A&P Exam 3

Chapter 12

1. Classes of Neurons

• Unipolar Neurons: Have one process that serves as both axon and dendrite. In humans, these are often referred to as “pseudo-unipolar” cells. They are primarily sensory neurons with cell bodies located in ganglia and are responsible for receiving sensory information.

• Bipolar Neurons: Possess two processes, one axon and one dendrite, extending from opposite ends of the cell body. These neurons are relatively rare and are mostly found in the olfactory epithelium and the retina.

• Multipolar Neurons: Include all neurons that are not unipolar or bipolar, characterized by having one axon and multiple dendrites. They are the most common type of neuron and include various subtypes, such as Purkinje cells in the cerebellum and pyramidal cells with pyramid-shaped cell bodies.

2. Astrocytes

Astrocytes are star-shaped glial cells in the central nervous system (CNS) that primarily provide support to neurons. They have processes extending from their main cell body, which interact with neurons, blood vessels, and the surrounding connective tissue (pia mater). Their supporting roles include maintaining chemical concentrations in the extracellular space, removing excess signaling molecules, responding to tissue damage, and contributing to the blood-brain barrier. The BBB is crucial in regulating what substances can enter the CNS, allowing specific nutrients like glucose but restricting many other molecules, which complicates drug delivery to the brain. Astrocytes help ensure that neuronal functions are facilitated by maintaining homeostasis and protecting the brain tissue from potential harm.

3. Satellite and Schwan Cells

types of glial cells found in the PNS

Satellite cells: found in sensory and autonomic ganglia, surrounding neuron cell bodies and providing support. They create a protective layer that maintains the ganglia's environment and manage nutrient and waste exchange between neurons and their extracellular space. By regulating ion balance and controlling neurotransmitter levels, they influence neuronal excitability and signaling efficiency. In response to damage or inflammation, satellite cells can activate to support neuron survival and regeneration, which is crucial in conditions like nerve injury or neuropathic pain. Additionally, they contribute to immune responses within the PNS, helping to minimize inflammatory

Schwann cells: insulate axons with myelin. Schwann cells only encase a portion of a single axon segment. The nucleus and cytoplasm of a Schwann cell are found at the edge of the myelin sheath, ensuring proper insulation of the axon. crucial for the efficient transmission of electrical signals along the axon.

4. Gated Channels

1. Ligand-Gated Channels: These channels open when a signaling molecule, or ligand, binds to the extracellular region of the channel. This binding allows specific ions, such as sodium, calcium, and potassium, to cross the membrane, altering the membrane's charge. This type of channel is often associated with neurotransmitter activity in the nervous system.

2. Mechanically Gated Channels: respond to physical distortion of the cell membrane, such as pressure, touch, or temperature. They are involved in somatosensation, allowing ions to enter the cell when mechanical changes occur in the surrounding tissue.

3. Voltage-Gated Channels: open in response to changes in the electrical properties (voltage) of the membrane. Typically, when the inner membrane becomes less negatively charged, these channels allow ions to cross, contributing to the generation of action potentials.

4. Leakage Channels: open and close at random, contributing to the resting transmembrane voltage of cells. They allow ions to diffuse across the membrane intermittently and play a role in maintaining ion concentrations across the membrane without an activation event.

5. Action Potentials

Action potentials are crucial electrical signals that allow communication within the nervous system. They occur when the membrane potential of a neuron changes significantly enough to trigger a rapid depolarization phase. This begins when specific sodium channels open in response to a stimulus, allowing sodium ions to flow into the cell. As sodium enters, the interior of the cell becomes less negative, moving toward a positive charge.

Following the depolarization, potassium channels open, allowing potassium ions to exit the cell, resulting in repolarization. This phase brings the membrane potential back down. However, potassium may exit the cell past the resting state, leading to a brief phase called hyperpolarization, where the interior of the cell becomes even more negative than its typical resting state.

Importantly, action potentials are all-or-nothing events; they either occur fully if the threshold is reached or not at all. Once initiated, action potentials are uniform in their magnitude, meaning no larger signal is produced regardless of the strength of the stimulus. This is why stronger stimuli result in more frequent action potentials rather than larger individual ones.

6. Refractory Periods

The refractory period refers to the phase during and immediately after an action potential when it is difficult or impossible to initiate a new action potential. It is divided into two phases: the absolute refractory period and the relative refractory period.

• Absolute Refractory Period: During this phase, it is impossible to trigger another action potential, regardless of the strength of the stimulus. This is due to the inactivation gate of the voltage-gated Na+ channels being closed after they have opened during depolarization. Once the membrane potential drops below -55 mV, the Na+ channels can only return to a resting conformation, allowing for potentially new action potentials, but these would require a stronger-than-normal stimulus to overcome the effects of hyperpolarization.

