Nervous System: Organization and Function

General Introduction to the Nervous System

The nervous system is a complex network of nerves and cells that carry messages to and from the brain and spinal cord to various parts of the body. This document provides a structured overview of its anatomy, physiology, and functional organization.

Anatomy

Anatomy, derived from "anatome" meaning "to dissect," is the study of the form, structure, position, and interrelationships of body parts.

• Macroscopic Anatomy: Study of structures visible to the naked eye.

• Microscopic Anatomy: Study of structures requiring magnification.

  • Cytology: Study of cells.

  • Histology: Study of tissues.

Physiology

Physiology is the study of how the body parts function. Examples include renal physiology, cardiac physiology, and neurophysiology.

• Anatomy provides a static view, while physiology reveals the dynamic nature of functions.

• These two sciences are inseparable, embodying the principle of structure-function relationship.

Levels of Human Body Organization

The human body is organized into six main levels:

1. Chemical Level

2. Cellular Level

3. Tissue Level

4. Organ Level

5. Organ System Level

6. Organismal Level

The 11 Systems of the Human Body

The human body comprises eleven major organ systems:

• Cardiovascular System

• Endocrine System

• Muscular System

• Integumentary System

• Skeletal System

• Nervous System

• Urinary System

• Digestive System

• Reproductive System

• Lymphatic System

• Respiratory System

1. General Organization of the Nervous System

The nervous system (NS) can be broadly divided into:

• Central Nervous System (CNS): Comprises the encephalon (brain, cerebellum, brainstem) and the spinal cord.

• Peripheral Nervous System (PNS): Consists of the nerves.

• Somatic Nervous System (SNS) (also known as the cerebrospinal system): Responsible for voluntary motor control, sensation in limbs, perception, and language.

• Autonomic Nervous System (ANS) (also known as the vegetative nervous system): Regulates vital involuntary functions (e.g., internal environment). It is further divided into the sympathetic and parasympathetic nervous systems.

1.1. Encephalon

The encephalon is divided into five main regions, each containing several structures:

1.2. Spinal Cord

The spinal cord (rachis) extends through the vertebral canal of the spinal column down to the first lumbar vertebra (L1). It is continuous with the encephalon at the foramen magnum of the skull. The central part of the spinal cord, involved in reflexes, is composed of gray matter. Peripherally, the spinal cord consists of ascending tracts of white matter carrying nerve impulses to the encephalon and descending tracts carrying impulses away from it. Thirty-one pairs of spinal nerves originate from the spinal cord.

In cross-section, the gray matter presents four horns:

• Anterior (ventral) horns

• Posterior (dorsal) horns

• Lateral horns (present in thoracic and upper lumbar segments)

2. Neuronal Histology and Physiology

Nervous tissue is highly cellular. For example, the CNS has less than 20% extracellular space, meaning its cells are extremely close and intricately intertwined. Despite its complexity, nervous tissue is composed of only two main types of cells:

1. Neurons: Excitable nerve cells that produce and transmit electrical signals.

2. Glial cells (gliocytes): Smaller cells that surround and protect neurons.

Both cell types form the structures of the central and peripheral nervous systems.

2.1. Glial Cells (Gliocytes)

• Approximately 10 times more numerous than neurons.

• These cells surround neurons and perform multiple functions:

  • Immune support.

  • Myelin synthesis.

  • Maintaining synaptic integrity ("tightness").

• Several types exist: astrocytes, oligodendrocytes, Schwann cells, microglia.

• Cerebral tumors often originate from these cells.

Gliocytes of the CNS

CNS gliocytes form the neuroglia or "nerve glue." Some researchers also include PNS gliocytes in neuroglia. Like neurons, most gliocytes have branched processes extending from a central cell body. However, gliocytes are much smaller than neurons, and their nuclei stain more readily. They are nine times more numerous than neurons in the CNS and constitute about half the mass of the encephalon.

The most abundant gliocytes are astrocytes, star-shaped cells.

• Astrocytes: Provide structural support, regulate the chemical environment, and form the blood-brain barrier.

• Oligodendrocytes: Form myelin sheaths around axons in the CNS, increasing the speed of nerve impulse transmission.

• Microglia: Immune cells of the CNS, acting as phagocytes to remove debris and pathogens.

• Ependymal cells: Line the ventricles of the brain and the central canal of the spinal cord, producing and circulating cerebrospinal fluid (CSF).

