The Autonomic Nervous System - Chapter 14A

Human Anatomy and Physiology: The Autonomic Nervous System

Overview of the Autonomic Nervous System (ANS)

  • The autonomic nervous system (ANS) is a component of the peripheral nervous system.

  • Functions:

    • Consists of motor neurons that innervate:

    • Smooth muscles

    • Cardiac muscle

    • Glands

    • Makes adjustments to ensure optimal support for body activities, e.g., shunting blood to areas that need it, adjusting heart rate, blood pressure, and digestive processes.

    • Operates via subconscious control.

  • Also referred to as the involuntary nervous system or general visceral motor system.

Structural Organization of the ANS

  • Figure 14.1 illustrates the place of the ANS in the structural organization of the nervous system.

Effectors of the Nervous System

  • The somatic nervous system (SNS) specifically innervates skeletal muscles.

  • The ANS innervates:

    • Cardiac muscle

    • Smooth muscle

    • Glands

Efferent Pathways and Ganglia

  • The Somatic Nervous System (SNS):

    • Cell body located in the central nervous system (CNS).

    • A single, thick myelinated group A axon extends through spinal or cranial nerves directly to skeletal muscle.

  • The Autonomic Nervous System (ANS):

    • Uses a two-neuron chain:

    • Preganglionic neuron:

      • Cell body located in the CNS.

      • Thin and lightly myelinated preganglionic axon extends to a ganglion.

    • Postganglionic (ganglionic) neuron:

      • Located outside the CNS.

      • Cell body synapses with the preganglionic axon in the autonomic ganglion.

      • Nonmyelinated postganglionic axon extends to the effector organ.

Neurotransmitter Effects in the Nervous System

  • Somatic Nervous System:

    • All somatic motor neurons release acetylcholine (ACh).

    • Effect is always stimulatory.

  • Autonomic Nervous System (ANS):

    • Preganglionic fibers release ACh.

    • Postganglionic fibers can release norepinephrine or ACh at effectors.

    • The effect can be either stimulatory or inhibitory, depending on the type of receptors present.

Comparative Anatomy of the SNS and ANS

  • Figure 14.2 shows the comparison of motor neurons in the somatic and autonomic nervous systems.

Divisions of the Autonomic Nervous System

  • The ANS has two main divisions:

    • Parasympathetic Division:

    • Promotes maintenance functions and conserves energy.

    • Known as the “rest-and-digest” system.

    • Sympathetic Division:

    • Mobilizes the body during activity.

    • Known as the “fight-or-flight” system.

  • Both divisions provide dual innervation to all visceral organs, causing opposite effects, maintaining homeostasis through dynamic antagonism.

Role of the Parasympathetic Division

  • Functions to keep body energy use low during maintenance activities.

  • Processes Directed by the Parasympathetic:

    • Digestion

    • Diuresis

    • Defecation

  • Example: A person reading and relaxing after a meal shows:

    • Low blood pressure, heart rate, and respiratory rates.

    • High gastrointestinal activity.

    • Constricted pupils and lenses accommodated for close vision.

Role of the Sympathetic Division

  • Mobilizes the body in times of activity.

  • Effects include:

    • Increased heart rate

    • Dry mouth

    • Cold, sweaty skin

    • Dilated pupils

  • During vigorous physical activity:

    • Blood shunted to skeletal muscles and heart.

    • Bronchioles dilate.

    • Liver releases glucose.

Key Anatomical Differences Between ANS Divisions

  1. Sites of Origin:

    • Parasympathetic: Craniosacral origin (brain and sacral spinal cord).

    • Sympathetic: Thoracolumbar origin (thoracic and lumbar regions of spinal cord).

  2. Relative Lengths of Fibers:

    • Parasympathetic: Long preganglionic and short postganglionic fibers.

    • Sympathetic: Short preganglionic and long postganglionic fibers.

  3. Location of Ganglia:

    • Parasympathetic: Ganglia located in or near visceral effector organs.

    • Sympathetic: Ganglia located close to the spinal cord.

Cranial Part of the Parasympathetic Division

  • Cell bodies located in the brain stem.

  • Preganglionic fibers run in:

    • Oculomotor Nerves (III): Control smooth muscles of the eye, cause pupils to constrict.

    • Facial Nerves (VII): Stimulate saliva production through activation of large glands in the head (nasal, lacrimal, submandibular, and sublingual).

    • Glossopharyngeal Nerves (IX): Stimulate parotid salivary glands by synapsing in the otic ganglia.

    • Vagus Nerves (X): Account for approximately 90% of all preganglionic parasympathetic fibers in the body, serving all thoracic and abdominal viscera, with fibers arising from the medulla and synapsing in terminal ganglia (intramural ganglia) within walls of target organs.

Sacral Part of the Parasympathetic Division

  • Originates from neurons in segments S2-S4 serving pelvic organs and distal half of the large intestine.

  • Axons travel in ventral root of spinal nerves, branching off to form pelvic splanchnic nerves.

  • Synapse with ganglia in the pelvic floor (inferior hypogastric plexus) and intramural ganglia in the walls of:

    • Distal half of large intestine

    • Urinary bladder

    • Ureters

    • Reproductive organs

Sympathetic Division Characteristics

  • The sympathetic division is more complex and innervates more organs than the parasympathetic division.

  • Some structures innervated exclusively by the sympathetic division include:

    • Sweat glands

    • Arrector pili muscle of hair follicles

    • Smooth muscles of all blood vessels.

  • This division is referred to as the thoracolumbar division, with preganglionic neurons located in spinal cord segments T1-L2, forming lateral horns of the spinal cord.

Neurotransmitters in the ANS

  • Major neurotransmitters of the ANS include acetylcholine (ACh) and norepinephrine (NE):

    • ACh, released by cholinergic fibers, is found in:

    • All autonomic preganglionic axons

    • All parasympathetic postganglionic axons.

    • NE released by adrenergic fibers occurs in almost all sympathetic postganglionic axons, with the exception of those at sweat glands (which release ACh).

  • The effects of neurotransmitters depend on whether they bind to cholinergic or adrenergic receptors.

Cholinergic Receptors and Functionality

  • Two types of cholinergic receptors that bind ACh:

    • Nicotinic Receptors:

    • Found on all postganglionic neurons (sympathetic and parasympathetic).

    • Found on hormone-producing cells of the adrenal medulla.

    • Present in sarcolemma of skeletal muscle at the neuromuscular junction.

    • Always have a stimulatory effect, opening ion channels and depolarizing postsynaptic cells.

    • Muscarinic Receptors:

    • Found on all effector cells stimulated by postganglionic cholinergic fibers.

    • Effects of ACh can vary (either inhibitory or excitatory) based on the receptor type of the target organ.

    • Example: Binding of ACh to cardiac muscle cells slows heart rate, while binding to intestinal smooth muscle cells increases motility.

Adrenergic Receptors in Sympathetic Responses

  • Two major classes of adrenergic receptors that respond to NE or epinephrine:

    • Alpha (α) Receptors:

    • Subclassified as α1 and α2.

    • Beta (β) Receptors:

    • Subclassified as β1, β2, and β3.

Tables Summarizing Effects and Comparisons

  • Table 14.1: Anatomical and physiological differences between the parasympathetic and sympathetic divisions.

  • Tables 14.5-1 & 14.5-2: Effects of the autonomic nervous divisions on various organs, providing a comparative analysis of functions and responses.