Introduction to the Nervous System and Nervous Tissue: Structured Study Notes

Overview of the Nervous System

General Functions

The nervous system is responsible for controlling perception, voluntary movement, consciousness, personality, learning, and memory. It also regulates homeostasis in conjunction with the endocrine system, affecting respiratory rate, blood pressure, body temperature, sleep/wake cycles, and blood pH.

• Perception and Experience: The nervous system interprets sensory information and shapes our interaction with the environment.

• Voluntary Movement: Directs skeletal muscle activity.

• Homeostasis: Maintains internal balance through rapid signaling.

Anatomical Divisions

The nervous system is divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

• CNS: Consists of the brain and spinal cord. The brain contains billions of neurons and is protected by the skull. The spinal cord extends from the foramen magnum to the lumbar vertebrae, enabling communication between the brain and the body.

• PNS: Comprises all nerves outside the CNS, including cranial nerves (12 pairs) and spinal nerves (31 pairs). Nerves are bundles of axons with blood vessels and connective tissue.

Functional Divisions

The nervous system is functionally categorized into sensory, integrative, and motor divisions.

• Sensory (Afferent) Division: Gathers information from internal and external environments via somatic (skeletal muscles, skin, special senses) and visceral (internal organs) subdivisions.

• Integrative Functions: Analyze and interpret sensory input, determining appropriate responses. Most sensory information is subconsciously disregarded.

• Motor (Efferent) Division: Executes responses via somatic (skeletal muscle, voluntary) and autonomic (smooth/cardiac muscle, glands, involuntary) subdivisions.

Nervous Tissue

Neurons: Structure and Function

Neurons are excitable cells that transmit electrical signals (action potentials). Each neuron consists of three main parts:

• Cell Body (Soma): Metabolically active region containing organelles for protein synthesis and energy production.

• Dendrites: Short, branched processes that receive input and transmit it toward the cell body.

• Axon: Single, long process that conducts action potentials away from the cell body. Includes axon hillock, axon collaterals, telodendria, axon terminals, and axolemma.

Axonal Transport

Substances move through the axon by slow (cytoskeletal proteins, 1–3 mm/day) or fast (vesicles, organelles, up to 400 mm/day) axonal transport, using motor proteins and ATP.

Functional Regions of Neurons

• Receptive Region: Dendrites and cell body

• Conducting Region: Axon

• Secretory Region: Axon terminal

Neuron Classification

• Structural: Multipolar (one axon, multiple dendrites), Bipolar (one axon, one dendrite), Pseudounipolar (one fused axon, two processes)

• Functional: Sensory (afferent), Interneurons (association), Motor (efferent)

Neuron Groupings

• CNS: Nuclei (cell bodies), Tracts (axons)

• PNS: Ganglia (cell bodies), Nerves (axons)

Neuroglia

Neuroglial cells provide structural support, protection, and environmental maintenance for neurons.

• CNS Types: Astrocytes (support, blood-brain barrier), Oligodendrocytes (myelination), Microglia (phagocytosis), Ependymal cells (cerebrospinal fluid)

• PNS Types: Schwann cells (myelination), Satellite cells (support)

The Myelin Sheath

Structure and Function

The myelin sheath is formed by Schwann cells (PNS) or oligodendrocytes (CNS), wrapping layers of membrane around axons.

• Insulation: Lipid-rich myelin prevents ion movement, increasing action potential speed.

• Myelinated vs. Unmyelinated: Myelinated axons conduct signals 15–20 times faster.

• Nodes of Ranvier: Gaps between myelinated segments where action potentials are regenerated.

• White Matter: Myelinated axons

• Gray Matter: Cell bodies, unmyelinated dendrites/axons

Electrophysiology of Neurons

Resting Membrane Potential

Neurons maintain a resting membrane potential (RMP) due to the distribution of ions across the plasma membrane.

• Voltage: Electrical gradient from charge separation

• Typical RMP: Slightly negative due to more potassium leaking out than sodium leaking in

• Polarization: Cell is polarized when voltage difference is not zero

Ion Channels and Gradients

Ion movement across the membrane relies on specific channels:

• Leak Channels: Always open, allow ions to flow down gradients

• Gated Channels: Open in response to stimuli

• Types: Ligand-gated (chemical), Voltage-gated (voltage), Mechanically-gated (mechanical)

Sodium-Potassium Pump

The sodium-potassium pump moves three sodium ions out and two potassium ions in per ATP hydrolyzed, maintaining gradients essential for RMP.

Changes in Membrane Potential

• Depolarization: Positive charges enter, making membrane less negative

• Repolarization: Return to RMP

• Hyperpolarization: Membrane becomes more negative than RMP

Local Potentials

Local (graded) potentials are small, reversible changes in membrane potential, useful for short-distance signaling.

