Fundamentals of the Nervous System: Structure, Function, and Physiology

Fundamentals of the Nervous System

Nervous System Overview

The nervous system is the primary controlling and communicating system of the body, responsible for rapid and precise regulation of bodily functions. It works in conjunction with the endocrine system, which is the second major controlling system.

• Sensory input: Monitoring stimuli occurring inside and outside the body.

• Integration: Interpretation of sensory input.

• Motor output: Response to stimuli by activating effector organs.

Internal Communication: The human body communicates via electrical and chemical signals, which are rapid and specific.

Organization of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord; integration and command center.

• Peripheral Nervous System (PNS): Paired spinal and cranial nerves; carries messages to and from the CNS.

Peripheral Nervous System (PNS): Two Functional Divisions

• Sensory (afferent) division: Carries impulses from skin, skeletal muscles, and joints to the brain.

• Visceral afferent fibers: Transmit impulses from visceral organs to the brain.

• Motor (efferent) division: Transmits impulses from the CNS to effector organs.

Motor Division: Two Main Parts

• Somatic nervous system: Voluntary control of skeletal muscles.

• Autonomic nervous system (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands.

• ANS subdivisions: Sympathetic and parasympathetic systems.

Histology of Nerve Tissue

Nerve tissue consists of two principal cell types:

• Neurons: Excitable cells that transmit electrical signals.

• Supporting cells (neuroglia or glia): Provide structural and functional support for neurons.

Supporting Cells: Neuroglia

• Astrocytes: Versatile, highly branched glial cells; anchor neurons, guide migration, and control chemical environment.

• Microglia and Ependymal cells: Microglia act as phagocytes; ependymal cells line central cavities of the brain and spinal cord.

• Oligodendrocytes, Schwann cells, Satellite cells: Oligodendrocytes and Schwann cells form myelin sheaths; satellite cells surround neuron cell bodies in the PNS.

Neurons (Nerve Cells)

Neurons are the structural units of the nervous system, specialized for conducting impulses.

• Cell body (perikaryon or soma): Contains nucleus and metabolic center.

• Processes: Extensions from the soma; include dendrites and axons.

• Dendrites: Receive input; convey graded potentials.

• Axons: Transmit impulses away from the cell body; may be myelinated or unmyelinated.

Myelin Sheath and Nodes of Ranvier

• Myelin sheath: Fatty, segmented sheath that insulates axons and increases transmission speed.

• Nodes of Ranvier: Gaps in the myelin sheath; facilitate rapid impulse conduction.

• Unmyelinated axons: Conduct impulses more slowly.

Regions of the Brain and Spinal Cord

• Gray matter: Mostly neuron cell bodies and unmyelinated fibers.

• White matter: Dense collections of myelinated fibers.

Neuron Classification

• Structural: Multipolar, bipolar, unipolar.

• Functional: Sensory (afferent), motor (efferent), interneurons (association neurons).

Neurophysiology

Neurons communicate via electrical impulses, which are generated by the movement of ions across the plasma membrane.

• Action potentials: All-or-none electrical signals propagated along axons.

• Electrical definitions:

  • Voltage (V): Potential energy generated by separated charge.

  • Current (I): Flow of electrical charge between two points.

  • Resistance (R): Hindrance to charge flow.

Role of Ion Channels

• Types: Leakage (always open), chemically gated, voltage-gated.

• Operation: Channels open/close in response to specific signals, allowing ions to move along electrochemical gradients.

Electrochemical Gradient

Movement of ions is driven by both chemical and electrical gradients, which together determine the direction and rate of ion flow.

Resting Membrane Potential (Vm)

The resting membrane potential is typically -70 mV, generated by differences in ion concentrations and maintained by the sodium-potassium pump.

• Equation: $V_m = -70\ \text{mV}$

Membrane Potentials: Signals

• Graded potentials: Short-lived, localized changes in membrane potential; decrease in intensity with distance.

• Action potentials: Principal means of neural communication; all-or-none events that travel long distances.

Action Potential Phases

• Resting state: All voltage-gated channels closed.

