CHAPTER 12
12-4 Explain how the resting membrane potential is established and maintained and how the membrane potential can change.
Key Concepts for Resting Membrane Potential
Resting Membrane Potential (RMP)
Definition: The electrical charge difference across the membrane of a resting cell, typically around -70 mV.
Why Important: Critical for the functioning of neurons and muscle cells, as it enables electrical signals to be generated (action potentials).
Ionic Composition
Extracellular Fluid (ECF): High concentration of Na+ and Cl–.
Intracellular Fluid (Cytosol): High concentration of K+ and negatively charged proteins.
Selective Permeability: The membrane allows K+ to pass more freely than Na+, which drives the resting potential closer to the equilibrium potential of K+ (~-90 mV).
Active Transport (Na+/K+ Pump)
Function: The Na+/K+ pump uses ATP to transport 3 Na+ ions out and 2 K+ ions into the cell.
Effect: This pump helps maintain the concentration gradients and stabilizes the resting membrane potential at -70 mV.
Ion Movement and Electrical Gradients
Chemical Gradient: Ions move from areas of high to low concentration (e.g., Na+ moves into the cell, K+ moves out).
Electrical Gradient: The inside of the membrane is negative relative to the outside, pulling positive ions (like Na+) inside.
Electrochemical Gradient: The combined influence of chemical and electrical gradients on ions. This contributes to the potential energy across the membrane.
Changes in Membrane Potential
Graded Potentials:
Localized changes in membrane potential, caused by gated ion channels opening (chemically, mechanically, or voltage-gated).
Depolarization: Na+ influx leads to more positive membrane potential.
Hyperpolarization: K+ efflux leads to a more negative membrane potential.
Action Potentials:
A large, rapid change in membrane potential, triggered by graded potentials that reach a threshold.
Propagates down axons and is key for nerve signal transmission.
How the Resting Membrane Potential Can Change
Opening/Closing of Ion Channels:
Chemically Gated Channels: Open in response to a neurotransmitter (e.g., acetylcholine).
Voltage-Gated Channels: Open in response to changes in membrane potential (e.g., in axons).
Mechanically Gated Channels: Open due to physical distortion (e.g., pressure).
Graded Potentials and Stimuli:
Depolarization: Caused by Na+ entering, making the inside more positive.
Hyperpolarization: Caused by K+ leaving, making the inside more negative.
Strength of Stimulus: A stronger stimulus will lead to a greater change in membrane potential.
Important Ion Equilibrium Potentials:
K+ equilibrium potential: ~ -90 mV (Resting potential close to this value).
Na+ equilibrium potential: ~ +66 mV (Less influential due to low permeability at rest).
Essential Takeaways:
Resting membrane potential is mainly determined by the gradients of Na+ and K+ across the membrane, with K+being the dominant factor due to higher permeability.
Graded potentials occur when gated ion channels open in response to stimuli and can lead to depolarization or hyperpolarization.
Action potentials are generated when graded potentials reach a threshold.
12-5 Describe the events involved in the generation and propagation of an action potential and the factors involved in determining the speed of action potential propagation.
12-5: Action Potential
🔹 What Is an Action Potential?
A rapid, propagated change in membrane potential across the entire excitable membrane.
Begins at the initial segment of the axon.
Triggered by a graded potential reaching threshold (−60 to −55 mV).
Follows the all-or-none principle: if threshold is reached, an identical action potential is always generated — no partial responses.
🔹 Phases of an Action Potential
Depolarization to Threshold
Graded depolarization opens voltage-gated Na⁺ channels.Activation of Na⁺ Channels / Rapid Depolarization
Na⁺ floods in → inside becomes positive.
Membrane potential shifts to +30 mV.
Inactivation of Na⁺ Channels / Activation of K⁺ Channels
Na⁺ channels close (inactivated); K⁺ channels open.
K⁺ leaves the cell → repolarization begins.
Return to Resting Potential / Hyperpolarization
K⁺ channels slowly close → excess K⁺ exits → hyperpolarization to −90 mV.as
Resting potential (−70 mV) is restored after all K⁺ channels close.
🔹 Refractory Periods
Absolute Refractory Period:
No new action potential can be triggered (Na⁺ channels are open/inactivated).Relative Refractory Period:
Possible to trigger another action potential, but only with a strong stimulus (Na⁺ channels have reset, but K⁺ channels still open).