• Depolarization: Moves membrane potential toward zero

• Hyperpolarization: Moves membrane potential away from zero

Action Potentials

Action potentials are rapid, uniform depolarizations and repolarizations, generated in trigger zones.

• Phases: Depolarization, repolarization, hyperpolarization

• All-or-None Principle: Action potentials occur fully or not at all

• Propagation: Action potentials travel from trigger zone to axon terminal, self-propagating in one direction

• Conduction Speed: Influenced by axon diameter and myelination (saltatory vs. continuous conduction)

Classification of Axons

Type	Diameter	Myelination	Speed	Function
Type A	Largest	Myelinated	Fastest (120 m/sec)	Sensory/motor for skeletal muscle and joints
Type B	Intermediate	Mostly myelinated	Slower (15 m/sec)	Autonomic efferent, some sensory
Type C	Smallest	Unmyelinated	Slowest (0.5–2 m/sec)	Pain, temperature, pressure

Neuronal Synapses

Types of Synapses

• Electrical Synapses: Direct ion flow via gap junctions; bidirectional and nearly instantaneous; found in programmed behaviors and muscle coordination.

• Chemical Synapses: Use neurotransmitters; unidirectional; allow variable signal intensities; involve synaptic vesicles, synaptic cleft, and neurotransmitter receptors.

Events at Chemical Synapse

1. Action potential arrives at axon terminal

2. Voltage-gated calcium channels open

3. Calcium influx triggers neurotransmitter release

4. Neurotransmitter binds to postsynaptic receptors

5. Ion channels open, generating local potential

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes membrane, moves closer to threshold

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes membrane, moves farther from threshold

Neural Integration and Summation

• Neural Integration: Postsynaptic neuron integrates all incoming signals

• Summation: Temporal (rapid, repeated input) and spatial (simultaneous input from multiple neurons)

Termination of Synaptic Transmission

• Neurotransmitter diffusion and absorption

• Enzymatic degradation

• Reuptake into presynaptic neuron

Neurotransmitters

Types of Neurotransmitter Receptors

• Ionotropic: Ligand-gated ion channels; direct ion movement

• Metabotropic: G-protein coupled; activate second messengers (e.g., cAMP)

Major Neurotransmitters

Group	Examples	Function
Acetylcholine	ACh	Excitatory/inhibitory; neuromuscular junction, CNS, ANS
Biogenic Amines	Norepinephrine, Epinephrine, Dopamine, Serotonin, Histamine	Regulation of homeostasis, cognition, mood, arousal
Amino Acids	Glutamate, Glycine, GABA	Glutamate (excitatory), Glycine/GABA (inhibitory)
Neuropeptides	Substance P, Opioids, Neuropeptide Y	Pain, feeding behaviors, nervous system depression

Functional Groups of Neurons

Neuronal Pools and Circuits

• Neuronal Pools: Groups of interneurons processing specific information

• Diverging Circuits: Single neuron communicates with multiple targets

• Converging Circuits: Multiple neurons converge on a single target

• Inhibitory Circuits: Negative feedback to prevent overexcitation

• Synaptic Fatigue: Weakening of transmission with prolonged excitation

Epileptic Seizures

• Result from abnormal, excessive electrical activity

• Symptoms range from mild to severe

• Treatment aims to restore inhibitory circuit function

Clinical Applications

Multiple Sclerosis

• Autoimmune attack on CNS myelin

• Progressive loss of myelin slows action potential propagation

• Symptoms: Sensory changes, motor dysfunction, cognitive alterations

Psychiatric Disorders and Treatments

• Schizophrenia: Excess dopamine; treated by blocking dopamine receptors

• Depressive Disorders: Deficiency in serotonin, norepinephrine, dopamine; treated with SSRIs

• Anxiety Disorders: Abnormal norepinephrine, serotonin, GABA; treated with antidepressants and GABA enhancers

• Bipolar Disorders: Abnormal mood episodes; treated by blocking sodium channels

Summary Table: Types of Ion Channels

Channel Type	Stimulus	Location
Ligand-gated	Chemical (neurotransmitter)	Dendrites, cell body
Voltage-gated	Change in membrane potential	Axon, axon terminal
Mechanically-gated	Mechanical force	Specialized sensory regions
Leak channels	None (always open)	Entire neuron
Sodium-potassium pump	ATP hydrolysis	Entire neuron

Additional info: These notes expand on brief points from the original slides, providing definitions, examples, and context for each major concept. Images included are directly relevant to neuron structure and ion channel function, reinforcing the explanations given.