• Relative Refractory Period: During this phase, a new action potential can be initiated, but only with a stronger stimulus than usual. This is because the membrane is still repolarizing, and potassium ions (K+) are flowing out, making the inside of the cell more negative than the resting potential. Thus, any incoming sodium (Na+) will face increased difficulty in depolarizing the membrane back to threshold.

7. Types of Summation

Summation: the process that different types of graded potentials in a neuron combine to influence whether or not an action potential will be generated. Graded potentials are local changes in membrane potential that can either be depolarizing or hyperpolarizing, and are influenced by the magnitude of the stimulus.

Types of Graded Potentials:

• Generator Potentials: Develop in the dendrites of sensory neurons and influence the generation of action potentials in the axon of the same cell.

• Receptor Potentials: Occur in sensory receptor cells (e.g., taste cells, photoreceptors) and cause neurotransmitter release at synapses with sensory neurons.

• Postsynaptic Potentials (PSPs): Result from synaptic inputs to a dendrite and can be either excitatory (EPSP) or inhibitory (IPSP).

  • EPSPs move the membrane potential towards threshold.

  • IPSPs move the membrane potential away from threshold.

The summation of these potentials can lead a neuron to reach its threshold potential. If the cumulative change in membrane voltage is positive, an action potential may be triggered.

Summation can be:

• Spatial Summation: Occurs when multiple graded potentials from different locations combine to reach threshold.

• Temporal Summation: Happens when multiple potential changes occur in quick succession at the same location, leading to a significant change in the membrane potential.

In neurons, summation generally occurs at the initial segment of the axon or the axon hillock, where there is a high density of voltage-gated Na+ channels, ready to initiate the action potential. Both spatial and temporal summation can work together to determine the neuron's firing.

Chapter 13

8. Cerebrospinal Fluid

Cerebrospinal fluid enters the blood circulation at the ___________

Cerebrospinal fluid primarily enters the blood circulation through arachnoid granulations in the dural venous sinuses, particularly the superior sagittal sinus. These structures are protrusions of the arachnoid matter that allow one-way flow of CSF from the subarachnoid space into the venous blood. In addition, CSF can also enter the lymphatic system through routes like the nasal cribriform plate and spinal nerve root sheaths. 

Formation of CSF
CSF is produced primarily in the ventricles of the brain by specialized structures called choroid plexuses. The choroid plexuses consist of ependymal cells that surround blood capillaries. These cells filter blood to create CSF, which is a clear solution composed mainly of water, small molecules, and electrolytes.

Circulation of CSF
Once formed, CSF flows from the lateral ventricles into the third ventricle through the interventricular foramina. From the third ventricle, CSF travels through the cerebral aqueduct into the fourth ventricle. The fourth ventricle is positioned between the cerebellum and the pons/upper medulla and narrows into the central canal of the spinal cord. Importantly, CSF also exits the fourth ventricle into the subarachnoid space via median and lateral apertures, allowing it to flow around the entirety of the CNS.

Functions of CSF
CSF serves multiple crucial functions:

1. Cushioning: It acts as a liquid cushion that protects the brain and spinal cord from physical trauma by absorbing shock.

2. Chemical Stability: CSF helps maintain the chemical environment of the CNS, facilitating nutrient transportation and waste removal. It picks up metabolic wastes from the nervous tissue and allows for their transport out of the CNS.

3. Buoyancy: By surrounding the brain and spinal cord, CSF reduces their effective weight, preventing excessive pressure on the underlying structures.

Reabsorption of CSF
Cerebrospinal fluid (CSF) is produced in the choroid plexuses of the lateral, third, and fourth ventricles. It flows through the interventricular foramina to the third ventricle, then through the cerebral aqueduct to the fourth ventricle, and finally into the subarachnoid space. CSF is reabsorbed into the bloodstream via arachnoid granulations in the superior sagittal sinus, maintaining volume and pressure. This process is crucial for preventing conditions like hydrocephalus, which results from an imbalance in CSF production and absorption leading to increased intracranial pressure. CSF provides cushioning, buoyancy, and chemical stability by transporting nutrients and removing waste, but does not provide ATP for impulse transmission, as ATP is generated within cells.

9. Basal Nuclei

A group of nuclei in the cerebrum for cognitive processing and the planning of movements. They help moderate activity in the nervous system by influencing movement likelihood based on overall activity levels. The major components of the basal nuclei include:

• Caudate Nucleus: Regulates voluntary movement, learning, and memory; involved in the coordination of motor planning.

• Putamen: Works with the caudate in motor control; key for modulating movements and motor learning.

• Globus Pallidus (Internal & External): Modulates and relays signals from the striatum to motor areas; the internal segment provides inhibitory output to control movement execution.

• Subthalamic Nucleus: Helps regulate basal ganglia output, inhibiting competing motor commands to fine-tune movement.

• Substantia Nigra: Particularly the pars compacta produces dopamine essential for smooth and coordinated movement; its loss is associated with Parkinson’s disease.

