Chapter 38: Nervous and Sensory Systems – Structure and Function

Chapter 38: Nervous and Sensory Systems

Introduction

The nervous and sensory systems are responsible for gathering, processing, and organizing information in animals. These systems enable organisms to perceive their environment, coordinate responses, and maintain homeostasis. This chapter explores the structure, function, and diversity of nervous and sensory systems across the animal kingdom.

Command and Control Center

• Information Processing: All nervous systems perform the essential functions of gathering, processing, and organizing information.

• Human Brain: The human brain contains approximately 100 billion neurons, which are organized into complex circuits.

• Neural Mapping: Connections between regions of the brain can be mapped using the expression of random combinations of colored proteins in neurons, allowing visualization of neural circuits.

Concept 38.1: Nervous Systems Consist of Circuits of Neurons and Supporting Cells

Simplest Nervous Systems

• Cnidarians (e.g., hydras, jellies): These are among the simplest animals with nervous systems.

• Nerve Net: In most cnidarians, interconnected nerve cells form a nerve net, which controls contraction and expansion of the gastrovascular cavity.

• Example: The hydra possesses a diffuse nerve net rather than a centralized nervous system.

Increasing Complexity in Nervous Systems

• Nerve Bundles: In more complex animals, axons of multiple nerve cells are bundled together to form nerves, which channel and organize information flow.

• Bilateral Symmetry: Animals with elongated, bilaterally symmetrical bodies have more specialized nervous systems, often with a central nervous system (CNS).

Cephalization and Centralization

• Cephalization: Evolutionary trend toward clustering of sensory neurons and interneurons at the anterior (head) end of the body.

• Flatworms: Nonsegmented worms have the simplest clearly defined CNS, consisting of a simple brain and longitudinal nerve cords.

• Segmented Systems: Annelids and arthropods have segmentally arranged clusters of neurons called ganglia.

Vertebrate Nervous System Organization

• CNS: Composed of the brain and spinal cord.

• PNS: Composed of nerves and ganglia outside the CNS.

• Spinal Cord: Runs lengthwise inside the vertebral column, conveying information to and from the brain and mediating reflexes.

• Gray Matter: Consists mainly of neuron cell bodies.

• White Matter: Consists of bundles of myelinated axons.

Supporting Cells: Glia

• Glial Cells (Glia): Non-neuronal cells that nourish, support, and regulate neurons.

• Types of Glia:

  • Astrocytes: Induce blood-brain barrier formation and regulate the CNS environment.

  • Oligodendrocytes (CNS) and Schwann Cells (PNS): Form myelin sheaths around axons.

  • Microglia: Act as immune cells in the CNS.

  • Ependymal Cells: Line ventricles and help circulate cerebrospinal fluid.

Organization of the Vertebrate Nervous System

• Ventricles and Cerebrospinal Fluid: The CNS contains fluid-filled spaces (ventricles in the brain, central canal in the spinal cord) filled with cerebrospinal fluid, which supplies nutrients, hormones, and removes wastes.

• Reflexes: The spinal cord can act independently of the brain to produce reflexes—automatic responses to certain stimuli.

The Peripheral Nervous System (PNS)

• Function: Transmits information to and from the CNS and regulates movement and internal environment.

• Afferent Neurons: Transmit sensory information to the CNS.

• Efferent Neurons: Transmit commands from the CNS to effectors (muscles, glands).

• Motor System: Carries signals to skeletal muscles (voluntary or involuntary).

• Autonomic Nervous System: Regulates smooth muscle, cardiac muscle, and glands (generally involuntary).

• Divisions of Autonomic System:

  • Sympathetic: "Fight-or-flight" responses.

  • Parasympathetic: "Rest-and-digest" functions.

  • Enteric: Controls digestive tract, pancreas, and gallbladder.

Concept 38.4: Sensory Receptors and Signal Transduction

Sensory Reception and Transduction

• Sensory Receptors: Specialized cells or structures that detect stimuli (internal or external).

• Sensory Transduction: Conversion of stimulus energy into a change in membrane potential (receptor potential).

• Receptor Potentials: Graded potentials whose magnitude varies with stimulus strength.

Transmission and Perception

• Transmission: Sensory information is transmitted as action potentials (nerve impulses) to the CNS.

• Perception: The brain interprets action potentials, constructing perceptions such as sight, sound, and touch based on the neural pathways activated.

Amplification and Adaptation

• Amplification: Strengthening of a sensory signal during transduction.

