Chapter 12

12.1 General Functions of the Nervous System

  • Coordinate voluntary and involuntary responses

    • Voluntary: moving your hand, speaking

    • Involuntary: heartbeat, breathing, digestion

  • Process sensory information

    • Receives input from senses (touch, vision, hearing) and integrates it

  • Maintain homeostasis

    • Regulates body temperature, blood pressure, pH, fluid balance

12.2 Divisions of the Nervous System

Structural Divisions:

  • Central Nervous System (CNS): Brain and spinal cord

    • Brain regions: cerebrum, cerebellum, brainstem

  • Peripheral Nervous System (PNS): Cranial nerves and spinal nerves connecting CNS to limbs and organs

Functional Divisions:

  • Somatic Nervous System (SNS): Controls voluntary movements of skeletal muscles

    • Example: lifting a book, walking

  • Autonomic Nervous System (ANS): Controls involuntary responses

    • Sympathetic (“fight or flight”): increases heart rate, dilates pupils, slows digestion

    • Parasympathetic (“rest and digest”): slows heart rate, constricts pupils, stimulates digestion

12.3 Glial Cells and Their Roles

Glial Cell

Location

Function

Example/Importance

Astrocytes

CNS

Support neurons, maintain blood-brain barrier

Repair brain injury, control ion balance

Microglia

CNS

Immune defense (phagocytosis)

Remove debris, fight infection

Oligodendrocytes

CNS

Produce myelin

Myelinate multiple axons → speeds up signals

Schwann Cells

PNS

Produce myelin

Myelinate single axons → aids regeneration

Ependymal Cells

CNS

Line ventricles, produce cerebrospinal fluid

Circulate CSF for protection and nutrient delivery

Extra detail: Myelin sheath has nodes of Ranvier where action potentials “jump,” speeding up signal conduction.

12.4 Structure of a Neuron

  • Dendrites: Receive incoming signals from other neurons

  • Cell body (soma): Contains nucleus and organelles; integrates signals

  • Axon: Transmits signals away from the cell body to other neurons or effectors

  • Axon terminals (synaptic boutons): Release neurotransmitters to communicate with the next cell

Tip: Visualize: Dendrites → Soma → Axon → Synapse

12.5 Electrical Excitability

  • Neurons generate action potentials by moving ions across the membrane:

    • Resting potential: ~–70 mV, maintained by Na⁺/K⁺ pump

    • Depolarization: Na⁺ channels open → inside becomes positive

    • Repolarization: K⁺ channels open → restores negative inside

    • Refractory period: neuron cannot fire again immediately

  • Threshold potential: The voltage that triggers an action potential (~–55 mV)

  • Action potentials allow fast, long-distance communication along axons

12.6 Neuron Classification

Structural Types:

  • Unipolar: One process; usually sensory neurons in PNS

  • Bipolar: Two processes; found in retina and olfactory system

  • Multipolar: Many processes; most common, e.g., motor neurons

Functional Types:

  • Sensory (afferent) neurons: Carry signals to CNS from senses

  • Motor (efferent) neurons: Carry commands from CNS to muscles/glands

  • Interneurons: Connect neurons within CNS; involved in processing and reflexes

12.7 Neurotransmitters

  • Chemical messengers released at synapses to transmit signals

  • Examples and functions:

    • Acetylcholine: Muscle contraction, learning, memory

    • Dopamine: Reward, mood, motor control

    • Serotonin: Mood, sleep, appetite

    • GABA: Main inhibitory neurotransmitter

    • Glutamate: Main excitatory neurotransmitter

12.8 Environmental Impact on the Nervous System

  • Nutrition: Malnutrition → impaired brain development

  • Toxins: Lead, alcohol → neural damage

  • Social interaction and learning: Strengthen neural connections (neuroplasticity)

  • Stress: Chronic stress can alter neurotransmitter levels and brain structure

  • Early experiences have long-term effects on cognitive and emotional development