Chapter 11: Functional Organization of the Nervous System

Functions of the Nervous System

• Maintaining Homeostasis: The nervous system is responsible for regulating and coordinating various body activities to ensure a state of internal balance.
• Receiving Sensory Input: The system monitors both internal and external stimuli through sensory receptors.
• Integrating Information: The Central Nervous System (CNS), composed of the brain and spinal cord, processes the received sensory input and initiates appropriate responses.
• Control of Activities: It directs the activities of muscles and glands throughout the body.
• Establishing and Maintaining Mental Activity: The nervous system manages complex processes including consciousness, thinking, memory, and emotion.

Functional Organization of the Nervous System

• Central Nervous System (CNS): Includes the brain and spinal cord.
• Peripheral Nervous System (PNS): Divided further into functional pathways:
  • Sensory (Afferent) Division: Relays sensory information to the CNS.
  • Motor (Efferent) Division: Relays signals from the CNS to effector organs.
    • Somatic Nervous System: Controls skeletal muscles.
    • Autonomic Nervous System (ANS): Controls smooth muscle, cardiac muscle, and glands.
      • Sympathetic Division: Involved in "fight or flight" responses.
      • Parasympathetic Division: Involved in "rest and digest" activities.

Structural and Functional Classification of Neurons

• Definition: A neuron or nerve cell is the basic functional unit of nervous tissue. Nerve cells consist of two main cell types: neuronal cells and glial cells.
• Functional Classification:
  • Sensory (Afferent) Neurons: Relay action potentials towards the Central Nervous System.
  • Association Neurons (Interneurons): Located within the CNS, specifically positioned between afferent and efferent neurons.
  • Motor (Efferent) Neurons: Relay action potentials away from the CNS to effectors like muscles or glands.
• Structural Classification:
  • Multipolar Neuron: Characterized by having more than $2$ projections directly off the cell body, including $1$ axon and multiple dendrites.
  • Bipolar Neuron: Possesses $2$ projections directly off the cell body ($1$ axon and $1$ dendrite).
  • Pseudo-unipolar Neuron: Features a single projection (the axon) extending directly from the cell body.
  • Anaxonic Neuron: Contains multiple dendrites but lacks an axon directly off of the cell body.

Anatomy of a Multipolar Neuron

• Neuron Cell Body (Soma): The central part of the neuron containing organelles. It specifically contains Nissl bodies, which are composed of rough endoplasmic reticulum.
  • Nucleus (plural: Nuclei): A collection of cell bodies located within the CNS.
  • Ganglion (plural: Ganglia): A collection of cell bodies located within the PNS.
• Dendrites: Short, highly branched extensions of the cell body. They often feature smaller extensions called dendritic spines. Dendrites receive information from other neurons or receptors and conduct electrical currents toward the cell body.
• Trigger Zone: The specific area of the neuron where an action potential is first generated.
• Axon: This projection arises from the cell body at the axon hillock and continues as the initial segment. It ends at the presynaptic terminal.
  • Tract: A collection of axons located within the CNS.
  • Peripheral Nerve: A collection of axons located within the PNS.
• Presynaptic Terminal: The end of the axon that contains synaptic vesicles filled with neurotransmitters. It innervates a muscle to form a neuromuscular junction (NMJ).

Glial Cells of the Nervous System

• Astrocytes (CNS): Star-shaped cells with processes that form "feet." These feet cover the surfaces of neurons, blood vessels, and the pia mater. They regulate the composition of extracellular brain fluid and produce chemicals that promote tight junctions, forming the Blood-Brain Barrier (BBB) which regulates substances reaching the CNS.
• Ependymal Cells (CNS): These cells line the brain ventricles and the central canal of the spinal cord. They possess cilia that help circulate Cerebrospinal Fluid (CSF) through brain cavities.
• Choroid Plexus: Found within specific regions of the ventricles in the brain and brainstem, these structures produce CSF.
• Microglia (CNS): Specialized macrophages that respond to inflammation. They phagocytize and remove necrotic tissue, microorganisms, and foreign substances.
• Satellite Cells (PNS): These cells surround neuron cell bodies in sensory and autonomic ganglia. They provide support, nutrients, and protection, notably from heavy-metal poisons.
• Oligodendrocytes (CNS): Form insulating myelin sheaths by wrapping cytoplasmic extensions around axons. A single oligodendrocyte can myelinate portions of several different axons, increasing the velocity of action potential conduction.
• Schwann Cells (PNS): Wrap around a portion of only one axon to form a myelin sheath. They also increase conduction velocity. The gaps between adjacent Schwann cells are called Nodes of Ranvier.