🔹 Ion Movement Summary
Depolarization: Influx of Na⁺
Repolarization: Efflux of K⁺
Restoration: Na⁺/K⁺ exchange pump restores gradients (3 Na⁺ out, 2 K⁺ in, uses 1 ATP)
🔹 Propagation of Action Potentials
Continuous Propagation
Occurs in unmyelinated axons
Slower (1 m/sec), step-by-step activation of each segment
Saltatory Propagation
Occurs in myelinated axons
Faster and energy-efficient
AP "jumps" from node to node (Nodes of Ranvier)
🔹 Factors Affecting Speed
Axon Diameter: Larger = less resistance = faster
Myelination: Myelinated = faster (enables saltatory conduction)
🔹 Axon Types
Type | Myelinated? | Diameter | Speed | Function |
|---|---|---|---|---|
A Fibers | Yes | Large | ~120 m/sec | Fast sensory & motor (e.g. muscle) |
B Fibers | Yes | Medium | ~18 m/sec | Intermediate sensory |
C Fibers | No | Small | ~1 m/sec | Slow sensory (e.g. pain, temp) |
12-6: Describe the Structure of a Synapse and Explain the Mechanism Involved in Synaptic Activity
Structure of a Synapse
A synapse is the specialized site where a neuron communicates with another cell, which could be another neuron, a muscle cell, or a gland cell. It consists of:
Presynaptic neuron: the neuron that sends the signal.
Postsynaptic neuron (or cell): the cell that receives the signal.
Synaptic cleft: the small gap between the presynaptic and postsynaptic membranes.
Synaptic vesicles: located in the axon terminal of the presynaptic neuron, these contain neurotransmitters.
Neurotransmitter receptors: found on the postsynaptic membrane, these bind neurotransmitters and trigger a response.
There are two types of synapses:
Electrical synapses: The presynaptic and postsynaptic membranes are physically connected by gap junctions, allowing ions to pass directly between cells. This allows very fast communication and is found in limited areas like parts of the brain, the eye, and ciliary ganglia.
Chemical synapses: More common, these involve neurotransmitters that cross the synaptic cleft to communicate between cells. The presynaptic cell releases neurotransmitters, and the postsynaptic cell receives them.
Mechanism of Synaptic Activity (Chemical Synapse)
Chemical synapses work through a multi-step process:
Arrival of Action Potential: An action potential reaches the axon terminal of the presynaptic neuron, causing the membrane to depolarize.
Calcium Ion Influx: Voltage-gated calcium (Ca²⁺) channels open, and Ca²⁺ enters the axon terminal.
Neurotransmitter Release: The influx of calcium triggers exocytosis of synaptic vesicles, releasing neurotransmitters (like acetylcholine, ACh) into the synaptic cleft.
Binding to Receptors: Neurotransmitters diffuse across the cleft and bind to receptors on the postsynaptic membrane. For example, ACh binds to chemically gated cation channels, causing Na⁺ to enter the postsynaptic cell.
Postsynaptic Response: The inflow of Na⁺ causes depolarization (graded potential) in the postsynaptic cell. If the depolarization reaches threshold, an action potential is generated.
Termination: ACh is broken down by the enzyme acetylcholinesterase (AChE) into acetate and choline. The choline is reabsorbed by the presynaptic terminal and reused to synthesize more ACh.
Other Key Points
Synaptic delay: A brief delay (0.2–0.5 ms) occurs due to the time needed for neurotransmitter release and binding. Fewer synapses = faster response.
Synaptic fatigue: If stimulation is too intense and sustained, neurotransmitters can't be recycled fast enough, weakening the synapse's response.
12-7 Describe the major types of neurotransmitters and neuromodulators and discuss theireffects on postsynaptic membranes.
🧠 Neurotransmitters and Neuromodulators: Overview
Neurotransmitters are chemical messengers that influence neurons and other cells.
Neuromodulators are chemicals that modify how neurotransmitters affect the postsynaptic cell. They often act more slowly and have longer-lasting effects.
📌 Types of Neurotransmitters
1. By Function
Excitatory neurotransmitters
Depolarize the postsynaptic membrane
Promote action potentials
Inhibitory neurotransmitters
Hyperpolarize the postsynaptic membrane
Suppress action potentials
⚠ The effect depends on the receptor, not just the neurotransmitter.
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📚 Major Classes of Neurotransmitters
🔷 Biogenic Amines
Norepinephrine (NE): Excitatory; found in the brain and ANS.
Dopamine: Can be excitatory or inhibitory; involved in Parkinson’s disease and the reward system (e.g., drug addiction).
Serotonin: Affects mood, attention, and emotion.
GABA (gamma-aminobutyric acid): Inhibitory; functions mainly in the CNS.
🔶 Amino Acids
Examples: Glutamate, Aspartate (excitatory)
🧬 Neuropeptides (Neuromodulators)
Small peptide chains that modify neuron activity.
Opioids (like endorphins, enkephalins, dynorphins): natural pain relievers; bind to same receptors as morphine.
💨 Dissolved Gases
Nitric oxide (NO) and Carbon monoxide (CO)
Diffuse across membranes and affect cells by binding to intracellular enzymes.
⚙ Mechanisms of Action
➤ Direct Effects
Neurotransmitter binds to chemically gated ion channels.
Example: ACh, glutamate, aspartate open sodium channels → immediate effect on membrane potential.
➤ Indirect Effects via G Proteins
Neurotransmitter activates a G protein, which then activates second messengers (e.g., cAMP).
Slower but more complex and long-lasting.
Examples: E, NE, dopamine, serotonin, histamine
➤ Indirect Effects via Intracellular Enzymes
Lipid-soluble gases like NO or CO diffuse into the cell and activate enzymes inside the neuron.