Gliocytes of the PNS

The two types of gliocytes found in the PNS are ganglion gliocytes and neurolemmocytes (Schwann cells). They differ mainly in their location.

• Ganglion gliocytes (satellite cells): Flattened cells that surround the cell bodies of neurons located in ganglia. They are thought to regulate the chemical environment of associated neurons.

• Neurolemmocytes (Schwann cells): Form myelin sheaths that wrap around large axons in the PNS, functionally similar to oligodendrocytes. They play a crucial role in the regeneration of peripheral nerve fibers.

2.2. Neurons

Neurons, or nerve cells, are the structural and functional units of the nervous system. These highly specialized cells transmit messages as nerve impulses between different parts of the body. Neurons possess several key characteristics:

1. Extreme Longevity: Neurons can live and function optimally for over 100 years if adequately nourished.

2. Amitotic: Neurons have lost their ability to undergo mitosis, which is incompatible with their function as communication links. Consequently, they are not replaced if destroyed.

3. High Metabolic Rate: Neurons require a continuous and abundant supply of oxygen and glucose. They cannot survive for more than a few minutes without oxygen.

A neuron is responsible for the generation, processing, and propagation of information. It is a highly communicative cell, with each neuron potentially receiving information from 100,000 other neurons and sending information to another 100,000 neurons.

Neurons have a distinctive shape with numerous extensions:

• One axon (which branches).

• Numerous dendrites.

The gray matter is where neuron cell bodies are located, while the white matter consists of their extensions (axons).

Classification of Neurons

Neurons can be classified based on their structure or function.

Structural Classification

Based on the number of processes emerging from the cell body:

• Multipolar Neurons: Possess many processes (numerous dendrites and a single axon). They are the most abundant type, primarily found in the CNS (e.g., motor neurons, interneurons).

• Bipolar Neurons: Have one dendrite and one axon emerging from the cell body. They are rare, found in certain sensory organs (e.g., olfactory mucosa, retina).

• Unipolar (Pseudounipolar) Neurons: Have a single short process that emerges from the cell body and divides T-like into proximal and distal branches. They are mainly found in the PNS (e.g., sensory neurons in dorsal root ganglia).

Functional Classification

Based on the direction of nerve impulse propagation:

• Motor Neurons (Efferent): Conduct impulses from the CNS to effectors (muscles or glands). Most are multipolar.

• Sensory Neurons (Afferent): Transmit impulses from sensory receptors towards the CNS. Most are unipolar, with some bipolar neurons in specific sensory organs.

• Interneurons (Association Neurons): Conduct impulses within the CNS, connecting sensory and motor neurons. Most are multipolar and form complex neural chains.

2.3. Resting Potential of the Neuron

• Neurons are delimited by a membrane.

• Ion concentrations (especially Na+, Cl-, K+) differ across the membrane.

• This concentration difference creates a resting potential, an electrical voltage, typically around -70 mV.

• The opening of ion channels in the membrane allows ion exchange, modifying the potential, leading to depolarization.

• If depolarization is sufficiently strong, it triggers an action potential: a propagating wave of depolarization.

2.4. Information in the Nervous System

• Information in the NS is encoded by electrical signals: repeated action potentials.

• These signals travel along neuronal processes like current in electrical wires.

• Signal propagation is facilitated and accelerated by myelin, a substance covering axons.

• At the junction between two neurons (synapse), the electrical signal is converted into a chemical signal.

2.5. Propagation of Electrical Signal

The electrical signal propagates along the axon, often insulated by a myelin sheath. Myelin allows for saltatory conduction, where the action potential "jumps" between nodes of Ranvier, significantly increasing transmission speed.

2.6. The Synapse

The synapse is the specialized junction where information is exchanged between two neurons.

• The two neurons do not physically touch; there is a gap called the synaptic cleft.

• The arrival of an electrical signal (action potential) at the presynaptic terminal triggers the release of chemical substances called neurotransmitters (Nt) into the synaptic cleft.

• Examples of neurotransmitters include acetylcholine, dopamine, GABA, and serotonin.

• These neurotransmitters bind to receptors on the postsynaptic neuron, causing the opening of ion channels and subsequent membrane depolarization, thereby generating a new electrical signal.

2.7. The Neuromuscular Junction (Motor End Plate)

Muscle contraction is initiated by the arrival of an electrical signal carried by nerves.

• Each muscle receives signals from a specific nerve (it is innervated by that nerve).

