Chapter 11: Introduction to the Nervous System and Nervous Tissue

11.1 Nervous System Overview

The nervous system is responsible for detecting information, processing it, and directing appropriate responses. It is essential for sensation, movement, consciousness, personality, learning, and memory, and works closely with the endocrine system to maintain homeostasis.

• Functions of the Nervous System:

  • Sensory input: Detects changes inside or outside the body (e.g., seeing a soccer ball).

  • Integration: The central nervous system (CNS) processes information and determines a response (e.g., deciding to kick the ball).

  • Motor output: Directs muscles or glands to act (e.g., leg muscles contract to kick).

• Structural Divisions:

  • CNS: Brain and spinal cord. The brain is protected by the skull; the spinal cord by the vertebral canal. The brain connects to the spinal cord at the foramen magnum.

  • PNS: Cranial nerves, spinal nerves, and their branches. Nerves are bundles of axons with blood vessels and connective tissue. Cranial nerves connect to the brain; spinal nerves to the spinal cord.

• Functional Divisions:

  • Sensory (afferent): Carries information toward the CNS. Includes somatic sensory (skin, muscles, bones, joints, special senses) and visceral sensory (internal organs).

  • Motor (efferent): Carries information away from the CNS. Includes somatic motor (skeletal muscle, voluntary) and visceral motor/ANS (smooth muscle, cardiac muscle, glands, involuntary).

  • Effectors: Muscles and glands that carry out responses.

Key Terms: Afferent (arrives at CNS), efferent (exits CNS), somatic (body wall/muscles), visceral (organs).

11.2 Neurons

Neurons are the primary signaling cells of the nervous system, specialized for receiving input, conducting electrical signals, and communicating with target cells.

• Neuron Structure:

  • Dendrites: Branching processes that receive input and carry electrical changes toward the cell body.

  • Cell body (soma): Contains the nucleus, maintains the neuron, and synthesizes proteins. Nissl bodies (clusters of ribosomes and rough ER) and mitochondria are abundant.

  • Axon: Single process that generates and conducts action potentials away from the cell body. May branch (axon collaterals).

  • Axon terminals: Secretory regions that release neurotransmitters to communicate with target cells.

• Specialized Regions: Axon hillock (connection to soma), initial segment (trigger zone), telodendria (terminal branches), synaptic knobs (axon terminals), axolemma (axon membrane), axoplasm (axon cytoplasm), neurofibrils (support), microtubules (transport).

• Axonal Transport: Movement of materials within the axon. Slow transport (proteins away from soma), fast transport (ATP-powered, bidirectional). Anterograde (toward terminals), retrograde (toward soma).

• Neuron Classification:

  • By structure:

    • Multipolar: One axon, many dendrites (motor neurons, interneurons).

    • Bipolar: One axon, one dendrite (special sensory neurons).

    • Pseudounipolar: One process splits into two branches (general sensory neurons).

  • By function: Sensory (to CNS), interneurons (within CNS), motor (from CNS to effectors).

Key Point: Dendrites receive, soma maintains, axon conducts, terminals release.

11.2 Glia, Myelin, and Repair

Glial cells support neurons, myelin insulates axons to speed signal conduction, and nerve repair is limited, especially in the CNS.

• Myelin: Layers of glial cell membrane rich in lipids; insulates axons, reduces current leakage, and increases conduction speed. Internodes are myelinated segments; Nodes of Ranvier are gaps.

• White matter: Mainly myelinated axons. Gray matter: Mainly cell bodies, dendrites, unmyelinated axons.

• Regeneration:

  • CNS: Limited; inhibitory factors and astrocyte scar tissue prevent regrowth.

  • PNS: Possible if soma survives. Steps: Wallerian degeneration (distal axon/myelin degenerates), proximal end sprouts, Schwann cells form regeneration tube, one sprout grows through, may reconnect and remyelinate.

11.3 Electrical Basics

Neurons are excitable cells that respond to stimuli with electrical changes and conduct these changes along their membranes.

• Resting Membrane Potential: Typically about -70 mV (inside negative). Maintained by ion gradients (Na+ higher outside, K+ higher inside) and the Na+/K+ pump.

• Ion Channels:

  • Leak channels: Always open.

  • Ligand-gated: Open when a chemical binds.

  • Voltage-gated: Open with membrane voltage changes.