These nuclei work together to control and refine movement, learning, and other motor behaviors.

10. Horns of Spinal Cord

In cross-section, the gray matter of the spinal cord resembling an H-shape. subdivided into regions called horns:

• Posterior Horn: Responsible for sensory processing.

• Anterior Horn: Sends out motor signals to skeletal muscles. containing multipolar motor neurons, sometimes referred to as the ventral horn.

• Lateral Horn: Found only in the thoracic, upper lumbar, and sacral regions, it contains motor neurons of the autonomic nervous system region of the spinal cord in the thoracic, upper lumbar, and sacral regions that is the central component of the sympathetic division of the autonomic nervous system.

  Here's an easy way to remember the spinal horns:

  • Dorsal Horn: Think of "Dorsal = Data." The dorsal horn is where sensory (data) information enters the spinal cord.

  • Ventral Horn: Think of "Ventral = Volition" or "Ventral = Motor." The ventral horn contains motor neurons that send commands to muscles for movement.

  • Lateral Horn: Think of "Lateral = Lives" (or "Lateral = Autonomic"). This horn, found in the thoracic and upper lumbar regions, houses neurons that control the autonomic (involuntary) functions like the fight-or-flight response.

  Using this mnemonic: Data (dorsal) → Volition (ventral) → Lives (lateral) helps reinforce the main roles of each horn in the spinal cord.

• The gray horns of the spinal cord mainly contain neuron cell bodies, which are crucial for processing motor and sensory information. Each horn has a distinct function, with the anterior horn involved in motor signaling and the posterior horn in sensory processing.

11. Connective Tissue of Nerves

Nerves in the peripheral nervous system consist of bundles of axons integrated with connective tissues and blood vessels for nourishment. The primary layers of connective tissue in nerves include:

• Epineurium: The outermost layer of fibrous connective tissue that surrounds the nerve.

• Perineurium: This layer encases individual fascicles within the nerve, providing additional protection and organization.

• Endoneurium: The innermost layer surrounding each individual axon with loose connective tissue.

12. Posterior and Anterior Rami

Example Question: Which of the following carry sensory input from interoceptors of the body walls and limbs to the spinal nerve? Answer: Anterior

Example Question: Which ramus innervates the skin and muscles of the back? Answer: the posterior ramus.

Anterior ramus: is larger, motor and sensory fibers to the anterolateral body wall and the limbs. brings sensory in and motor out

posterior ramus: smaller. Exclusively innervates the intrinsic muscles and the skin of the back

“Anterior = Front/Arms/limbs” and “Posterior = Back.”

13. Nerve Plexuses

networks of nerve fibers that arise from spinal nerves and provide pathways for sensory and motor functions in the peripheral nervous system. Four main nerve plexuses in the human body:

1. Cervical Plexus: branches into nerves supplying the posterior neck and head, and includes the phrenic nerve that innervates the diaphragm.

2. Brachial Plexus: Formed from C4 through T1, starts the nerves of the arms, including the radial nerve, which is associated with the axillary, ulnar, and median nerves.

3. Lumbar Plexus: arises from all lumbar spinal nerves and generates nerves that innervate the pelvic region and the anterior leg. A notable nerve from this plexus is the femoral nerve, which branches into the saphenous nerve serving the anterior lower leg.

4. Sacral Plexus: Originating from lower lumbar nerves L4 and L5, along with sacral nerves S1 to S4, the most significant is the sciatic nerve, which is a combination of the tibial and fibular nerves and is commonly associated with sciatica due to nerve compression or irritation.

14. Phrenic Nerve

phrenic nerve: systemic nerve from the cervical plexus that innervates the diaphragm

Which nerve originates in the cervical plexus and innervates the diaphragm?

Which nerve, if damaged, would interfere with the ability to breathe?

The phrenic nerve originates from the cervical plexus, is essential for breathing because it provides the sole motor innervation to the diaphragm. This nerve runs from the neck, over the anterior scalene muscle, and descends into the thoracic cavity, where it serves not only to trigger contraction of the diaphragm during inspiration but also carries sensory fibers from parts of the pericardium and pleura. Damage to the phrenic nerve can result in diaphragmatic paralysis, significantly impairing respiratory function and making it critical for survival. Remember, when asked which nerve originates in the cervical plexus and innervates the diaphragm—or which nerve, if damaged, would interfere with breathing—the correct answer is the phrenic nerve.

Chapter 14

15. Types of Receptors

Which of the following receptors monitor the position of skeletal muscles and joints?

• The receptors that monitor the position of skeletal muscles and joints are called proprioceptors.

1. General Sensory Receptors

• Somatic Sensory Receptors: Cover sensations from the body’s surface and deep tissues.

  • Mechanoreceptors: Detect mechanical forces (pressure, stretch, vibration).

  • Thermoreceptors: temperature changes (warmth and cold).

  • Nociceptors: Detect damaging stimuli (pain).

