Nervous System II: Structure, Protection, and Peripheral Organization

Nervous System Organization

Central and Peripheral Nervous Systems

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord, while the PNS includes cranial and spinal nerves that connect the CNS to the rest of the body.

• CNS: Integrates sensory information and coordinates motor output.

• PNS: Transmits sensory input to the CNS and motor output from the CNS to effectors.

• PNS Divisions:

  • Somatic Sensory: Sensations from skin, muscles, joints.

  • Autonomic Sensory: Sensations from internal organs.

  • Somatic Motor: Voluntary control of skeletal muscles.

  • Autonomic Motor: Involuntary control of smooth muscle, cardiac muscle, and glands (includes sympathetic and parasympathetic divisions).

  • Enteric Motor: Controls gastrointestinal tract.

The Spinal Cord

Function and Structure

The spinal cord is a major conduit for sensory and motor information between the body and the brain. It also mediates spinal reflexes.

• Function:

  • Transmits sensory information from the PNS to the brain.

  • Sends motor signals from the brain to the PNS.

  • Responsible for spinal reflexes (automatic responses to stimuli).

• Location:

  • Extends from the inferior part of the medulla oblongata.

  • Passes through the foramen magnum and the vertebral foramen of each vertebra, forming the vertebral canal.

  • At L1 vertebra, the spinal cord ends and becomes the cauda equina (bundle of spinal nerves floating in CSF).

  • The tip of the spinal cord is called the conus medullaris.

Gray Matter vs White Matter

The spinal cord contains both gray and white matter, each with distinct roles.

• Gray Matter:

  • Shaped like an H or butterfly.

  • Divided into horns:

    • Lateral Horns: Present only in thoracic region; associated with sympathetic division of ANS.

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

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

• White Matter:

  • Surrounds gray matter.

  • Composed of myelinated axons (tracts) carrying information up and down the spinal cord.

  • Divided into columns:

    • Dorsal (posterior) columns

    • Ventral (anterior) columns

    • Lateral columns

Protection of the CNS

Bones, Meninges, and Cerebrospinal Fluid (CSF)

The CNS is protected by physical and chemical barriers.

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

• Meninges: Three layers of connective tissue covering the brain and spinal cord:

  • Dura mater: Tough outer layer; attached to skull bones and forms dural venous sinuses.

  • Arachnoid mater: Middle layer; beneath it is the subarachnoid space filled with CSF.

  • Pia mater: Thin inner layer; follows the surface of the brain and spinal cord.

• Spaces:

  • Epidural space: Between dura and bone; filled with fat (spinal cord) or potential space (brain).

  • Subdural space: Potential space under dura mater.

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

Cerebrospinal Fluid (CSF)

Definition and Function

Cerebrospinal fluid (CSF) is a clear, colorless fluid filtered from the blood, filling the ventricles and subarachnoid space.

• Functions:

  • Cushions and protects the brain and spinal cord.

  • Removes waste products.

  • Provides a stable chemical environment.

  • Delivers nutrients to CNS tissues.

Production and Circulation

• Production:

  • Produced in the ventricles of the brain by ependymal cells lining the ventricles.

  • Choroid plexus (network of capillaries) filters fluid from blood through ependymal cells to create CSF.

• Circulation:

  • CSF flows from lateral ventricles → interventricular foramen → 3rd ventricle → cerebral aqueduct → 4th ventricle → central canal (spinal cord) and subarachnoid space (brain and spinal cord).

• Removal:

  • CSF is absorbed into the venous system via arachnoid villi protruding into the superior sagittal sinus.

  • Excess CSF drains into the internal jugular vein.

Clinical Note: Hydrocephalus

• Hydrocephalus: Excess CSF builds up in the ventricles, increasing pressure on the brain. Often caused by blockage (e.g., in the cerebral aqueduct), preventing CSF from circulating and draining properly.

The Peripheral Nervous System

Nerves and Spinal Nerves

The PNS consists of bundles of myelinated axons called nerves, which can be sensory, motor, or mixed.

• Nerve: Bundle of myelinated axons in the PNS. Can contain axons of sensory neurons, motor neurons, or both.

• Spinal Nerve: Bundle of axons going to/from the spinal cord. Most are mixed nerves (contain both sensory and motor axons).

• Afferent neurons: Sensory neurons sending information to the CNS.

• Efferent neurons: Motor neurons sending information from the CNS to effectors.

Spinal Nerve Structure

• Spinal nerves branch off the spinal cord and enter/exit via the intervertebral foramen.

• Sensory neurons: Dendrites in the periphery; cell bodies in the dorsal root ganglion; axons enter the spinal cord via the dorsal root into the dorsal horn.

• Motor neurons: Cell bodies in the ventral horn; axons exit the spinal cord via the ventral root.

Summary Table: CNS and PNS Organization

Division	Main Components	Function
CNS	Brain, Spinal Cord	Integration, coordination, reflexes
PNS	Cranial Nerves, Spinal Nerves	Sensory input, motor output
Somatic Motor (PNS)	Skeletal muscles	Voluntary movement
Autonomic Motor (PNS)	Sympathetic, Parasympathetic	Involuntary control of smooth/cardiac muscle, glands

Key Terms and Definitions

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): Nerves outside the CNS.

• Meninges: Three protective layers around the CNS (dura mater, arachnoid mater, pia mater).

• Cerebrospinal Fluid (CSF): Fluid that cushions, nourishes, and protects the CNS.

• Gray Matter: Neuron cell bodies and dendrites.

• White Matter: Myelinated axons (tracts).

• Spinal Nerve: Mixed nerve carrying sensory and motor information to/from the spinal cord.

• Dorsal Root Ganglion: Cluster of sensory neuron cell bodies in the PNS.

• Arachnoid Villi: Structures that return CSF to the venous system.

• Hydrocephalus: Condition of excess CSF in the ventricles.

Relevant Equations

• CSF Flow Pathway:

$\text{Lateral ventricle} \rightarrow \text{Interventricular foramen} \rightarrow \text{3rd ventricle} \rightarrow \text{Cerebral aqueduct} \rightarrow \text{4th ventricle} \rightarrow \text{Central canal/subarachnoid space}$

Additional info: These notes expand on the anatomical and physiological organization of the nervous system, focusing on the CNS and PNS, their protection, and the role of CSF. The content is suitable for psychology students studying biological psychology and neuroanatomy.