Chapter 12: Nervous System Study Guide

Primary Functions of the Nervous System

The nervous system performs three essential functions to maintain homeostasis and interact with the environment:

• Collection of Information: Specialized structures known as receptors monitor changes in both the external and internal environments. These changes are termed stimuli. For example, sensory receptors located in the skin detect and relay information regarding touch.

• Processing and Evaluation of Information: Upon receiving sensory input, the brain and spinal cord analyze the data to determine the specific responses required by the body.

• Initiation of Response to Information: The brain and spinal cord initiate motor output transmitted via nerves to effectors. Effectors include glands and various types of muscle tissue, specifically skeletal, cardiac, and smooth muscle.

Anatomical Subdivisions of the Nervous System

The nervous system is divided into two primary anatomical components:

1. Central Nervous System (CNS)

   • Consists of the brain (associated with cranial nerves) and the spinal cord (associated with spinal nerves).

   • It is physically protected by the cranium (skull) and the vertebral column.

   • Its primary role is to carry out processing and integrative functions.

2. Peripheral Nervous System (PNS)

   • Includes all nerves leading to and from the CNS.

   • Serves as the pathway for signal input and output.

   • Consists of nerves and bundles of neuron processes.

   • Connects the CNS to the rest of the body, including sense organs, muscles, and glands.

   • Responsible for both sensory (inbound) and motor (outbound) functions.

Functional Divisions of the Peripheral Nervous System

The PNS is further categorized based on the direction of information flow:

• Sensory Nervous System (Afferent Nervous System): Receives information from receptors and transmits it toward the CNS.

  • Somatic Sensory Neurons: Detect stimuli that are consciously perceived. Receptors are found in the eyes, nose, tongue, ears, skin, joints, skeletal muscles, and proprioceptors (which monitor body position).

  • Visceral Sensory Neurons: Detect stimuli that are not consciously perceived. These receptors are located within blood vessels and internal organs, such as those detecting the stretching of an organ wall.

• Motor Nervous System (Efferent Nervous System): Initiates and transmits motor output from the CNS to effectors.

  • Somatic Motor Neurons: Transmit output to voluntary skeletal muscles, which are consciously controlled (e.g., the action of pressing a car's accelerator).

  • Autonomic Motor Neurons: Transmit output without conscious control to cardiac muscle, smooth muscle, and glands within organ walls.

Cells of the Nervous Tissue

Nervous tissue is comprised of two distinct cell types: neurons and glial cells.

• Neurons: Highly specialized cells that transmit electrical signals, including graded and action potentials, and facilitate communication through the release of neurotransmitters.

• Glial Cells (Neuroglia): These cells support and protect neurons.

  • Found in both the CNS and PNS.

  • They are smaller than neurons and retain the ability to divide (mitosis).

  • They do not transmit electrical signals.

Types of Glial Cells

CNS Glial Cells

• Astrocytes: The most abundant type. They have a star-like shape with perivascular feet. Their functions include forming the blood-brain barrier, regulating interstitial fluid composition, providing structural support, assisting neuronal development, altering synaptic activity, and occupying spaces left by dying neurons.

• Ependymal Cells: Epithelial cells lining the fluid-filled spaces of the CNS. They form the choroid plexus, which produces cerebrospinal fluid (CSF).

• Microglia: The smallest percentage of CNS glia. These are phagocytic cells that respond to infection.

• Oligodendrocytes: Large cells with bulbous bodies that insulate axons to form the myelin sheath via myelination. These cells are the specific target of the autoimmune disease multiple sclerosis.

PNS Glial Cells

• Satellite Cells: Flat cells surrounding neuronal cell bodies within a ganglion. They separate the cell bodies from interstitial fluid, provide insulation, and regulate the exchange of nutrients and waste.

• Neurolemmocytes (Schwann Cells): Elongated, flat cells that wrap around and insulate axons to form the myelin sheath in the PNS.