• Each nerve can send signals to multiple muscles.

• The junction between a nerve and a muscle is called the neuromuscular junction or motor end plate.

• When a nerve impulse arrives, the neuron releases acetylcholine, which binds to receptors on the muscle fibers, triggering their contraction.

• Curare is a substance that causes paralysis by blocking the motor end plate.

2.8. Neurotransmitters

Over 50 substances are known or suspected to be neurotransmitters. A chemical substance must meet specific criteria to be classified as a neurotransmitter:

1. It must be present in the presynaptic nerve terminal and released upon adequate neuronal stimulation. Some neurotransmitters are synthesized and stored in vesicles within the terminal, while others are formed in the cell body and transported to the terminals.

2. It must mimic the effect of endogenous neurotransmitters, producing ion fluxes and EPSPs or IPSPs when applied experimentally to the postsynaptic membrane.

3. A natural process must exist to inactivate or remove the substance from the synapse.

Neurotransmitters are classified by their chemical structure and function.

Classes of Neurotransmitters

2.9. Reflex Activity

Many regulatory mechanisms in the body are stimulus-response sequences called reflexes. In its strictest sense, a reflex is a rapid, predictable motor response to a stimulus. Most reflexes are neither learned, premeditated, nor voluntary; they are, in a way, integrated into the physiology of the nervous system.

To remember the function of each cranial nerve: "Sahara sablonneux (et) Mer Morte: deux mondes de silence (et) déserts de mouvants mirages." Words starting with "S" denote sensory nerves, "M" denote motor nerves, and "D" (for "deux") denote mixed (both sensory and motor) nerves.

— Mamadou Djenapo, biological sciences student

2.10. Elements of a Reflex Arc

Reflexes occur in specific neural pathways called reflex arcs.

1. Receptor: The structure on which the stimulus acts.

2. Sensory Neuron (Afferent): Carries afferent impulses to the CNS (usually the spinal cord).

3. Integration Center: In the simplest reflex arcs, this can be a single synapse between a sensory neuron and a motor neuron (monosynaptic reflexes). Complex reflexes involve chains of neurons and multiple synapses (polysynaptic reflexes). The integration center is always located in the CNS.

4. Motor Neuron (Efferent): Propagates efferent impulses from the integration center to an effector organ (muscle or gland).

5. Effector: A muscle cell (myocyte) or glandular cell that responds to efferent impulses in a characteristic way (by contraction or secretion).

Example: Osteotendinous Reflexes

In an osteotendinous reflex:

• Peripheral excitation stimulates a sensory neuron, which enters the spinal cord via the posterior root (nucleus in the ganglion).

• In the posterior horn, it synapses with a second, association neuron, which remains in the gray matter.

• This relays to a motoneuron in the gray matter.

• The motoneuron exits via the anterior root/motor nerve/muscle, causing contraction.

3. Functioning of the Central Nervous System

The CNS is responsible for integrating information, coordinating activity, and controlling the body. It includes the brain and spinal cord, protected by meninges, cerebrospinal fluid, and the blood-brain barrier.

3.1. The Meninges

The meninges are three membranes that surround the central nervous system (brain and spinal cord).

• They delimit a space where cerebrospinal fluid (CSF) circulates: the subarachnoid space.

• From exterior to interior, the three meninges are: the dura mater, the arachnoid mater, and the pia mater.

• Meningitis: Inflammation of the meninges, most often due to an infectious agent (virus, bacterium).

3.2. The Encephalon

The encephalon, or brain, is the control center of the nervous system. It is characterized by various fissures and lobes.

• Cerebral Cortex: The superficial layer of gray matter, responsible for all conscious activities and many unconscious ones.

• Cerebral Lobes: The brain is divided into distinct lobes, each associated with specific functions:

  • Frontal Lobe: Planning, decision-making, voluntary movement.

  • Parietal Lobe: Sensory processing, spatial awareness.

  • Temporal Lobe: Auditory processing, memory, language comprehension.

  • Occipital Lobe: Visual processing.

3.3. Ventricles and Cerebrospinal Fluid (CSF)

• Ventricles: Interconnected cavities within the brain.

• Approximately 0.65 L of CSF is produced daily by the choroid plexuses.

• CSF can be collected via lumbar puncture.

• It is a clear fluid, "like rock water."

• A cloudy appearance indicates certain types of meningitis.

• If CSF flow is obstructed, it can lead to hydrocephalus.

