Nervous System: Control and Communication

Overview of the Nervous System

• The nervous system serves as the body's control center, facilitating communication among organ systems.
• It performs sensory input, integration, and motor output functions.
• Sensory input involves receiving stimuli via numerous sensory receptors throughout the body.
• Integration processes the input stimuli and determines the necessary actions.
• Motor output activates effector organs to produce a response.

Composition of Nervous Tissue

• Nervous tissue consists of densely packed neurons and neuroglia.
• Neurons (nerve cells) are excitable cells that respond to stimuli by conducting impulses to transmit signals.
• Neuroglia (glial cells) are supportive cells that provide nutrition, insulation, and aid in signal transmission.

Types of Neuroglia and Their Functions

• Astrocytes are located in the CNS (brain and spinal cord).
• Astrocytes regulate synapse function, support neurons metabolically, and protect against toxic compounds.
• Ependymal cells line cavities in the brain, central canal, and spinal cord.
• Ependymal cells produce cerebrospinal fluid to fill ventricles and the central canal.
• Microglial cells are found throughout the brain and spinal cord.
• Microglial cells remove bacterial cells and cellular debris and help with injury repair, regulate brain development, and neuronal networks.
• Oligodendrocytes are myelinating cells in the CNS, providing myelin membranes that insulate CNS axons.
• Satellite cells are located in the PNS.
• Satellite cells supply nutrients to surrounding neurons, provide structural support, and act as protective, cushioning cells.
• Schwann cells are in the PNS.
• Schwann cells wrap around axons to form a myelin sheath, aiding nerve impulse conduction and regeneration when nerves are damaged.

Neuron Structure

• All neurons have a soma (cell body) that supports life, contains the nucleus, and houses organelles like mitochondria.
• Neurons have processes that extend from the cell body, including dendrites and an axon.
• Dendrites are the main receptor of signals, serving as the input region.
• The axon generates and transmits nerve impulses, acting as the conducting region and is also known as a nerve fiber.
• A ganglion is a collection of nerve cell bodies located in the body, excluding the brain and spinal cord.
• Nerves are bundles of axons that extend from the brain and spinal cord to the rest of the body.
• Axon terminals are the end of the axon that releases neurotransmitters at a synapse when a nerve impulse is received.
• The myelin sheath covers long axons to protect and electrically insulate them, increasing nerve impulse transmission speed.
• Nodes of Ranvier are unmyelinated gaps in the myelin sheath that enhance nerve signal conduction velocity.

Neuron Classification by Structure

• Neurons are classified by structural differences, based on the number of processes extending from the cell body.
• Multipolar neurons have more than 3 processes (1 axon and 2 or more dendrites); 99% of neurons are multipolar.
• Bipolar neurons have 2 processes (1 axon and 1 dendrite on opposite ends of the cell), found in a few special sense organs.
• Unipolar neurons have 1 process that divides like a T from the cell body, located in the ganglia in the PNS.

Neuron Classification by Function

• Neurons are classified by functional differences, based on how an impulse travels through a neuron regarding the brain and spine.
• Sensory neurons (afferent neurons) transmit information from sensory receptors to the CNS and are mostly unipolar.
• Motor neurons (efferent neurons) transport information from the CNS to the rest of the body and are mostly multipolar.
• Interneurons (association neurons) are housed in the CNS and transport information between sensory and motor neurons, mostly multipolar.

Organization of the Nervous System

• The nervous system comprises the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord, serving as the integration and control center.
• The PNS includes spinal and cranial nerves and acts as a communication system between the CNS and the rest of the body.
• The PNS has sensory (afferent) and motor (efferent) divisions.
• The motor division includes the somatic nervous system (voluntary) and the autonomic nervous system (involuntary).
• The autonomic nervous system is divided into the parasympathetic (rest and digest) and sympathetic (fight or flight) systems.

Central Nervous System (CNS)

• The brain is protected by the skull and surrounded by meninges and cerebrospinal fluid, cushioning it from injury.
• Ventricles are hollow, fluid-filled cavities in the brain containing the choroid plexus, which produces cerebrospinal fluid.
• The cerebrum is the largest part of the brain, divided into left and right hemispheres and four lobes.
• The cerebrum functions in learning, speech, emotion, reasoning, vision, hearing, and fine movements.
• The surface of the cerebrum is the cerebral cortex, arranged in folds to increase surface area.
• The cerebellum is under the cerebrum and maintains posture and balance, coordinating timing and patterns for smooth movements.
• The brainstem is at the base of the cerebrum and anterior to the cerebellum.
• The brainstem includes the medulla oblongata, midbrain, and pons.
• The brainstem relays information between the rest of the brain and the spinal cord, coordinating automatic functions such as respiration, circulation, and digestion.

