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Central Nervous System and Autonomic Nervous System Overview

Motor Neurons in the CNS

• Lower Motor Neurons: Located in the spinal cord; involved in signal transmission.

• Efferent Pathways: Signals exit the CNS through the ventral root to reach peripheral ganglia.

Ganglia Definition

• Ganglion: A collection of neuron cell bodies located in the Peripheral Nervous System (PNS).

Structure of Autonomic Pathways

• Two Neurons System:

  • Preganglionic Neuron: Originates in the CNS, myelinated, and synapses in a ganglion.

  • Postganglionic Neuron: Located in the ganglion, unmyelinated, and connects to the effector organ.

• Preganglionic Fiber: Use acetylcholine (ACh) at synapse.

• Postganglionic Fiber: Varies in neurotransmitter (often norepinephrine for sympathetic actions).

Divisions of the Autonomic Nervous System (ANS)

Sympathetic Nervous System

• Function: Activates fight or flight response.

• Responses:

  • Increases heart rate and respiration.

  • Vasodilation of blood vessels to skeletal muscles.

  • Activates sweat glands and causes goosebumps (evolutionarily protective).

  • Suppresses digestive activities.

Parasympathetic Nervous System

• Function: Promotes rest and digest response.

• Responses:

  • Decreases heart rate and respiration.

  • Promotes digestion and energy storage processes.

  • Causes pupil constriction.

Neurotransmitters in the ANS

• Acetylcholine (ACh): Used in all preganglionic neurons, both sympathetic and parasympathetic.

• Postganglionic Effects: Varies depending on division; norepinephrine is common in sympathetic pathways but ACh can be used in certain parasympathetic pathways.

Anatomy of the Sympathetic Nervous System

Sympathetic Chain Ganglia

• Located alongside the vertebral column, containing clusters of cell bodies.

• Connected to spinal nerves via two communicating rami:

  • White Ramus: Contains myelinated preganglionic fibers.

  • Gray Ramus: Contains unmyelinated postganglionic fibers.

Pathways of the Sympathetic Nervous System

• Four different pathways for sympathetic fibers:

  1. Same Level Pathway: Preganglionic fibers synapse with postganglionic fibers at the same spinal cord level.

  2. Ascending Pathway: Fibers ascend to higher ganglia before synapsing.

  3. Descending Pathway: Fibers descend to lower ganglia before synapsing.

  4. Splanchnic Nerves: Skip the sympathetic chain ganglia and synapse on distant ganglia in the abdomen/pelvis.

Sympathetic vs. Parasympathetic Responses

• Sympathetic: Prepare body for physical exertion (e.g., increased heart rate).

• Parasympathetic: Restorative functions during safety and relaxation (e.g., increased digestive activity).

Location of Autonomic Neurons

• Preganglionic Neurons: Located in the lateral horn of the spinal cord, specific to T1 to L2 for sympathetic neurons.

• Thoracolumbar Division: Referring to the sympathetic division based on the location of preganglionic cell bodies.

Representation of Fiber Types

• Myelinated Preganglionic: Travel through white rami.

• Unmyelinated Postganglionic: Travel through gray rami to reach their destinations.