Chapter 13
13-1 Describe the basic structural and organizational characteristics of the nervous system.
📍 1. Central Nervous System (CNS)
Includes:
Brain
Spinal cord
Functions:
Acts as the processing center for sensory data and motor commands
Responsible for intelligence, memory, learning, and emotion
📍 2. Peripheral Nervous System (PNS)
Includes:
Cranial nerves (from the brain)
Spinal nerves (from the spinal cord)
Functions:
Carries sensory input from receptors to the CNS
Delivers motor output from the CNS to effectors (muscles, glands, etc.)
⚡ Reflexes
Reflexes are quick, automatic responses to specific stimuli.
They help protect the body and maintain homeostasis.
➤ Spinal Reflexes
Controlled entirely by the spinal cord—no need for the brain’s input.
Example:
Dropping a hot pan: your spinal reflex causes you to release it before the pain signal reaches your brain.
🔄 Flow of Information in the Nervous System
Sensory Input
Comes from sensory receptors
Travels through cranial or spinal nerves
Processing Centers (Reflex Centers)
Located in either:
The spinal cord (spinal reflexes)
The brain (cranial reflexes)
Motor Output
Sent via:
Cranial nerves to facial/head effectors
Spinal nerves to muscles, glands, adipose tissue
13-2 Discuss the structure and functions of the spinal cord and describe the three meningeal layers that surround the central nervous system.
🧠 Structure and Function of the Spinal Cord
✅ Main Functions
Transmits sensory and motor information between the brain and the body.
Integrates and processes spinal reflexes.
Gives rise to 31 pairs of spinal nerves.
✅ Basic Structure
Length: ~18 inches (45 cm)
Width: ~0.5 inch (14 mm)
Regions: Cervical, Thoracic, Lumbar, Sacral
Stops growing at age 4—vertebral column continues growing
Grooves:
Posterior median sulcus – shallow, on back
Anterior median fissure – deeper, on front
Central canal contains cerebrospinal fluid (CSF)
✅ Special Regions
Cervical Enlargement: Supplies upper limbs
Lumbosacral Enlargement: Supplies lower limbs
Conus Medullaris: Tapered end of the spinal cord
Cauda Equina: "Horse’s tail" of nerve roots below the conus
Filum Terminale: Fibrous thread anchoring the spinal cord to the coccyx
🔁 Spinal Roots and Nerves
➤ Roots
Anterior (ventral) root: Motor neuron axons
Posterior (dorsal) root: Sensory neuron axons
Dorsal Root Ganglia: Contain sensory neuron cell bodies
➤ Spinal Nerves
Formed where anterior and posterior roots join
Mixed nerves: contain both sensory and motor fibers
Branch into:
Posterior ramus: Skin/muscles of the back
Anterior ramus: Skin/muscles of body wall and limbs
🧬 The Three Meningeal Layers
These protect the spinal cord and carry blood supply. They are continuous with the cranial meninges.
1. Dura Mater (Outer Layer)
Tough, fibrous, made of dense collagen
Extends from brain to sacrum and fuses with the coccygeal ligament
Epidural space (between vertebrae and dura): contains fat and connective tissue
Subdural space: potential space just beneath the dura
2. Arachnoid Mater (Middle Layer)
Made of:
Arachnoid membrane (thin, web-like)
Arachnoid trabeculae (collagen and elastic fibers)
Subarachnoid space (between arachnoid and pia): filled with CSF
Site of lumbar puncture/spinal tap
3. Pia Mater (Innermost Layer)
Thin, delicate layer of collagen and elastic fibers
Directly adheres to the spinal cord surface
Contains blood vessels
Denticulate ligaments anchor pia to dura and prevent side-to-side motion
13-3 Explain the roles of white matter and gray matter in processing and relaying sensory information and motor commands.
White matter and gray matter play distinct but complementary roles in processing and relaying sensory information and motor commands within the central nervous system (CNS).
Gray Matter
Gray matter is composed of neuron cell bodies, neuroglia, and unmyelinated axons. It is found in the central portion of the spinal cord and forms an "H" or butterfly shape. This area is functionally organized into nuclei, which are groups of neuron cell bodies with specific roles:
Posterior horns contain somatic and visceral sensory nuclei—they process incoming sensory information from receptors.
Anterior horns contain somatic motor nuclei, which issue outgoing motor commands to skeletal muscles.
Lateral horns, present only in thoracic and lumbar regions, house visceral motor nuclei, which send commands to organs and glands (autonomic motor output).
Gray commissures allow axons to cross sides of the spinal cord, enabling coordination between the two sides of the body.
White Matter
White matter consists primarily of myelinated and unmyelinated axons, which are organized into tracts that connect different parts of the CNS:
It is divided into three columns: posterior, anterior, and lateral white columns.
Each column contains tracts, which are bundles of axons that carry similar types of information:
Ascending tracts carry sensory information from the spinal cord to the brain.
Descending tracts carry motor commands from the brain to the spinal cord.
The anterior white commissure allows axons to cross from one side of the spinal cord to the other, promoting bilateral coordination.
