29 - CNS: Structure, Function, and Voluntary Motor Control

CNS: Gray and White Matter

Overview of CNS Structure

The central nervous system (CNS) is composed of the brain and spinal cord, which are organized into regions of gray matter and white matter. These regions have distinct cellular compositions and functions essential for neural processing and communication.

• Gray matter: Contains neuron cell bodies, dendrites, and axon terminals. It is the site of synaptic integration and processing.

• White matter: Composed primarily of myelinated axons, which facilitate rapid transmission of electrical signals across different CNS regions.

• Oligodendrocytes: Glial cells responsible for myelinating axons in the CNS, increasing conduction velocity.

Functional Organization

• Gray matter forms the outer cortex of the brain and the central region of the spinal cord.

• White matter forms internal tracts in the brain and surrounds the gray matter in the spinal.

• Tracts in white matter are classified as association fibers (connect regions within the same hemisphere), commissural fibers (connect hemispheres), and projection fibers (connect cortex with lower CNS regions).

Spinal Cord Structure and Nerves

Origin of Spinal and Cranial Nerves

The spinal cord is the main conduit for information between the brain and the peripheral nervous system. It gives rise to 31 pairs of spinal nerves, each serving specific body regions.

• Spinal nerves: 31 pairs, each emerging from the spinal cord and innervating specific body segments.

• Cranial nerves: 12 pairs, with 10 originating from the brainstem, responsible for sensory and motor functions of the head and neck.

Dermatomes & Myotomes

Functional Segmentation

Spinal nerves serve distinct regions of the body, which can be mapped as dermatomes and myotomes.

• Dermatomes: Areas of skin supplied by sensory fibers of a single spinal nerve.

• Myotomes: Groups of muscles innervated by motor fibers of a single spinal nerve.

• These maps are clinically important for diagnosing nerve injuries and neurological disorders.

Spinal Cord (Cross Section) & Spinal Nerves

Functional Halves of Gray Matter

The spinal cord's gray matter is divided into dorsal and ventral regions, each with distinct functions.

• Dorsal (posterior) horn: Responsible for sensory processing; receives input from sensory neurons.

• Ventral (anterior) horn: Responsible for motor output; contains motor neurons that project to muscles.

• Spinal nerves: Mixed nerves carrying both sensory (afferent) and motor (efferent) fibers.

Spinal Cord: Tracts

White Matter Tracts

White matter in the spinal cord is organized into tracts that carry information to and from the brain.

• Ascending tracts: Carry sensory information from the body to the brain.

• Descending tracts: Carry motor commands from the brain to the body.

Spinal Cord: Ascending & Descending Tracts

Pathways of Neural Transmission

Neural signals travel through specific pathways in the spinal cord to reach their destinations.

• Ascending tracts: Example: Pain receptor signals travel via the spinothalamic tract to the thalamus and somatosensory cortex.

• Descending tracts: Example: Motor commands from the motor cortex travel via the corticospinal tract to skeletal muscles.

9.5: Voluntary Motor Control

Steps in Voluntary Movement

• 1) Idea: Initiation of movement in the limbic system(hippocampus + amygdala) and prefrontal association areas(planning, decision-making, problem-solving, moderating social behavior, and controlling emotions, essentially coordinating complex, goal-directed actions by integrating sensory, motor, and emotional information.

  • based on pas experiences, emotions, motivations, and sensory input -> INTEGRATE info into an intention & initial plan.

2) Program: Motor commands are organized in the supplementary motor area, premotor area, and primary motor cortex.

3) Execution: Motor signals travel through pyramidal and extrapyramidal tracts to activate motor neurons and skeletal muscles.

• .

4) Feedback: Sensory systems, cerebellum, thalamus, basal nuclei, and brainstem provide feedback to refine and adjust movement.

Process Flow

• Stimulus detected by sensory receptor

• Afferent neuron transmits signal to CNS

• Integration center processes information

• Efferent neuron carries command to effector (muscle/gland)

• Response occurs via pyramidal tracts(gross motor) & extrapyramidal tracts(tweaks for balance, posture, proprioreceptions VIA. input to brainstem from skin receptors, eyes, ears, proprioreceptors, vestibular apparatus/middle ear- 3 dimensional axis that tells you where you are) with feedback for adjustment.

Functional Areas of Cerebrum

Specialized Cortical Regions

The cerebrum contains distinct functional areas responsible for sensory perception, motor control, and higher cognitive functions.

• Primary motor cortex: Controls voluntary movements.

• Premotor cortex: Coordinates complex movements.

• Prefrontal association area: Involved in planning, reasoning, and voluntary behavior.

• Primary somatosensory cortex: Processes tactile and proprioceptive information.

• Visual, auditory, and sensory association areas: Integrate and interpret sensory input.

• Broca's area: Speech formation.

• Wernicke's area: Language comprehension.

• Limbic association cortex: Emotion, learning, and memory.

Summary Table: CNS Gray vs. White Matter

Feature	Gray Matter	White Matter
Main Components	Neuron cell bodies, dendrites, synapses	Myelinated axons
Location in Brain	Cortex (outer layer)	Internal tracts
Location in Spinal Cord	Central region	Surrounds gray matter
Function	Processing, integration	Signal transmission

Additional info:

• Motor control and sensory processing in the CNS occur rapidly, allowing for quick and coordinated voluntary movements.

• Damage to specific tracts or functional areas can result in characteristic neurological deficits, useful for clinical diagnosis.

• Parnell processing

• basal neuclei: feedback control of voluntary movements & select purposeful over unwanted movements??

• Parkinson's disease-> lack of dopamine (receptor that coordinates movements & feelings).\

CONFUSED:

• spinal cord ascending and descending tracts Figure 9.19 a,b