Nervous System Functions and Organization

Nervous System Basics

• Two control centers regulate homeostasis: Nervous and Endocrine
• Nervous system is the primary system for fast responses, targeting specific areas, using neurotransmitters
• Endocrine system produces general effects slowly, using hormones

Major Functions

• Sensation: The body uses millions of sensory receptors to detect internal and external changes (stimuli)
  • External stimuli examples: Temperature, light, sound
  • Internal stimuli examples: Internal environment (hormone regulation), pressure (blood pressure), pH (blood acidity), CO2, electrolytes (Na+, K+, Ca+2)
• Integration: Sensory input is converted into electrical signals (nerve impulses) transmitted to the brain for interpretation (thought, sensation, or memory)
• Action: The nervous system responds by sending signals to effectors (muscles or glands) causing an action or motor output

Organization of the Nervous System

• Two principal divisions: Central Nervous System (CNS) and Peripheral Nervous System (PNS)
• CNS consists only of the brain and spinal cord

Peripheral Nervous System

• Two main components: Nerves and Ganglia
• Nerves from the brain and spinal cord
  • 12 pairs of cranial nerves emerge from the brain, transmitting information to organs, glands, or muscles.
  • 31 pairs of spinal nerves emerge from the spinal cord, transmitting information to organs, glands, or muscles.
• Ganglia: Clusters of nerve cell bodies resembling knots
  • PNS is divided into somatic and autonomic nervous systems.

Somatic Nervous System

• Also known as the voluntary nervous system.
• Controls voluntary skeletal (muscles) movements
• Neurotransmitter: Acetylcholine

Autonomic Nervous System

• Involuntary system that maintains homeostasis
• Controls smooth muscle, cardiac muscle, glands
• Two parts: Sympathetic and parasympathetic nervous systems

Sympathetic Nervous System

• "Fight or Flight" response
• Examples: Increased heart rate, increased force of heart contraction, dilation of vessels to the heart, pupils dilate, constriction of blood vessels in skin and viscera, rapid and deeper breathing, blood sugar rise, blood vessels in lungs dilate, adrenaline, other hormones.

Parasympathetic Nervous System

• "Rest and Digest" response, opposite of sympathetic
• Examples: Digestion dominates over sympathetic stimulation

Nervous Tissue

• Composed of two cell types: Neuroglia and neurons
• Derived from ectoderm

Neuroglia (Glial Cells)

• Supporting and protective portion of nervous tissue
• Much more numerous than neurons. (more)

Glial Cell Types

• Astrocytes: Support, nutrient exchange
• Ependymal cells: Formation and circulation of cerebrospinal fluid
• Microglia: Protection, phagocytosis (immunity)
• Oligodendrocytes: Form myelin sheaths around axons in the CNS
• Schwann cells: Form myelin sheaths around axons in the PNS, nerve regeneration
• Satellite cells: Support nerve cell bodies within ganglia

Blood-Brain Barrier

• Neuroglia (particularly astrocytes) form a wall around the blood vessels, preventing the access of many substances

Neurons (Nerve Cells)

• Conductive, excitable portion of the nervous system
• Communicate with other neurons, muscles, or glands.
• Highly specialized, cannot be replaced if destroyed.
• Receive, integrate, and transmit action potentials at the cell body (soma), axon, and dendrites

Neuron Parts

• Cell Body (Soma): Contains nucleus, organelles; major region for nutrition & metabolism.
• Dendrites: Branching extensions that receive signals and transmit toward the cell body.
• Axon: A single, long extension that transmits signals away from the cell body, ending at axon terminals.
• Axon Terminals: Enlarged structures at axon ends containing neurotransmitters.

Myelin Sheath Detail

• Myelin is a fatty substance that insulates axons for faster signal transmission
  • PNS axons are myelinated by Schwann cells and possess a neurilemma
  • CNS axons are myelinated by oligodendrocytes and lack a neurilemma/ regeneration
• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are generated

Nerve Impulse (Action Potential)

• Excitability: Ability to respond to a stimulus
• Conductivity: Ability to transmit an impulse
• Cell at rest is negatively charged(-70 mV) and polarized. An action potential can occur if a stimulus is strong enough (threshold).
• Sodium rushes into the cell (depolarization): inside becomes positively charged
• Potassium rushes out of the cell (repolarization): inside becomes negatively charged again
• Resting condition is restored once the sodium/potassium pumps work

Conduction of Nerve Impulses Along a Neuron

• Action potential propagates (moves) down the neuron, from the axon hillock to the terminal (axon terminals).
• Saltatory Conduction: In myelinated axons, the action potential jumps from node to node, faster than in unmyelinated axons.
• Refractory Period: A brief period after an action potential during which a neuron cannot be stimulated again.

Functional Classifications of Neurons

• Afferent (Sensory): Carry impulses from sensory receptors to the CNS.
• Efferent (Motor): Carry impulses from the CNS to effectors (muscles or glands).
• Association (Interneurons): Connect sensory and motor neurons in the CNS

Structural Classifications of Neurons

• Multipolar: Many dendrites, one axon (typical in the brain and spinal cord).
• Bipolar: One dendrite, one axon (in sensory systems like the eye and ear).
• Unipolar: Only one process extending from the cell body; usually sensory neurons.

Synapse

• Junction between two neurons or a neuron and an effector
• Synaptic Transmission is the communication across the synapse
  • Neurotransmitters are released from the presynaptic neuron's axon terminals
  • Neurotransmitter binds to receptors of the postsynaptic membrane (neuron, muscle, or gland).
  • Excitatory neurotransmitters cause depolarization (making the inside of a cell more positive).
  • Inhibitory neurotransmitters cause hyperpolarization (inside becomes more negative).

Common Neurotransmitters

• Acetylcholine: Generally excitatory; found in neuromuscular junctions and in various autonomic synapses.
• Norepinephrine: Excitatory or inhibitory, in the cardiovascular system
• Epinephrine: Excitatory or inhibitory; related to behavior and mood (related to adrenaline)
• Dopamine: Excitatory; related to emotional responses, decreased levels related to Parkinson's Disease.
• Serotonin: Inhibitory; regulation in temperature, sensations (perception, mood, sleep)
• GABA (Gamma-aminobutyric acid): Inhibitory; in the CNS

Reflex Arc

• A simple, automatic response to a stimulus
  • Five components: sensory receptor, sensory neuron, processing center, motor neuron, and effector.

Explore the nervous system's vital roles in sensation, integration, and action. Learn how it swiftly regulates homeostasis through neurotransmitters, contrasting with the endocrine system's slower hormonal effects. Discover the central nervous system's divisions.