  • Proprioceptors: Monitor body position, movement, and muscle tension

  • Baroreceptors: monitor changes in blood pressure.

• Interoceptors: Monitor internal conditions (e.g., organ function, blood chemistry, blood pressure).

• The receptors that monitor the digestive, respiratory, cardiovascular, urinary, and reproductive systems are interoceptors. 

  Explanation:

  • Interoceptors:

    These receptors detect stimuli within the body, including organs like the heart, lungs, stomach, kidneys, and reproductive organs

2. Special Sensory Receptors

• Photoreceptors: Located in the retina to convert light into neural signals (vision).

• (Also include receptors for hearing, taste, smell, and balance, which are grouped as special senses.)

16. Photoreception

What changes do visual pigments undergo during photoreception?

Answer: Activation, bleaching, and reassembly.

Activation: When light hits a visual pigment molecule like rhodopsin, it undergoes a conformational change. This activated form then triggers a cascade of events leading to the transmission of visual information to the brain.

Bleaching: once activated, the pigment is "bleached," meaning it loses its ability to absorb light.

Reassmbly: regeneration process involves the reassembly of the pigment with the 11-cis-retinal molecule.

specialized cells located in the retina that play a critical role in vision by changing their membrane potential in response to light stimuli. two main types of photoreceptors:

• Rods: dark vision. highly sensitive to light, allowing for vision in low-light conditions. A notable feature of rods is that they can trigger an action potential from a single photon, making them essential for night vision.

• Cones: light vision. responsible for color vision and function best in brighter light conditions. The activation of different cones allows the brain to perceive a variety of colors based on the relative activity of these cones.

The human lens focuses light on the photoreceptor cells by changing shape. 

Explanation: The lens of the eye is flexible and can adjust its shape to focus light on the retina, depending on the distance of the object being viewed. This process is called accommodation. When focusing on near objects, the lens becomes thicker, and when focusing on distant objects, it becomes thinner

17. Auditory Ossicles

Which of the following is true about ossicles?

The correct answer is: They transmit movement of the tympanic membrane to the inner ear.

Explanation: The ossicles (malleus, incus, and stapes) are three small bones in the middle ear that function to transmit vibrations from the eardrum (tympanic membrane) to the oval window, which leads to the inner ear.

The auditory ossicles—malleus, incus, and stapes—are three small bones in the middle ear that transmit sound vibrations from the tympanic membrane to the inner ear. The malleus attaches to the tympanic membrane and connects to the incus, which then articulates with the stapes. The stapes connects to the oval window of the cochlea, where sound waves are converted into neural signals, amplifying vibrations along the chain of ossicles. While the ossicles move as a unit, their movements are complex, primarily involving the stapes moving back and forth to transmit vibrations. Hair cells in the cochlea detect sound vibrations and generate electrical signals. The ossicles do not produce receptor potentials; they simply stimulate the hair cells. The vestibular system regulates balance in the inner ear, independently of the ossicles. The middle ear connects to the pharynx via the Eustachian tube, helping equalize air pressure across the tym

The correct answer is The tympanic membrane to the oval window. 

Explanation:

• The auditory ossicles (malleus, incus, and stapes) form a chain that connects the tympanic membrane to the oval window of the inner ear. 

• The tympanic membrane (eardrum) is the outer part of the middle ear and vibrates in response to sound waves. 

• The oval window is a membrane-covered opening that connects the middle ear to the inner ear. 

• The ossicles transmit these vibrations from the tympanic membrane to the oval window. 

18. Tectorial and Basilar Membranes

What do the hair cells of the spiral organ press against when the basilar membrane moves?

When the basilar membrane moves, the hair cells of the spiral organ press against the tectorial membrane.

• Basilar Membrane: A flexible, ribbon-like structure in the cochlea of the inner ear that vibrates when sound waves hit it. Different parts of the basilar membrane vibrate for different sound frequencies, which helps you distinguish between low and high pitches.

• Tectorial Membrane: A gel-like layer that sits above the hair cells in the cochlea. When the basilar membrane moves in response to sound, the tectorial membrane moves relative to the hair cells, bending them and initiating the conversion of sound into electrical signals for the brain.

The structure that overlies the organ of Corti is the tectorial membrane. 

Explanation: The organ of Corti is a sensory organ within the cochlea, and the tectorial membrane is a gel-like structure that sits directly above it, essentially acting as a roof over the organ of Corti. 

19. Olfactory receptors

Olfactory receptors send axons through the cribriform plate and synapse on neurons in the?

