Neurophysiology: The Brain & Sensory Systems

Learning Objectives

• Understand the divisions of the nervous system.

• Identify the locations and functions of major brain regions involved in sensation.

• Describe how different types of sensory information are detected and coded, including the concept of receptive fields.

• Trace the pathways carrying sensory information from the body to the brain.

• Explain how somatosensory information is mapped in the brain.

• Describe the process of taste perception.

Divisions of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. It acts as the main control center for processing and integrating information.

• Brain: Responsible for higher functions such as thought, sensation, and movement.

• Spinal Cord: Conducts signals to and from the brain and controls reflex activities.

Peripheral Nervous System (PNS)

The PNS connects the CNS to the rest of the body and is divided into several functional subdivisions:

• Afferent Division: Carries sensory information from receptors to the CNS.

• Efferent Division: Carries motor commands from the CNS to effectors (muscles and glands).

• Enteric Nervous System: A specialized network involved in the control of digestion.

Major Brain Areas Involved in Sensation

The Brainstem

The brainstem is the most primitive part of the brain and is essential for basic life functions.

• Controls vital functions such as breathing, heart rate, and blood pressure.

• Acts as a conduit for sensory and motor pathways between the body and higher brain regions.

• Composed of three main parts:

  • Medulla oblongata

  • Pons

  • Midbrain

The Diencephalon

The diencephalon is located deep within the brain and includes several important structures:

• Thalamus: Acts as a relay station for sensory information traveling to the cerebral cortex.

• Hypothalamus: Regulates homeostasis and behavioral drives (e.g., hunger, thirst, reproduction).

• Pituitary and Pineal Glands: Involved in hormone secretion and circadian rhythms.

The Cerebrum

The cerebrum is the largest part of the brain, responsible for higher cognitive functions, voluntary movement, and sensory perception.

• Divided into two hemispheres connected by the corpus callosum.

• The cerebral cortex is the outer layer, highly folded to increase surface area (gyri and sulci).

• Four main lobes:

  • Frontal lobe: Motor control, decision making, problem solving.

  • Parietal lobe: Sensory perception and integration.

  • Temporal lobe: Auditory processing, memory.

  • Occipital lobe: Visual processing.

The Limbic System

The limbic system is a group of structures involved in emotion, motivation, and memory.

• Includes the amygdala (emotion), hippocampus (memory), and cingulate cortex.

Sensory Systems Overview

Types of Sensory Systems

• Special Senses: Vision, hearing, taste, smell, equilibrium (balance).

• Somatic Senses: Touch, temperature, pain, itch, proprioception (body position).

Types of Sensory Receptors

Sensory receptors are specialized cells or structures that detect specific types of stimuli:

• Mechanoreceptors: Detect pressure, vibration, and stretch.

• Thermoreceptors: Detect changes in temperature.

• Photoreceptors: Detect light (in the retina).

• Chemoreceptors: Detect chemical stimuli (e.g., taste, smell).

• Nociceptors: Detect pain (potentially damaging stimuli).

Sensory Transduction and Coding

Sensory transduction is the process by which a stimulus is converted into an electrical signal in the nervous system.

• A stimulus activates a receptor, generating a graded potential.

• If the graded potential reaches threshold, it triggers action potentials in the sensory neuron.

• The frequency and pattern of action potentials encode information about stimulus intensity and duration.

Sensory Receptor Adaptation

• Tonic receptors: Respond continuously to a stimulus (e.g., pain receptors).

• Phasic receptors: Respond only when the stimulus changes (e.g., touch receptors that adapt quickly).

Receptive Fields and Sensitivity

A receptive field is the specific area monitored by a sensory neuron. The size and density of receptive fields determine sensitivity:

• Smaller, densely packed receptive fields = higher sensitivity (e.g., fingertips).

• Larger, sparsely distributed receptive fields = lower sensitivity (e.g., back).

Body regions with more sensory neurons and smaller receptive fields have greater tactile acuity.

Somatosensory Pathways

Somatosensory information (touch, proprioception, pain, temperature) is transmitted from the body to the brain via a series of neurons:

1. Primary afferent neurons: Sensory receptors in the skin or muscles send signals to the spinal cord or brainstem.

2. Secondary afferent neurons: Relay information to the thalamus.

3. Tertiary afferent neurons: Project from the thalamus to the somatosensory cortex for conscious perception.

The Somatosensory Cortex

The somatosensory cortex contains a map of the body called the sensory homunculus:

• Body regions with greater sensitivity (e.g., hands, lips) occupy larger areas of the cortex.

• Less sensitive regions (e.g., back, legs) occupy smaller areas.

Sensing Taste (Gustation)

Taste Buds and Taste Perception

• Taste buds contain 50–100 taste receptor cells, each sensitive to one of five basic tastes: sweet, salty, sour, bitter, umami.

• Other taste sensations (e.g., fat) are under investigation.

• Taste receptor cells transduce chemical signals from food into electrical signals.

• Signals are transmitted via primary gustatory neurons to the brain, where taste is perceived and integrated with emotional and memory centers.

Essential Terminology

• Special senses: Senses with specialized organs (vision, hearing, taste, smell, equilibrium).

• Somatic senses: Senses from the body (touch, temperature, pain, proprioception).

• Chemoreceptor: Detects chemical stimuli.

• Thermoreceptor: Detects temperature changes.

• Mechanoreceptor: Detects mechanical changes (pressure, stretch).

• Nociceptor: Detects pain.

• Sensory adaptation: Decrease in response to a constant stimulus.

• Receptive field: Area monitored by a single sensory neuron.

• Sensory homunculus: Cortical map representing the relative sensitivity of body regions.