• Sensory Adaptation: Decrease in responsiveness to a continued stimulus.

Types of Sensory Receptors

• Mechanoreceptors: Detect physical deformation (pressure, touch, stretch, motion, sound).

• Electromagnetic Receptors: Detect light, electricity, and magnetism.

• Thermoreceptors: Detect heat and cold.

• Pain Receptors (Nociceptors): Detect stimuli that could damage tissues.

• Chemoreceptors: Detect chemical stimuli (solute concentration, specific molecules).

Mechanoreceptors

• Function: Sense mechanical changes such as pressure, vibration, and stretch.

• Examples: Cat whiskers, human skin touch receptors.

Electromagnetic, Thermo-, and Pain Receptors

• Electromagnetic Receptors: Used by some animals to detect prey or navigate using Earth's magnetic field.

• Thermoreceptors: Detect temperature changes; important for thermoregulation.

• Pain Receptors: Trigger defensive responses; prostaglandins increase sensitivity (inhibited by aspirin/ibuprofen).

Chemoreceptors

• General Chemoreceptors: Detect overall solute concentration.

• Specific Chemoreceptors: Detect specific molecules (e.g., taste, smell).

• Taste: Humans detect five types of tastants: sweet, sour, salty, bitter, umami. Taste buds are found in papillae on the tongue.

Concept 38.5: Hearing and Equilibrium

Mechanoreceptors in Hearing and Balance

• Invertebrates: Use statocysts for gravity detection and body hairs for sound detection.

• Mammals: Hearing and equilibrium organs are closely associated in the inner ear.

Hearing in Mammals

• Sound Waves: Vibrating objects create pressure waves in air, which are transduced by the ear into nerve impulses.

• Ear Structure: The tympanic membrane (eardrum) vibrates, transmitting sound via three middle ear bones to the oval window of the cochlea.

• Cochlea: Pressure waves travel through the cochlea, causing the basilar membrane and attached hair cells to vibrate.

• Hair Cells: Bending of hair cells opens/closes ion channels, changing membrane potential and generating action potentials.

• Pitch and Volume: Pitch is determined by the region of the basilar membrane that vibrates; volume by the amplitude of sound waves.

Equilibrium

• Utricle and Saccule: Contain otoliths that detect gravity and linear movement.

• Semicircular Canals: Detect rotational movement in any direction.

Concept 38.6: Visual Receptors and Vision

Evolution of Visual Perception

• Photoreceptors: All light-detecting organs contain photoreceptors with light-absorbing pigment molecules.

• Simple Eyes: Flatworms have ocelli (simple eyespots) that detect light direction and intensity.

• Compound Eyes: Insects and crustaceans have compound eyes with many ommatidia, effective at detecting movement.

• Single-Lens Eyes: Found in vertebrates and some invertebrates; function like a camera, focusing light onto a retina.

The Vertebrate Visual System

• Light Pathway: Light enters the eye, passes through the lens, and strikes photoreceptors (rods and cones) in the retina.

• Retina: Contains layers of neurons; rods detect light intensity (black and white), cones detect color.

• Signal Processing: In the dark, rods and cones release glutamate; in light, they hyperpolarize and reduce glutamate release, affecting bipolar cells and ultimately ganglion cells whose axons form the optic nerve.

• Optic Chiasm: Optic nerves cross near the cerebral cortex; visual information from each visual field is processed in the opposite brain hemisphere.

• Color Vision: Most vertebrates have good color vision; humans have three types of cones (red, green, blue). Nocturnal animals have more rods for night vision.

Type of Sensory Receptor	Stimulus Detected	Example
Mechanoreceptor	Pressure, touch, vibration, stretch	Cat whiskers, human skin
Electromagnetic receptor	Light, electricity, magnetism	Photoreceptors in eyes, electroreceptors in fish
Thermoreceptor	Heat, cold	Human skin, snake pit organs
Pain receptor (nociceptor)	Potentially damaging stimuli	Free nerve endings in skin
Chemoreceptor	Chemical concentration, specific molecules	Taste buds, olfactory cells

Key Equations and Concepts

• Action Potential Transmission: The frequency of action potentials encodes stimulus intensity.

• Receptor Potential: Graded change in membrane potential in response to a stimulus.

• Pitch Discrimination: The basilar membrane's varying stiffness allows different regions to respond to different frequencies.

Additional info: Some details, such as the specific types of glial cells and the structure of the retina, were expanded for academic completeness and clarity.