CSF Composition

• Cells: 1 to 5 cells/µL

• Protein: 0.2 to 0.4 g/L

• Glucose: 0.5 g/L

• Germs: 0 (normally sterile)

3.4. Afferent and Efferent Pathways

• Afferent Pathways: Nerve pathways carrying information from the periphery towards the brain.

• Efferent Pathways: Nerve pathways carrying information from the brain towards the periphery.

• Decussation: These pathways cross over at the level of the brainstem, meaning the right brain receives/sends information from/to the left side of the body, and vice versa.

3.5. Major and Minor Hemispheres

• The functions of the two cerebral hemispheres are not entirely symmetrical.

• One is referred to as the major (or dominant) hemisphere, and the other as the minor (or non-dominant) hemisphere.

• The major hemisphere is typically the left in right-handed individuals and the right in left-handed individuals.

• This distinction primarily concerns the parietal lobe:

  • Major Hemisphere: Primarily responsible for language.

  • Minor Hemisphere: Involved in integrating body schema (spatial awareness).

3.6. The Cortex and Cortical Areas

The cortex is a layer of gray matter (neuronal cell bodies) on the surface of the brain. It is responsible for all conscious activities and many unconscious ones.

Cortical Areas: Each function is processed by a specific area of the cortex.

• Voluntary Movements: Motor area (posterior part of the frontal lobe).

• Sensations: Sensory area (anterior part of the parietal lobe).

• Integration of Body Schema: Parietal lobe of the minor hemisphere.

• Language: Parietal lobe of the major hemisphere.

• Vision: Occipital lobe.

Therefore, the location of a lesion (tumor, infarct) can be inferred from neurological symptoms.

3.7. Cortical Areas

Specific regions of the cerebral cortex are dedicated to distinct functions, forming a functional map of the brain.

3.8. The Motor and Sensory Cortical Areas

These areas exhibit somatotopic representation, meaning specific body parts are mapped to specific regions of the cortex.

• Within the motor area, each zone is responsible for the motor control of a particular body part.

• The ascending parietal gyrus contains the psychosensory area, involved in the gnosis (recognition) of present and past sensations (memory of perceptions).

3.9. Electrical Activity of the Cortex

• Cortical electrical activity can be evaluated by electroencephalography (EEG).

• In epilepsy, there is anarchic, disorganized, and diffuse electrical activity.

3.10. The Basal Ganglia (Central Gray Nuclei)

These are islands of gray matter located deep within the brain, performing diverse functions:

• Control of involuntary movements (e.g., substantia nigra).

• Integration of stimuli, association of different functions (e.g., thalamus).

• Regulation of body temperature, various vegetative functions, and hormonal secretions (e.g., hypothalamus).

• Memory formation (e.g., amygdala).

3.11. The Cerebellum

• Located beneath the occipital lobe, behind the brainstem.

• Connected to the brain by the cerebellar peduncles.

• Functions include:

  • Coordination of movements.

  • Maintenance of posture and balance (walking and standing).

  • Regulation of muscle tone.

3.12. The Brainstem

• Located in front of the cerebellum, below the cerebrum, and above the spinal cord.

• Contains vital gray matter nuclei.

• Serves as a crucial pathway for all afferent and efferent signals between the spinal cord and the brain.

• Functions include:

  • Maintenance of consciousness.

  • Regulation of biological cycles.

  • Control of respiration and heart rate.

• It is the emergence point for most cranial nerves.

3.13. The Spinal Cord

• Part of the CNS.

• Continuous with the brainstem.

• Contained within the vertebral canal, delimited by the vertebrae.

• Shorter than the vertebral column, ending at the second lumbar vertebra.

• Emits nerve roots that exit the vertebral canal to form various nerves.

• There are 8 cervical, 12 thoracic, 5 lumbar, and 5 sacral roots, plus 1 coccygeal pair.

3.14. The Spinal Cord (Detailed)

• Acts as a relay between the brain and nerves.

• Receives information from peripheral receptors (pain, limb position, etc.) and relays it to the brain for integration.

• Also receives information from the brain (movement commands, etc.) and sends it to effectors (muscles).

• Involved in certain reflexes: information from the periphery generates a response without passing through the brain.

3.15. Voluntary Movements

• The command for movement originates from neurons in the motor cortical area.

• The axons of these neurons descend through the white matter, cross over at the brainstem (decussation), and then descend into the spinal cord to the level corresponding to the target muscle.