Summary
In essence, gray matter processes and integrates information, determining how the body responds, while white matter acts as the highway system, transmitting that information to and from the brain. Sensory input enters through the posterior horns, is processed in gray matter nuclei, and responses are sent out via the anterior or lateral horns, with white matter tracts ensuring that signals reach their proper destinations.
13-4 Describe the major components of a spinal nerve, describe a nerve plexus, and relate the distribution pattern of spinal nerves to the regions they innervate.
1. Major Components of a Spinal Nerve:
Spinal nerves are mixed nerves that contain both sensory and motor fibers. Each spinal nerve is wrapped in three connective tissue layers:
Epineurium: The outermost layer; a tough sheath of collagen fibers that surrounds the entire nerve.
Perineurium: The middle layer; surrounds groups of axons (called fascicles).
Endoneurium: The innermost layer; surrounds individual axons and their Schwann cells.
Spinal nerves emerge from the spinal cord via the junction of anterior (motor) and posterior (sensory) roots, and then branch into:
Posterior ramus: Serves the muscles and skin of the back.
Anterior ramus: Serves the limbs and anterior/lateral body wall.
Rami communicantes: Connect spinal nerves to the sympathetic trunk.
2. Nerve Plexus:
A nerve plexus is a complex network of interwoven anterior rami from adjacent spinal nerves. These plexuses allow nerves from multiple spinal segments to innervate the same body part. The main plexuses are:
Cervical Plexus (C1–C5) – Innervates the neck, scalp, and diaphragm (includes the phrenic nerve for diaphragm control).
Brachial Plexus (C5–T1) – Innervates the pectorals and upper limbs (includes radial, median, ulnar, musculocutaneous, and axillary nerves).
Lumbar Plexus (T12–L4) – Innervates the lower abdomen, anterior thigh (includes femoral and obturator nerves).
Sacral Plexus (L4–S4) – Innervates the pelvis, buttocks, and lower limbs (includes sciatic and pudendal nerves).
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3. Distribution Pattern of Spinal Nerves:
Spinal nerves follow a segmental distribution and correspond to areas of the body called dermatomes, which are regions of skin innervated by specific spinal nerve pairs.
Dermatomes help map sensory loss or nerve damage.
Peripheral nerves can also suffer localized damage (called peripheral neuropathies) which affect dermatomes.
Example: In shingles, a viral infection affects the spinal nerve and creates a painful rash along the affected dermatome.
Chapter 14 Section I
14-1 Name the major brain regions, vesicles, and ventricles, and describe the locations and functions of each.
Major Brain Regions and Their Functions
Cerebrum
Location: Largest and most superior part of the brain.
Function: Conscious thought, memory storage and processing, voluntary muscle movement, and sensory interpretation.
Divisions: Frontal, parietal, temporal, occipital lobes, and insula.
Structures:
Cerebral Cortex: Outer gray matter layer involved in higher functions.
Gyri, Sulci, and Fissures: Increase surface area.
Corpus Callosum: Connects left and right hemispheres.
Basal Nuclei: Coordinate movement.
Limbic System: Emotions, memory, and motivation.
Cerebellum
Location: Posterior and inferior to cerebrum.
Function: Coordinates skeletal muscle contractions, balance, and posture.
Features: Two hemispheres, cerebellar cortex, and vermis (central band).
Diencephalon
Location: Beneath the cerebrum.
Function: Sensory relay, hormone production, and autonomic control.
Components:
Thalamus: Relays sensory input to cerebral cortex.
Hypothalamus: Controls hormones, homeostasis, emotion.
Epithalamus: Contains pineal gland (melatonin production).
Pituitary Gland: Major endocrine gland attached to hypothalamus.
CVOs: Circumventricular organs (monitor blood chemistry).
Midbrain (Mesencephalon)
Location: Between pons and diencephalon.
Function: Processes auditory/visual data; maintains consciousness; coordinates reflexive motor responses.
Structures:
Cerebral Peduncles
Corpora Quadrigemina
Substantia Nigra
Red Nuclei
Reticular Formation (consciousness, muscle tone).
Pons (Part of Metencephalon)
Location: Superior to medulla oblongata.
Function: Connects parts of brain, relays signals from cerebrum to cerebellum; helps regulate breathing.
Associated Cranial Nerves: Trigeminal, Abducens, Facial, Vestibular branch of Vestibulocochlear.
Medulla Oblongata (From Myelencephalon)
Location: Connects brain to spinal cord.
Function: Relays sensory info, controls autonomic functions (HR, BP, digestion).
Cranial Nerves: Vestibulocochlear and Hypoglossal.
Functional Centers: Respiratory, cardiovascular, vomiting, sneezing, coughing.
