Motor Pathways & Autonomic Nervous System: Study Notes

Motor Pathways & Autonomic Nervous System

Introduction

The nervous system is divided into the somatic and autonomic divisions, each responsible for controlling different aspects of body function. The somatic nervous system (SNS) governs voluntary movements, while the autonomic nervous system (ANS) regulates involuntary physiological processes. This guide covers the structure and function of motor pathways, the differences between somatic and autonomic systems, and the anatomy of the sympathetic chain.

Pyramidal (Corticospinal) Motor Pathways

Overview

The pyramidal pathways, primarily the corticospinal tracts, are responsible for voluntary control of skeletal muscles. These pathways originate in the cerebral cortex and descend through the brainstem and spinal cord.

• Upper Motor Neurons (UMNs): Originate in the primary motor cortex (precentral gyrus) and descend through the internal capsule, brainstem, and spinal cord.

• Lower Motor Neurons (LMNs): Located in the anterior horn of the spinal cord; their axons exit the CNS to innervate skeletal muscles.

• Decussation: Most corticospinal fibers cross (decussate) at the medullary pyramids, forming the lateral corticospinal tract. A smaller portion remains uncrossed as the ventral corticospinal tract.

Key Structures:

• Precentral Gyrus: Location of primary motor cortex.

• Internal Capsule: White matter tract carrying motor signals from cortex to brainstem.

• Medullary Pyramids: Site of decussation for most corticospinal fibers.

• Spinal Cord: Contains both lateral and ventral corticospinal tracts.

Example: Voluntary movement of the fingers is controlled by the lateral corticospinal tract, which decussates at the medulla and synapses with LMNs in the spinal cord.

Somatic vs. Autonomic Nervous System

Structural and Functional Differences

The somatic and autonomic nervous systems differ in their control, efferent pathways, effector organs, and effects on target tissues.

	SOMATIC	AUTONOMIC
Control	Voluntary	Involuntary
Efferents	1 neuron from spinal cord; cell body in ventral grey horn or cranial nerve nuclei in brainstem	2 neurons: 1st cell body in lateral grey horn or cranial nerve nuclei in brainstem; 2nd cell body in peripheral ganglia
Effector Organs	Skeletal muscle	Smooth & cardiac muscle; secretory glands
Effect	Excitation	Excitation or inhibition

Example: The somatic system controls biceps contraction, while the autonomic system regulates heart rate and digestion.

Autonomic Nervous System (ANS)

General Organization

The ANS is divided into sympathetic and parasympathetic divisions, each with distinct anatomical and functional characteristics.

• Preganglionic Neuron: Cell body in CNS (lateral horn of spinal cord or brainstem); axon is myelinated.

• Postganglionic Neuron: Cell body in autonomic ganglion; axon is unmyelinated and projects to effector organ.

• Ganglion: A collection of neuron cell bodies outside the CNS.

Pathway:

• Preganglionic axon exits CNS → synapses in autonomic ganglion → postganglionic axon innervates target organ.

Sympathetic Division

The sympathetic division prepares the body for emergency situations ('fight or flight').

• Origin: Thoracolumbar region (T1-L2) of spinal cord.

• Ganglia: Sympathetic chain (paravertebral ganglia) and collateral ganglia.

• Preganglionic fibers: Short; postganglionic fibers are long.

• Exception: Preganglionic fibers to the adrenal medulla synapse directly with chromaffin cells, releasing neurotransmitters into the bloodstream.

Functions:

• Increases heart rate and blood pressure

• Dilates pupils and airways

• Decreases gastrointestinal activity

Parasympathetic Division

The parasympathetic division conserves energy and promotes 'rest and digest' activities.

• Origin: Craniosacral region (brainstem and sacral spinal cord S2-S4).

• Ganglia: Located near or within target organs.

• Preganglionic fibers: Long; postganglionic fibers are short.

Functions:

• Decreases heart rate

• Constricts pupils and airways

• Stimulates gastrointestinal activity

Sympathetic Chain and Collateral Ganglia

Sympathetic Chain (Paravertebral Ganglia)

The sympathetic chain consists of paired ganglia running alongside the vertebral column, connected by nerve fibers. It allows sympathetic fibers to be distributed throughout the body.

• Location: Lateral to the vertebral bodies from the cervical to sacral region.

• Function: Distributes sympathetic innervation to visceral organs and blood vessels.

Collateral Ganglia

Collateral (prevertebral) ganglia are located anterior to the vertebral column and supply abdominal and pelvic organs.

• Main Collateral Ganglia: Celiac, superior mesenteric, and inferior mesenteric ganglia.

• Function: Innervate abdominal viscera via splanchnic nerves.

Neuroendocrine Component: Adrenal Medulla

The adrenal medulla acts as a modified sympathetic ganglion. Preganglionic sympathetic fibers stimulate chromaffin cells to release epinephrine and norepinephrine into the bloodstream.

• Function: Rapid, widespread sympathetic response.

Functional Effects of Sympathetic and Parasympathetic Stimulation

Comparison Table

Structure	Sympathetic	Parasympathetic
Heart	↑ heart rate, ↑ force contraction (↑ blood pressure)	↓ heart rate, ↓ force contraction (↓ blood pressure)
Eye	Dilates pupil	Constricts pupil
Airways	↑ airway diameter	↓ airway diameter
Blood vessels	↓ GIT blood flow, ↑ skeletal muscle blood flow	Not innervated
GIT Glands	Inhibition (e.g., ↓ insulin production)	Stimulation (e.g., ↑ insulin production)

Review Questions (Sample)

Motor Pathways

• Where are the cell bodies of upper motor neurons for the lateral corticospinal tract located? Answer: Primary motor cortex (precentral gyrus).

• What structure carries the axons of upper motor neurons through the midbrain? Answer: Cerebral peduncles.

• Where do upper motor neurons of the lateral corticospinal tract decussate? Answer: Pyramids of the medulla oblongata.

• Where do upper motor neurons of the ventral corticospinal tract decussate? Answer: At the level of the spinal cord segment where they terminate.

• What structure houses the cell bodies of lower motor neurons? Answer: Anterior (ventral) horn of the spinal cord.

Sympathetic Effects

• Heart: Increases heart rate and force of contraction.

• Pupil of the eye: Dilates pupil.

• Airways of the lungs: Dilates airways.

• Blood vessels supplying the gut: Constricts, reducing blood flow.

• Gastrointestinal glands: Inhibits secretion.

Key Terms

• Upper Motor Neuron (UMN): Neuron originating in the motor cortex and projecting to the spinal cord.

• Lower Motor Neuron (LMN): Neuron with cell body in the spinal cord, projecting to skeletal muscle.

• Ganglion: Cluster of neuron cell bodies outside the CNS.

• Preganglionic/ Postganglionic Neuron: First and second neurons in a two-neuron autonomic pathway.

• Sympathetic Chain: Series of ganglia adjacent to the vertebral column.

• Collateral Ganglia: Prevertebral ganglia supplying abdominal organs.

Additional info: The notes are based on standard textbook content (Marieb, Martini, Snell) and are suitable for exam preparation in college-level Anatomy & Physiology courses.