Organization of Nervous Tissue

• Gray Matter:
  • Composed of unmyelinated axons, cell bodies, and dendrites.
  • Location: In the spinal cord, it is located toward the center; in the brain (cortex), it is located in the outer peripheral region.
• White Matter:
  • Composed of myelinated axons.
  • Location: In the spinal cord (tracts), it is in the outer peripheral region; in the brain, it is located toward the center.
• Terminology Comparison:
  • CNS: Cell body clusters are nuclei; myelinated axon bundles are nerve tracts.
  • PNS: Cell body clusters are ganglia; axon bundles with connective tissue are peripheral nerves.

Membrane Potentials and the Action Potential

• Resting Membrane Potential:
  • The extracellular environment is positively charged relative to the intracellular environment.
  • Voltage-gated $Na^+$ ion channels are closed.
  • Voltage-gated $K^+$ ion channels (possessing one gate) are closed.
• Depolarization:
  • Voltage-gated $Na^+$ channels open, leading to an influx of $Na^+$ ions.
  • The neuron becomes more positive inside.
  • Voltage-gated $K^+$ channels remain closed initially.
• Repolarization:
  • Voltage-gated $Na^+$ channels close.
  • Voltage-gated $K^+$ channels open, allowing an efflux of $K^+$ ions.
  • The neuron becomes more negative again.
• Hyperpolarization (After-potential): Occurs due to the slow closing of voltage-gated $K^+$ channels.
• Return to Resting Potential: The $Na^+/K^+$ pump resets the potential by moving $3$ $Na^+$ ions out of the cell and moving $2$ $K^+$ ions back into the cell.

Action Potential Propagation and Refractory Periods

• Refractory Period: A time during which the sensitivity of an area to further stimulation decreases.
  • Absolute Refractory Period: Complete insensitivity to another stimulus occurs from the start of the action potential until near the end of repolarization. No second action potential can be produced regardless of stimulus size.
  • Relative Refractory Period: A second action potential can be initiated, but only by a stimulus that is stronger than the standard threshold.
• Propagation Types:
  • Continuous Conduction: Occurs in unmyelinated axons. An action potential at one site causes an action potential at the adjacent site. Conduction is one-way because the previous site is in a refractory period.
  • Saltatory Conduction: Occurs in myelinated axons. The action potential is conducted (jumps) from one Node of Ranvier to the next.

Conduction Speed and Nerve Fiber Types

• Factors Affecting Speed:
  • Myelination: Conduction is faster in myelinated axons than in non-myelinated axons. In myelinated fibers, speed is influenced by the thickness of the myelin sheath.
  • Axon Diameter: Large-diameter axons conduct faster than small-diameter ones due to greater surface area and a higher number of voltage-gated channels.
• Nerve Fiber Classifications:
  • Type A: Large-diameter, myelinated. Conduction speed is $15 \text{ to } 120\,m/s$. Found in motor neurons for skeletal muscles and most sensory neurons.
  • Type B: Medium-diameter, lightly myelinated. Conduction speed is $3 \text{ to } 15\,m/s$. Part of the Autonomic Nervous System (ANS).
  • Type C: Small-diameter, unmyelinated. Conduction speed is $2\,m/s$ or less. Part of the ANS.