Developmental Brain Vesicles
Primary Vesicle | Secondary Vesicle | Adult Brain Structure | Associated Ventricles |
|---|---|---|---|
Prosencephalon | Telencephalon | Cerebrum | Lateral ventricles |
Diencephalon | Thalamus, Hypothalamus, Epithalamus | Third ventricle | |
Mesencephalon | Mesencephalon | Midbrain | Cerebral aqueduct |
Rhombencephalon | Metencephalon | Pons, Cerebellum | Part of fourth ventricle |
Myelencephalon | Medulla Oblongata | Part of fourth ventricle |
Brain Ventricles and CSF Flow
Lateral Ventricles (1st & 2nd): Found in the cerebral hemispheres.
Third Ventricle: In the diencephalon.
Cerebral Aqueduct: Connects third to fourth ventricle; runs through midbrain.
Fourth Ventricle: Located between pons and cerebellum; connects to central canal of spinal cord and subarachnoid space.
14-2 Explain how the brain is protected and supported, and discuss the formation, circulation,and function of cerebrospinal fluid.
Physical Protection of the Brain
Cranial Bones: The skull forms a hard, protective outer shell to shield the brain from external injury.
Cranial Meninges: Three layers of connective tissue that cushion and protect the brain:
Dura Mater: The outermost layer, tough and durable, with two sublayers:
Periosteal layer (attached to the skull)
Meningeal layer (facing the brain)
Dural folds (falx cerebri, tentorium cerebelli, falx cerebelli) help stabilize the brain.
Arachnoid Mater: Middle layer, contains the subarachnoid space filled with CSF.
Pia Mater: Innermost layer, adheres tightly to the brain surface and is held in place by astrocytes.
Biochemical Isolation
Blood–Brain Barrier (BBB):
Formed by tight junctions between endothelial cells of brain capillaries.
Astrocytes regulate what substances can pass into brain tissue.
Blocks harmful substances but allows O₂, CO₂, and lipid-soluble compounds.
Breaks in the BBB exist in:
Hypothalamus
Posterior pituitary
Pineal gland
Choroid plexus
Blood–CSF Barrier:
Formed by ependymal cells of the choroid plexus.
Controls exchange between blood and CSF, maintaining chemical balance.
Cerebrospinal Fluid (CSF)
Formation:
Produced by the choroid plexus in the ventricles using blood plasma.
Involves ependymal cells filtering blood and adjusting CSF composition.
~500 mL of CSF is produced daily.
Circulation:
CSF is secreted by the choroid plexus in the lateral, third, and fourth ventricles.
It flows through:
Lateral ventricles → third ventricle → fourth ventricle
Then through median and lateral apertures into the subarachnoid space
Circulates around the brain, spinal cord, and cauda equina
Arachnoid villi and granulations return CSF to the superior sagittal sinus for absorption into the bloodstream.
Functions:
Cushions the brain, protecting it from trauma.
Supports the brain’s weight by creating buoyancy.
Transports nutrients, chemical messengers, and waste products.
Maintains chemical stability in the CNS environment.
Blood Supply to the Brain
Delivered by the internal carotid and vertebral arteries.
Drained by internal jugular veins.
The brain uses 20% of the body’s oxygen supply despite being only 2% of body weight.
Interruptions can lead to serious consequences (e.g., stroke, confusion, death of neurons).
14-3 Describe the anatomical differences between the medulla oblongata and the spinal cord, and identify the main components and functions of the medulla oblongata.
Anatomical Differences Between the Medulla Oblongata and the Spinal Cord:
Location:
Medulla Oblongata: Located in the brainstem, just superior to the spinal cord and inferior to the pons.
Spinal Cord: Extends from the medulla oblongata down through the vertebral canal.
Structure:
Medulla Oblongata:
Contains more complex neural structures.
Has olives and pyramids, which are bulges formed by tracts of motor fibers.
Gray matter is broken into distinct nuclei for processing various autonomic and sensory functions.
Spinal Cord:
Has a more straightforward cylindrical shape.
Central gray matter (in an "H" shape) is surrounded by white matter.
Cranial Nerves:
Medulla Oblongata: Origin or termination site for cranial nerves VIII–XII.
Spinal Cord: Gives rise to spinal nerves, not cranial nerves.
Function:
Medulla Oblongata: Regulates vital autonomic functions (e.g., heart rate, blood pressure, respiration), and is a relay station for sensory/motor pathways.
Spinal Cord: Primarily transmits signals between the body and brain; also controls basic reflexes.
Main Components of the Medulla Oblongata:
Pyramids:
Contain descending corticospinal tracts (motor pathways).
Site of decussation (crossing over) of motor fibers, which is why each hemisphere of the brain controls the opposite side of the body.
Olives (Olive-shaped structures):
Contain the inferior olivary nuclei, which relay signals from the spinal cord to the cerebellum, assisting with motor learning and coordination.
Nuclei of Cranial Nerves VIII–XII:
Involved in sensory and motor control of the face, throat, and some visceral organs.
Autonomic Nuclei:
Cardiovascular centers: Regulate heart rate and blood vessel diameter.
Respiratory rhythmicity centers: Control the basic rhythm of breathing.
Reflex centers: Coordinate coughing, sneezing, swallowing, and vomiting.
Relay Stations:
Gracile and cuneate nuclei: Relay somatic sensory information to the thalamus.
Associated with touch, pressure, vibration, and proprioception.
