Integration and Coordination: The Nervous System Comprehensive Study Notes

INTRODUCTION TO THE NERVOUS SYSTEM

• Definition: It is a communication and control network that allows an organism to interact rapidly and adaptively with both its external and internal environments.
• Process Flow:     - Receives sensory information from receptors.     - Integrates this information with previously obtained data embedded in the system.     - Issues commands to effector organs, which include muscles and glands, to execute a response.

FUNCTIONS OF THE NERVOUS SYSTEM

• Sensation: Detection of external and internal stimuli, including:     - Vision     - Hearing     - Smell     - Taste     - Touch
• Transmitting Information: Moving signals through the system.
• Processing Information: Integrating and analyzing data.
• Producing a Response: Executing actions such as movement.

ORGANIZATION OF THE NERVOUS SYSTEM

• Central Nervous System (CNS):     - Consists of the brain and the spinal cord.
• Peripheral Nervous System (PNS):     - Consists of nerves and ganglia (small groups of neurons).     - Function: Innervates all parts of the body and provides an interface between the environment and the CNS.
• Transitions: The transition between the CNS and PNS occurs on the dorsal and ventral rootlets.

ANATOMICAL CLASSIFICATION OF NERVOUS SYSTEMS

• Central Nervous System (CNS): Brain and spinal cord.
• Peripheral Nervous System (PNS): All nerves and sensory structures outside the brain and spinal cord.     - Somatic: Provides voluntary control of skeletal muscles.     - Autonomic: Provides involuntary control of glands and smooth muscle.         - Sympathetic: Identified with the "fight or flight" response.         - Parasympathetic: Identified with the "rest and digest" response.

FUNCTIONAL CLASSIFICATION

• Sensory (Afferent) Division: Brings information into the nervous system. Information is transmitted to progressively higher levels, eventually reaching the cerebral cortex.
• Motor (Efferent) Division: Carries information out of the nervous system to the periphery.

CELLULAR COMPONENTS OF THE NERVOUS SYSTEM

• Composition: The system is composed of cells, connective tissue, and blood vessels.
• Major Cell Types:     - Neurons: Nerve cells, defined as the structural and functional units of the nervous system.     - Glia (Neuroglia): Known as "nerve glue."

STRUCTURE OF THE NEURON

• Main Cellular Compartments:     - Cell Body (Soma/Perikaryon): Irregularly shaped, covered by a cell membrane and containing neuroplasm.     - Dendrites: Variable number of processes extending from the soma.     - Axon: A single long process.

COMPONENTS OF THE NERVE CELL BODY (SOMA)

• Nucleus:     - Centrally placed; usually one per cell.     - Contains one or two prominent nucleoli.     - Nerve cells cannot multiply like other cells.
• Nissl Bodies:     - Small basophilic granules concerned with protein synthesis.     - Present in the soma and dendrites, but absent in the axon and axon hillock.     - Dendrites are distinguished from axons by the presence of these granules.     - Chromatolysis: The process where Nissl bodies fragment and disappear during fatigue or injury.
• Neurofibrils:     - Thread-like structures forming a network in the soma and processes.     - Comprised of microfilaments and microtubules.
• Mitochondria:     - Present in both the soma and the axon.     - Act as the powerhouse of the nerve cell, producing $ATP$.
• Golgi Apparatus:     - Similar to other cells; processes and packs proteins into granules.

DENDRITES AND AXONS

• Dendrites:     - Branched processes that may be single or many.     - Contain Nissl granules and neurofibrils.     - Transmit impulses toward the nerve cell body.     - Usually shorter than the axon.
• Axon:     - Each neuron has only one axon.     - Arises from the axon hillock, which is devoid of Nissl granules.     - Transmits impulses away from the nerve cell body.     - Can extend for long distances (up to $1\,meter$).

ORGANIZATION OF NERVES AND INTERNAL AXON STRUCTURE

• Connective Tissue Sheaths:     - Epineurium: Tubular sheath covering the whole nerve.     - Perineurium: Covers each bundle of nerve fibers (fasciculus).     - Endoneurium: Covers each individual nerve fiber (axon).     - Neurilemma: Membrane covering the axis cylinder of the nerve fiber.
• Internal Axon Structure:     - Axoplasm: Central core of cytoplasm containing mitochondria, neurofibrils, and axoplasmic vesicles.     - Axolemma: Tubular sheath membrane covering the axoplasm; a continuation of the soma cell membrane.     - Axis Cylinder: The combination of the axoplasm and axolemma.

MYELINATED NERVE FIBERS

• Myelin Sheath:     - A thick lipoprotein sheath that acts as insulation.     - Formed by Schwann cells in the neurilemma.     - Responsible for the white color of nerve fibers.
• Node of Ranvier: Regular intervals where the myelin sheath is absent.
• Internode: The segment of nerve fiber between two nodes.
• Functions of Myelin:     - Faster Conduction: Facilitates saltatory conduction where impulses jump from node to node.     - Insulation: Restricts impulses to a single fiber and prevents stimulation of neighbors.

