Chapter 7: Peripheral Nervous System: Efferent Division - Comprehensive Study Guide

Overview of the Peripheral Nervous System: Efferent Division

  • The Peripheral Nervous System (PNS) serves as the critical communication link through which the Central Nervous System (CNS) controls the activities of muscles and glands.

  • The Efferent Division of the PNS is divided into two primary branches:

    • Autonomic Nervous System (ANS): The involuntary branch of the PNS. It innervates cardiac muscle, smooth muscle, most exocrine glands, some endocrine glands, and adipose tissue.

    • Somatic Nervous System: The branch subject to voluntary control. it innervates skeletal muscle.

  • Sensory information flows in via the afferent division, information processing occurs in the CNS (incorporating higher-order functions like memory and learning), and motor commands are sent out through the efferent division to effectors.

  • Effectors and Innervation:

    • Skeletal Muscle: Controlled by the Somatic nervous system.

    • Smooth Muscle, Cardiac Muscle, Glands: Controlled by the Autonomic nervous system (divided into Sympathetic and Parasympathetic divisions).

The Autonomic Nervous System (ANS) Structure

  • Autonomic Nerve Pathway: Extends from the CNS to an innervated organ.

  • Two-Neuron Chain: Unlike the somatic system, the ANS uses a two-neuron circuit to reach its target:

    • Preganglionic Fiber: The first neuron, which has its cell body in the CNS. Its axon synapses with the cell body of the second neuron in a ganglion.

    • Postganglionic Fiber: The second neuron, which has its cell body in a ganglion and innervates the effector organ.

  • Neurotransmitters: The primary chemical messengers used in the ANS are Acetylcholine (ACh\text{ACh}) and Norepinephrine (NE\text{NE}).

Subdivisions of the Autonomic Nervous System

  • The ANS is divided into the Sympathetic and Parasympathetic nervous systems, which often produce opposite effects on the same organ to maintain homeostasis.

  • Sympathetic Nervous System:

    • Origin: Fibers originate in the thoracic and lumbar regions of the spinal cord (thoracolumbar division).

    • Preganglionic Fibers: Most are short. They release Acetylcholine (ACh\text{ACh}).

    • Ganglia: Located in the sympathetic ganglion chain (near the spinal cord) or collateral ganglia (midway between the cord and effectors).

    • Postganglionic Fibers: Long fibers that release Norepinephrine (NE\text{NE}). These are termed adrenergic fibers.

    • Dominance: Dominates during stressful, emergency, or exercise situations ("Fight, flight, or fright"). It prepares the body for strenuous physical activity.

  • Parasympathetic Nervous System:

    • Origin: Fibers originate from the cranial (brain) and sacral areas of the CNS (craniosacral division).

    • Preganglionic Fibers: Long fibers that release Acetylcholine (ACh\text{ACh}).

    • Ganglia: Terminal ganglia located in or near the effector organs.

    • Postganglionic Fibers: Very short fibers that release Acetylcholine (ACh\text{ACh}). These are termed cholinergic fibers.

    • Dominance: Dominates in quiet, relaxed situations ("Rest-and-digest"). It promotes body-maintenance activities like digestion.

Physiological Effects of Autonomic Stimulation

  • Heart:

    • Sympathetic: Increased heart rate and increased force of contraction of the whole heart.

    • Parasympathetic: Decreased heart rate and decreased force of contraction of the atria only.

  • Blood Vessels:

    • Sympathetic: Constriction.

    • Parasympathetic: Dilation of vessels supplying the penis and clitoris only.

  • Lungs:

    • Sympathetic: Dilation of bronchioles (airways); possible inhibition of mucus secretion.

    • Parasympathetic: Constriction of bronchioles; stimulation of mucus secretion.

  • Digestive Tract:

    • Sympathetic: Decreased motility (movement); contraction of sphincters to prevent forward movement; inhibition of digestive secretions.

    • Parasympathetic: Increased motility; relaxation of sphincters to permit movement; stimulation of digestive secretions.

  • Urinary Bladder:

    • Sympathetic: Relaxation.

    • Parasympathetic: Contraction (emptying).

  • Eye:

    • Sympathetic: Dilation of the pupil and adjustment for far vision.

    • Parasympathetic: Constriction of the pupil; adjustment for near vision.

  • Metabolism:

    • Sympathetic (Liver): Glycogenolysis (glucose release).

    • Sympathetic (Adipose): Lipolysis (fatty acid release).

    • Parasympathetic: No effect on liver or adipose cells.

  • Exocrine Glands:

    • Exocrine Pancreas: Sympathetic inhibits secretion; Parasympathetic stimulates secretion.

    • Sweat Glands: Sympathetic stimulates most sweat glands (via cholinergic fibers); Parasympathetic stimulates only some.

    • Salivary Glands: Sympathetic stimulates a small volume of thick, mucus-rich saliva; Parasympathetic stimulates a large volume of watery, enzyme-rich saliva.

  • Endocrine Glands:

    • Adrenal Medulla: Sympathetic stimulations release epinephrine and norepinephrine.

    • Endocrine Pancreas: Sympathetic inhibits insulin and stimulates glucagon; Parasympathetic stimulates both insulin and glucagon.

  • Genitals:

    • Sympathetic: Ejaculation and orgasmic contractions.

    • Parasympathetic: Erection (via vasodilation).

