Anatomy & Physiology: Central Nervous System and Spinal Cord Study Notes

Central Nervous System (CNS)

Overview of the CNS

The central nervous system consists of the brain and spinal cord, serving as the main control center for the body. It processes sensory information, coordinates voluntary and involuntary responses, and is responsible for higher mental functions.

• Brain: Divided into several regions including the cerebrum, cerebellum, and brainstem.

• Spinal Cord: Connects the brain to peripheral nerves and mediates reflexes.

Spinal Cord Anatomy and Physiology

Gray and White Matter

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

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

• White Matter: Composed of myelinated axons organized into tracts that carry information up and down the spinal cord.

• Posterior Gray Horns: Contain sensory neurons. Damage to these cells can result in loss of sensory function below the level of injury.

Functions of the Spinal Cord

• Reflex Center: Mediates simple reflexes such as the stretch reflex.

• Conduction Pathway: Transmits sensory and motor information between the body and brain.

Spinal Cord Tracts

Tracts are bundles of axons in the CNS. They are classified as ascending (sensory) or descending (motor).

• Ascending Tracts: Carry sensory information to the brain (e.g., spinothalamic tract, dorsal columns).

• Descending Tracts: Carry motor commands from the brain to the body (e.g., corticospinal tract).

Decussation of Tracts

Many tracts cross (decussate) from one side of the CNS to the other, often in the medulla oblongata or spinal cord. This explains why each hemisphere of the brain controls the opposite side of the body.

• Lateral Corticospinal Tract: Decussates in the medullary pyramids.

• Anterior Corticospinal Tract: Decussates at the level of the spinal cord.

Brain Anatomy and Functions

Major Brain Regions

• Cerebrum: Responsible for higher mental functions, voluntary movement, and sensory perception.

• Cerebellum: Coordinates movement and balance.

• Brainstem: Includes the midbrain, pons, and medulla oblongata; controls vital functions such as breathing and heart rate.

Functional Areas of the Brain

• Primary Visual Cortex: Processes visual information; located in the occipital lobe.

• Auditory Cortex: Processes auditory information; located in the temporal lobe.

• Association Areas: Integrate sensory and motor information for complex processing.

• Prefrontal Cortex: Involved in complex mental functions, personality, and decision-making.

Ventricular System and Cerebrospinal Fluid (CSF)

• Ventricles: Cavities within the brain that produce and circulate CSF.

• CSF Functions: Cushions the brain, removes waste, and provides nutrients.

• CSF Formation: Produced by the choroid plexus in the ventricles.

Brain Nuclei and Cranial Nerves

• Basal Nuclei: Involved in movement regulation.

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

• Example: The oculomotor nerve (CN III) controls most eye movements.

Neural Pathways and Synapses

Motor and Sensory Pathways

• Upper Motor Neurons: Originate in the brain and synapse on lower motor neurons in the spinal cord.

• Lower Motor Neurons: Project from the spinal cord to muscles.

• Synapse: The junction where neurons communicate, often using neurotransmitters.

Fasciculus Cuneatus and Gracilis

• Fasciculus Cuneatus: Carries fine touch and proprioception from the upper body.

• Fasciculus Gracilis: Carries similar information from the lower body.

Clinical Applications

Spinal Tap (Lumbar Puncture)

• Definition: A procedure to collect CSF from the subarachnoid space, usually between L3-L4 or L4-L5 vertebrae.

• Purpose: Diagnose infections, hemorrhage, or neurological diseases.

Effects of Spinal Cord Damage

• Posterior Gray Horn Damage: Loss of sensory function below the lesion.

• Anterior Horn Damage: Loss of motor function.

Sample Table: Comparison of Ascending and Descending Tracts

Tract Type	Direction	Main Function	Example
Ascending	Spinal cord to brain	Sensory (touch, pain, proprioception)	Spinothalamic tract
Descending	Brain to spinal cord	Motor (voluntary movement)	Corticospinal tract

Key Terms and Definitions

• Decussation: The crossing of nerve fibers from one side of the CNS to the other.

• Reflex: An automatic, rapid response to a stimulus.

• Association Fibers: Connect different parts of the same cerebral hemisphere.

• Commissural Fibers: Connect the two hemispheres (e.g., corpus callosum).

• Projection Fibers: Connect the cortex with lower brain regions and spinal cord.

Equations and Formulas

• Nernst Equation (for membrane potential):

$E = \frac{RT}{zF} \ln \left( \frac{[\text{ion outside}]}{[\text{ion inside}]} \right)$

• Ohm's Law (for neural current):

$V = IR$

Summary

• The CNS is essential for integrating sensory input, coordinating movement, and higher mental functions.

• Damage to specific regions of the spinal cord or brain leads to predictable deficits.

• Understanding the structure and function of neural pathways is crucial for diagnosing and treating neurological disorders.

Additional info: Some explanations and definitions were expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.