The Nervous System — Pathways and Higher-Order Functions (Chapter 16 Study Notes)

Introduction

Overview of Nervous System Information Flow

The nervous system maintains a continuous flow of information between the brain, spinal cord, and peripheral nerves. This communication is essential for integrating sensory input, motor output, and higher-order functions. Subtle forms of interaction, feedback, and regulation link higher centers with various components of the brain stem, ensuring coordinated function.

• Central Nervous System (CNS): Composed of the brain and spinal cord.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS.

• Integration: Higher centers in the brain interact with the brain stem for regulation and feedback.

Sensory and Motor Pathways

Nerve Tracts Connecting CNS and PNS

Nerve pathways, known as tracts, connect the CNS and PNS. These tracts are classified as ascending (sensory) or descending (motor) pathways, and their names often indicate their origin and destination.

• Ascending (Sensory) Pathways: Carry sensory information from the body to the brain.

• Descending (Motor) Pathways: Transmit motor commands from the brain to effectors (muscles and glands).

Sensory Pathways

Sensory pathways transmit information about touch, pressure, pain, temperature, and proprioception. They typically involve three neurons:

• First-order neuron: Delivers sensory information to the CNS.

• Second-order neuron: An interneuron located in the spinal cord or brain stem.

• Third-order neuron: Carries information from the thalamus to the cerebral cortex.

Main sensory pathways include:

• Posterior column pathway: Fine touch, vibration, and proprioception.

• Spinothalamic pathway: Pain, temperature, crude touch, and pressure.

• Spinocerebellar pathway: Proprioceptive input to the cerebellum.

Principal Ascending Sensory Pathways Table

Pathway/Tract	Sensation	First Order	Second Order	Third Order	Site of Decussation	Final Destination
Posterior Column Pathway	Fine touch, vibration, proprioception	Dorsal root ganglia of spinal nerves	Nucleus gracilis/cuneatus of medulla oblongata	Thalamic nuclei	Medulla oblongata	Primary sensory cortex
Spinothalamic Pathway	Pain, temperature, crude touch, pressure	Dorsal root ganglia of spinal nerves	Posterior horns of spinal cord	Thalamic nuclei	Spinal cord	Primary sensory cortex
Spinocerebellar Pathway	Proprioceptive input	Dorsal root ganglia of spinal nerves	Posterior horns of spinal cord	Not present	Spinal cord	Cerebellar cortex

Motor Pathways

Motor pathways transmit commands from the CNS to effectors. They usually contain two neurons:

• Somatic Nervous System (SNS):

  • Upper motor neuron: Located within the CNS.

  • Lower motor neuron: Extends from the CNS to the effector (skeletal muscle).

• Autonomic Nervous System (ANS):

  • Preganglionic neuron: Originates in the CNS.

  • Ganglionic neuron: Located in autonomic ganglia, innervates visceral effectors.

Main motor pathways include:

• Corticospinal pathway: Voluntary control of skeletal muscles.

• Corticobulbar tracts: Control muscles of the face and neck.

• Medial and lateral pathways: Subconscious regulation of balance, muscle tone, and posture.

Principal Descending (Motor) Pathways Table

Pathway/Tract	Nucleus of Upper Motor Neuron	Site of Decussation	Action
Corticospinal Pathway	Primary motor cortex	Medulla oblongata (pyramidal decussation)	Conscious control of skeletal muscles
Medial Pathway	Vestibular nuclei, reticular formation, superior/inferior colliculi	Varies by tract	Subconscious regulation of balance, muscle tone, and posture
Lateral Pathway	Red nucleus	Midbrain	Subconscious regulation of upper limb muscle tone and movement

Higher-Order Functions

Characteristics of Higher-Order Functions

Higher-order functions are complex processes performed by the cerebral cortex. They involve both conscious and unconscious information processing and are subject to modification and learning over time.

• Performed by the cerebral cortex

• Involve complex interconnections within and between cortical areas

• Include conscious and unconscious processing

• Not hardwired; can be modified (learning)

Functional Areas of the Cerebral Cortex

The cerebral cortex contains specialized regions for processing sensory and motor information, as well as integrative functions.

• Primary motor cortex: Controls voluntary movements.

• Primary sensory cortex: Receives somatic sensory information.

• Association areas: Interpret incoming data or coordinate motor responses.

Integrative Regions of the Cerebral Cortex

• General interpretive area (Wernicke's area): Receives information from all sensory association areas; usually present in the left hemisphere.

• Speech center (Broca's area): Regulates patterns of breathing and vocalization.

• Prefrontal cortex: Coordinates information from secondary and special association areas; performs abstract intellectual functions.

Hemispheric Specialization

The left and right hemispheres of the brain have specialized functions:

• Left hemisphere: Language, mathematical computation, logical reasoning.

• Right hemisphere: Spatial visualization, analysis by touch, artistic and musical abilities.

Memory

Memory is the process of accessing stored information acquired through experience. It is categorized as short-term or long-term, and involves consolidation processes.

• Short-term memory: Lasts seconds to hours.

• Long-term memory: Can last for years; stored in the cerebral cortex.

• Memory consolidation: Conversion from short-term to long-term memory, involving the amygdaloid body and hippocampus.

Reticular Activating System (RAS)

The RAS is a network within the brainstem that regulates states of consciousness and alertness by integrating sensory input and modulating cortical activity.

States of Awareness Table

Level or State	Description
Conscious States	Includes alertness, disorientation, confusion, and normal consciousness
Unconscious States	Includes sleep, stupor, and coma

Aging and the Nervous System

Age-Related Anatomical Changes

As individuals age, several anatomical changes occur in the nervous system, affecting its function and efficiency.

• Reduction in brain size and weight

• Reduction in the number of neurons

• Decrease in blood flow to the brain

• Changes in synaptic organization

• Intracellular and extracellular changes in CNS neurons

Example:

Age-related decline in memory and cognitive function is often associated with reduced synaptic density and changes in neurotransmitter levels.

Additional info: The study of nervous system pathways and higher-order functions is essential for understanding sensory perception, motor control, learning, memory, and the impact of aging on neural processes.