2-8 Autonomic Motor Nervous System 1

Peripheral Nervous System (PNS) Overview

• PNS Components:

  • Motor neurons and sensory neurons that connect the central nervous system (CNS) with limbs and organs.

  • Includes somatic (voluntary) and autonomic (involuntary) nervous system components.

Motor Neurons Types

• Lower Motor Neuron (LMN):

  • Connects CNS (spinal cord/brain) to skeletal muscles.

  • Acetylcholine (ACh) is the neurotransmitter.

  • Myelinated and has a large diameter causing fast conduction speed.

• Autonomic Motor Neurons:

  • Regulate smooth and cardiac muscle and glandular functions.

  • Typically consist of two neurons:

    • Preganglionic Neuron: Originates in CNS and synapses in PNS ganglia.

    • Postganglionic Neuron: Synapses on the target effector cell.

Pathways of Motor Neurons

• Comparison of Neuron Pathways in PNS:

  • Number of Neurons:

    • Autonomic: 2 neurons (preganglionic and postganglionic).

    • Somatic: 1 neuron directly to effectors.

  • Neurotransmitter:

    • Somatic: Acetylcholine (ACh) only.

    • Autonomic: Mainly ACh or norepinephrine (NE).

  • Receptor Types:

    • Nicotinic receptors for somatic motor neurons and various receptors for autonomic fibers.

  • Effects on Target Cells:

    • Somatic: Always excitatory.

    • Autonomic: Can be excitatory or inhibitory, depending on the neuroeffector junction.

Somatic Motor Neuron Pathway

• Pathway Details:

  • Begins in the brain or spinal cord and travels to the skeletal muscle.

  • Utilizes ligand-gated sodium (Na+) channels linked to nicotinic ACh receptors, resulting in muscle contraction.

Neuromuscular Junction

• Components of NMJ:

  • Axon terminals at the muscle cell surface, motor end plate, and Schwann cell sheaths.

  • ACh is released into the synaptic cleft and binds to nicotinic receptors on muscle, opening Na+ channels, leading to depolarization.

Autonomic Nervous System Divisions

• **Divisions:

  • Sympathetic Division:**

    • Involved in 'fight or flight' response.

    • Preganglionic neurons originate in thoracic and lumbar regions of the spinal cord.

  • Parasympathetic Division:

    • Controls 'rest and digest' functions.

    • Preganglionic neurons originate in cranial and sacral regions.

Autonomic Pathways

• Functional Effects of Autonomic Neurons:

  • Preganglionic fibers synapse at ganglia, then postganglionic reach target effector.

  • Sympathetic: Can cause increased heart rate and bronchodilation; mobilizes energy.

  • Parasympathetic: Slows heart rate and increases digestion; conserves energy.

Homeostasis and Responses

• Balance Between Sympathetic and Parasympathetic:

  • Parasympathetic Dominance:

    • During rest, promotes digestion, decreases heart rate.

  • Sympathetic Dominance:

    • Increases heart rate and redistributes blood flow during stress.

Specific Neuron Types and Functions

• Sympathetic Neurons:

  • Short preganglionic axons and long postganglionic axons, facilitating a rapid response.

• Parasympathetic Neurons:

  • Long preganglionic axons and short postganglionic axons, localizing effects closely to target organs.

Adrenal Gland Role

• Adrenal Medulla:

  • Releases epinephrine (80%) and norepinephrine (20%) under sympathetic activation, enhancing the fight-or-flight response without a postganglionic neuron.