• They synapse with a spinal cord neuron (motoneuron).

• The axon of the motoneuron extends to the neuromuscular junction (motor end plate).

• The pyramidal tract (or pyramidal fasciculus) is the pathway taken by motor commands.

4. The Peripheral Nervous System (PNS)

The PNS is crucial for transmitting information as nerve impulses from the periphery to the brain, and from the brain to the periphery.

4.1. General Characteristics of the PNS

• Nerves are composed of bundles of axons.

• Nerves are white cords that branch throughout the body.

4.2. Sensory Receptors

Sensory receptors are structures designed to react to changes in the environment, i.e., stimuli. Generally, stimulation of a sensory receptor by a sufficiently strong stimulus generates local depolarizations or graded potentials, which in turn trigger action potentials (nerve impulses) in afferent nerve fibers leading to the CNS.

Classification by Stimulus Type

Receptors are divided into 5 classes based on the stimuli they detect:

1. Mechanoreceptors: Respond to mechanical factors like touch, pressure, vibrations, and stretch, which deform them or adjacent tissues.

2. Thermoreceptors: Respond to changes in temperature.

3. Photoreceptors: (e.g., in the retina) React to light energy.

4. Chemoreceptors: Sensitive to dissolved chemical substances.

5. Nociceptors (noci = harm): React to potentially harmful stimuli, and the sensory information they transmit is interpreted as pain by the brain.

Classification by Anatomical Location

Based on their anatomical location or the stimuli they react to, receptors are divided into 3 classes:

1. Exteroceptors: Sensitive to stimuli from the external environment. These include cutaneous receptors for touch, pressure, pain, and temperature, as well as most receptors of special sense organs.

2. Interoceptors (Visceroceptors): React to stimuli produced within the internal environment (viscera and blood vessels). Stimuli detected by interoceptors reach brain structures but often remain unconscious.

3. Proprioceptors: Like interoceptors, they react to internal stimuli, but are found only in skeletal muscles, tendons, joints, ligaments, and the connective tissue covering bones and muscles. They provide information about body position and movement.

4.3. Structure of a Nerve

A nerve is a cord-like organ belonging to the peripheral nervous system. Nerves vary in size but not in composition: they are all formed by parallel bundles of peripheral axons (myelinated and unmyelinated) surrounded by layered connective tissue coverings.

• Each axon, with its myelin sheath or neurolemma, is surrounded by a thin layer of loose connective tissue called the endonerve (endoneurium).

• Axons are grouped into fascicles by a thicker connective tissue sheath, the perinerve (perineurium).

• Finally, all fascicles are enveloped by a tough fibrous sheath, the epinerve (epineurium).

Neuronal processes constitute only a small fraction of the nerve; most of its mass is made up of myelin and protective connective tissue coverings. Nerves also contain blood vessels and lymphatic vessels.

• Nerves are formed from nerve roots emerging from the spinal cord.

• Each nerve contains:

  • Motor fibers: Axons of motoneurons.

  • Sensory fibers.

  • Potentially autonomic fibers: Innervating sweat glands, vascular and visceral motility.

4.4. Classification of Nerves

Nerves are classified according to the type of nerve impulses they transmit: sensory information or motor commands. Nerves containing both sensory and motor nerve fibers (transmitting impulses towards and from the CNS) are called mixed nerves.

• Nerves that transmit impulses only towards the CNS are sensory (afferent) nerves.

• Nerves that conduct impulses only from the CNS are motor (efferent) nerves.

Most nerves are mixed; exclusively sensory or motor nerves are extremely rare.

Mixed nerves often include fibers from both the somatic and autonomic (visceral) nervous systems. These fibers can be classified, according to the region they innervate, as somatic afferent, somatic efferent, visceral afferent (autonomic), and visceral efferent.

Depending on whether nerves emerge from the encephalon or the spinal cord, peripheral nerves are classified as cranial nerves and spinal nerves.

Cranial Nerves

Twelve pairs of cranial nerves emerge from the encephalon through various foramina in the skull. The first two pairs originate in the forebrain, and the others in the brainstem. With the exception of the vagus nerves, which extend into the thoracic and abdominal cavities, cranial nerves only innervate structures of the head and neck.

In most cases, the names of cranial nerves indicate the main structures they innervate or their primary functions. Cranial nerves are numbered (traditionally with Roman numerals) from the rostral to the caudal end.

• Three cranial nerves (I, II, and VIII) have only sensory function.

