19  Neurobiology and Information Processing

19.1 Learning Objectives

By the end of this chapter, you should be able to:

  1. Describe the cellular components of nervous systems (neurons and glia) and their functions
  2. Explain the molecular and ionic basis of the resting membrane potential and action potentials
  3. Trace the steps of chemical and electrical synaptic transmission
  4. Distinguish the structure and function of major divisions of the vertebrate nervous system
  5. Describe how sensory systems transduce physical and chemical stimuli into neural signals
  6. Explain the neural control of movement through motor systems and reflexes
  7. Outline the neural basis of learning, memory, and higher cognitive functions in selected models
  8. Apply principles of neurobiology to understand neurological disorders and brain-machine interfaces

19.2 Introduction

The nervous system is the biological information processing network that allows organisms to sense their environment, integrate signals, and generate adaptive responses. From simple nerve nets to complex brains, nervous systems embody the principles of information flow in biology. This chapter explores how specialized cells called neurons use electrical and chemical signals to communicate, how neural circuits process information, and how these processes give rise to behavior, learning, and consciousness. Understanding neurobiology bridges cellular mechanisms with organismal behavior and cognitive function.

19.3 Cellular Components of Nervous Systems

19.3.1 Neurons: The Signaling Units

Basic structure:

  • Cell body (soma): Contains nucleus and organelles
  • Dendrites: Receive signals (input zone)
  • Axon: Conducts signals (conducting zone)
  • Axon terminals: Transmit signals (output zone)
  • Synapse: Junction between neurons

Functional classification:

  • Sensory (afferent): Carry information toward CNS
  • Motor (efferent): Carry commands away from CNS
  • Interneurons: Connect neurons within CNS (90% of neurons)

Structural classification:

  • Multipolar: Multiple dendrites, one axon (most common)
  • Bipolar: One dendrite, one axon (sensory systems)
  • Unipolar (pseudounipolar): Single process that divides (sensory neurons)

19.3.2 Glial Cells: The Support Cells

Central nervous system glia:

  • Astrocytes: Support, regulate extracellular environment, form blood-brain barrier
  • Oligodendrocytes: Form myelin sheaths (insulation)
  • Microglia: Immune surveillance and phagocytosis
  • Ependymal cells: Line ventricles, produce cerebrospinal fluid (CSF)

Peripheral nervous system glia:

  • Schwann cells: Form myelin sheaths
  • Satellite cells: Support cell bodies in ganglia

19.3.3 Myelination and Signal Conduction

Myelin: Lipid-rich wrapping that insulates axons

  • Nodes of Ranvier: Gaps between myelin segments
  • Saltatory conduction: Action potentials “jump” between nodes (faster)

Conduction velocity factors:

  • Axon diameter: Larger = faster
  • Myelination: Myelinated = faster
  • Temperature: Warmer = faster (within physiological range)

19.4 Electrical Properties of Neurons

19.4.1 Resting Membrane Potential

Definition: Electrical charge difference across membrane (~-70 mV)

Ionic basis:

  • Na⁺/K⁺ ATPase: Pumps 3 Na⁺ out, 2 K⁺ in (establishes gradients)
  • Potassium leak channels: Allow K⁺ to diffuse out (main determinant)
  • Sodium leak channels: Allow some Na⁺ to diffuse in

Nernst equation: Calculates equilibrium potential for single ion

  • \(E_{ion} = \frac{RT}{zF} \ln \frac{[ion]_{out}}{[ion]_{in}}\)

Goldman-Hodgkin-Katz equation: Calculates membrane potential considering multiple ions

19.4.2 Graded Potentials

Characteristics:

  • Variable magnitude: Proportional to stimulus strength
  • Decremental: Decrease with distance
  • Local: Don’t travel far
  • Summation: Can add together (temporal and spatial)

Types: Receptor potentials, synaptic potentials, pacemaker potentials

19.4.3 Action Potentials

Definition: All-or-none electrical signal that travels along axon

Threshold: ~-55 mV (must be reached to trigger)

Phases:

  1. Resting state: All voltage-gated channels closed
  2. Depolarization: Na⁺ channels open → Na⁺ influx
  3. Peak: Na⁺ channels inactivate, K⁺ channels open
  4. Repolarization: K⁺ efflux
  5. Hyperpolarization (afterpotential): K⁺ channels close slowly
  6. Refractory periods:
    • Absolute: Cannot fire another AP (Na⁺ channels inactivated)
    • Relative: Harder to fire (membrane hyperpolarized)

Voltage-gated ion channels:

  • Na⁺ channels: Fast activation and inactivation
  • K⁺ channels: Slower activation, no inactivation
  • Ca²⁺ channels: Important at synapses

Propagation: Local currents depolarize adjacent membrane

19.5 Synaptic Transmission

19.5.1 Types of Synapses

Electrical synapses:

  • Gap junctions: Direct cytoplasmic connection
  • Fast, bidirectional
  • Synchronize activity (cardiac muscle, some neurons)

Chemical synapses:

  • Synaptic cleft: ~20-40 nm gap
  • Unidirectional
  • Amplification, modulation possible

19.5.2 Chemical Synaptic Transmission

Steps:

  1. AP arrives at presynaptic terminal
  2. Voltage-gated Ca²⁺ channels open
  3. Ca²⁺ influx triggers vesicle fusion
  4. Neurotransmitter release by exocytosis
  5. Diffusion across cleft
  6. Binding to postsynaptic receptors
  7. Postsynaptic response (EPSP or IPSP)
  8. Termination (reuptake, degradation, diffusion)

19.5.3 Neurotransmitters

Small molecule neurotransmitters:

  • Acetylcholine (ACh): Neuromuscular junction, autonomic NS, CNS
  • Biogenic amines: Dopamine, norepinephrine, epinephrine, serotonin, histamine
  • Amino acids: Glutamate (excitatory), GABA (inhibitory), glycine (inhibitory)

Neuropeptides: Endorphins, substance P, oxytocin (modulatory)

Gases: Nitric oxide (NO), carbon monoxide (CO) (retrograde signals)

19.5.4 Postsynaptic Potentials

Excitatory postsynaptic potential (EPSP): Depolarization (makes AP more likely)

Inhibitory postsynaptic potential (IPSP): Hyperpolarization (makes AP less likely)

Receptor types:

  • Ionotropic: Ligand-gated ion channels (fast, ~1 ms)
  • Metabotropic: G-protein coupled receptors (slow, seconds to minutes)

Integration: Spatial and temporal summation determines if threshold reached

19.6 Organization of Nervous Systems

19.6.1 Evolution of Nervous Systems

Simple systems: Nerve nets (cnidarians), nerve cords (flatworms)

Complex systems: Ganglia, brains, specialized regions

19.6.2 Vertebrate Central Nervous System

Brain development: Neural tube → forebrain, midbrain, hindbrain

Major brain regions:

  • Forebrain: Cerebrum (cerebral cortex, basal ganglia, limbic system), thalamus, hypothalamus
  • Midbrain: Visual and auditory reflexes, motor control
  • Hindbrain: Pons, medulla oblongata, cerebellum
  • Brainstem: Midbrain + pons + medulla (vital functions)

Spinal cord: Reflex pathways, ascending/descending tracts

Ventricles and CSF: Circulation, cushioning, waste removal

19.6.3 Peripheral Nervous System

Somatic nervous system: Voluntary control of skeletal muscles

Autonomic nervous system: Involuntary control of glands, organs, smooth/cardiac muscle

  • Sympathetic: “Fight or flight”
  • Parasympathetic: “Rest and digest”
  • Enteric: “Second brain” of gut

Cranial and spinal nerves

19.6.4 Blood-Brain Barrier

Function: Selective barrier protects CNS

Structure: Tight junctions between endothelial cells

Transport: Selective carriers for nutrients

Circumventricular organs: Areas without BBB (monitor blood)

19.7 Sensory Systems

19.7.1 General Principles

Sensory transduction: Convert stimulus energy to neural signals

Receptor types: Mechanoreceptors, thermoreceptors, photoreceptors, chemoreceptors, nociceptors