Correct answer: Olfactory bulb

Olfactory receptor neurons are located in the olfactory epithelium within the superior nasal cavity, consisting of bipolar sensory neurons. Each neuron has dendrites that extend into the mucus lining. Odorant molecules, inhaled through the nose, dissolve in this mucus and bind to proteins that transport them to the olfactory dendrites. These odorant-protein complexes bind to G protein-coupled receptor proteins in the dendrite membrane, generating a graded membrane potential in the neurons. The axon of each olfactory neuron extends through an olfactory foramen in the cribriform plate of the ethmoid bone, connecting to the olfactory bulb in the brain. These axons then project to the primary olfactory cortex and other brain structures, linking specific smells to long-term memories and emotional responses. Additionally, olfactory neurons are subject to regular replacement to maintain function in response to potential damage from airborne toxins.

20. Flexor Reflex

Which reflex complements the flexor reflex by activating contralateral muscles?

Answer: The reflex that complements the flexor reflex by activating contralateral muscles is the crossed extensor reflex.

Explanation: When the flexor reflex is triggered on one leg (like withdrawing from a hot stove), the crossed extensor reflex simultaneously activates the extensor muscles on the opposite leg to maintain balance and stability.

The flexor reflex involves the rapid contraction of flexor muscles to withdraw a limb from harmful stimuli, such as touching something hot. Nociceptors detect the painful stimulus and send sensory impulses to the spinal cord, where they are processed through a polysynaptic pathway involving interneurons. These interneurons activate motor neurons in the ventral horn, causing the flexor muscles to contract and pull the limb away while inhibiting antagonistic extensor muscles to prevent resistance. This reflex is often accompanied by the crossed extensor reflex, where the extensor muscles of the opposite leg contract to maintain balance. Thus, the flexor reflex, a withdrawal reflex, plays a critical role in protecting the body

21. Corticospinal Tracts

Which spinal tract consists of a small bundle of descending fibers and connects the cerebral cortex to the spinal cord?

Answer: The spinal tract that consists of a small bundle of descending fibers and connects the cerebral cortex to the spinal cord is the Anterior corticospinal tract.

The corticospinal tracts are significant descending pathways connecting the cerebral cortex to the spinal cord, essential for controlling voluntary movements of skeletal muscles. There are two main types:

• Lateral Corticospinal Tract: The larger portion crosses (decussates) at the medulla, controlling fine motor movements in the distal limbs. It is critical for skilled voluntary movements, especially in the arms and legs. the descending spinal tract that crosses to the opposite side of the body within the pyramids of the medulla oblongata

• Anterior Corticospinal Tract: A smaller tract that does not decussate in the medulla; it crosses at the spinal cord level and primarily controls axial muscles, coordinating posture and trunk movements bilaterally.
  Neurons in the primary motor cortex, known as Betz cells, send axons down these tracts. Disorders affecting these pathways can lead to motor impairments, highlighting their role in voluntary movement

Chapter 15

22. Ganglionic Neurons

• Role in the ANS: They are the relay neurons in autonomic ganglia, transmitting signals from the central nervous system (preganglionic fibers) to the target organs via postganglionic fibers.

• Sympathetic Division (Thoracolumbar System):

  • Location: Approximately 23 ganglia along the vertebral column (3 cervical, 12 thoracic, 4 lumbar, 4 sacral).

  • Fiber Characteristics:

    • Preganglionic fibers are short.

    • Postganglionic fibers are long, unmyelinated.

  • Function: Activate responses like increased heart rate, vasoconstriction, and blood flow redirection during stress.

• Parasympathetic Division (Craniosacral System):

  • Location: Preganglionic neurons reside in the brainstem and sacral spinal cord.

  • Fiber Characteristics:

    • Preganglionic fibers are long.

    • Postganglionic fibers are short because ganglia are near or within target organs.

  • Function: Promote "rest and digest" activities, such as lowering heart rate and enhancing digestion.

• Synaptic Transmission: At the ganglia, both divisions use acetylcholine and nicotinic receptors for preganglionic to postganglionic signaling. After this, postganglionic neurons release either acetylcholine (parasympathetic) or norepinephrine (sympathetic) to activate target organs.

This network of ganglionic neurons is essential for controlling involuntary functions and maintaining homeostasis throughout the body.

Ganglionic neurons do NOT innervate skeletal muscle. 

Explanation:

Ganglionic neurons are part of the autonomic nervous system, which controls involuntary functions like smooth muscle contraction, gland secretion, and blood pressure regulation. However, skeletal muscle is under the control of the somatic nervous system, which is responsible for voluntary movements. Therefore, ganglionic neurons do not directly innervate skeletal muscle. 

23. Ganglionic Sympathetic Neurons

Specialized ganglionic sympathetic neurons that release hormones into the bloodstream are found within the

Answer: Specialized ganglionic sympathetic neurons that release hormones into the bloodstream are found within the adrenal medulla.

Explanation: The adrenal medulla is a part of the adrenal gland that functions like a modified sympathetic ganglion, releasing hormones like epinephrine and norepinephrine directly into the bloodstream when stimulated by the sympathetic nervous system.