• Four cranial nerves (III, VII, IX, and X) include parasympathetic nerve fibers that innervate smooth muscles, cardiac muscle, and glands.

• All cranial nerves with motor function also contain afferent nerve fibers from proprioceptors of the muscles they innervate.

Spinal Nerves

Thirty-one pairs of spinal nerves, each containing thousands of nerve fibers, emerge from the spinal cord and innervate all parts of the body, except the head and certain neck regions. All spinal nerves are mixed.

Spinal nerves are named after their point of emergence from the spinal cord:

• 8 pairs of cervical nerves (C1 to C8).

• 12 pairs of thoracic nerves (T1 to T12).

• 5 pairs of lumbar nerves (L1 to L5).

• 5 pairs of sacral nerves (S1 to S5).

• 1 pair of tiny coccygeal nerves (Co).

4.5. Ganglia

Ganglia are clusters of neuron cell bodies associated with PNS nerves. Ganglia associated with afferent nerves contain cell bodies of sensory neurons. Ganglia linked to efferent nerves contain cell bodies of autonomic motor neurons, as well as a particular variety of integration neurons.

4.6. Plexuses

All spinal nerves, with the exception of T2 to T12, are characterized by their ventral rami branching and intertwining into complex plexuses.

• Plexuses are found in the cervical, brachial, lumbar, and sacral regions, primarily innervating the limbs.

• Only the ventral rami of spinal nerves form plexuses; the dorsal rami do not.

5. The Autonomic (Vegetative) Nervous System

5.1. Generalities

• Independent of voluntary control, it regulates homeostasis.

• Controls the activity of smooth muscles (bronchi, intestines, blood vessels, etc.) and cardiac muscle.

• Regulated by the hypothalamus.

5.2. Comparison: Somatic Nervous System vs. Autonomic Nervous System

Both systems include motor nerve fibers, but they differ in three key aspects:

• Their effectors.

• Their efferent pathways.

• The responses their neurotransmitters elicit in target organs.

Somatic Nervous System

• Axons of somatic motor neurons extend from the CNS directly to effectors (skeletal muscle cells).

• These axons are generally heavily myelinated.

• Somatic motor neurons release acetylcholine (ACh), whose effect is always stimulatory.

Autonomic Nervous System

• Axons of most preganglionic neurons emerge from the CNS and synapse with a postganglionic neuron in a peripheral autonomic ganglion.

• Some sympathetic preganglionic axons synapse with cells of the adrenal medulla.

• Axons of postganglionic neurons extend from the ganglia to effectors (cardiac muscle cells, smooth muscle cells, glands).

• Preganglionic axons are lightly myelinated; postganglionic axons are unmyelinated.

• All autonomic preganglionic axons release ACh.

• All parasympathetic postganglionic axons release ACh.

• Most sympathetic postganglionic axons release norepinephrine (NE).

• Upon stimulation, adrenal medulla cells release NE and epinephrine (adrenaline) into the bloodstream.

• Autonomic effects are either stimulatory or inhibitory, depending on the postganglionic neurotransmitter released and the protein receptors on the effectors.

5.3. Anatomy of the Autonomic Nervous System

The sympathetic and parasympathetic nervous systems are distinguished by:

• Their Origins: Parasympathetic nerve fibers emerge from the encephalon and the sacral region of the spinal cord, while sympathetic nerve fibers originate in the thoracolumbar region of the spinal cord.

• Length of Their Nerve Fibers: Preganglionic fibers are long, and postganglionic fibers are short in the parasympathetic system; the reverse is true in the sympathetic system.

• Location of Their Ganglia: Most parasympathetic ganglia are located within the visceral organs (effectors) or very close to them (intramural or extramural ganglia), while sympathetic ganglia are found near the spinal cord (sympathetic trunk ganglia) and in front of the spinal cord (prevertebral ganglia).

5.4. Physiology of the Autonomic Nervous System

Two major systems:

• Sympathetic Nervous System:

  • Neurotransmitter (Nt) = Norepinephrine.

  • "Stress system," enabling mobilization of reserves.

  • Activation leads to: tachycardia (increased heart rate), hyperglycemia (increased blood sugar), bronchial dilation, etc.

• Parasympathetic Nervous System:

  • Neurotransmitter (Nt) = Acetylcholine.

  • Activation leads to: iris constriction, salivation, bradycardia (slowed heart rate), etc.

5.5. Neurotransmitters and Receptors