Functions of the Medulla Oblongata:
Autonomic regulation: Controls vital body functions such as breathing, heart rate, and digestion.
Sensory and motor relay: Pathways between the brain and spinal cord pass through here, including those for voluntary muscle control.
Cranial nerve function: Facilitates sensation and movement in the head and neck.
Reflex control: Manages reflexes like sneezing, coughing, swallowing, and vomiting.
Chapter 16 Section I
16.1 Compare the organization of the autonomic nervous system with that of the somatic nervous system, and name the divisions and major functions of the A N S.
Somatic Nervous System (SNS):
Type of control: Voluntary
Effector targets: Skeletal muscles
Neural pathway:
A single motor neuron extends from the central nervous system (CNS) directly to the skeletal muscle.
Cell body located in the CNS (spinal cord or brainstem).
Function: Controls conscious movement, posture, and reflexes involving skeletal muscles.
Autonomic Nervous System (ANS):
Type of control: Involuntary
Effector targets: Visceral effectors—smooth muscle, cardiac muscle, glands, and adipose tissue
Neural pathway:
Uses two motor neurons in sequence:
Preganglionic neuron: Cell body in CNS; axon (preganglionic fiber) exits the CNS.
Postganglionic neuron: Cell body in an autonomic ganglion; axon (postganglionic fiber) extends to the effector.
Function: Regulates unconscious activities like heart rate, digestion, respiratory rate, and homeostasis.
Control center: Mainly regulated by the hypothalamus, which acts like the "command center" for autonomic output.
Divisions of the ANS and Their Major Functions:
1. Sympathetic Division ("Fight or Flight"):
Purpose: Prepares the body for emergency or high-stress situations.
Key Effects:
Increases alertness and energy availability
Raises heart rate and blood pressure
Dilates airways to improve oxygen intake
Mobilizes energy by stimulating glycogen and fat breakdown
Reduces non-essential functions (like digestion and urination)
2. Parasympathetic Division ("Rest and Digest"):
Purpose: Conserves energy and supports bodily functions during rest.
Key Effects:
Slows heart rate and reduces blood pressure
Stimulates digestion and nutrient absorption
Enhances glandular secretions (saliva, digestive enzymes)
Promotes waste elimination (urination and defecation)
Increases motility and blood flow in the gastrointestinal tract
Interaction Between Divisions:
Opposing Effects: Common in organs like the heart—sympathetic increases heart rate, parasympathetic decreases it.
Independent Control: Some organs (e.g., sweat glands) are controlled by only one division.
Cooperation: Divisions may act together during complex processes like sexual arousal and reproduction.
16.2 Describe the structures and functions of the sympathetic division of the autonomic nervous system.
Overview: Sympathetic Division (Thoracolumbar Division)
The sympathetic division is part of the autonomic nervous system responsible for the "fight-or-flight" response.
It is called the thoracolumbar division because its preganglionic neurons originate from spinal cord segments T1–L2.
These neurons have short preganglionic fibers and long postganglionic fibers.
Major Structures of the Sympathetic Division
Preganglionic Neurons
Located in the lateral horns of spinal cord segments T1–L2.
Axons leave via anterior roots and enter white rami communicantes (myelinated fibers).
Ganglia (Where Preganglionic Neurons Synapse)
Sympathetic Chain Ganglia: Found on either side of the vertebral column. Each ganglion controls effectors in the head, neck, thoracic cavity, and limbs. Preganglionic fibers may synapse here.
Includes 3 cervical, 10–12 thoracic, 4–5 lumbar, 4–5 sacral, and 1 coccygeal ganglion.
Collateral Ganglia: Located anterior to the vertebral column. These control organs in the abdominopelvic cavity. Main ganglia include:
Celiac Ganglion: Innervates the stomach, liver, gallbladder, pancreas, and spleen.
Superior Mesenteric Ganglion: Supplies the small intestine and most of the large intestine.
Inferior Mesenteric Ganglion: Supplies kidneys, bladder, lower large intestine, and reproductive organs.
Adrenal Medullae: Function as modified sympathetic ganglia inside each adrenal gland. Instead of releasing neurotransmitters at synapses, they secrete hormones (epinephrine and norepinephrine) into the bloodstream.
Sympathetic Fiber Pathways
Preganglionic fibers (short) synapse in one of the three ganglia.
Postganglionic fibers (long) extend to target organs.
The white rami communicantes carry preganglionic fibers into the ganglia.
The gray rami communicantes carry postganglionic fibers back to spinal nerves for distribution to the body.
Functions and Effects of the Sympathetic Division
Sympathetic activation occurs in crisis situations, triggering widespread body changes:
Increases alertness and energy levels
Elevates heart rate, blood pressure, and respiration
Dilates airways
Enhances muscle tone
Mobilizes energy reserves (e.g., glycogen breakdown, fat release)
The adrenal medulla’s hormone release results in a longer-lasting and more widespread effect than neural innervation alone.