Ganglionic sympathetic neurons, also known as sympathetic postganglionic neurons, are second-order neurons of the sympathetic division of the autonomic nervous system. Their cell bodies reside in sympathetic ganglia, including paravertebral and prevertebral ganglia, and they receive cholinergic input from preganglionic sympathetic neurons originating in the lateral horn of the thoracolumbar spinal cord . At the ganglion synapse, preganglionic fibers release acetylcholine that binds to nicotinic receptors on ganglionic neurons, generating action potentials. The axons of these neurons extend to peripheral target tissues, often releasing norepinephrine to mediate “fight or flight” responses like increased heart rate and vasoconstriction, while those innervating sweat glands continue to use acetylcholine as their neurotransmitter. One preganglionic neuron can influence multiple ganglionic neurons, ensuring coordinated sympathetic output for rapid stress responses. Understanding this setup is crucial for clinical conditions such as hypertension and autonomic dysregulation. Postganglionic fibers innervating the body wall or thoracic cavity originate from neurons in the sympathetic chain ganglia, situated alongside the spinal cord. The preganglionic neuron synapses onto the postganglionic neuron, which then innervates target tissues in the body wall or thoracic cavity.

24. Parasympathetic Nerves

Preganglionic fibers of parasympathetic pelvic nerves synapse on neurons located within What ganglia.

Answer: Preganglionic fibers of parasympathetic pelvic nerves synapse on neurons located within intramural ganglia.

Explanation: Intramural ganglia: are located within the walls of the organs they innervate. This means that the preganglionic fibers from the pelvic nerves travel to the target organs and synapse on postganglionic neurons that are embedded within the organ tissue.

The parasympathetic system, also referred to as the craniosacral system, has its preganglionic neurons located in the brain stem nuclei and the lateral horn of the sacral spinal cord. Preganglionic fibers from the cranial region travel in cranial nerves, while those from the sacral region move in spinal nerves, targeting terminal ganglia located near or within the target organs. Often called intramural ganglia, these ganglia allow postganglionic fibers to project only a short distance to the specific target tissue within the organ. In the parasympathetic system, preganglionic fibers are long, and postganglionic fibers are short due to their proximity to the target effectors.

The cranial components include important nuclei, such as the Edinger–Westphal nucleus, part of the oculomotor complex, that control activities like pupil size through the ciliary ganglion. In terms of secretion, the salivatory nuclei influence salivary glands through the facial and glossopharyngeal nerves, and tear production is regulated by parasympathetic fibers in the facial nerve. The dorsal nucleus of the vagus nerve regulates functions in the thoracic and abdominal cavities, primarily affecting the heart, bronchi, and esophagus, as well as digestive organs including the stomach and gall bladder. The postganglionic fibers activated by the vagus nerve are sometimes integrated into the structures of the organs they innervate, such as the mesenteric plexus of the digestive tract.

25. Postganglionic Fibers

Example Question: "Which of the following is NOT true about the parasympathetic division of the autonomic nervous system?"

• Preganglionic neurons are located in the brainstem and sacral region of the spinal cord.

• Ganglionic neurons are located in ganglia within or near effectors.

• The actions of the parasympathetic division are more localized than those of the sympathetic division.

Answer: The statement that is NOT true is: “Preganglionic fibers are relatively short, and postganglionic fibers are relatively long.”

Explanation: In the parasympathetic division, preganglionic neurons (found in the brainstem and sacral spinal cord) have long fibers that extend to ganglia located near or within target organs. This setup means that the postganglionic fibers are short. This arrangement supports highly localized control of functions such as digestion and heart rate. In contrast, the sympathetic division has short preganglionic fibers and long, unmyelinated postganglionic fibers that travel further to reach targets. Additionally, parasympathetic postganglionic fibers release acetylcholine (affecting muscarinic receptors), while sympathetic postganglionic fibers typically release norepinephrine (except in certain cases like sweat glands).

he effect NOT associated with the action of postganglionic sympathetic fibers is decreased heart rate. 

Explanation:

• Postganglionic sympathetic fibers:

  These nerve fibers release norepinephrine, which generally triggers the "fight or flight" response, leading to increased heart rate, dilation of pupils, and reduced blood flow to the skin to prioritize blood delivery to muscles during stressful situations. 

Features of Postganglionic Fibers:

• Location:

  • Sympathetic: Reside in chain or collateral ganglia, then extend long distances to target organs.

  • Parasympathetic: Located in terminal ganglia that are near or within the target organ.

• Fiber Length:

  • Sympathetic: Generally long, to reach distant targets.

  • Parasympathetic: Typically short, due to proximity of ganglia to effectors.

• Myelination:

  • Most postganglionic fibers are unmyelinated, which slows signal conduction compared to preganglionic fibers.

• Neurotransmitters Released:

  • Sympathetic: Predominantly norepinephrine (with exceptions, such as those innervating sweat glands which use acetylcholine).

  • Parasympathetic: Acetylcholine, acting on muscarinic receptors.