The Synapse

• Definition: A junction between two cells where action potentials in one cell (presynaptic) cause action potentials in another cell (postsynaptic).
• Junction Types: Nerve to nerve, nerve to organ (e.g., muscle), or nerve to gland (e.g., sweat gland).
• Functional Components:
  • Presynaptic Terminal: The cell transmitting the signal.
  • Postsynaptic Terminal: The target cell receiving the signal.
• Synapse Types:
  • Electrical Synapse: Cells are connected by gap junctions containing protein tubes called connexons. This allows graded current to flow directly between cells. Found in cardiac and smooth muscle where coordinated contraction is essential.
  • Chemical Synapse: Signals are transmitted via neurotransmitters like Acetylcholine (ACh) or Epinephrine.

Features of Chemical Synapses and Neurotransmitters

• Chemical Synapse Components:
  • Presynaptic Terminal: The end of the presynaptic axon.
  • Synaptic Cleft: The physical space between the presynaptic and postsynaptic membranes.
  • Postsynaptic Membrane: The target membrane of other neurons, muscles, or glands.
  • Neurotransmitters (NT): Released from synaptic vesicles via action potentials; they bind to ligand-gated channels on the postsynaptic membrane.
• Major Classes of Neurotransmitters:
  • Acetylcholine (ACh): The most common and best understood.
  • Biogenic Amines (Catecholamines and Indoleamines): Includes Serotonin (5-HT), Dopamine, and Norepinephrine (NE).
  • Amino Acids: Includes GABA (Gamma-Aminobutyric Acid), Glycine, and Glutamate.
  • Neuropeptides: Short amino acid chains including Substance P and Endorphins.
  • Gases and Lipids: Gasotransmitters like Nitric oxide (NO) and Carbon monoxide; Lipids include Endocannabinoids.

Removal of Neurotransmitters from the Cleft

• Acetylcholine (ACh) Removal:
  1. ACh molecules release from receptors.
  2. The enzyme Acetylcholinesterase splits ACh into Choline and Acetic acid.
  3. Choline is taken up by the presynaptic terminal.
  4. Choline combines with Acetyl-CoA to re-form ACh.
  5. Other molecules diffuse away into the extracellular fluid.
• Norepinephrine (NE) Removal:
  1. NE is released and then taken back up by the presynaptic terminal.
  2. Once inside, NE is repackaged into synaptic vesicles.
  3. The enzyme Monoamine oxidase (MAO) breaks down some NE molecules.

Neuromodulation and Postsynaptic Responses

• Neuromodulators: Chemical messengers secreted by neurons that influence the likelihood of an action potential in the postsynaptic cell. Neurons can secrete more than $100$ types.
• Axoaxonic Synapses: The axon of one neuron synapses with the presynaptic terminal of another; common in the CNS.
• Presynaptic Inhibition: Reduction in the amount of neurotransmitter released (e.g., Endorphins inhibiting pain).
• Presynaptic Facilitation: Increase in the amount of neurotransmitter released (e.g., Glutamate facilitating Nitric oxide production).
• Postsynaptic Potentials:
  • Excitatory Postsynaptic Potential (EPSP): Leads to depolarization; stimulatory response that may reach threshold.
  • Inhibitory Postsynaptic Potential (IPSP): Leads to hyperpolarization; inhibitory response that moves the potential farther from threshold.

Summation and Neuronal Circuits

• Summation: Since a single postsynaptic potential is usually insufficient to reach threshold, multiple potentials combine at the trigger zone.
  • Spatial Summation: Graded potentials from different dendrites (Action Potentials $1$ and $2$) summate at the trigger zone.
  • Temporal Summation: Multiple action potentials arrive in close succession at a single presynaptic membrane. The resulting graded potentials summate to reach threshold.
  • Combined Summation: Integration of both EPSPs and IPSPs to determine if threshold is reached.
• Neuronal Pathways and Circuits:
  • Serial Pathway: Simple input along one single pathway.
  • Convergent Pathways: Many neurons synapse with a smaller number of neurons (e.g., data synthesis in the brain).
  • Divergent Pathways: A small number of presynaptic neurons synapse with a large number of postsynaptic neurons (e.g., broadcasting information to multiple brain regions).
  • Reverberating Circuit: Outputs cause reciprocal activation, essential for rhythmic activities like breathing.
  • Parallel After-discharge Circuit: Neurons stimulate several parallel neurons that eventually converge on a common output cell, used for complex data processing.