action potential can be initiated.

Refractory Periods: Absolute Refractory Period

• No new action potential can be initiated because the sodium channels are inactivated.

Refractory Periods: Relative Refractory Period

• A stronger-than-normal stimulus is required to elicit a new action potential.

Ceasing Neural Transmission

• Diffusion: Neurotransmitters can diffuse out of the synaptic cleft.
• Reuptake: Neurotransmitters are reabsorbed by the presynaptic neuron for reuse.
• Degradation: Enzymes break down neurotransmitters.

Types of Memory: Immediate Memory

• Duration: A few seconds.
• Used for retrieval and processing of information for immediate use.
• The memory is fleeting and does not involve long-term synaptic changes.

Types of Memory: Short-Term Memory

• Mechanism: Post-Tetanic Potentiation (PTP).
• Definition: A temporary increase in synaptic strength.
• Follows rapid stimulation of the presynaptic neuron.
• Used when recalling information over a few minutes to a few hours.
• Subsequent stimulations to the postsynaptic neuron yield a stronger response.

Types of Memory: Long-Term Memory

• The capacity to store information for an extended period.

Types of Memory: Declarative Memory

• Involves facts and events.
• Episodic memory: personal experiences.
• Semantic memory: general knowledge.

Types of Memory: Procedural Memory

• Involves skills and tasks.

Synaptic Plasticity: Chemical Synapses

• Highly Modifiable: These synapses are predominantly used in the nervous system due to their ability to change and adapt.

Synaptic Plasticity: Electrical Synapses

• Immediate Response: They are essential for coordinating synchronized responses.

Mechanisms of Synaptic Plasticity: Post-Tetanic Potentiation (PTP)

• Form of short-term plasticity.
• Involves rapid stimulation.

Mechanisms of Synaptic Plasticity: Long-Term Potentiation (LTP)

• A long-lasting increase in synaptic strength upon repeated high-frequency stimulation.

Mechanisms of Synaptic Plasticity: Receptors Involved

• AMPA Receptors: activated first and are crucial for initial synaptic response.
• NMDA Receptors: Require higher stimulation and allow calcium ions to enter the postsynaptic neuron and initiate long-term changes.
• Result: Increased postsynaptic neuron which leads to a more robust synaptic response that can last hours, days, or longer.

Neural Circuits: Converging Circuits

• Multiple inputs converge to produce a single output.
• Salivation example: Stimuli converge to activate salivary glands.

Neural Circuits: Diverging Circuits

• A single input spreads to multiple outputs.
• Walking example: A single thought initiates movement across multiple muscles.

Neural Circuits: Reverberating Circuits

• Circuits continue to fire after the initial stimulus.
• Breathing example: A circuit perpetuates inspiratory signals.

Neural Circuits: Parallel After-Discharge Circuits

• Input travels across multiple pathways with varying delays, resulting in a prolonged output.
• Higher functions such as mathematical reasoning are hypothesized to use these circuits.