Study Guide: Nervous System – Spinal Cord, CNS Sensation, PNS, Reflexes, and Autonomic Nervous System

The Spinal Cord

Regions and Structure of the Spinal Cord

The spinal cord is a major component of the central nervous system (CNS), responsible for transmitting neural signals between the brain and the rest of the body. It consists of distinct regions and contains both gray and white matter.

• Gray Matter: Contains neuronal cell bodies, dendrites, and unmyelinated axons. Organized into dorsal (posterior), ventral (anterior), and lateral horns.

• White Matter: Composed of myelinated axons organized into ascending (sensory) and descending (motor) tracts.

• Ascending Tracts: Carry sensory information to the brain.

• Descending Tracts: Transmit motor commands from the brain to the body.

Layers of Meninges:

• Dura Mater: Outermost, tough protective layer.

• Arachnoid Mater: Middle, web-like layer.

• Pia Mater: Innermost, delicate layer adhering to the spinal cord.

Dorsal vs. Ventral Roots:

• Dorsal Roots: Carry sensory (afferent) information into the spinal cord.

• Ventral Roots: Carry motor (efferent) information out to muscles and glands.

Cauda Equina: A bundle of spinal nerves and roots at the lower end of the spinal cord, resembling a horse's tail.

Role of the CNS in Sensation

Pathways and Sensory Homunculus

The CNS processes sensory information through specific pathways and regions.

• Dorsal Column-Medial Lemniscus: Transmits fine touch, vibration, and proprioception.

• Anterolateral System: Conveys pain, temperature, and crude touch.

• Sensory Homunculus: A cortical map representing the body regions according to the density of sensory receptors.

Example: The hand occupies a large area on the sensory homunculus due to its high sensitivity.

Role of CNS in Voluntary Movement

Basal Ganglia and Cerebellum

Voluntary movement is coordinated by several CNS structures.

• Basal Ganglia: Involved in the initiation and regulation of movement.

• Cerebellum: Coordinates timing, precision, and accuracy of movements.

Example: Damage to the cerebellum can result in ataxia, a loss of coordination.

Overview of the Peripheral Nervous System (PNS)

Cranial and Spinal Nerves

The PNS connects the CNS to limbs and organs via nerves.

• Cranial Nerves: Twelve pairs, each with specific sensory, motor, or mixed functions.

• Spinal Nerves: Thirty-one pairs, named according to their vertebral level.

Cranial Nerve	Type	Main Function
Olfactory (I)	Sensory	Smell
Optic (II)	Sensory	Vision
Oculomotor (III)	Motor	Eye movement
Additional info: ...	...	...

Reflexes

Reflex Arc and Classification

Reflexes are rapid, involuntary responses to stimuli, mediated by neural pathways called reflex arcs.

• Components of a Reflex Arc: Receptor, sensory neuron, integration center, motor neuron, effector.

• Monosynaptic Reflex: Direct communication between sensory and motor neuron (e.g., patellar reflex).

• Polysynaptic Reflex: Involves one or more interneurons (e.g., withdrawal reflex).

Example: The crossed extensor reflex helps maintain balance when withdrawing from a painful stimulus.

Autonomic Nervous System (ANS)

Sympathetic vs. Parasympathetic Divisions

The ANS regulates involuntary physiological functions and is divided into sympathetic and parasympathetic branches.

• Sympathetic Division: Prepares the body for 'fight or flight' responses; short preganglionic and long postganglionic axons; releases norepinephrine at effectors.

• Parasympathetic Division: Promotes 'rest and digest' activities; long preganglionic and short postganglionic axons; releases acetylcholine at effectors.

Division	Preganglionic Axon Length	Postganglionic Axon Length	Main Neurotransmitter	General Function
Sympathetic	Short	Long	Norepinephrine	Fight or flight
Parasympathetic	Long	Short	Acetylcholine	Rest and digest

Example: Sympathetic activation increases heart rate, while parasympathetic activation decreases it.

Additional info: The study guide covers topics from chapters 11–14 and 15 (Nervous System and Special Senses) and chapter 16 (Autonomic Nervous System) of a typical Anatomy & Physiology curriculum.