  • Mechanically gated: Open with pressure/stretch/vibration.

• Voltage Changes:

  • Depolarization: Membrane becomes less negative (e.g., -70 to -60 mV).

  • Repolarization: Returns toward resting potential after depolarization.

  • Hyperpolarization: Membrane becomes more negative than rest (e.g., -70 to -80 mV).

• Local (Graded) Potentials: Short-distance, variable-size signals that can depolarize or hyperpolarize. Fade with distance and reverse when stimulation stops. Occur in dendrites and soma.

Key Equation:

• Na+/K+ pump:

11.3 Action Potentials

Action potentials are all-or-none electrical signals that travel along axons when the membrane reaches threshold.

• Refractory Periods:

  • Absolute: No new action potential possible (Na+ channels not reset).

  • Relative: Stronger stimulus needed (Na+ channels reset, K+ channels open).

• Propagation: Action potential triggers the next segment. Continuous conduction (unmyelinated axons) vs. saltatory conduction (myelinated axons, faster).

• Conduction Speed: Increased by larger axon diameter and myelination. A fibers: largest, myelinated, fastest. B fibers: intermediate. C fibers: smallest, unmyelinated, slowest.

11.4 Neuronal Synapses

Synapses are specialized junctions where neurons communicate with other cells, either electrically or chemically.

• Chemical Synaptic Transmission Steps:

  1. Action potential arrives at presynaptic terminal.

  2. Voltage-gated Ca2+ channels open; Ca2+ enters.

  3. Ca2+ triggers vesicle fusion and neurotransmitter release (exocytosis).

  4. Neurotransmitter diffuses across cleft, binds postsynaptic receptors.

  5. Receptors alter ion channel activity, producing postsynaptic potential.

• EPSP (Excitatory Postsynaptic Potential): Depolarizes, increases firing probability (Na+ or Ca2+ enters).

• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes, decreases firing probability (K+ leaves or Cl- enters).

• Summation:

  • Temporal: Rapid, repeated input from one neuron.

  • Spatial: Combined input from several neurons.

• Termination: Neurotransmitters removed by diffusion, enzymatic breakdown, or reuptake.

11.5 Neurotransmitters

Neurotransmitters are chemicals that transmit signals across synapses. The effect depends on the receptor type, not just the transmitter.

• Major Neurotransmitter Groups:

  • Acetylcholine (ACh): Made from choline + acetyl-CoA. Used at neuromuscular junction, CNS, ANS. Broken down by acetylcholinesterase (AChE).

  • Biogenic amines/monoamines: Derived from amino acids. Includes catecholamines (norepinephrine, epinephrine, dopamine), serotonin, histamine.

  • Amino acids: Glutamate (major excitatory in brain), GABA (major inhibitory in brain), glycine (major inhibitory in spinal cord).

  • Neuropeptides: Chains of amino acids (e.g., substance P for pain, endorphins for pain relief, neuropeptide Y for hunger).

• Neuromodulation: Neuromodulators adjust transmitter release, receptor sensitivity, or gene activity, often via metabotropic pathways. Effects can be long-lasting and widespread.

Key Point: The receptor determines the response; the same transmitter can excite or inhibit depending on the receptor.

11.6 Groups of Neurons

Neurons function in groups (pools) and circuits, allowing complex information processing and integration.

• Neuronal Pools: Groups of interneurons in the CNS working together for a common function. Input neurons branch to contact pool members; summation of inputs determines firing.

• Neural Circuits:

  • Diverging: One input branches to many targets (amplifies, spreads information).

  • Converging: Many inputs meet on one target (combines information, allows spatial summation).

• Inhibitory Circuits: Negative feedback and neuromodulators prevent overexcitation.

• Synaptic Fatigue: Prolonged activity weakens transmission; postsynaptic sensitivity may decline (downregulation).

Integration: Pools use EPSPs and IPSPs; their sum determines if action potentials are generated and passed onward.

Summary Table: CNS vs. PNS Structures

Key Equations and Concepts

• Resting Membrane Potential:

• Na+/K+ Pump:

• Action Potential Threshold:

• Depolarization Peak:

Quick Reference: Major Neurotransmitters

Study Tips

• Copy one small topic at a time and explain it in your own words.

• Practice recall by covering answers and testing yourself.

• Focus on understanding key terms and their relationships.