General Characteristics of Neurons

Neurons possess three primary functional characteristics:

• Excitability: Associated with dendrites and the cell body. This is the ability to respond to a stimulus (chemical, stretch, or pressure change) by causing a local voltage change in the resting membrane potential as ions move across the plasma membrane.

• Conductivity: Associated with the axon. It involves voltage changes propagated along the plasma membrane as voltage-gated channels open in sequence.

• Secretion: Associated with the synaptic knob. Neurons secrete neurotransmitters in response to conductive activity.

Additional characteristics include:

• Longevity: Most neurons must last a lifetime.

• Loss of Mitotic Ability: During fetal development, most neurons lose the ability to divide. Exceptions include neurons in the hippocampus and the olfactory epithelium of the nose.

Anatomical Features of the Neuron

The Cell Body (Soma)

• Includes the nucleus (with chromatin and a nucleolus for ribosome synthesis) and cytoplasm, specifically called the perikaryon.

• Contains standard organelles: endoplasmic reticulum, Golgi apparatus, ribosomes, and mitochondria.

• Chromatophilic Substance (Nissl Bodies): Dark-staining clusters of ribosomes. These account for the gray matter in the CNS. The axon hillock is the only part of the soma lacking this substance.

Processes

• Dendrites: Short, tapering, uninsulated processes branching from the soma that receive input.

• Axon: A long process originating at the axon hillock. The cytoplasm is the axoplasm and the membrane is the axolemma.

  • Axon Collaterals: Side branches of the axon.

  • Terminal Extensions: Extensive branching at the distal end.

  • Synaptic Knobs (Bulbs/Boutons): Expanded tips containing synaptic vesicles filled with neurotransmitters. These end at a synapse.

  • Myelin Sheath: Insulation formed by neurolemmocytes or oligodendrocytes.

  • Neurofibril Nodes (Nodes of Ranvier): Uninsulated gaps between myelin segments. Axons and myelin contribute to the white matter of the CNS.

Cytoskeleton

• Microfilaments: Actin protein meshwork supporting the plasma membrane.

• Intermediate Filaments (Neurofilaments): Form bundles called neurofibrils that maintain shape and extend into processes.

• Microtubules: Involved in axonal transport. The protein tau stabilizes these; its dysfunction is associated with Alzheimer disease.

Neuron Transport

Transport within a neuron can be categorized by direction and speed:

• Anterograde Transport: Movement from the cell body toward the synaptic knobs.

• Retrograde Transport: Movement from the synaptic knobs toward the cell body.

• Fast Axonal Transport: Occurs along microtubules using motor proteins kinesin and dynein. These proteins split $ATP$ for energy. This transport can be anterograde (moving vesicles, organelles, glycoproteins) or retrograde (moving used vesicles for recycling or harmful agents).

• Slow Axonal Transport: Also called axoplasmic flow. It results from the continuous flow of axoplasm and occurs only in the anterograde direction. It moves enzymes, cytoskeletal components, and new axoplasm for regenerating axons.

Structural Categories of Neurons

1. Multipolar: Multiple processes (one axon, many dendrites). Includes all motor neurons and most interneurons. This is the most common structural type.

2. Bipolar: Two processes (one axon, one dendrite). Found in limited areas like the retina and olfactory epithelium.

3. Unipolar (Pseudounipolar): A single short process that branches like a T. These start as bipolar neurons during development and then fuse. They include most sensory neurons and consist of a peripheral process (dendrites to cell body) and a central process (cell body to CNS).

4. Anaxonic: Only dendrites, no axon. Includes certain interneurons.

Functional Classification of Neurons

• Sensory (Afferent): Conduct signals toward the CNS. Most are unipolar.

• Motor (Efferent): Conduct signals away from the CNS to effectors. All are multipolar.

• Interneurons (Association Neurons): Located entirely within the CNS. They receive, process, and store information. Approximately $99\%$ of all neurons are interneurons.

Structure of Nerves and Ganglia

The Nerve as an Organ

A nerve is an organ composed of axons, connective tissue, and blood vessels. Axons are bundled into fascicles.

