The Nervous System Part 2: Receptors, Glia, Neurons, and the Reflex Arc

THE PERIPHERAL NERVOUS SYSTEM (PNS) FUNCTIONS

• The Peripheral Nervous System (PNS) is categorized into two primary functional systems that manage data flow to and from the Central Nervous System (CNS):

  • Afferent System: Responsible for receiving sensory data from various internal and external receptors. This system transmits the gathered information to the CNS for integration and processing.

  • Efferent System: Responsible for receiving processed messages and instructions from the CNS. It transmits these signals to effector cells (such as muscles or glands) to initiate an action or response.

SENSORY RECEPTORS AND MODALITIES

• Receptors are specialized structures located within organs that detect environmental changes and specific stimuli.

• Photoreceptors: Detect light stimuli.

• Thermoreceptors: Detect changes in temperature.

• Chemoreceptors: Detect changes in chemical composition.

• Mechanoreceptors: Detect physical touch and pressure through mechanical distortion.

• Nociceptors: Act as a critical warning system by detecting damaging stimuli which are interpreted as pain.

THE SPECIAL SENSES: VISION AND HEARING

• Vision:

  • Photoreceptors are located in the retina of the eye.

  • Rods: Responsible for detecting light and dark (contrast).

  • Cones: Responsible for detecting color, specifically Red, Green, and Blue wavelengths.

  • The vision process involves converting light stimuli into electrochemical impulses for brain processing.

• Hearing:

  • Pinna: The outer ear structure that focuses sound waves into the ear canal.

  • Tympanic Membrane: Also known as the eardrum, it vibrates in response to sound waves.

  • Ossicles: Three tiny bones named the Malleus, Incus, and Stapes. These bones transfer vibrations from the tympanic membrane to the oval window.

  • Oval Window: A membrane where vibrations from the ossicles are converted into fluid vibrations within the cochlea.

  • Cochlea: Contains the basilar membrane which is lined with hair cells. These hair cells bend in response to fluid movement, transmitting signals to the auditory nerve.

  • Semicircular Canals: Fluid-filled structures in the inner ear essential for maintaining balance.

THE PHYSICAL AND CHEMICAL SENSES: TOUCH, TASTE, AND SMELL

• Touch (Mechanoreception):

  • Mechanoreceptors detect physical forces. These forces distort proteins in the plasma membrane of the receptor, which alters ion flow.

  • This alteration in ion flow generates action potentials.

  • Free nerve endings: Specialized for detecting light touch.

  • Pacinian Corpuscles: Specialized for detecting deep pressure and vibrations.

  • Ruffini Endings: Specialized for detecting deep pressure.

• Taste (Chemoreception):

  • Chemoreceptors utilize membrane receptor proteins to bind specific molecules such as flavor molecules, $O_2$, $CO_2$, and $H^+$ ions.

  • Signals travel to two main brain areas: the cerebral cortex (for taste identification) and the brain stem (associated with emotional responses to pleasant or unpleasant tastes).

• Smell (Olfactory System):

  • Olfactory receptors possess hairs that project into a layer of mucus within the nose.

  • Airborne molecules must dissolve into the mucus layer to be detected by these receptors.

NOCICEPTION, PAIN PERCEPTION, AND NEUROPHARMACOLOGY

• Nociceptors:

  • These are the most numerous types of receptors in the body.

  • They detect damaging stimuli that the brain interprets as pain, eliciting a reflex response to remove the stimulus.

  • Pain can be classified as localized or delocalized.

• Pain Pathway:

  • Perception occurs when substantia gelatinosa (SG) cells are stimulated, sending a message to the brain.

• Analgesia (Pain Suppression):

  • Endorphins: Natural neuropeptides acting as internal painkillers. They bind to receptors on Substance P neurons in the SG of the spinal cord to inhibit pain signaling.

  • Opiates: Drugs such as heroin, morphine, and codeine function by binding to these same endorphin receptors to suppress pain.

HIERARCHICAL STRUCTURE OF THE NERVOUS SYSTEM

• Central Nervous System (CNS): Consists of the Brain and Spinal Cord (interneurons). It acts as the center for input integration and output generation.

• Peripheral Nervous System (PNS): Divided into Sensory (Afferent) and Motor (Efferent) pathways.

  • Somatic Nervous System: Controls voluntary movements by acting on skeletal muscles.

