Nervous System II: Spinal Cord, CNS Protection, Peripheral and Autonomic Nervous System

Nervous System Organization

Central and Peripheral Nervous System

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each division has distinct structural and functional roles in processing and transmitting information throughout the body.

• CNS: Composed of the brain and spinal cord; responsible for integrating sensory information and coordinating motor output.

• PNS: Consists of cranial and spinal nerves; transmits sensory input to the CNS and motor output from the CNS to effectors.

• Sensory Divisions: Somatic sensations (from skin, muscles, joints) and autonomic sensations (from internal organs).

• Motor Divisions: Somatic motor (skeletal muscle control), autonomic motor (sympathetic and parasympathetic control of smooth/cardiac muscle and glands), and enteric motor (gastrointestinal tract).

Division	Function	Target
Somatic Motor	Voluntary movement	Skeletal muscles
Autonomic Motor	Involuntary control	Cardiac/smooth muscle, glands
Enteric Motor	GI tract regulation	Digestive organs

The Spinal Cord

Function and Role

The spinal cord is a major conduit for information between the brain and the rest of the body. It is essential for both sensory transmission and motor control, as well as for mediating spinal reflexes.

• Sensory Transmission: Sends sensory information from the PNS to the brain.

• Motor Transmission: Sends motor signals from the brain to the PNS.

• Spinal Reflexes: Direct, rapid responses to stimuli that do not require brain input.

• Analogy: The spinal cord functions like a highway, efficiently routing information between the brain and peripheral structures.

Location and Structure

The spinal cord extends from the inferior aspect of the medulla oblongata and passes through the foramen magnum and the vertebral foramen of each vertebra, forming the vertebral canal.

• Foramen Magnum: Large opening at the base of the skull through which the spinal cord passes.

• Vertebral Canal: Formed by stacking vertebral foramina; houses and protects the spinal cord.

Cauda Equina and Conus Medullaris

At the level of L1 vertebra, the spinal cord terminates as the conus medullaris and "breaks up" into a bundle of nerve roots called the cauda equina.

• Cauda Equina: Collection of lumbar and sacral nerve roots floating in cerebrospinal fluid (CSF).

• Conus Medullaris: Tapered end of the spinal cord.

• Clinical Application: The cauda equina region allows for lumbar punctures (spinal taps) to sample CSF.

Spinal Cord: Gray vs White Matter

Gray Matter

Gray matter in the spinal cord is shaped like an H or butterfly and is divided into horns:

• Lateral Horns: Present only in the thoracic region; associated with the sympathetic division of the autonomic nervous system.

• Dorsal Horns: Located posteriorly; entry point for sensory neurons.

• Ventral Horns: Located anteriorly; exit point for motor neurons.

White Matter

White matter surrounds the gray matter and consists of myelinated axons organized into columns (tracts) that carry information up and down the spinal cord.

• Dorsal (Posterior) Columns: Carry sensory information to the brain.

• Ventral (Anterior) Columns: Carry motor information from the brain.

• Lateral Columns: Contain both sensory and motor tracts.

Region	Function
Dorsal Horn	Sensory input
Ventral Horn	Motor output
Lateral Horn	Sympathetic autonomic control

Protection of the CNS

Bones, Meninges,

The CNS is protected by multiple layers:

• Bones: Skull, vertebrae, and sacrum provide rigid protection.

• Meninges: Three layers of connective tissue that cover the brain and spinal cord.

The Meninges

The meninges consist of three layers:

• Dura Mater: Tough, outer layer; attached to the skull and vertebrae. In the brain, forms dural venous sinuses for blood drainage.

• Arachnoid Mater: Thin, middle layer; follows the dura. The subarachnoid space beneath is filled with CSF.

• Pia Mater: Very thin, inner layer; closely follows the surface of the brain and spinal cord.

Meningeal Spaces

• Epidural Space: Between dura mater and bone; filled with fat in the spinal cord, potential space in the brain.

• Subdural Space: Between dura and arachnoid mater; potential space.

• Subarachnoid Space: Between arachnoid and pia mater; actual space filled with CSF.

Space	Location	Contents
Epidural	Spinal cord/brain	Fat (spinal cord), potential space (brain)
Subdural	Under dura	Potential space
Subarachnoid	Under arachnoid	CSF

The Peripheral Nervous System (PNS)

Structure and Function

The PNS consists of cranial and spinal nerves that connect the CNS to limbs and organs. It is responsible for transmitting sensory and motor information.

• Nerve: A bundle of myelinated axons in the PNS.

• Axons: Can be from sensory neurons (somatic or visceral) or motor neurons (somatic or autonomic).

• Nerve Types: Sensory only, motor only, or mixed (both sensory and motor).

Nerve Type	Axon Composition
Sensory Only	Axons of sensory neurons
Motor Only	Axons of motor neurons
Mixed	Both sensory and motor axons

Autonomic Nervous System (ANS)

Sympathetic and Parasympathetic Divisions

The ANS regulates involuntary functions of organs and tissues. It is divided into:

• Sympathetic Division: Prepares the body for 'fight or flight' responses; increases heart rate, dilates pupils, inhibits digestion.

• Parasympathetic Division: Promotes 'rest and digest' activities; decreases heart rate, stimulates digestion, contracts pupils.

Physiological Effects: Each division has specific effects on target tissues and organs, maintaining homeostasis.

Example: Autonomic Control of the Heart

• Sympathetic: Increases heart rate and force of contraction.

• Parasympathetic: Decreases heart rate.

Additional info: The enteric nervous system is sometimes considered a third division of the ANS, primarily regulating gastrointestinal function.