• Functional Role:

  • Relay signals from ganglionic neurons to effector organs, modulating organ function in a coordinated manner.

• Synaptic Structure:

  • Their endings form diffuse connections with target cells, often creating a network of influence rather than a discrete synaptic end bulb seen at neuromuscular junctions.

26. Sympathetic and Parasympathetic Activation

Parasympathetic vs. Sympathetic Activity

Parasympathetic System:

• Active When: The body is in a "rest and digest" state—relaxed, after eating, or during sleep.

• Key Effects:

  • Cardiovascular: Slows heart rate; lowers blood pressure.

  • Digestive: Increases salivation, digestive enzyme and gastric acid secretion, and promotes gut motility.

  • Respiratory: Causes mild bronchoconstriction (less oxygen demand).

  • Ocular: Constricts pupils for near vision.

  • Urinary/Energy: Stimulates bladder contraction and lowers metabolic rate.

Sympathetic System:

• Active When: The body faces "fight or flight" situations—threats, intense stress, physical exertion, loud noises, or adverse environmental conditions (e.g., extreme temperature, dehydration).

• Key Effects:

  • Cardiovascular: Increases heart rate, contractility, and blood pressure; constricts vessels of digestive organs while dilating vessels of skeletal muscles.

  • Respiratory: Dilates bronchi to maximize oxygen intake.

  • Metabolic: Releases catecholamines (epinephrine and norepinephrine), elevates blood glucose via glycogenolysis and gluconeogenesis, and enhances lipolysis.

  • Ocular: Dilates pupils for improved peripheral vision.

  • Other: Decreases GI activity, increases sweating, and inhibits bladder function to focus on immediate survival.

Answer to the Practice Question: During the "rest and digest" state, the parasympathetic nervous system is especially active.

27. Visceral and Sympathetic Reflexes

Which of the following is a sympathetic reflex?

       Increase in heart rate, Constriction of the pupils, Sexual arousal, or Defecation

The correct answer is Increase in heart rate.

Explanation: The sympathetic nervous system is responsible for the "fight-or-flight" response, which includes increasing heart rate to prepare the body for immediate action.

Pupil constriction is a parasympathetic response that allows more light during near focusing. Sexual arousal is primarily governed by the parasympathetic system, although the sympathetic system affects aspects like ejaculation. Defecation is also controlled by the parasympathetic system, facilitating bowel movements.

Visceral reflexes are autonomic pathways regulating internal organ functions to maintain homeostasis. Sensory receptors in organs detect changes (e.g., stretch, pressure) and send signals to integration centers in the CNS, which process this input and command autonomic responses. These reflexes adjust vital functions like heart rate and blood pressure automatically, such as the baroreceptor reflex, which stabilizes blood pressure by altering heart rate.

Sympathetic reflexes are activated in response to stressors, sparking the “fight or flight” response. They employ a two-neuron chain: preganglionic neurons in the thoracolumbar region send signals to postganglionic neurons in sympathetic chain or prevertebral ganglia, leading to rapid systemic changes such as increased heart rate, bronchodilation, and redirected blood flow to skeletal muscles.

Overall, visceral reflexes maintain organ balance and fine-tune functions, while sympathetic reflexes invoke immediate systemic responses to threats.

he visceral reflex NOT coordinated by the medulla oblongata is pupillary. 

Explanation:

The medulla oblongata is primarily responsible for regulating vital functions like breathing, heart rate, and blood pressure, including reflexes related to swallowing, coughing, and baroreceptor activity. 

Key points about the medulla oblongata:

• Function: It acts as the control center for many autonomic nervous system functions. 

• Related reflexes: Swallowing, coughing, vomiting, sneezing. 

• Pupillary control: This function is primarily regulated by the brainstem midbrain region, specifically the oculomotor nucleus.

Chapter 12: Neurons and Cellular Properties

• Classes of Neurons:

  • Functional: Sensory (afferent), motor (efferent), and interneurons (connecting neurons).

  • Morphological: Multipolar (most common in the CNS for motor neurons), bipolar (in sensory systems like the retina), and unipolar/pseudounipolar (typically in sensory ganglia).

• Astrocytes:

  • Star-shaped glial cells in the CNS.

  • Provide structural support, maintain the blood-brain barrier, regulate neurotransmitter levels, and help repair the brain after injury.

• Satellite and Schwann Cells:

  • Satellite Cells: Surround neuronal cell bodies in peripheral ganglia, maintaining a regulated microenvironment and supporting metabolic needs.

  • Schwann Cells: Form the myelin sheath around peripheral nerve axons, facilitating rapid electrical conduction.

• Gated Channels:

  • Ion channels that open/close in response to specific stimuli (voltage, ligand, or mechanical changes).

  • Essential for initiating and propagating electrical signals in neurons.