16.3 Describe the types of neurotransmitters and receptors and explain their mechanisms of action.
1. Neurotransmitters in the Sympathetic Nervous System:
Acetylcholine (ACh)
Released by: Sympathetic preganglionic neurons
Acts on: Nicotinic receptors on ganglionic neurons
Effect: Always excitatory, causing the ganglionic neuron to fire
Norepinephrine (NE)
Released by: Most sympathetic postganglionic (ganglionic) neurons
Acts on: Adrenergic receptors (alpha and beta types) on target organs
Effect: Depends on the receptor type (can be excitatory or inhibitory)
Epinephrine (E)
Released by: Adrenal medulla (as a hormone into the bloodstream)
Acts on: Both alpha and beta receptors
Effect: Widespread and long-lasting sympathetic responses
Some Postganglionic Neurons Release ACh
These are called cholinergic neurons
Found in the body wall, skin, brain, and skeletal muscles
Their effects depend on the receptor types they interact with
2. Receptors and Their Mechanisms:
Adrenergic Receptors (Respond to NE and E)
These are G-protein-coupled receptors, meaning they activate intracellular pathways like cAMP to produce effects.
Alpha (α) Receptors
α1 Receptors
Found on: Smooth muscle cells
Effect: Excitatory (e.g., vasoconstriction)
Mechanism: Activates enzymes via G proteins to increase intracellular calcium
α2 Receptors
Found on: Preganglionic sympathetic neurons
Effect: Inhibitory (lowers cAMP)
Function: Helps regulate sympathetic activity (feedback inhibition)
Beta (β) Receptors
All beta receptors increase cAMP, but their effects vary by subtype:
β1 Receptors
Found in: Heart, kidneys
Effect: Increases heart rate and force, stimulates renin release
β2 Receptors
Found in: Lungs, blood vessels of skeletal muscles
Effect: Relaxes smooth muscle (e.g., bronchodilation)
β3 Receptors
Found in: Adipose tissue
Effect: Stimulates lipolysis (breakdown of fat)
16.5 Describe the mechanisms of parasympathetic neurotransmitter release and their effects on target organs and tissues.
1. Neurotransmitter Involved: Acetylcholine (ACh)
All parasympathetic neurons release acetylcholine (ACh).
The effects vary based on:
The type of cholinergic receptor (nicotinic or muscarinic).
The intracellular signaling pathways (second messengers) they activate.
Effects are localized and short-lived because:
ACh is rapidly broken down by acetylcholinesterase (AChE) at the synapse.
Any ACh that diffuses away is inactivated by tissue cholinesterase.
2. Cholinergic Receptors in the Parasympathetic Division
A. Nicotinic Receptors
Location:
On ganglionic neurons in both sympathetic and parasympathetic ganglia.
Also found at neuromuscular junctions in the somatic nervous system (SNS).
Effect of ACh Binding: Always excitatory
Causes depolarization and firing of the neuron or muscle fiber.
Mechanism: Ligand-gated ion channels—open to allow ion flow.
B. Muscarinic Receptors
Location:
At neuroeffector junctions in parasympathetic division (organs, glands, smooth muscle).
Also found in some sympathetic targets (e.g., sweat glands).
Effect of ACh Binding: Can be excitatory or inhibitory
Depends on the type of G protein and enzymes activated.
Mechanism: G protein-coupled receptors (slower onset, longer duration)
3. Toxins Affecting Parasympathetic Receptors
Nicotine
Binds to: Nicotinic receptors
Targets: Autonomic ganglia and skeletal muscle junctions
Effect: Can cause overstimulation leading to coma or death if in high doses
Muscarine
Produced by: Certain poisonous mushrooms
Binds to: Muscarinic receptors
Targets: Parasympathetic neuroeffector sites
Effect: Can dangerously enhance parasympathetic activity (e.g., slowed heart rate, constricted pupils, increased gland secretion)
Summary of Parasympathetic Effects:
Slows heart rate
Stimulates digestion and glandular secretions
Promotes energy storage
Constricts pupils
Contracts urinary bladder and promotes urination
16.6 Compare and contrast the sympathetic and parasympathetic nervous systems.
🔷 Sympathetic Nervous System
General Function: “Fight or flight” – prepares the body for stressful or emergency situations.
Structure:
Two sympathetic chain ganglia, three collateral ganglia, and two adrenal medullae
Short preganglionic fibers, long postganglionic fibers
Extensive divergence (1 preganglionic fiber can synapse with many postganglionic neurons)
Neurotransmitters:
Preganglionic: Acetylcholine (ACh)
Postganglionic: Primarily norepinephrine (NE) (adrenergic)
Effects:
Widespread and long-lasting
Varies based on type of receptor and second messenger system involved
🔷 Parasympathetic Nervous System
General Function: “Rest and digest” – conserves energy and promotes housekeeping activities during rest.