Adaptation: Decreased response to constant stimulus

  • Phasic receptors: Adapt quickly (smell, touch)
  • Tonic receptors: Adapt slowly (pain, body position)

19.7.2 Specific Sensory Systems

Vision:

  • Photoreceptors: Rods (dim light) and cones (color)
  • Retinal processing: Horizontal, bipolar, amacrine cells
  • Visual pathways: Retina → thalamus → visual cortex

Audition:

  • Sound transduction: Hair cells in cochlea
  • Frequency coding: Basilar membrane tonotopy
  • Auditory pathways: Cochlea → brainstem → thalamus → auditory cortex

Somatosensation:

  • Touch: Mechanoreceptors (Merkel, Meissner, Pacinian, Ruffini)
  • Pain: Nociceptors (fast vs. slow pain)
  • Temperature: Thermoreceptors
  • Pathways: Dorsal column-medial lemniscus (touch), spinothalamic (pain/temp)

Chemical senses:

  • Taste: Taste buds (sweet, salty, sour, bitter, umami)
  • Smell: Olfactory receptors → olfactory bulb

Vestibular system: Balance, head position, acceleration

19.8 Motor Systems

19.8.1 Spinal Reflexes

Reflex arc: Sensor → afferent neuron → integration → efferent neuron → effector

Stretch reflex (monosynaptic): Muscle spindle → muscle contraction

Withdrawal reflex (polysynaptic): Pain receptor → flexor contraction, extensor inhibition

19.8.2 Brain Motor Areas

Primary motor cortex: Voluntary movement execution

Premotor cortex: Movement planning

Supplementary motor area: Complex sequences

Basal ganglia: Movement initiation, smoothing

Cerebellum: Coordination, timing, motor learning

19.8.3 Descending Motor Pathways

Pyramidal (corticospinal): Direct cortical control of movement

Extrapyramidal: Indirect control via brainstem

Motor neuron pools: Alpha motor neurons → skeletal muscle

19.9 Neural Basis of Behavior

19.9.1 Learning and Memory

Hebbian plasticity: “Cells that fire together wire together”

Synaptic plasticity:

  • Long-term potentiation (LTP): Increased synaptic strength (glutamate, NMDA receptors)
  • Long-term depression (LTD): Decreased synaptic strength

Memory systems:

  • Declarative (explicit): Facts, events (hippocampus-dependent)
  • Non-declarative (implicit): Skills, habits, conditioning
  • Working memory: Temporary storage (prefrontal cortex)

19.9.2 Biological Rhythms

Circadian rhythms: ~24-hour cycles (suprachiasmatic nucleus)

Sleep stages: REM (rapid eye movement) and NREM (non-REM)

Sleep functions: Memory consolidation, neural repair, energy conservation

19.9.3 Emotion and Motivation

Limbic system: Amygdala (emotion), hippocampus (memory), hypothalamus (motivation)

Reward system: Dopamine pathways (ventral tegmental area → nucleus accumbens)

19.9.4 Language and Cognition

Language areas: Broca’s (production), Wernicke’s (comprehension)

Lateralization: Left hemisphere (language, logic), right (spatial, emotional)

Consciousness: Neural correlates, theories (integrated information, global workspace)

19.10 Disorders and Diseases

19.10.1 Neurodegenerative Diseases

Alzheimer’s disease: Amyloid plaques, neurofibrillary tangles, memory loss

Parkinson’s disease: Dopamine neuron loss, movement problems

Huntington’s disease: Genetic, chorea, cognitive decline

Amyotrophic lateral sclerosis (ALS): Motor neuron degeneration

19.10.2 Other Disorders

Stroke: Ischemic (blockage) or hemorrhagic (bleeding)

Epilepsy: Abnormal electrical activity, seizures

Multiple sclerosis: Autoimmune demyelination

Mental health disorders: Depression, anxiety, schizophrenia (neural correlates)

