Nervous System: CNS and PNS

• The nervous system, along with the endocrine system, is crucial for maintaining body homeostasis.
• Its cells use electrical signals for rapid and specific communication, leading to immediate responses.

Functions of the Nervous System

• Using sensory receptors to monitor environmental changes
• Processing sensory input to make decisions
• Activating muscles or glands to effect responses

Divisions of the Nervous System

• The nervous system has two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
• The CNS includes the brain and spinal cord
• The CNS serves as the integration and command center, interpreting sensory data and dictating responses based on experience, reflexes, and current conditions.
• The PNS consists of nerves extending from the brain and spinal cord.
• Spinal nerves carry impulses to and from the spinal cord
• Cranial nerves carry impulses to and from the brain.

Functional Subdivisions of the PNS

• Sensory (Afferent): Nerve fibers carry impulses from sensory receptors throughout the body to the CNS.
• Somatic afferent fibers send impulses from the skin, skeletal muscles, and joints.
• Visceral afferent fibers transmit impulses from the visceral organs.
• Motor (Efferent): Transmits impulses from the CNS to effector organs like muscles and glands.
• Somatic Nervous System: Motor nerve fibers conduct impulses from the CNS to skeletal muscles and is voluntary.
• Autonomic Nervous System: Motor nerve fibers regulate smooth muscles, cardiac muscles, and glands, functioning involuntarily.
• The Autonomic Nervous System (ANS) has sympathetic and parasympathetic divisions, which usually have opposite effects on visceral organs.

Neuroglial Cells

• Both the CNS and PNS are composed of neurons and supporting cells.

• Supporting cells form the nervous tissue's scaffolding, assisting, segregating, and insulating neurons, each with specific functions.

• CNS contains astrocytes, microglia, ependymal cells, and oligodendrocytes.

• PNS contains Schwann cells and satellite cells.

• Supporting cells are 10-50 times more numerous than neurons, they can reproduce throughout life

• Astrocytes: Star-shaped cells abundant in the CNS, making up nearly half of its volume; they brace neurons, anchor them to their nutrient supply, form a barrier between capillaries and neurons, control the chemical environment, and recycle neurotransmitters.

• Microglia: Small, ovoid cells in the CNS with long processes; specialized macrophages protect the CNS.

• Ependymal Cells: Line the brain and spinal cord's central cavity, forming cerebrospinal fluid (CSF); their cilia circulate the CSF.

• Oligodendrocytes: Line thicker neuron fibers in the CNS, wrapping extensions around nerve fibers to create myelin sheaths.

• Schwann Cells: Form myelin sheaths around larger nerve fibers in the PNS, similar to oligodendrocytes. They also act as phagocytes to remove deteriorating cell debris for nerve regeneration.

• Satellite Cells: closely related to Schwann cells that control the chemical environment of neurons in the PNS.

Neurons

• Neurons are structural units which conduct nerve impulses from one area to another.
• Neurons are amitotic, have high metabolic rates, and require oxygen and glucose.
• Most neurons have three parts: a receptive/input region, a conducting component, and a secretory/output component.

Neuron Cell Body

• The neuron cell body is the biosynthetic center, containing organelles except for centrioles.
• Rough ER, known as Nissl bodies, is prominent in protein and membrane production.
• Golgi apparatus encircles the nucleus.
• Mitochondria are scattered.
• The cell body's plasma membrane is receptive to information.
• Nuclei are clusters of neuron cell bodies in the CNS, while ganglia are collections in the PNS.

Neuron Processes

• Neuron processes are cytoplasmic extensions, abundant in the PNS.

• Neuron processes bundled in the CNS are tracts, while in the PNS, they are nerves.

• Dendrites: Short, branched extensions containing hundreds clustered on the cell body that act as receptive regions.

• Dendrites conduct electrical signals (graded potentials) toward the cell body.

• Axon: Each neuron has a single axon arising from the axon hillock; it tapers into a uniform slender process.

• Long axons are called nerve fibers.

• Larger diameter axons conduct impulses quickly.

• Axons branch into axon collaterals.

• The distal ends of axons have telodendria, ending in axonal terminals.

• Axons function as a conducting component, generating and transmitting nerve impulses away from the cell body.

• Neurotransmitters either excite or inhibit other neurons.