• Epineurium: Thick, dense irregular connective tissue surrounding the whole nerve.

• Perineurium: Dense irregular connective tissue wrapping each fascicle; supports blood vessels.

• Endoneurium: Areolar connective tissue surrounding and electrically insulating each individual axon.

• Mixed Nerves: Contain both sensory and motor axons. Most nerves are mixed.

• Vascularization: Capillaries within the endoneurium facilitate the exchange of oxygen, glucose, and waste.

Ganglia

Clusters of neuron cell bodies in the PNS, appearing as swellings on a nerve.

• Posterior (Dorsal) Root Ganglia: Associated with sensory neurons enter the spinal cord.

• Autonomic Ganglia: Extend to autonomic effectors (cardiac/smooth muscle/glands).

Synapses

A synapse is the functional connection between a neuron and another cell.

Chemical Synapse

• Components: Presynaptic neuron (signal producer), synaptic cleft (gap), and postsynaptic neuron (signal receiver).

• Transmission: Neurotransmitters in vesicles undergo exocytosis from the synaptic knob upon calcium entry, diffuse across the cleft, and bind to receptors on the postsynaptic membrane.

• Synaptic Delay: The time required for neurotransmitter release, diffusion, and binding.

Electrical Synapse

• Neurons are physically bound by gap junctions. Electrical signals pass directly with no synaptic delay. Found in specific areas of the brain and eyes.

Synaptic Knob Dynamics

• A calcium gradient is maintained by pumps ($Ca^{2+}$ is higher outside than inside).

• Action potentials trigger voltage-gated $Ca^{2+}$ channels.

• $Ca^{2+}$ entry triggers vesicle fusion and neurotransmitter release into the cleft.

Membrane Proteins: Channels and Pumps

Pumps

Maintain concentration gradients via active transport.

• $Na^+/K^+$ Pump: Moves $3\,Na^+$ out and $2\,K^+$ in. Requires $2/3$ of a neuron's energy expenditure.

• $Ca^{2+}$ Pump: Located in synaptic knobs.

Channels

Facilitate diffusion down concentration gradients.

• Leak Channels: Always open (e.g., $Na^+$ and $K^+$ leak channels).

• Chemically Gated Channels: Open in response to neurotransmitter binding (e.g., $K^+$ or $Cl^-$ channels). Chemically gated cation channels allow both $Na^+$ in and $K^+$ out, though more $Na^+$ enters than $K^+$ leaves.

• Voltage-Gated Channels: Open in response to electrical changes. Examples include $Na^+$, $K^+$, and $Ca^{2+}$ channels.

  • $Na^+$ channels have three states: Resting (activation gate closed, inactivation gate open), Activation (both gates open), and Inactivation (inactivation gate closed, activation gate open).

• Modality-Gated Channels: Found in sensory receptors; respond to specific stimuli like pressure or chemicals.

Functional Segments of a Neuron

1. Receptive Segment (Soma and Dendrites): Contains chemically gated channels (cation, $K^+$, and $Cl^-$). No voltage-gated channels.

2. Initial Segment (Axon Hillock): Contains voltage-gated $Na^+$ and $K^+$ channels. Site of summation.

3. Conductive Segment (Axon): Contains voltage-gated $Na^+$ and $K^+$ channels for signal propagation.

4. Transmissive Segment (Synaptic Knob): Contains voltage-gated $Ca^{2+}$ channels and $Ca^{2+}$ pumps.

Electrical Principles and Resting Membrane Potential (RMP)

Ohm's Law

• Voltage: Potential energy representing the difference in charge between two areas.

• Current: Movement of charged particles.

• Resistance: Opposition to the movement of charge.

Resting Membrane Potential (RMP)

• The RMP of a typical neuron is $-70\,mV$ (range: $-40\,mV$ to $-90\,mV$).

• Inside the cell: High concentration of $K^+$, negatively charged proteins, and phosphate ions (e.g., $ATP$).

