The Organization and Function of the Nervous System

Organization and Evolution of Nervous Systems

Basic Structure of Nervous Systems

The nervous system is composed of circuits of neurons and supporting cells, with increasing complexity across animal phyla. Its organization reflects evolutionary adaptations to different lifestyles and behaviors.

• Nerve Nets: The simplest nervous systems, found in cnidarians (e.g., jellyfish), consist of interconnected neurons forming diffuse nerve nets.

• Nerves: In more complex animals, axons of multiple neurons are bundled into nerves, channeling information along specific routes.

• Radial Nerves: Echinoderms (e.g., sea stars) possess radial nerves connected to a central nerve ring.

Cephalization and Bilateral Symmetry

• Cephalization: The clustering of sensory organs and neurons at the anterior end (head) is a hallmark of bilaterally symmetrical animals.

• Flatworms: Exhibit a simple central nervous system (CNS) with a small brain and longitudinal nerve cords.

• Peripheral Nervous System (PNS): Composed of neurons carrying information into and out of the CNS.

Segmented Nervous Systems

• Annelids and Arthropods: Have segmentally arranged clusters of neurons called ganglia.

• Molluscs: Nervous system complexity correlates with lifestyle; sessile forms have simple systems, while active forms (e.g., octopuses) have sophisticated brains.

Vertebrate Nervous System

• CNS: Composed of the brain and spinal cord.

• PNS: Composed of nerves and ganglia.

• Regional Specialization: Both systems exhibit specialized regions for distinct functions.

Structure and Function of the Vertebrate Nervous System

Development and Anatomy

• The CNS develops from the hollow dorsal nerve cord.

• The central canal of the spinal cord and brain ventricles arise from this structure and are filled with cerebrospinal fluid, which supplies nutrients, hormones, and removes waste.

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons.

• White Matter: Consists of bundles of myelinated axons.

Spinal Cord Functions

• Conveys information to and from the brain.

• Generates basic patterns of locomotion.

• Produces reflexes—automatic responses to stimuli (e.g., knee-jerk reflex).

Peripheral Nervous System (PNS)

• Transmits information between the CNS and the body.

• Afferent Neurons: Carry information to the CNS.

• Efferent Neurons: Carry information away from the CNS.

Efferent Components of the PNS

• Motor System: Controls skeletal muscles; can be voluntary or involuntary.

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

• Enteric Nervous System: Controls digestive tract, pancreas, and gallbladder independently.

Divisions of the Autonomic Nervous System

• Sympathetic Division: Prepares the body for "fight-or-flight" (arousal, energy generation).

• Parasympathetic Division: Promotes "rest-and-digest" functions, antagonistic to sympathetic effects.

Division	Main Function	Neurotransmitter (Postganglionic)
Sympathetic	Arousal, energy generation	Norepinephrine
Parasympathetic	Calming, rest-and-digest	Acetylcholine

Additional info: Both divisions use acetylcholine at the preganglionic synapse.

Glial Cells

• Glia: Support, nourish, and regulate neurons.

• Radial Glia: Guide neuron migration during development; can act as stem cells.

• Astrocytes: Form the blood-brain barrier, restricting entry of substances into the brain; can also act as stem cells.

Regional Specialization of the Vertebrate Brain

Major Brain Regions

• Forebrain: Processes olfactory input, regulates sleep, learning, and complex processing.

• Midbrain: Coordinates routing of sensory input.

• Hindbrain: Controls involuntary activities and coordinates motor activities.

Relative sizes of these regions vary among vertebrates, reflecting functional importance.

Human Brain Structure

• Cerebrum: Controls skeletal muscle contraction; center for learning, emotion, memory, and perception.

• Cerebral Cortex: Outer layer of the cerebrum; vital for perception, voluntary movement, and learning.

• Corpus Callosum: Thick band of axons connecting the two cerebral hemispheres.

• Cerebellum: Coordinates movement, balance, and motor learning.

• Diencephalon: Gives rise to thalamus, hypothalamus, and epithalamus.

• Hypothalamus: Regulates homeostasis, biological clock, and endocrine functions.

• Brainstem: Includes midbrain, pons, and medulla oblongata; controls automatic functions (e.g., breathing, heart rate).

Brain Function: Arousal, Sleep, and Biological Clocks

Arousal and Sleep

• Arousal: State of awareness of the external world.