NEURILEMMA (SHEATH OF SCHWANN)

• Structure: Thin membrane surrounding the axis cylinder; contains Schwann cells with flattened/elongated nuclei.
• Positioning:     - In non-myelinated fibers: Surrounds axolemma continuously.     - In myelinated fibers: Covers the myelin sheath; at nodes of Ranvier, it invaginates to reach the axolemma via finger-like processes.

CLASSIFICATION OF NEURONS

• Based on Number of Poles:     - Unipolar: One process.     - Bipolar: Two processes.     - Multipolar: Multiple dendrites and one axon.     - Pseudounipolar: Often associated with sensory reception.
• Based on Function:     - Motor (Efferent): Carry impulses from CNS to effector organs (muscles, glands). Characterized by long axons and short dendrites.     - Sensory (Afferent): Carry impulses from periphery to CNS. Characterized by short axons and long dendrites.
• Based on Axon Length:     - Golgi Type I: Long axons reaching remote peripheral organs; soma located in the CNS.     - Golgi Type II: Short axons; located in the cerebral cortex and spinal cord.

NEUROGLIA (GLIAL CELLS)

• General Characteristics: Supporting cells; non-excitable; do not transmit action potentials. They possess a low density of fast voltage-gated sodium channels, high potassium permeability, and morphological specialization.

Central Neuroglial Cells (CNS)

• Astrocytes: Star-shaped cells present throughout the brain.     - Fibrous Astrocytes: Located mainly in white matter. Form the blood-brain barrier (BBB) via tight junctions with capillary membranes. Their processes cover nerve cells and synapses.     - Protoplasmic Astrocytes: Located mainly in gray matter; processes run between nerve cell bodies.     - Functions: Form supporting networks; regulate BBB entry; maintain ECF chemical environment ($K^{+}$ spatial buffering); clear neurotransmitters; provide $Ca^{2+}$ and $K^{+}$.
• Microglia: Smallest glial cells derived from monocytes (macrophages of the CNS). They engulf microorganisms and debris via phagocytosis at injury sites.
• Oligodendrocytes: Also called oligodendroglia. Provide myelination for CNS nerve fibers and semi-stiff connective support.

Peripheral Neuroglial Cells (PNS)

• Schwann Cells: Major glial cells in PNS. Function in myelination, nerve regeneration, and phagocytic removal of debris.
• Satellite Cells: Located on the exterior surface of PNS neurons. Provide physical support and regulate chemical environment.

THE SYNAPSE

• Definition: Junction between two neurons.
• Anatomical Classification:     - Axoaxonic: Axon to axon.     - Axodendritic: Axon to dendrite.     - Axosomatic: Axon to cell body.
• Functional Classification:     - Electrical Synapse: Physiological continuity via gap junctions. Direct ion exchange allows for no delay. Common in cardiac muscle, smooth muscle of intestine, and eye lens epithelial cells.     - Chemical Synapse: No continuity; features a synaptic cleft. Transmission occurs via chemical neurotransmitters.
• Functional Types:     - Excitatory: Transmit impulses.     - Inhibitory: Inhibit impulse transmission.

EXCITATORY SYNAPTIC TRANSMISSION

1. Arrival of action potential in the axon terminal.
2. Opening of calcium channels in the presynaptic membrane.
3. Influx of $Ca^{2+}$ ions from the ECF into the terminal.
4. Opening of vesicles and release of Acetylcholine ($Ach$).
5. Passage of $Ach$ through the synaptic cleft.
6. Formation of $Ach$-receptor complex.
7. Opening of sodium channels and influx of $Na^{+}$ ions from ECF.
8. Development of Excitatory Postsynaptic Potential (EPSP).     - Properties of EPSP: Non-propagated; does not obey the all-or-none law.
9. Opening of sodium channels in the initial segment of the axon.
10. Influx of $Na^{+}$ and development of action potential.
11. Spread of action potential through the postsynaptic neuron axon.

INHIBITORY SYNAPTIC FUNCTION

• Five Types:     1. Postsynaptic (Direct) inhibition.     2. Presynaptic (Indirect) inhibition.     3. Negative feedback (Renshaw cell) inhibition.     4. Feedforward inhibition.     5. Reciprocal inhibition.
• Postsynaptic/Direct Inhibition Mechanism:     - Release of inhibitory neurotransmitters (e.g., $GABA$, dopamine, glycine).     - Receptor complex opens ligand-gated potassium or chloride channels.     - $K^{+}$ efflux and $Cl^{-}$ influx cause hyperpolarization (more negativity inside).     - Result: Inhibitory Postsynaptic Potential (IPSP).
• Presynaptic/Indirect Inhibition: Majorly in the spinal cord via modulatory neurons; reduces action potential magnitude.

PROPERTIES OF THE SYNAPSE

• One-Way Conduction (Bell-Magendie Law): Impulses only travel from presynaptic to postsynaptic neurons.
• Synaptic Delay: Short delay due to neurotransmitter release, passage across the cleft, and action on postsynaptic ionic channels.
• Fatigue: Caused by depletion of neurotransmitter stores (e.g., $Ach$).
• Summation: Fusion of effects or increase in EPSP.     - Spatial: Many presynaptic terminals stimulated simultaneously.     - Temporal: Single presynaptic terminal stimulated repeatedly.

