Study Guide: Nervous System – Spinal Cord, 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 tracts, each with specialized functions.

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

• White Matter: Composed of myelinated axons forming 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.

• Three Layers of Meninges: Dura mater (outer), arachnoid mater (middle), pia mater (inner).

• Dorsal Root: Contains sensory (afferent) fibers.

• Ventral Root: Contains motor (efferent) fibers.

Example: The dorsal column-medial lemniscus pathway transmits fine touch and proprioceptive information.

CNS in Sensation

Pathways and Sensory Homunculus

The central nervous system processes sensory input through specific pathways and regions, such as the sensory homunculus.

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

• Anterolateral System: Carries pain and temperature information.

• Sensory Homunculus: A cortical map representing the body’s sensory input distribution.

Example: The sensory homunculus in the postcentral gyrus shows which body parts have greater sensory representation.

Voluntary Movement

Basal Ganglia and Cerebellum

Voluntary movement is coordinated by several CNS structures, including the basal ganglia and cerebellum.

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

• Cerebellum: Coordinates timing and precision of movements.

Example: Damage to the basal ganglia can result in movement disorders such as Parkinson’s disease.

Peripheral Nervous System (PNS)

Cranial and Spinal Nerves

The PNS consists of nerves that connect the CNS to limbs and organs. It includes cranial and spinal nerves.

• Cranial Nerves: 12 pairs, each with specific sensory, motor, or mixed functions.

• Spinal Nerves: 31 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: ...	...	...

Example: The vagus nerve (X) controls heart rate, digestion, and other autonomic functions.

Reflexes

Reflex Arc and Classification

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

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

• Monosynaptic Reflex: Involves a single synapse (e.g., knee-jerk reflex).

• Polysynaptic Reflex: Involves multiple synapses (e.g., withdrawal reflex).

• Crossed Extensor Reflex: Maintains balance during withdrawal.

Example: The patellar reflex is a classic monosynaptic reflex used in neurological exams.

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.

• Parasympathetic Division: Promotes 'rest and digest' activities.

• Preganglionic Axons: Sympathetic are short; parasympathetic are long.

• Postganglionic Axons: Sympathetic are long; parasympathetic are short.

• Neurotransmitters: Sympathetic releases norepinephrine; parasympathetic releases acetylcholine.

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: The sympathetic division increases heart rate and dilates pupils, while the parasympathetic division slows heart rate and stimulates digestion.

Additional info: The study notes above expand on the exam topics by providing definitions, examples, and tables for comparison, ensuring a comprehensive review for Anatomy & Physiology students.