• Depolarization: Na+ channels open, membrane potential becomes less negative.

• Repolarization: Na+ channels close, K+ channels open, membrane potential returns to resting value.

• Undershoot: Excessive efflux of K+ causes hyperpolarization.

Action Potential: Role of Sodium-Potassium Pump

• Restores ionic conditions after repolarization.

• Equation: $3\ \text{Na}^+\ \text{out} / 2\ \text{K}^+\ \text{in}$ per ATP hydrolyzed

Propagation of Action Potential

• Action potential moves along the axon toward the axon terminals.

• Propagation is unidirectional due to refractory periods.

Threshold and All-or-None Principle

• Action potentials are generated only if membrane depolarization reaches threshold (typically 15-20 mV above resting potential).

• All-or-none phenomenon: Action potentials either occur completely or not at all.

Coding for Stimulus Intensity

• Stronger stimuli generate action potentials more frequently.

Refractory Periods

• Absolute refractory period: No new action potential can be generated.

• Relative refractory period: A new action potential can be generated with a stronger stimulus.

Conduction Velocities of Axons

• Myelinated axons conduct impulses faster than unmyelinated axons.

• Saltatory conduction: Impulse jumps from node to node, increasing speed.

Multiple Sclerosis (MS)

• Autoimmune disease affecting young adults.

• Symptoms: Visual disturbances, weakness, loss of muscular control, urinary incontinence.

• Myelin sheaths in CNS are destroyed, leading to impaired nerve impulse conduction.

Synapses

• Junctions mediating information transfer from one neuron to another or to an effector cell.

• Electrical synapses: Direct flow of ions between cells; less common.

• Chemical synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

Synaptic Cleft: Information Transfer

• Neurotransmitter released from presynaptic neuron binds to receptors on postsynaptic membrane, causing excitatory or inhibitory effects.

Termination of Neurotransmitter Effects

• Neurotransmitters are removed by reuptake, enzymatic degradation, or diffusion away from the synaptic cleft.

Synaptic Delay

• Time required for neurotransmitter release, diffusion, and binding; typically 0.3-5.0 ms.

Postsynaptic Potentials

• Excitatory postsynaptic potentials (EPSPs): Graded potentials that can initiate an action potential.

• Inhibitory postsynaptic potentials (IPSPs): Cause membrane to become more negative, reducing likelihood of action potential.

Summation

• EPSPs can summate temporally or spatially to induce an action potential.

Neurotransmitters

• Chemicals used for neural communication; over 50 identified.

• Types: Acetylcholine, biogenic amines, amino acids, peptides, novel messengers.

Chemical Neurotransmitters

Type	Examples	Main Functions
Acetylcholine (ACh)	Released at neuromuscular junction	Muscle activation
Biogenic amines	Dopamine, norepinephrine, serotonin, histamine	Mood, emotional behaviors, biological clock
Amino acids	GABA, glycine, aspartate, glutamate	Major inhibitory/excitatory signals in CNS
Peptides	Substance P, endorphins	Pain signals, natural opiates
Novel messengers	ATP, NO, CO	Fast excitatory responses, regulation of cGMP

Neurotransmitters: Acetylcholine

• Best understood; released at neuromuscular junction.

• Degraded by acetylcholinesterase (AChE).

Neurotransmitters: Biogenic Amines

• Include catecholamines (dopamine, norepinephrine, epinephrine) and indolamines (serotonin, histamine).

• Distributed in the brain; play roles in emotional behavior and biological clock.

Neurotransmitters: Amino Acids

• Include GABA, glycine, aspartate, glutamate.

• Major neurotransmitters in the CNS.

Neurotransmitters: Peptides

• Include substance P (pain signals), endorphins (natural opiates), somatostatin, cholecystokinin.

Novel Messengers

• ATP, nitric oxide (NO), carbon monoxide (CO).

• Fast excitatory responses; NO activates guanylyl cyclase, CO regulates cGMP in the brain.

Additional info: These notes cover the essential structure and function of the nervous system, including cellular components, neurophysiology, and neurotransmitter classification, suitable for college-level Anatomy & Physiology students.