  • Autonomic Nervous System: Controls involuntary functions by acting on smooth muscles, cardiac muscles, and glands.

  • Sympathetic Division: Prepares the body for action/stress ("fight or flight" response).

  • Parasympathetic Division: Returns the body to normal resting levels and slows body activities ("rest and digest").

  • Enteric System: Specifically controls the digestive tract, pancreas, and gallbladder.

THE AUTONOMIC NERVOUS SYSTEM: SYMPATHETIC VS. PARASYMPATHETIC

• Physiological Effects:

  • Pupils: Sympathetic dilates; Parasympathetic constricts.

  • Salivation: Sympathetic inhibits; Parasympathetic stimulates.

  • Heart: Sympathetic accelerates; Parasympathetic inhibits/slows.

  • Bronchi: Sympathetic relaxes; Parasympathetic constricts.

  • Digestion: Sympathetic inhibits activity; Parasympathetic stimulates activity.

  • Glucose/Bile: Sympathetic stimulates glucose release by the liver; Parasympathetic stimulates the gallbladder.

  • Adrenal Gland: Sympathetic triggers secretion of epinephrine and norepinephrine from the kidneys.

  • Bladder: Sympathetic relaxes; Parasympathetic contracts.

  • Rectum: Sympathetic contracts; Parasympathetic relaxes.

• Neurotransmitters: The sympathetic system is notably affected by epinephrine and norepinephrine.

GLIAL CELLS AND NEURONAL SUPPORT

• Glial Cells: Non-conducting cells that provide structural support and protection for neurons.

  • They digest dead cells and provide essential nutrients to neurons.

  • They are responsible for the formation of the myelin sheath.

• Astrocytes:

  • Provide structural and metabolic support.

  • Regulate extracellular concentrations of ions and neurotransmitters to clear the synaptic cleft.

• Myelin-Producing Glia:

  • Oligodendrocytes: Produce myelin in the CNS.

  • Schwann Cells: Produce myelin in the PNS.

NEURONAL ANATOMY AND SIGNAL TRANSMISSION

• Dendrites: Highly branched extensions that receive signals and transmit them toward the cell body (soma).

• Cell Body (Soma): Contains the nucleus and integrates signals.

• Axon Hillock: The region where the axon joins the cell body; often where signals are initiated.

• Axon: A long extension that transmits signals away from the soma toward other cells.

• Myelin Sheath: An insulating cover composed of fat-containing cells. It covers the axon to speed up electrochemical signal transmission.

• Synaptic Terminals: The ends of the axon that form connections with other cells.

• Synapse: The communication point between neurons where neurotransmitters are released into the synaptic cleft.

• Presynaptic vs. Postsynaptic: The presynaptic cell sends the signal, and the postsynaptic cell receives it.

FUNCTIONAL CLASSIFICATION OF NEURONS

• Sensory (Afferent) Neurons: Sense information from the environment and send signals to the CNS. Note: In these neurons, dendrites do not typically branch directly from the cell body.

• Motor (Efferent) Neurons: Relay information from the CNS to effector cells such as muscles, organs, or glands. They interface with muscles at the neuromuscular junction.

• Interneurons: Primarily found in the brain and spinal cord. They connect sensory and motor neurons, sorting and integrating incoming signals to form an appropriate outgoing response.

THE REFLEX ARC AND NEURAL INTEGRATION

• Definition: A neural pathway that controls a reflex, allowing for extremely fast, unconscious reactions controlled by the spinal cord before the brain processes the message.

• Components of a Reflex Arc:

1. Receptor: Detects the stimulus (e.g., thermal pain receptor in a finger).

2. Afferent Pathway: Signal travels through the sensory neuron toward the spinal cord.

3. Integrating Center: Located in the spinal cord; involves interneurons (excitatory and inhibitory).

4. Efferent Pathway: Motor neurons transmit the response signal toward the effectors.

5. Effector: The muscle or gland that performs the action (e.g., muscle contraction).

• Example: Withdrawal Reflex (Hot Surface):

  • When touching a hot object, the signal travels to the spinal cord.

  • To facilitate movement, muscles work in opposing pairs: an excitatory signal causes the biceps (flexor) to contract, while an inhibitory signal causes the triceps (extensor) to relax.

  • Simultaneously, an ascending pathway sends the information to the brain after the reflex has already occurred.