Autonomic Nervous System

Overview of Autonomic Nervous System

  • The discussion transitions to the autonomic nervous system (ANS) following previous topics including the spinal cord, cranial nerves, and motor neurons.

  • The lecturer cites the complexity of the ANS, indicating its two major divisions: parasympathetic and sympathetic systems.

Key Definitions

  • Autonomic Nervous System (ANS): A complex interconnected system that controls involuntary body functions.

  • Divisions of the ANS:

    • Parasympathetic: Known as "rest and digest"; responsible for conserving energy and maintaining bodily functions.

    • Sympathetic: Known as "fight or flight"; prepares the body for stressful or emergency situations.

Comparison of Nervous Systems

Somatic vs Autonomic Nervous Systems

  • Somatic Nervous System (SNS):

    • Responsible for skeletal muscle control.

    • Composed of sensory and motor functions.

    • Somatic motor neurons: Innervate skeletal muscle fibers for voluntary movements.

    • Structure: Single neuron pathway extends from the spinal cord to muscle fibers.

  • Autonomic Nervous System (ANS):

    • Involuntary control affecting smooth muscle, cardiac muscle, and glands.

    • Structure: Comprised of two neurons:

    • Preganglionic neuron (presynaptic): First neuron, originating in the central nervous system (either brain or spinal cord).

    • Postganglionic neuron (postsynaptic): Second neuron, exists in the peripheral nervous system, and innervates the effector organs.

Reflex Arc and Neuron Types

  • Reflex Arc:

    • The pathway followed during a reflex, initiated by a sensory receptor.

  • Neurons in Reflex Arcs:

    • Unipolar sensory neurons lead to the interpretation of impulses at the spinal cord and back out through motor neurons.

Detailed Neuronal Pathways

Somatic Neurons

  • A single lower motor neuron projects directly from the spinal cord to the effector muscle fiber.

  • Classification for motor neurons includes differentiation between anterior and posterior ramus based on direction and target muscular regions.

Autonomic Neurons

Preganglionic and Postganglionic Neurons

  • The presynaptic neuron synapses at ganglia in the peripheral nervous system, whereas the postsynaptic neuron extends to effector tissues.

  • Significant point: The ANS engages two neurons, enhancing complexity in regulatory functions compared to the SNS.

Location of Neurons

  • Preganglionic Neurons:

    • Located in the lateral horn of the spinal cord.

  • Postganglionic Neurons:

    • Reside in ganglia close to or within the target organ in the periphery.

Functional Differences of ANS Divisions

Parasympathetic Division

  • Characterized by long preganglionic and short postganglionic fibers.

  • Originates from the brainstem and sacral regions of the spinal cord, thus termed craniosacral division.

Sympathetic Division

  • Exhibits short preganglionic and long postganglionic fibers.

  • Originates from thoracic and lumbar regions, hence termed thoracolumbar division.

  • More complex due to additional pathways and ganglia involved in signaling.

Neurotransmitter Functions

  • All preganglionic neurons in both divisions release acetylcholine (ACh), which is excitatory (leads to an excitatory postsynaptic potential or EPSP).

  • Postganglionic neurons vary in their neurotransmitter release:

    • Parasympathetic: Releases acetylcholine, also leads to EPSP.

    • Sympathetic: Primarily releases norepinephrine and occasionally epinephrine; effects can be excitatory or inhibitory depending on the receptor type activated.

Receptor Types

  • Nicotinic Receptors: Post-synaptic receptors for acetylcholine on postsynaptic neurons leading to EPSPs.

  • Muscarinic Receptors: Activated by acetylcholine, leading to various effects (both excitatory and inhibitory based on tissue type).

  • Adrenergic Receptors: Targeted by norepinephrine, eliciting diverse effects depending on whether they are alpha (α) or beta (β) receptors.

Regulatory Control of the Autonomic Nervous System

Central Nervous System Involvement

  • Cerebrum: Involved in conscious activities and initiating autonomic responses based on conditional stimuli.

  • Hypothalamus: Key regulatory center for autonomic control and hormonal balance, coordinating responses to maintain homeostasis.

  • Brainstem: Houses cardiac and respiratory centers, integrating autonomic responses with environmental adjustments.

Conclusion

  • The lecture stresses the importance of understanding the relationships between these nervous system divisions.

  • Future topics include sensory systems like vision, hearing, taste, and ongoing discussions on the cardiovascular system.