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.