Peripheral Nervous System (PNS)

• The autonomic nervous system includes the sympathetic division (fight or flight responses).
• The sympathetic division focuses on what the body needs to do immediately.

Sensory Nerve Receptors

• Sensory nerve receptors take in stimuli from physical environments and send them to the brain to interpret and respond.
• Mechanoreceptors detect mechanical forces like vibration, pressure, stretch, and touch.
• Thermoreceptors detect changes in temperature.
• Photoreceptors detect light.
• Chemoreceptors detect chemicals.
• Nociceptors detect pain.

Reflexes

• Stimuli are received by receptors, transmitted through nerves to the spinal cord and brain, all within the sensory division.
• Stimuli can trigger an action potential (AP) that sends signals to the motor division, resulting in a reflex.
• A reflex is an automatic reaction to stimuli.
• Innate reflexes are rapid, predictable motor responses to a startling stimulus.
• Learned reflexes result from practice, repetition, or experience.

Reflex Arc Components

• Reflexes occur over specific neural pathways called reflex arcs.
• Receptor: Site of stimulus.
• Sensory neuron: Transmits impulse from PNS to CNS.
• Integration center: "Decodes" the signal at a synapse.
• Motor neuron: Conducts impulses to an effector organ.
• Effector: Responds by contracting (muscle cell) or secreting (gland).

Vocab

• Soma (cell body): The main part of a neuron that contains the nucleus and organelles.
• Dendrites: Branch-like extensions of a neuron that receive signals from other neurons.
• Axon: A long, slender projection of a neuron that transmits electrical impulses.
• Ganglion: A cluster of neuron cell bodies in the peripheral nervous system.
• Nerve: A bundle of axons that transmit signals between the brain, spinal cord, and body.
• Axon terminal: The endpoint of an axon where neurotransmitters are released.
• Myelin sheath: A fatty layer that insulates axons and speeds up nerve impulses.
• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal transmission.
• Sensory neurons: Neurons that carry signals from sensory receptors to the central nervous system.
• Motor neurons: Neurons that transmit signals from the central nervous system to muscles or glands.
• Interneurons: Neurons that connect sensory and motor neurons within the central nervous system.
• Resting membrane potential: The electrical charge difference across a neuron's membrane at rest.
• Graded potential: A small, localized change in membrane potential that varies in strength.
• Threshold: The minimum membrane potential needed to trigger an action potential.
• Action potential: A rapid electrical signal that travels along a neuron.
• Nerve impulse: The transmission of an action potential along a nerve fiber.
• Depolarization: The process of a neuron's membrane potential becoming more positive.
• Repolarization: The restoration of a neuron's negative membrane potential after depolarization.
• Hyperpolarization: A change in membrane potential that makes a neuron more negative than its resting state.
• Synapse: The junction between two neurons where communication occurs.
• Meninges: Protective membranes that surround the brain and spinal cord.
• Ventricle: Fluid-filled cavities in the brain that produce and circulate cerebrospinal fluid.
• Cerebrum: The largest part of the brain responsible for higher cognitive functions.
• Cerebellum: The brain region that coordinates movement and balance.
• Brain stem: The structure that connects the brain to the spinal cord and controls vital functions.
• Neurotransmitter: A chemical messenger that transmits signals across a synapse.
• Hormone: A chemical released by glands that regulates physiological processes.
• Mechanoreceptor: A sensory receptor that detects touch, pressure, and vibration.
• Thermoreceptor: A sensory receptor that responds to temperature changes.
• Photoreceptor: A sensory receptor in the eye that detects light.
• Chemoreceptor: A sensory receptor that detects chemical stimuli, such as taste or smell.
• Nociceptor: A pain receptor that detects harmful stimuli.
• Reflex: An automatic, involuntary response to a stimulus.
• Sclera: The white, protective outer layer of the eye.
• Cornea: The transparent front part of the eye that helps focus light.
• Choroid: The vascular layer of the eye that supplies nutrients and oxygen.
• Ciliary body: A structure that controls the shape of the lens and produces aqueous humor.
• Exocrine: Glands that release secretions through ducts, such as sweat glands.
• Endocrine: Glands that release hormones directly into the bloodstream.
• Humoral stimuli: Hormone release triggered by changes in blood levels of certain substances.
• Neural stimuli: Hormone release triggered by nerve signals.
• Hormonal stimuli: Hormone release triggered by other hormones.
• Permissiveness: When one hormone enhances the effects of another hormone.
• Synergism: When two hormones work together for a stronger effect.
• Antagonism: When one hormone opposes the action of another hormone.
• Iris: The colored part of the eye that controls the size of the pupil.
• Pupil: The opening in the iris that regulates the amount of light entering the eye.
• Suspensory ligament: Fibers that hold the lens in place and adjust its shape for focusing.
• Retina: The light-sensitive layer of the eye that contains photoreceptors.
• Optic nerve: The nerve that carries visual information from the retina to the brain.
• Fovea centralis: A small pit in the retina responsible for sharp central vision.
• Optic disc: The blind spot in the retina where the optic nerve exits the eye.
• Rods: Photoreceptors in the retina that detect low light and peripheral vision.
• Cones: Photoreceptors in the retina that detect color and fine details.
• Lens: A transparent structure that focuses light onto the retina.
• Vitreous humor: The gel-like substance that fills the interior of the eye.
• Aqueous humor: The fluid that nourishes and maintains pressure in the front of the eye.
• Odor: A scent detected by olfactory receptors.
• Olfactory epithelium: The sensory tissue in the nasal cavity that detects smells.
• Taste buds: Clusters of sensory cells on the tongue that detect flavors.
• Papillae: Small bumps on the tongue that contain taste buds.
• Gustatory epithelial cells: The receptor cells responsible for detecting taste stimuli.
• Basal epithelial cells: Cells that replace worn-out gustatory cells.

