Nervous System Overview: CNS vs PNS

Central Nervous System (CNS) vs. Peripheral Nervous System (PNS)

• CNS Structure: Brain (cerebrum, cerebellum, brainstem) and spinal cord, protected by skull, vertebrae, meninges, and cerebrospinal fluid (CSF)
• PNS Structure: Neural tissue outside the CNS, including cranial nerves (12 pairs), spinal nerves (31 pairs), and sensory and motor fibers
• CNS Function: Integrating and controlling center, interprets sensory input, plans and coordinates motor output, and stores information.
• PNS Function: Sensory division transmits sensory input from receptors to CNS; motor division transmits motor commands from CNS to effectors.

Sensory (Afferent) vs. Motor (Efferent) Components

• Sensory (Afferent) Input: Carries signals from sensory receptors to CNS.
  • Subdivisions:
    • Somatic sensory (external stimuli e.g., pain, temp, pressure)
    • Visceral sensory (internal stimuli e.g., organ stretch, chemical changes)
• Motor (Efferent) Output: Carries commands from CNS to muscles/glands.
  • Subdivisions:
    • Somatic motor (voluntary; skeletal muscle activation)
    • Autonomic motor (involuntary; smooth muscle, cardiac muscle, and gland regulation)

Nervous System as a Control System: Functional Steps

• Step 1: Sensory Receptors: Detect stimuli (e.g., photoreceptors in eyes, mechanoreceptors in skin)
• Step 2: Afferent Pathways: Sensory neurons send signals to CNS
• Step 3: Control Center (CNS): Processes input and formulates responses (e.g., reflexes in spinal cord or higher brain processing)

Somatic vs. Autonomic Motor Divisions

• Somatic Motor Division:
  • Single neuron pathway
  • Voluntary control of skeletal muscles
  • Neurotransmitter: Acetylcholine (ACh)
• Autonomic Motor Division:
  • Two-neuron chain (preganglionic and postganglionic neurons)
  • Involuntary control of smooth/cardiac muscle and glands
  • Divided into Sympathetic (prepares body for stress, "fight or flight") and Parasympathetic (promotes rest and recovery, "rest and digest")

Neuron Anatomy

• Components: Cell body (soma), dendrites, axon
  • Soma: Contains nucleus and organelles, chromatophilic substance (Nissl bodies) for protein synthesis
  • Dendrites: Receive incoming signals, highly branched
  • Axon: Transmits action potentials to other neurons or effectors; action potential initiates in axon hillock

Types of Neurons

• Sensory (Afferent): Transmit sensory input to CNS; pseudounipolar structure.
• Interneurons (Association Neurons): Connect sensory and motor neurons; entirely within CNS.

Neuroglial (Glial) Cells

• CNS Glial Cells:
  • Astrocytes (blood-brain barrier, regulate extracellular environment)
  • Oligodendrocytes (myelinate CNS axons)
  • Microglia (phagocytes, remove debris)
  • Ependymal cells (line ventricles, produce and circulate cerebrospinal fluid (CSF))
• PNS Glial Cells:
  • Schwann cells (myelinate PNS axons, aid in repair)
  • Satellite cells (support cell bodies in ganglia)

Myelination

• CNS: Oligodendrocytes wrap multiple axons
• PNS: Schwann cells wrap a single axon; nodes of Ranvier enable saltatory conduction

Resting Membrane Potential (RMP)

• Established by sodium-potassium ATPase pump (3 Na⁺ out, 2 K⁺ in) and K⁺ leak channels
• Key Concepts:
  • Depolarization: Membrane potential becomes more positive (Na⁺ influx)
  • Repolarization: Membrane potential returns to resting state (K⁺ efflux)
  • Hyperpolarization: Membrane potential becomes more negative

Graded vs. Action Potentials

• Graded Potentials: Localized changes in membrane potential, vary in size
• Action Potentials: Rapid, all-or-none depolarization, propagate along axons

Synaptic Transmission: Steps

• Action potential arrives at axon terminal
• Voltage-gated Ca²⁺ channels open
• Synaptic vesicles release neurotransmitter into cleft
• Neurotransmitter binds to postsynaptic receptors
• Excitatory/Inhibitory Postsynaptic Potentials (EPSPs/IPSPs): Depolarizing/Hyperpolarizing signals

Spinal Cord Anatomy and Reflexes

• Gross Anatomy: Cervical/Lumbar enlargements (motor neurons for limbs), Conus Medullaris (tapered end of spinal cord), Cauda Equina (nerve roots extending below spinal cord)
• Reflex Arc Components: Receptor → Sensory neuron → Integration center → Motor neuron → Effector

Memory and Alzheimer's Disease

• Memory: Short-term (temporary synaptic increase), long-term (synaptic remodeling, new dendritic spines)
• Alzheimer's Disease: Amyloid plaques, tau protein tangles, neuron death, impaired memory and cognition.

Explore the structures and functions of the Central Nervous System (CNS) and Peripheral Nervous System (PNS) in this comprehensive quiz. Test your knowledge on the sensory and motor components, including their subdivisions and roles within the nervous system.