Neurodevelopmental disorders: Autism spectrum, ADHD

19.11 Chapter Summary

19.11.1 Key Concepts

  1. Neurons and glia form the cellular basis of nervous systems
  2. Resting potential results from ion gradients maintained by pumps and channels
  3. Action potentials are all-or-none signals propagated along axons
  4. Synapses transmit signals chemically or electrically between neurons
  5. Nervous systems are organized hierarchically from simple to complex
  6. Sensory systems transduce physical and chemical stimuli into neural codes
  7. Motor systems generate and control movement through reflexes and voluntary pathways
  8. Neural plasticity underlies learning, memory, and adaptation
  9. Neural disorders reveal important principles of normal function

19.11.2 Action Potential Phases

Phase Membrane Potential Na⁺ Channels K⁺ Channels
Resting -70 mV Closed Closed
Depolarization Rising toward +30 mV Open Closed
Repolarization Falling toward -70 mV Inactivated Open
After-hyperpolarization Below -70 mV Closed Closing slowly

19.11.3 Major Neurotransmitters

Neurotransmitter Type Main Functions Disorders
Glutamate Excitatory amino acid Learning, memory Epilepsy, excitotoxicity
GABA Inhibitory amino acid Reducing neuronal excitability Anxiety, epilepsy
Acetylcholine Ester Muscle contraction, learning, attention Myasthenia gravis, Alzheimer’s
Dopamine Biogenic amine Reward, movement, motivation Parkinson’s, schizophrenia
Serotonin Biogenic amine Mood, sleep, appetite Depression, anxiety

19.11.4 Brain Regions and Functions

Region Main Structures Primary Functions
Forebrain Cerebral cortex, thalamus, hypothalamus Cognition, sensation, homeostasis
Midbrain Tectum, tegmentum Visual/auditory reflexes, motor control
Hindbrain Pons, medulla, cerebellum Vital functions, coordination, balance
Spinal Cord Gray matter, white matter Reflexes, pathway to/from brain

19.11.5 Sensory Receptor Types

Stimulus Receptor Type Example Locations
Light Photoreceptor Retina
Sound Mechanoreceptor Cochlea
Touch/Pressure Mechanoreceptor Skin
Temperature Thermoreceptor Skin, hypothalamus
Chemicals Chemoreceptor Taste buds, olfactory epithelium
Pain Nociceptor Throughout body

19.12 Review Questions

19.12.1 Level 1: Recall and Understanding

  1. Describe the structural components of a typical neuron and their functions.
  2. What ions are primarily responsible for the resting membrane potential, and how are their gradients maintained?
  3. List the steps in chemical synaptic transmission from action potential arrival to postsynaptic response.
  4. Name the major divisions of the vertebrate nervous system and their general functions.
  5. What are the main differences between ionotropic and metabotropic receptors?

19.12.2 Level 2: Application and Analysis

  1. A neuron has a resting potential of -70 mV. If GABA (which opens Cl⁻ channels) is applied, what happens to the membrane potential? What if glutamate (which opens Na⁺ channels) is applied instead?
  2. Explain why action potentials are said to be “all-or-none” and how intensity of stimulus is encoded in the nervous system.
  3. Trace the neural pathway for a visual stimulus from the retina to the visual cortex.
  4. Compare and contrast the sympathetic and parasympathetic divisions of the autonomic nervous system.
  5. How does myelination increase the speed of action potential conduction, and what disease disrupts this process?

19.12.3 Level 3: Synthesis and Evaluation

  1. Design an experiment to test whether long-term potentiation (LTP) is necessary for a specific type of learning.
  2. Evaluate the hypothesis that consciousness emerges from specific neural circuits rather than being a property of all neural activity.
  3. How might understanding the neural basis of addiction lead to more effective treatments?
  4. Propose a model for how different brain regions might work together to produce a coordinated movement like reaching for an object.

