The Nervous System: CNS and PNS

• The nervous system is organized into several major divisions to perform its various functions.
• These divisions include the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System (CNS)

• The CNS consists of the brain and spinal cord.
• The brain serves as the control center for most bodily functions.
• The spinal cord relays signals between the brain and the peripheral nerves.

Peripheral Nervous System (PNS)

• The PNS includes all neural tissue outside the brain and spinal cord.
• It is divided into the somatic nervous system and the autonomic nervous system.

Somatic Nervous System

• Controls voluntary movements via skeletal muscle.
• Contains sensory neurons that relay information to the CNS.

Autonomic Nervous System (ANS)

• Regulates involuntary functions.
• Affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils, micturition, and sexual arousal.
• It operates without conscious control.
• The ANS has two main branches: the sympathetic and parasympathetic nervous systems.

Sympathetic Nervous System

• Often referred to as the "fight or flight" system.
• Activates physiological responses during stress or danger.
• Increases heart rate, blood pressure, and respiration.

Parasympathetic Nervous System

• Often referred to as the "rest and digest" system.
• Conserves energy and promotes relaxation.
• Decreases heart rate and blood pressure, stimulates digestion.

Basic Functions of Synapses

• Synapses are the junctions where neurons communicate with each other or with other cells.
• Synaptic transmission is fundamental to neural communication.
• Electrical synapses involve direct electrical connections between cells.
• Chemical synapses involve the release of neurotransmitters to transmit signals.

Chemical Synapses - Overview

• Pre-synaptic neuron releases neurotransmitter.
• Neurotransmitter diffuses across synaptic cleft.
• Neurotransmitter binds to receptors on post-synaptic neuron.
• Binding leads to a change in the post-synaptic neuron's membrane potential.

Neurotransmitters

• Neurotransmitters are chemical messengers that transmit signals across a synapse.
• They are synthesized in the neuron and stored in vesicles.
• Vesicles fuse with pre-synaptic membrane to release neurotransmitter.
• Neurotransmitter binds to receptors on the post-synaptic cell.
• The action of neurotransmitters can be excitatory or inhibitory, depending on the receptor and ion channels affected.

Transmitter Substances

• A variety of substances serve as neurotransmitters, each with specific effects.
• These can be categorized into small-molecule transmitters and neuropeptides.

Small-Molecule, Rapidly Acting Transmitters

• Synthesized in the cytosol of the presynaptic terminal.
• Packaged into vesicles in the Golgi apparatus.
• Released into the synaptic cleft upon stimulation.
• Examples include: Acetylcholine, Norepinephrine, Epinephrine, Dopamine, Serotonin, Histamine, GABA, Glycine, and Glutamate

Acetylcholine

• Released by neurons in the autonomic nervous system and motor neurons.
• Involved in muscle contraction and parasympathetic functions.

Norepinephrine

• Secreted by many neurons whose cell bodies are located in the brain stem and hypothalamus.
• Activates excitatory receptors, but mainly activates inhibitory receptors.

Epinephrine

• Secreted by the adrenal medulla.
• Affects many of the same receptors as norepinephrine.

Dopamine

• Found in regions of the brain that control movement, motivation, and reward.

Serotonin

• Involved in mood regulation, sleep, and appetite.

Histamine

• Involved in arousal and attention.

GABA (Gamma-Aminobutyric Acid)

• The primary inhibitory neurotransmitter in the brain.

Glycine

• An inhibitory neurotransmitter in the spinal cord and brainstem.

Glutamate

• The primary excitatory neurotransmitter in the CNS.

Neuropeptides

• Large-molecule, slow-acting transmitters.
• Synthesized in the cell body of the neuron.
• Transported to the nerve terminals via axonal transport.
• Stored in dense-core vesicles.
• Released more slowly than small-molecule transmitters.
• Have more prolonged actions on the postsynaptic neuron.
• Examples include: TRH, LHRH, Somatostatin, Substance P, Enkephalins, Endorphins, Neuropeptide Y

Synthesis and Release of Neurotransmitters - Details

• Small-molecule transmitters are synthesized in the presynaptic terminal.
• Enzymes needed for synthesis are produced in the neuronal cell body and transported to the terminal.
• Neuropeptides are synthesized in the cell body and transported to the terminal in vesicles.
• Release is triggered by the influx of calcium ions into the presynaptic terminal.
• Calcium influx causes the vesicles to fuse with the presynaptic membrane and release the neurotransmitter into the synaptic cleft.

Removal of Neurotransmitters

• Essential for proper synaptic function to prevent overstimulation.
• Mechanisms include: Diffusion, Reuptake, Degradation

Diffusion

• Neurotransmitter diffuses away from the synaptic cleft.

Reuptake

• Neurotransmitter is transported back into the presynaptic terminal or into nearby glial cells.

Degradation

• Neurotransmitter is broken down by enzymes in the synaptic cleft.
• For example, acetylcholinesterase breaks down acetylcholine.

Receptor Binding and Postsynaptic Effects

• Neurotransmitters bind to specific receptors on the postsynaptic membrane.
• These receptors can be ionotropic or metabotropic.

Ionotropic Receptors

• Ligand-gated ion channels.
• Binding of the neurotransmitter directly opens or closes the ion channel.
• Results in a rapid change in membrane potential.

Metabotropic Receptors

• G-protein-coupled receptors.
• Binding of the neurotransmitter activates a G protein, which then modulates ion channels or intracellular signaling cascades.
• Slower but more prolonged effects.
• Modulation of Intracellular Signaling

Examples of intracellular signaling pathways modulated by G-protein-coupled receptors

• Adenylyl cyclase/cAMP pathway
• Phospholipase C/IP3/DAG pathway.
• These pathways can affect gene expression, protein synthesis, and other cellular processes.