Structure:
Originates from cranial nerves III, VII, IX, and X and sacral spinal segments (S2–S4)
Ganglia located in or near target organs
Long preganglionic fibers, short postganglionic fibers
Minimal divergence (more targeted response)
Neurotransmitters:
All neurons release ACh (cholinergic)
Effects:
Localized and short-lived
Controlled primarily by AChE and tissue cholinesterase breaking down ACh quickly
✅ Key Differences:
Feature | Sympathetic | Parasympathetic |
|---|---|---|
Origin | Thoracolumbar (T1–L2) | Craniosacral (III, VII, IX, X, S2–S4) |
Fiber Length | Short preganglionic, long post | Long preganglionic, short post |
Divergence | High (widespread effects) | Low (localized effects) |
Neurotransmitter (Post) | Mostly norepinephrine | Acetylcholine |
Ganglia Location | Near spinal cord | In or near target organs |
Overall Effect | Widespread, long-lasting | Targeted, brief |
16.9 Explain how memories are created, stored, and recalled; distinguish among the levels of consciousness and unconsciousness; and describe how neurotransmitters influence brain function.
Memory Formation:
Fact Memories: Specific bits of information (e.g., names, dates).
Skill Memories: Learned motor behaviors that become automatic with repetition (e.g., riding a bike).
Brain Regions Involved:
Amygdaloid body and hippocampus: Essential for memory consolidation.
Nucleus basalis: Plays a role in memory storage and retrieval, linking with the hippocampus and cerebral cortex.
Cerebral cortex: Stores most long-term memories, and different areas are responsible for sensory and motor memories.
Short-Term vs. Long-Term Memory:
Short-Term Memory (STM):
Holds small bits of information briefly.
Can be converted to long-term memory through memory consolidation (repetition helps this process).
Long-Term Memory (LTM):
Two types:
Secondary memories: Fade over time and require effort to recall.
Tertiary memories: Do not fade and are typically stable.
Memory Consolidation and Access:
Memory engram: A neural circuit that forms when an experience is repeated. This takes at least an hour to form and is influenced by factors like stimulus nature, intensity, and frequency.
Synaptic Changes:
Increased neurotransmitter release: Frequently active synapses release more neurotransmitters.
Facilitation at synapses: Repeated activation causes depolarization, making future activation easier.
Formation of additional synaptic connections: Repeated neural communication strengthens connections, leading to memory storage.
Consciousness and Unconsciousness
States of Consciousness:
Varying degrees of consciousness are controlled by CNS activity. Wakefulness indicates the ongoing activity level.
Deep Sleep (NREM): Body and brain activity decrease, and restorative functions are enhanced.
REM Sleep: Active dreaming occurs, and the brain's EEG resembles the awake state.
Arousal:
Reticular formation plays a key role in waking up from sleep.
Reticular Activating System (RAS): Involved in promoting wakefulness by activating the cerebral cortex.
Sleep–Wake Cycle Regulation: RAS is regulated by two groups of brainstem nuclei: one promoting wakefulness (via norepinephrine), and the other promoting deep sleep (via serotonin).
Neurotransmitter Influence on Brain Function
Changes in neurotransmitter levels can significantly affect brain functions and behaviors:
Serotonin: Regulates sensory interpretation and emotional states. Changes in serotonin levels are linked to conditions like depression and anxiety.
Huntington’s Disease: Destruction of ACh-secreting and GABA-secreting neurons causes degeneration in basal nuclei and frontal lobes, leading to difficulty controlling movements and intellectual decline.
Drugs like caffeine or nicotine can enhance memory consolidation by stimulating the CNS.
16.10 Summarize the effects of aging on the nervous system and give examples of interactions between the nervous system and other organ systems.
Aging affects the nervous system through gradual anatomical and physiological changes that start around age 30 and accumulate over time. By age 65, about 85% of individuals experience some degree of change in mental performance and central nervous system (CNS) function.
Anatomical Changes:
Brain size and weight decrease, especially in the cerebral cortex, resulting in narrower gyri, wider sulci, and larger subarachnoid spaces.
Neuronal loss occurs, primarily in cortical areas, though brainstem nuclei are generally preserved.
Reduced blood flow to the brain due to arteriosclerosis (fatty deposits in arteries), which increases the risk of strokes (cerebrovascular accidents).
Synaptic changes include the loss of connections and reduced neurotransmitter production.
Cellular debris accumulates: neurons gather lipofuscin, and form neurofibrillary tangles and amyloid plaques, especially in memory-related brain regions. These are associated with neurodegenerative conditions like Alzheimer's disease.
Functional Changes:
Memory declines, with difficulties in memory consolidation and retrieving secondary memories.
Sensory impairments occur, such as reduced hearing, vision, balance, smell, and taste.
Slower reaction times, weakened reflexes, and reduced motor control precision are common.
Interactions with Other Organ Systems:
Nervous system changes impact musculoskeletal coordination, leading to decreased balance and increased fall risk.
The cardiovascular system is affected via reduced cerebral blood flow, increasing stroke risk.
The endocrine system is influenced as neurological control of hormone release may weaken.
Aging in the digestive and urinary systems may be exacerbated by diminished autonomic nervous regulation.
Although most elderly individuals maintain adequate nervous system function, some may experience senile dementia or Alzheimer’s disease, characterized by memory loss, amnesia, and emotional instability.