• Axons lack Nissl bodies and depend on the cell body for protein and membrane component renewal, decaying if damaged.

• Substances travel bidirectionally within the axon.

• Axons are grouped in the nervous system. Axons grouped in the CNS are tracts, while in the PNS, they are nerves.

Myelin Sheath and Neurilemma

• Many nerve fibers are covered by a myelin sheath, which protects and insulates fibers.
• Myelinated fibers transmit nerve impulses rapidly.
• Myelin sheaths are associated with axons; dendrites are unmyelinated.
• Schwann cells in the PNS form myelin sheaths by wrapping around the axon and forming a neurilemma.
• Nodes of Ranvier are gaps between adjacent Schwann cells where axon collaterals can emerge.
• Oligodendrocytes form myelin sheaths in the CNS, wrapping around up to 60 axons simultaneously.
• Nodes of Ranvier are present in these myelin sheaths.
• White matter includes regions containing dense collections of myelinated fibers while Gray matter consists of nerve cell bodies and unmyelinated fibers.

Neuron Classification

• Structurally neurons are classified by the number of processes that extend from their body:
• Multipolar: Has 3+ processes.
• Bipolar: Has 2 processes (axon and dendrite).
• Unipolar: Has a single, short process that divides in a T-shape.
• Functionally, neurons are classified by the direction in which the nerve impulse travels relative to the CNS:
• Sensory (afferent): Transmits impulses toward the CNS.
• Motor (efferent): Carries impulses away from the CNS.
• Sensory neurons are generally unipolar with cell bodies in sensory ganglia located outside the CNS.
• Motor neurons are multipolar with cell bodies located inside the CNS.

Action Potential Generation and Transmission

• Neurons are excitable.
• An action potential is produced and conducted when a neuron is stimulated.
• A localized charge separation exists across the plasma membrane due to differing ion concentrations, especially Na+ and K+.
• The inside of the cell is negatively charged more K+ and protein anions
• The outside is positively charged more Na+.
• Voltage measures potential energy from separated charge.
• Current is the flow of electrical charge (ions)
• Resistance is the opposition to flow.
• Ion flow is influenced by membrane protein channels.
• Leakage channels stay open.
• Gated channels are close until stimulated to open and can be chemically-gated or voltage-gated.
• Resting membrane potential is -70 mV, with the inside of the cell being more negative.
• Leakage channels allow more K+ to leave and less Na+ to enter, while the Na+ - K+ pump maintains the separation of charge.
• Graded potentials are short-lived, localized membrane potential changes and are triggered by stimuli that open gated ion channels.
• The resulting ion flow makes the inside of the cell membrane more positive and the outside more negative.
• An action potential is a brief reversal of membrane potential (about 100 mV total voltage change -70 mV to +30 mV) that sends signals over distances, generated by ion flows through gated Na+ and K+ channels.
• At rest, both channels are closed.
• Depolarization (via graded potentials) opens Na+ channels, and Na+ influx makes the inside more positive (+30 mV), but these channels close quickly.
• Repolarization begins as slower K+ channels open and K+ exits, making the inside more negative again, resulting in hyperpolarization, a very negative state.
• The Na+ - K+ pump restores ions to the resting state.
• Action potential propagates as local current depolarizes adjacent areas, opening voltage-gated Na+ and K+ channels.
• Graded potentials in the dendrites and cell body spread toward the axon hillock.
• If the sum of graded potentials exceeds the threshold, an action potential is generated and propagated down the axon.
• The action potential is all-or-none.
• The intensity of the stimulus is indicated by the number of action potentials generated (frequency).
• Myelinated axons keep the current inside, and action potentials are generated at the nodes of Ranvier, resulting in rapid (saltatory) conduction.