• Action Potentials:

  • All-or-none electrical impulses that follow a sequence: depolarization (influx of Na⁺), repolarization (efflux of K⁺), and sometimes hyperpolarization.

  • Driven by voltage-gated ion channels.

• Refractory Periods:

  • Absolute refractory period: Neuron cannot fire another action potential, ensuring one-way propagation.

  • Relative refractory period: A stronger-than-normal stimulus is required to initiate another action potential.

• Types of Summation:

  • Temporal Summation: Repeated stimulation at a single synapse over time boosts the postsynaptic potential.

  • Spatial Summation: Simultaneous stimulation from multiple synapses on a neuron combines to reach the threshold.

Chapter 13: Central Nervous System and Nerve Anatomy

• Cerebrospinal Fluid (CSF):

  • Produced in the choroid plexuses of the ventricles.

  • Flows through ventricles to the subarachnoid space and is reabsorbed via arachnoid granulations.

  • Cushions the brain, provides buoyancy, and removes waste.

• Basal Nuclei (Basal Ganglia):

  • A group of deep nuclei (including the caudate, putamen, and globus pallidus) involved in regulating and coordinating movement.

• Horns of the Spinal Cord:

  • Dorsal Horn: Contains sensory neurons.

  • Ventral Horn: Contains motor neurons.

  • Lateral Horn: Present in segments with autonomic output (sympathetic).

• Connective Tissue of Nerves:

  • Endoneurium: Surrounds individual nerve fibers.

  • Perineurium: Encloses bundles of nerve fibers (fascicles).

  • Epineurium: Outermost layer covering the entire nerve.

• Posterior (Dorsal) and Anterior (Ventral) Rami:

  • Posterior Rami: Innervate the skin and muscles of the back.

  • Anterior Rami: Innervate the anterolateral body wall and limbs.

• Nerve Plexuses:

  • Complex, intertwined networks of nerves (e.g., brachial, lumbosacral plexuses).

  • Provide motor and sensory innervation to the limbs.

• Phrenic Nerve:

  • Originates from the cervical plexus (C3–C5).

  • Innervates the diaphragm; essential for breathing.

Chapter 14: Sensory Systems and Motor Reflexes

• Types of Receptors:

  • Mechanoreceptors (pressure, stretch, vibration), Thermoreceptors (temperature), Chemoreceptors (chemical changes), Nociceptors (pain), Photoreceptors (light), and Proprioceptors (body position).

• Photoreception:

  • Process by which rods and cones in the retina convert light stimuli into neural signals for vision.

• Auditory Ossicles:

  • Three small bones in the middle ear (malleus, incus, stapes) that transmit sound vibrations from the tympanic membrane to the oval window of the inner ear.

• Tectorial and Basilar Membranes:

  • Basilar Membrane: Supports the organ of Corti in the cochlea and is key to sound frequency discrimination.

  • Tectorial Membrane: Contacts hair cells, facilitating their deflection and initiation of auditory signals.

• Olfactory Receptors:

  • Located in the nasal epithelium; detect odorant molecules using G-protein coupled receptor mechanisms, critical for the sense of smell.

• Flexor Reflex:

  • A withdrawal reflex initiated by painful stimuli.

  • Involves activation of flexor muscles to retract a limb, while inhibitory interneurons suppress antagonistic muscles.

  • Often coupled with a crossed extensor reflex to maintain balance.

• Corticospinal Tracts:

  • Descending motor pathways from the cerebral cortex to the spinal cord, crucial for voluntary motor control, especially for fine movements.

Chapter 15: Autonomic Nervous System and Reflex Integration

• Ganglionic Neurons:

  • Neurons located within autonomic ganglia that relay signals from preganglionic to postganglionic fibers.

• Ganglionic Sympathetic Neurons:

  • Postganglionic neurons located in sympathetic ganglia (paravertebral or prevertebral) that release norepinephrine (except in sweat glands) to evoke fight-or-flight responses.

• Parasympathetic Nerves:

  • Part of the craniosacral division; typically characterized by long preganglionic fibers and short postganglionic fibers, they mediate rest-and-digest functions.

• Postganglionic Fibers:

  • Nerve fibers that extend from autonomic ganglia to target organs, executing the final responses in both sympathetic and parasympathetic systems.

• Sympathetic and Parasympathetic Activation:

  • Sympathetic: Triggers fight-or-flight responses—elevated heart rate, bronchodilation, vasoconstriction in non-essential areas, mobilization of energy stores.

  • Parasympathetic: Promotes rest-and-digest responses—decreased heart rate, increased digestive activity, energy conservation.

• Visceral and Sympathetic Reflexes:

  • Visceral Reflexes: Autonomic reflexes that regulate vital functions (e.g., the baroreceptor reflex controlling blood pressure).

  • Sympathetic Reflexes: A subset that engage the sympathetic system (e.g., the withdrawal reflex in response to pain) typically involve a polysynaptic pathway to elicit rapid, systemic effects.