Study Notes on the Efferent Division of the PNS

Peripheral Nervous System (PNS) Overview

Efferent Division

  • The efferent division of the PNS refers to the pathway that transmits signals from the central nervous system (CNS) to effector organs, such as muscles and glands.

Lecture Outline

  1. Somatic versus Autonomic Motor Divisions

    • Compare the anatomy, neurotransmitters, and receptors of the somatic motor and autonomic motor divisions.

  2. Sympathetic and Parasympathetic Divisions

    • Compare and contrast the anatomy and chemical communication of the sympathetic and parasympathetic branches of the autonomic nervous system (ANS).

    • Describe the physiological role of the autonomic division and its branches.

    • Describe the structure and secretions of the adrenal medulla.

  3. Neuromuscular Junctions

    • Describe the structure of the neuromuscular/neuroeffector junction and the sequence of events that occur at a neuromuscular junction.

Effector Organs

  • Effector organs consist of muscle and gland tissue.

  • The PNS is divided into:

    • Afferent Division: Sends information to the CNS from peripheral stimuli.

    • Efferent Division: Transmits output from the CNS to peripheral effector organs.

  • The efferent division of the PNS includes:

    • Somatic Nervous System: Innervates skeletal muscles.

    • Autonomic Nervous System: Innervates smooth muscle, cardiac muscle, and glands.

Somatic and Autonomic Motor Divisions

Comparisons

Feature

Somatic Motor Division

Autonomic Motor Division

Type of Control

Voluntary, reflexive

Involuntary, reflexive

Neurons in Pathway

One motor neuron

Two motor neurons: preganglionic, postganglionic

Location of Cell Bodies

Ventral horn of spinal cord

Preganglionic – Brain and spinal cord; Postganglionic – Autonomic ganglia

Effectors

Skeletal Muscle

Smooth and Cardiac Muscle

Effector Response

Stimulatory response

Stimulatory or Inhibitory response

Neurotransmitter Released

Acetylcholine (ACh)

ACh or Norepinephrine (NE)

Axon Properties

Thick, well-myelinated

Preganglionic – thin, lightly myelinated; Postganglionic – thinner, unmyelinated

Autonomic Nervous System

  • The autonomic nervous system consists of a chain of two motor neurons that innervate smooth and cardiac muscle as well as glands:

    • Preganglionic Fiber: Connects the CNS to the autonomic ganglion.

    • Postganglionic Fiber: Connects the autonomic ganglion to effector organs.

  • Dual Innervation: Organs are innervated by both branches of ANS, providing precise control over the organ's activity, leading to either excitatory or inhibitory effects.

Divisions of the Autonomic Nervous System

Sympathetic Division

  • Dominates in emergency or stressful situations (known as “fight-or-flight”).

  • Prepares the body for intense physical activity.

  • Features:

    • Overall Function: Prepare the body for intense activity.

    • Preganglionic Cell Bodies: Found in the lateral horns of T1-L2; referred to as the thoracolumbar division.

    • Preganglionic Axons: Short, with many branches.

    • Postganglionic Cell Bodies: Sympathetic trunk ganglia and collateral ganglia located anterior to the vertebral column.

    • Postganglionic Axons: Long in length.

    • Rami Communicans: White rami (T1-L2) and gray rami (associated with all spinal nerves).

    • Degree of Response: Mass activation; widespread effects.

    • Neurotransmitters: Preganglionic – ACh, Postganglionic – NE.

Parasympathetic Division

  • Dominates in quiet, relaxed situations (known as “rest-and-digest”).

  • Promotes body maintenance activities, such as digestion.

  • Features:

    • Overall Function: Promotes body maintenance.

    • Preganglionic Cell Bodies: Located in the brainstem and lateral horns in the sacral region of the spinal cord; referred to as the craniosacral division.

    • Preganglionic Axons: Long, with few or no branches.

    • Postganglionic Cell Bodies: Terminal ganglia close to the target organ and intramural ganglia located within the target organ.

    • Postganglionic Axons: Short in length.

    • Rami Communicans: Not applicable.

    • Degree of Response: Local activation; specific effects.

    • Neurotransmitters: Preganglionic – ACh, Postganglionic – ACh.

Neuromuscular Junctions

Structure and Events

  • The neuromuscular junction is the junction where an axon terminal of a motor neuron forms a connection with a muscle cell.

  • Sequence of Events in Neuromuscular Junction:

    1. An action potential travels down a motor neuron to the axon terminal.

    2. The local action potential triggers the opening of voltage-gated Ca$^{2+}$ channels, causing Ca$^{2+}$ to enter the axon terminal.

    3. Ca$^{2+}$ influx triggers the exocytosis of acetylcholine (ACh).

    4. ACh diffuses across the synaptic cleft and binds to nicotinic receptors on the motor end plate of the muscle cell membrane.

    5. This binding opens cation channels, leading to a net influx of Na$^{+}$ ions into the muscle cell and a smaller efflux of K$^{+}$ ions.

    6. The result is an end-plate potential (EPP) that depolarizes the membrane.

    7. If the depolarization reaches threshold, voltage-gated Na$^{+}$ channels open, triggering an action potential that is propagated throughout the muscle fiber.

    8. ACh is broken down by the enzyme acetylcholinesterase to terminate the muscle response.