Synapse

• The nervous system relies on the flow of information through neuron chains connected by synapses.
• Synapses typically occur between the axonal endings of one neuron and the dendrites or cell bodies of others.
• Less common synapse sites are: between axons, between dendrites, or between dendrites and cell bodies.
• A presynaptic neuron sends impulses
• A postsynaptic neuron receives electrical activity.
• Neurons act as both presynaptic and postsynaptic, communicating in both directions.
• Neurons have thousands of axonal terminals making synapses, activated by a similar amount of other neurons.
• In the body periphery, postsynaptic cells may be other neurons or effector cells.
• Neuromuscular junctions are synapses between neurons and muscle cells.
• Neuroglandular junctions are synapses between neurons and gland cells.
• Synapses are electrical or chemical.
• Electrical Synapses: Gap junctions which contain protein channels interconnecting the cytoplasm of adjacent neurons.
• Neurons are electrically coupled
• Transmission is rapid
• Allow activity synchronizing of neurons.
• Help produce stereotyped movements.
• Abundant in non-nervous cardiac and smooth muscle tissues and allow sequential and rhythmic excitation.
• Chemical Synapses: Specialized for neurotransmitter release and reception.
• Neurotransmitters open or close ion channels to influence membrane permeability and potential.
• Chemical synapses includes the knoblike axonal terminal of the presynaptic neuron (synaptic vesicles containing neurotransmitters) and a receptor region of the postsynaptic neuron on a dendrite or cell body.
• Chemical synapses prevent the direct transmission of nerve impulses from one neuron to another, instead transmitting the signals across chemical synapses as a chemical event which relies on the release, diffusion, and neurotransmitter receptor binding for unidirectional communication.
• The chemical synapse functions: action potential arrives at axon terminal, calcium channels open and calcium ions enter the axon terminal, calcium entry causes the neurotransmitter-containing vesicles to release their contents by exocytosis, neurotransmitter diffuses across synaptic cleft and binds to receptors on the postsynaptic membrane, this binding opens ion channels resulting in graded potentials, and neurotransmitter effects are terminated.

Termination of Neurotransmitter Effects

• Neurotransmitter bound to a receptor keeps producing effects and blocks new messages;
• Effects last briefly and are terminated through degradation by enzymes, re-uptake into presynaptic terminals, or diffusion away from the synapse.

EPSP and IPSP

• Chemical synapses can be excitatory or inhibitory depending on how they affect the postsynaptic neuron membrane potential.
• At excitatory synapses, neurotransmitter binding causes depolarization of the postsynaptic membrane known as excitatory postsynaptic potentials (EPSPs).
• Inhibitory synapses result in hyperpolarization of the postsynaptic membrane known as inhibitory postsynaptic potential (IPSP).
• A single EPSP or IPSP doesn't have much effect
• Signals accumulate at the axon hillock
• An action potential is only generated if the threshold is above the cumulative signals.
• Summation can be temporal (at different times) or spatial (at the same time).
• More EPSPs make an action potential more likely, while IPSPs counteract EPSPs.

Neurotransmitters and their Receptors

• Neurotransmitters facilitate neuronal communication for body regulation such as sleep, hunger, memory, anger or joy etc.
• These chemicals are chemically or functionally classified.
• Chemical Structures:
• Acetylcholine (ACh): Released at neuromuscular junctions (somatic nervous system) and by autonomic nervous system neurons. Synthesized and enclosed in synaptic vesicles within axonal terminals.
• Biogenic Amines: Include catecholamines (dopamine, norepinephrine, epinephrine) and indolamines (serotonin, histamine). Present in the brain, plays a role in regulating emotion and the biological clock. Norepinephrine are released by some motor neurons of the autonomic nervous system.
• Amino Acids: Glycine, aspartate, glutamate, and GABA (gamma-aminobutyric acid).
• Peptides: Strings of amino acids like beta-endorphins and enkephalins reduce pain.
• ATP: Produces an excitatory response.
• Functional Classification:
• Excitatory and Inhibitory: Some are excitatory, some are inhibitory, some are both. ACh is excitatory at neuromuscular junctions with skeletal muscle and inhibitory on cardiac muscle.
• Ionotropic: Open ion channels, using ACh and amino acids
• Metabotropic: promote longer effects by acting through intracellular second messenger molecules, such as cyclic AMP.

Neural Integration

• Neurons function in groups.
• Integration enables the parts to fuse for an operating whole.

Types of Circuits

• Patterns of synaptic connections are circuits that determine functional capabilities.
• Circuits:
• Diverging: One fiber triggers increasing responses.
• Converging: A pool receives inputs to concentrate them.
• Reverberating: Collateral synapses cause impulses to reverberate.
• Parallel After-Discharge: Fibers stimulate neurons in parallel.

Patterns of Neural Processing

• Processing is both serial and parallel.
• Serial processing: Input travels along one pathway.
• Parallel processing: Input travels along multiple routes.