THE ACTION POTENTIAL (AP)

• Definition: Abrupt/sudden reversal in resting membrane potential (RMP) in response to a threshold stimulus.
• Stimulus Types: Electrical, Mechanical, Chemical.
• Key Terms:     - Depolarization: Change to a less negative potential.     - Repolarization: Return toward RMP.     - Hyperpolarization: Potential becomes more negative than RMP.

STAGES OF THE ACTION POTENTIAL

1. Resting State: Membrane at RMP.
2. Threshold: Stimulus depolarizes the membrane to threshold ($\approx -55\,mV$).
3. Depolarization Stage: Rapid opening of voltage-gated $Na^{+}$ channels and influx of $Na^{+}$. Membrane potential approaches the equilibrium potential for $Na^{+}$ (the apex is the "overshoot").
4. Repolarization Stage: $Na^{+}$ inactivation gates close; voltage-gated $K^{+}$ channels open slowly. $K^{+}$ efflux occurs.
5. Afterhyperpolarization: $K^{+}$ conductance exceeds resting levels, potential becomes more negative than initial RMP. $Na^{+}, K^{+}-ATPase$ (sodium pump) reestablishes gradients.

PROPERTIES OF ACTION POTENTIALS

• "All-or-None" Law: If threshold is reached, an AP is generated at full amplitude; if not reached, no AP occurs. Stimulus intensity does not change AP amplitude.
• Frequency: Increased stimulus intensity increases the frequency of APs, not the amplitude (e.g., skin mechanoreceptors).
• Refractory Periods:     - Absolute: No AP possible regardless of intensity; occurs during depolarization due to closed $Na^{+}$ inactivation gates.     - Relative: AP only possible with a supra-threshold stimulus; occurs during repolarization.
• Accommodation: Slow depolarization causes $Na^{+}$ inactivation gates to close automatically, preventing an AP. This is observed in hyperkalemia, where high serum $K^{+}$ depolarizes skeletal muscle, causing weakness because inactivation gates remain closed.
• Saltatory Conduction: AP "jumps" between nodes of Ranvier, allowing conductance without decrement.

CENTRAL NERVOUS SYSTEM LAYERS AND COVERINGS

• Brain and Spinal Cord Composition:     - Gray Matter: Consists of nerve cell bodies and proximal parts of nerve fibers.     - White Matter: Consists of the remaining parts of nerve fibers.
• Arrangement:     - Brain: Outer gray matter, inner white matter.     - Spinal Cord: Outer white matter, inner gray matter.
• Meninges:     - Dura Mater: Outer layer.     - Arachnoid Mater: Middle layer.     - Pia Mater: Inner layer on the surface of the brain.     - Functions: Protect the brain; support framework for vessels; enclose the subarachnoid space (vital CSF cavity).

DIVISIONS OF THE BRAIN

• Prosencephalon (Forebrain):     - Telencephalon: Cerebral hemispheres, basal ganglia, hippocampus, amygdala.     - Diencephalon: Thalamus, hypothalamus, metathalamus, subthalamus.
• Mesencephalon (Midbrain).
• Rhombencephalon (Hindbrain):     - Metencephalon: Pons and Cerebellum.     - Myelencephalon: Medulla oblongata.

BRAIN STEM AND CEREBELLUM FUNCTIONS

• Brain Stem: Composed of medulla, pons, and midbrain. Source of cranial nerves CN III–XII.     - Medulla: Extension of the spinal cord; contains centers for breathing, BP, swallowing, coughing, and vomiting.     - Pons: Regulates breathing, balance, and posture maintenance.     - Midbrain: Controls eye movements and contains auditory/visual relay nuclei.
• Cerebellum: Attached dorsal to the pons/medulla. Coordinates movement planning, execution, posture, and head/eye movements. Integrates position (spinal cord), motor (cortex), and balance (vestibular) data.

DIENCEPHALON AND CEREBRAL HEMI-SPHERES

• Thalamus: Relay station; processes almost all sensory input to the cortex and motor output to the stem/cord.
• Hypothalamus: Located ventral to the thalamus. Regulates body temperature, food intake, and water balance.
• Cerebral Cortex: Features four lobes (Frontal, Parietal, Temporal, Occipital) and specialized areas (Primary, Secondary, Tertiary).
• Deep Nuclei:     - Basal Ganglia: Caudate nucleus, putamen, and globus pallidus. Regulate movement via the thalamus/frontal cortex.     - Limbic System: Includes the hippocampus (memory) and amygdala (emotions).

SENSORY SYSTEMS AND PATHWAYS

1. First-Order Neuron: Primary sensory afferent; often the receptor cell itself (e.g., somatosensory/olfactory).
2. Second-Order Neuron: Located in relay nuclei (spinal cord or brain stem). Axons cross the midline (decussation) and ascend to the thalamus.
3. Third-Order Neuron: Typically resides in thalamic relay nuclei.
4. Fourth-Order Neuron: Located in the appropriate sensory area of the cerebral cortex.