Functions of the Nervous System

• Sensory Input: Detects stimuli from the environment and the body.
• Integration: Processes and interprets sensory information.
• Motor Output: Sends signals to muscles and glands for a response.
• The Process: Sensory receptors detect changes → Signals travel via sensory neurons → Brain/spinal cord processes information → Motor neurons send signals to effectors (muscles/glands) → Response occurs.

Types of Nervous Tissue Cells

• Neurons: Transmit electrical and chemical signals.
• Neuroglia (Glial Cells): Support, protect, and nourish neurons.

Signal Transmission Between Neurons

• Action Potential: Travels down the axon.
• Synapse: The signal reaches the axon terminal, triggering neurotransmitter release.
• Neurotransmitters: Bind to receptors on the next neuron, generating a response.
• Signal Strength: Increased by releasing more neurotransmitters or increasing receptor sensitivity.
• Role of Protein Channels in Nerve Impulses
• Na+/K+ Pump: Maintains resting membrane potential by pumping 3 Na⁺ out and 2 K⁺ in.
• Depolarization: Na⁺ channels open, allowing Na⁺ in, making the cell more positive.
• Repolarization: K⁺ channels open, allowing K⁺ out, restoring negativity.
• Hyperpolarization: Overshoot of repolarization before balance is restored.

Types of Gated Protein Channels

• Voltage-Gated: Open/close due to voltage changes (e.g., Na⁺ and K⁺ channels in action potentials).
• Ligand-Gated: Open when a specific neurotransmitter binds.
• Mechanically-Gated: Open in response to pressure or stretch (e.g., touch receptors).

Excitatory vs. Inhibitory Neurotransmitters

• Excitatory: Increase the likelihood of an action potential (e.g., glutamate, dopamine).
• Inhibitory: Decrease the likelihood of an action potential (e.g., GABA, serotonin).
• Mnemonic: "Excite = GO!" (activates neurons), "Inhibit = STOP!" (slows or stops signals).

Central vs. Peripheral Nervous System (CNS vs. PNS)

• CNS: Brain and spinal cord (processes and integrates info).
• PNS: Nerves outside the CNS (connects CNS to the body).

Main Brain Parts & Functions

• Cerebrum: Higher thinking, memory, voluntary movements.
• Cerebellum: Coordination, balance, fine motor control.
• Brain Stem: Controls automatic functions (breathing, heart rate).

Sensory vs. Motor Division of PNS

• Sensory (Afferent): Sends signals from receptors to the CNS.
• Motor (Efferent): Sends signals from CNS to muscles/glands.

Somatic vs. Autonomic Nervous System

• Somatic: Voluntary control (skeletal muscles).
• Autonomic: Involuntary control (organs, glands).

Parasympathetic vs. Sympathetic Nervous System

• Parasympathetic ("Rest & Digest"): Lowers heart rate, increases digestion.
• Sympathetic ("Fight or Flight"): Increases heart rate, slows digestion.
• Mnemonic: Sympathetic = Stress response, Parasympathetic = Peaceful state.

Innate vs. Learned Reflexes

• Innate: Automatic, built-in responses (e.g., knee-jerk reflex).
• Learned: Acquired through experience (e.g., driving, playing an instrument).

General vs. Special Senses

• General: Touch, pain, temperature (spread throughout the body).
• Special: Vision, hearing, taste, smell, equilibrium (specific organs).

Explore the nervous system's role as the body's control center, facilitating communication between organ systems. Learn about sensory input, integration, and motor output. Nervous tissue consists of neurons and neuroglia.