• Sleep: Active brain state with reduced conscious perception of external stimuli; essential for survival and memory consolidation.

• Reticular Formation: Network in the midbrain and pons controlling sleep and arousal, including REM sleep and dreaming.

Biological Clocks and Circadian Rhythms

• Circadian Rhythms: Daily cycles of biological activity, typically synchronized to light/dark cycles.

• Suprachiasmatic Nucleus (SCN): Cluster of hypothalamic neurons acting as the pacemaker for circadian rhythms.

Emotions and Functional Imaging

Limbic System and Emotions

• Limbic System: Includes amygdala, hippocampus, and parts of the thalamus; involved in emotion generation and experience.

• Amygdala: Key for storage and recall of emotional memories.

Functional Imaging

• PET (Positron-Emission Tomography): Displays metabolic activity using radioactive glucose.

• fMRI (Functional Magnetic Resonance Imaging): Detects brain activity via changes in local oxygen concentration.

Cerebral Cortex: Voluntary Movement and Cognition

Information Processing

• The cerebral cortex is divided into four lobes: frontal, temporal, occipital, and parietal, each with specialized functions.

• Receives input from sensory organs and somatosensory receptors (touch, pain, temperature, etc.).

• The thalamus directs sensory input to appropriate cortical areas.

• Primary sensory areas pass information to association areas for processing, then to the prefrontal cortex for planning actions.

• Somatosensory and motor cortices are organized by body region.

Language and Speech

• Broca's Area: In the frontal lobe; damage impairs speech production but not comprehension.

• Wernicke's Area: Damage impairs language comprehension but not speech production.

Lateralization of Cortical Function

• Lateralization: Specialization of the left and right hemispheres for different functions.

• Left: Language, math, logic.

• Right: Pattern recognition, spatial relations, nonverbal thinking.

• Hemispheres communicate via the corpus callosum.

Frontal Lobe Function

• Critical for executive functions, decision making, and emotional responses.

• Damage can impair these functions without affecting intellect or memory.

Evolution of Cognition

• Advanced cognition does not require a convoluted neocortex; birds achieve it with clustered nuclei in the pallium.

• Both birds and mammals evolved from ancestors with a pallium; mammals developed a layered cortex, birds retained clustered organization.

Memory, Learning, and Synaptic Plasticity

Development and Plasticity

• Nervous system formation involves regulated gene expression, neuron migration, and synapse elimination.

• Neuronal Plasticity: The nervous system's ability to remodel synaptic connections after birth; essential for learning and memory.

• Defects in plasticity may underlie disorders such as autism spectrum disorder.

Memory Types

• Short-Term Memory: Temporary links in the hippocampus.

• Long-Term Memory: Stable connections in the cerebral cortex; consolidation may occur during sleep.

Long-Term Potentiation (LTP)

• LTP is a lasting increase in synaptic strength, involving glutamate release and changes in postsynaptic glutamate receptors.

Nervous System Disorders

Overview

• Disorders include schizophrenia, depression, drug addiction, Alzheimer's disease, and Parkinson's disease.

• Both genetic and environmental factors contribute; family studies help distinguish their roles.

Schizophrenia

• Affects ~1% of the population; characterized by hallucinations and delusions.

• Strong genetic and environmental components.

Depression

• Major Depressive Disorder: Persistent lack of interest or pleasure.

• Bipolar Disorder: Alternating manic and depressive phases.

• Treated with drugs that increase biogenic amine activity in the brain.

Drug Addiction and the Reward System

• The brain's reward system uses dopamine to reinforce motivation with pleasure.

• Addictive drugs (e.g., cocaine, heroin, alcohol) increase dopamine activity.

Alzheimer's Disease

• Progressive dementia with confusion and memory loss; incidence increases with age.

• Associated with amyloid plaques, neurofibrillary tangles (tau protein), and brain tissue shrinkage.

• No cure; some drugs relieve symptoms.

• Tau protein accumulation also seen in chronic traumatic encephalopathy (CTE).

Parkinson's Disease

• Motor disorder caused by death of dopamine-secreting neurons in the midbrain.

• Symptoms: Muscle tremors, flexed posture, shuffling gait.

• Incidence increases with age; some genetic forms exist.

• Treated with L-dopa, a dopamine-related drug; not curable.

Future Directions in Brain Research

• The BRAIN Initiative (launched in 2014) aims to map brain circuits, measure their activity, and understand how this activity produces thought and behavior.