19.13 Key Terms

  • Neuron: Nerve cell specialized for electrical signaling
  • Action potential: All-or-none electrical signal propagated along axons
  • Resting membrane potential: Voltage difference across membrane at rest (~-70 mV)
  • Synapse: Functional connection between neurons
  • Neurotransmitter: Chemical messenger released at synapses
  • Dendrite: Neuronal process that receives signals
  • Axon: Neuronal process that conducts action potentials
  • Myelin: Insulating sheath around axons that increases conduction velocity
  • Neuroglia: Support cells in nervous system
  • Ion channel: Protein pore that allows ions to cross membrane
  • Sensory receptor: Specialized cell that transduces stimuli
  • Reflex: Automatic response to stimulus
  • Plasticity: Ability of nervous system to change
  • Central nervous system (CNS): Brain and spinal cord
  • Peripheral nervous system (PNS): Nerves outside CNS
  • Cerebral cortex: Outer layer of cerebrum involved in higher functions

19.14 Further Reading

19.14.1 Books

  1. Kandel, E. R., Schwartz, J. H., Jessell, T. M., Siegelbaum, S. A., & Hudspeth, A. J. (2012). Principles of Neural Science (5th ed.). McGraw-Hill.
  2. Purves, D., Augustine, G. J., Fitzpatrick, D., et al. (2018). Neuroscience (6th ed.). Oxford University Press.
  3. Bear, M. F., Connors, B. W., & Paradiso, M. A. (2020). Neuroscience: Exploring the Brain (4th ed.). Jones & Bartlett.

19.14.2 Scientific Articles

  1. Hodgkin, A. L., & Huxley, A. F. (1952). A quantitative description of membrane current and its application to conduction and excitation in nerve. Journal of Physiology, 117(4), 500-544.
  2. Hebb, D. O. (1949). The Organization of Behavior: A Neuropsychological Theory. Wiley.
  3. Bliss, T. V., & Lømo, T. (1973). Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path. Journal of Physiology, 232(2), 331-356.

19.14.3 Online Resources

  1. Neurotree: Academic genealogy of neuroscientists: https://neurotree.org
  2. Brain Facts (Society for Neuroscience): https://www.brainfacts.org
  3. Neuropod (Nature podcast): https://www.nature.com/neuropod
  4. Allen Brain Atlas: https://portal.brain-map.org

19.15 Quantitative Problems

  1. Resting Potential Calculation: Given: [K⁺]in = 150 mM, [K⁺]out = 5 mM, [Na⁺]in = 15 mM, [Na⁺]out = 150 mM, [Cl⁻]in = 10 mM, [Cl⁻]out = 125 mM Relative permeabilities: PK:PNa:PCl = 1:0.05:0.5 Temperature = 37°C
    1. Calculate the Nernst potential for each ion.
    2. Use the Goldman-Hodgkin-Katz equation to calculate the resting membrane potential.
  2. Synaptic Integration: A neuron receives three synaptic inputs:
    • Input A: EPSP of 5 mV (10 ms duration)
    • Input B: EPSP of 8 mV (15 ms after A starts)
    • Input C: IPSP of 4 mV (5 ms after B starts) The neuron’s resting potential is -65 mV, threshold is -55 mV.
    1. Plot the membrane potential over time.
    2. Does the neuron reach threshold? Why or why not?
    3. What would happen if Input C occurred 25 ms after A instead?
  3. Action Potential Propagation: An unmyelinated axon has a conduction velocity of 1 m/s. A myelinated axon of the same diameter conducts at 50 m/s.
    1. How long does it take for a signal to travel 1 meter in each axon?
    2. If a reflex requires signals to travel 0.5 m to spinal cord and back, what’s the minimum total time for each axon type?
    3. Multiple sclerosis demyelinates axons, reducing conduction velocity to 10% of normal. How would this affect the reflex time?

19.16 Case Study: Phantom Limb Sensations

Background: After amputation, many patients experience sensations (including pain) in the missing limb. This phenomenon reveals important principles of neural organization and plasticity.

Questions:

  1. How does the somatosensory cortex normally represent different body parts?
  2. What neural changes might explain phantom limb sensations?
  3. How has mirror therapy been used to treat phantom limb pain?
  4. What does this case reveal about the relationship between neural maps and subjective experience?
  5. How might this knowledge inform rehabilitation after nerve damage or spinal cord injury?

Data for analysis:

  • Cortical reorganization: Adjacent areas expand into territory of missing limb
  • Mirror neurons: Fire when performing or observing actions
  • Treatment efficacy: Mirror therapy reduces pain in ~60% of patients
  • fMRI studies: Show changes in cortical activation patterns

Next Chapter: Developmental Biology and Morphogenesis