Chapter 16: Sensory, Motor, and Integrative Systems

Introduction

This chapter explores the mechanisms by which the nervous system detects, processes, and integrates sensory information, and how it controls voluntary and involuntary movements. It also discusses higher-order functions such as sleep, memory, and language.

Sensation

Definition and Types of Sensation

• Sensation is the conscious or subconscious awareness of changes in the external or internal environment.

• Perception is the conscious interpretation of sensations, mainly by the cerebral cortex.

• Each type of sensation is called a sensory modality (e.g., touch, pain, vision, hearing).

• Sensory modalities are grouped into:

  • General senses: somatic (tactile, thermal, proprioceptive) and visceral (pressure, chemicals, stretch, nausea, hunger, temperature).

  • Special senses: smell, taste, vision, hearing, and equilibrium.

Types of Sensory Receptors

• Free nerve endings: Detect pain, temperature, tickle, itch, and some touch.

• Encapsulated nerve endings: Detect pressure, vibration, and some touch.

• Specialized receptor cells: Include gustatory receptor cells (taste), photoreceptors (vision), and hair cells (hearing and equilibrium).

Classification of Sensory Receptors

• By location and origin of stimuli:

  • Exteroreceptors: Respond to external stimuli (e.g., hearing, vision, touch).

  • Interoreceptors: Monitor internal environment (e.g., blood pressure, chemical changes).

  • Proprioceptors: Provide information about body position, muscle length, and movement of joints.

• By microscopic structure and type of stimulus detected (e.g., mechanoreceptors, thermoreceptors, nociceptors).

Somatic Sensations

Types of Somatic Sensations

• Tactile sensations: Touch, pressure, vibration, itch, tickle.

• Thermal sensations: Temperature changes.

• Pain sensations: Fast pain (acute, sharp, pricking) and slow pain (chronic, burning, aching, throbbing).

• Proprioceptive sensations: Awareness of body position and movement.

Pain Classification

• Superficial somatic pain: From skin receptors.

• Deep somatic pain: From skeletal muscles, joints, tendons, and fascia.

• Visceral pain: From internal organs, often referred to skin overlying the organ.

Proprioceptors

• Muscle spindles: Located in skeletal muscles; monitor muscle length and participate in stretch reflexes.

• Tendon organs: Located at the junction of tendon and muscle; protect against overstretching.

• Joint kinesthetic receptors: In and around synovial joint capsules; respond to pressure, acceleration, and deceleration.

Summary Table: Receptors for Somatic Sensations

Somatic Sensory Pathways

Overview of Pathways

Somatic sensory pathways transmit information from receptors to the primary somatosensory area of the cerebral cortex and the cerebellum. Each pathway typically involves three neurons: first-order, second-order, and third-order neurons.

• First-order neurons: From receptors to spinal cord or brainstem.

• Second-order neurons: From spinal cord/brainstem to thalamus (decussate to opposite side).

• Third-order neurons: From thalamus to primary somatosensory cortex.

Major Somatic Sensory Pathways

• Posterior column–medial lemniscus pathway: Touch, pressure, vibration, proprioception from limbs, trunk, neck, and posterior head.

• Anterolateral (spinothalamic) pathway: Pain, temperature, itch, tickle from the body and posterior head.

• Trigeminothalamic pathway: Tactile, thermal, pain sensations from the face, nasal and oral cavities, and teeth.

• Spinocerebellar tracts: Proprioceptive impulses to the cerebellum for coordination, posture, and balance.

Pathway Details

• Posterior column–medial lemniscus pathway:

  • First-order neurons ascend ipsilaterally to the medulla, synapse with second-order neurons, which decussate and ascend to the thalamus. Third-order neurons project to the cortex.

• Anterolateral (spinothalamic) pathway:

  • First-order neurons synapse in the posterior gray horn. Second-order neurons decussate and ascend to the thalamus. Third-order neurons project to the cortex.

• Trigeminothalamic pathway:

  • First-order neurons synapse in the pons and medulla. Second-order neurons decussate and ascend to the thalamus. Third-order neurons project to the cortex.

• Spinocerebellar tracts:

  • Convey proprioceptive information to the cerebellum without crossing over, allowing for coordination of movement.

Somatic Motor Pathways

Lower and Upper Motor Neurons

• Lower motor neurons (LMNs): Originate in the brainstem and spinal cord; innervate skeletal muscles via cranial and spinal nerves.

• Upper motor neurons (UMNs): Originate in the cerebral cortex or brainstem; provide input to LMNs directly or via local circuit neurons.

• Basal nuclei neurons: Modulate movement by influencing UMNs.

• Cerebellar neurons: Coordinate movement by influencing UMNs.

Motor Pathways

• Direct motor pathways (pyramidal tracts):

  • Lateral corticospinal tract: Precise, voluntary movements of distal limbs; 90% of fibers decussate in the medulla.

  • Anterior corticospinal tract: Movements of trunk and proximal limbs; 10% of fibers decussate at the spinal cord level.

  • Corticobulbar tract: Voluntary movements of head and neck via cranial nerves.

• Indirect motor pathways (extrapyramidal tracts):

  • Rubrospinal tract: Precise movements of upper limbs.

  • Tectospinal tract: Reflexive movements of head, eyes, trunk in response to visual/auditory stimuli.

  • Vestibulospinal tract: Posture and balance in response to head movements.

  • Lateral and medial reticulospinal tracts: Posture and muscle tone regulation.

Role of the Cerebellum

• Monitors intentions for movement.

• Monitors actual movement.

• Compares command signals with sensory information.

• Sends out corrective feedback to coordinate, smooth, and refine movements.

Integrative Functions of the Cerebrum

Wakefulness and Sleep

• Regulated by the reticular activating system (RAS).

• Includes REM sleep (rapid eye movement) and NREM sleep (non-rapid eye movement).

• Coma: State of unconsciousness with little or no response to stimuli.

Learning, Memory, and Language

• Memory types:

  • Immediate memory

  • Short-term memory

  • Long-term memory

• Language areas:

  • Wernicke’s area: Language comprehension.

  • Broca’s area: Motor aspects of speech.

Disorders

Parkinson’s Disease

• Typically affects individuals around age 60.

• Characterized by insufficient dopamine production.

• Symptoms include tremor, bradykinesia (slowness of movement), and hypokinesia (decreased movement).

Summary Table: Major Somatic Sensory and Motor Pathways

Example: Voluntary Movement

When you decide to pick up a pencil, the primary motor cortex sends signals via the lateral corticospinal tract to the muscles of your hand and fingers, allowing for precise movement. The cerebellum monitors and adjusts the movement for accuracy.

Additional info: The above notes expand on the slide content to provide definitions, pathway details, and clinical context for exam preparation.