  • Brain Activity:

    • Sympathetic: Increased alertness.

The Adrenal Medulla

  • The adrenal medulla is considered a modified part of the sympathetic nervous system.

  • It acts as a modified sympathetic ganglion that does not give rise to postganglionic fibers.

  • When stimulated by sympathetic preganglionic fibers, it secretes hormones directly into the blood.

  • Hormone Release Ratio:

    • Approximately 20%20\% of the hormone release is Norepinephrine (NE\text{NE}).

    • Approximately 80%80\% of the hormone release is Epinephrine (Adrenaline).

Autonomic Neurotransmitter Receptors

  • Tissues innervated by the ANS possess different receptor types for postganglionic chemical messengers.

  • Cholinergic Receptors (Bind to ACh\text{ACh}):

    • Nicotinic Receptors: Activated by nicotine. Found on postganglionic cell bodies of all autonomic ganglia and on the motor end plates of skeletal muscle fibers.

    • Muscarinic Receptors: Activated by the mushroom poison muscarine. Found on effector cell membranes, including smooth muscle, cardiac muscle, and glands.

  • Adrenergic Receptors (Bind to NE\text{NE} and Epinephrine):

    • Alpha (α\alpha) Receptors: Includes subtypes such as α1\alpha_1 and α2\alpha_2.

    • Beta (β\beta) Receptors: Includes subtypes β1\beta_1, β2\beta_2, and β3\beta_3 (found on lipid and bladder tissue).

Receptor-Specific Responses

  • The target organ response depends on the specific isoform of the adrenergic receptor present.

  • Example: Epinephrine Influence on Blood Vessels:

    • Epinephrine + α\alpha-Receptor (e.g., intestinal blood vessel) →\rightarrow Vessel constricts.

    • Epinephrine + β\beta-Receptor (e.g., skeletal muscle blood vessel) →\rightarrow Vessel dilates.

The Somatic Nervous System

  • Consists of the axons of motor neurons that originate in the ventral horn of the spinal cord or the brain stem.

  • End Point: Axons end directly on skeletal muscle.

  • Neurotransmitter: Motor neurons release ACh\text{ACh}, which binds to nicotinic receptors to stimulate muscle contraction.

  • Final Common Pathway: Motor neurons are the final pathway through which the CNS (spinal cord, motor cortex, basal nuclei, cerebellum, and brain stem) exerts control over skeletal muscle.

Comparison: Autonomic vs. Somatic Nervous Systems

  • Site of Origin:

    • ANS: Brain or spinal cord.

    • Somatic: Spinal cord (most) or brain (for head muscles).

  • Number of Neurons in Chain:

    • ANS: Two-neuron chain (preganglionic and postganglionic).

    • Somatic: Single neuron (motor neuron).

  • Type of Innervation:

    • ANS: Most organs are dually innervated by antagonistic branches.

    • Somatic: Effector organs (skeletal muscle) innervated only by motor neurons.

  • Neurotransmitter at Effector:

    • ANS: ACh\text{ACh} or NE\text{NE}.

    • Somatic: Only ACh\text{ACh}.

  • Effect on Effector:

    • ANS: Either stimulation or inhibition.

    • Somatic: Stimulation only.

  • Control Type:

    • ANS: Involuntary.

    • Somatic: Voluntary (with some subconscious coordination).

The Neuromuscular Junction (NMJ)

  • The axon terminal of a motor neuron forms a neuromuscular junction with a single muscle cell.

  • Step-by-Step Events:

    1. An action potential in a motor neuron propagates to the terminal button.

    2. Opening of voltage-gated Ca2+Ca^{2+} channels triggered by the action potential.

    3. Entry of Ca2+Ca^{2+} into the terminal button triggers the release of ACh\text{ACh} by exocytosis.

    4. ACh\text{ACh} diffuses across the cleft and binds with nicotinic receptor sites on the motor end plate of the muscle cell membrane.

    5. Binding opens chemically gated cation channels, leading to a large influx of Na+Na^+ and a smaller efflux of K+K^+.

    6. The resulting depolarization produces an end-plate potential (EPP).

    7. Local current flow occurs between the depolarized end plate and the adjacent membrane.

    8. Voltage-gated Na+Na^+ channels in the adjacent membrane open, reducing the potential to threshold and initiating an action potential that propagates throughout the muscle fiber.

    9. ACh\text{ACh} is destroyed by the enzyme acetylcholinesterase (AChE) on the motor end-plate membrane, terminating the response.

Comparison: Synapse vs. Neuromuscular Junction

  • Similarities:

    • Both involve two excitable cells separated by a narrow cleft.

    • Both store neurotransmitters in vesicles released via Ca2+Ca^{2+}-induced exocytosis.

    • Both involve neurotransmitters binding to receptors to open ion channels and alter membrane potential via graded potentials.

  • Differences:

    • Cells involved: Synapse is neuron-to-neuron; NMJ is motor neuron-to-skeletal muscle fiber.

    • Transmission ratio: NMJ has one-to-one transmission of action potentials; Synapse requires summation of EPSPs to reach threshold.

    • Nature of signal: NMJ is always excitatory (EPP); Synapse can be excitatory (EPSP) or inhibitory (IPSP).

    • Inhibition mechanism: Skeletal muscle cannot be inhibited at the NMJ; inhibition only occurs in the CNS via IPSPs on the motor neuron itself.