• Outside the cell (Interstitial Fluid): High concentration of $Na^+$ and $Cl^-$.

• Establishing RMP: $K^+$ diffusion out through leak channels is the most important factor. If only $K^+$ channels existed, the RMP would be $-90\,mV$. The difference to $-70\,mV$ is caused by a small amount of $Na^+$ leaking into the cell. The $Na^+/K^+$ pump maintains these gradients and contributes about $-3\,mV$ to the potential.

Graded vs. Action Potentials

Graded Potentials

• Small, short-lived voltage changes in the receptive segment.

• Intensity weakens with distance and varies based on the stimulus.

• Excitatory Postsynaptic Potential (EPSP): Depolarization (e.g., $-70\,mV$ to $-65\,mV$) caused by $Na^+$ entry.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarization (e.g., $-70\,mV$ to $-75\,mV$) or repolarization caused by $K^+$ exit or $Cl^-$ entry.

Action Potentials

• Self-propagating voltage spikes in the conductive segment.

• Follow the All-or-None Law: If threshold ($-55\,mV$) is reached, an action potential of constant intensity is triggered. If subthreshold, no signal is sent.

Generation of an Action Potential

1. Depolarization: Threshold ($-55\,mV$) is reached; voltage-gated $Na^+$ channels open. $Na^+$ rushes into the cell, shifting potential toward $+30\,mV$.

2. Repolarization: $Na^+$ channels inactivate; voltage-gated $K^+$ channels open slowly. $K^+$ flows out, making the interior negative again.

3. Hyperpolarization: $K^+$ channels stay open longer than necessary, making the interior more negative than the RMP (e.g., $-80\,mV$). The RMP is eventually restored.

Summation and Refractory Periods

Summation

Occurs in the initial segment to determine if threshold is reached:

• Spatial Summation: Multiple presynaptic neurons release neurotransmitters at different locations simultaneously.

• Temporal Summation: A single presynaptic neuron releases neurotransmitter repeatedly in a short period.

Refractory Periods

• Absolute Refractory Period: Onset of action potential until it returns near RMP. No stimulus can trigger a second signal. Ensures one-way travel.

• Relative Refractory Period: Follows the absolute period. A strong-than-normal stimulus can trigger a signal because the cell is hyperpolarized.

• Clinical Note: Local anesthetics work by blocking voltage-gated $Na^+$ channels, preventing nerve signals.

Velocity of Nerve Signals

Signal speed depends on:

1. Axon Diameter: Larger axons are faster.

2. Myelination: The most important factor.

• Continuous Conduction: Occurs in unmyelinated axons; sequential opening of channels along the entire length. Slowest fibers ($1$-$2$ seconds).

• Saltatory Conduction: Occurs in myelinated axons. Action potentials "jump" between neurofibril nodes (Nodes of Ranvier), where the fiber is exposed to ECF and dense with channels. Signals travel rapidly and stay strong.

Neurotransmitters and Neuromodulation

Neurotransmitter Classes

1. Acetylcholine (ACh): Released at neuromuscular junctions; can be excitatory or inhibitory.

2. Amino Acids: Glutamate, glycine, aspartate.

3. Monoamines: Derived from amino acids; includes catecholamines (norepinephrine, epinephrine, dopamine) and serotonin (regulating sleep/mood).

4. Neuropeptides: Chains of amino acids like enkephalins and somatostatin.

Neuromodulation

The use of chemicals to alter neuronal response:

• Facilitation: Increases response (e.g., by increasing neurotransmitter amount or receptors). Examples: Dopamine, Serotonin.

• Inhibition: Decreases response (e.g., via acetylcholinesterase, Nitric Oxide, or endocannabinoids).

Neuronal Pools

Interneurons are organized into complex circuits:

• Converging: Many inputs to one neuron.

• Diverging: One input to many neurons.

• Reverberating: Feedback loop for cyclical stimulation.

• Parallel-after-discharge: Input sent through several pathways to one common cell.