Chapter 11

Module 1.1 – Overview of the Nervous System

Primary Anatomical Divisions

The nervous system is divided into two main anatomical divisions, each with distinct structures and functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord.

  • Integrates sensory information and coordinates voluntary and involuntary responses.

  • Responsible for higher functions such as thought, memory, and emotion.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS.

  • Connects the CNS to limbs and organs.

  • Divided into cranial nerves (to and from the brain) and spinal nerves (to and from the spinal cord).

Functional Divisions of the PNS

• Sensory (Afferent) Division: Transmits sensory information to the CNS.

  • Somatic sensory: Information from skin, muscles, joints, and special senses (vision, hearing, taste, smell, balance).

  • Visceral sensory: Information from internal organs (heart, lungs, kidneys, stomach, urinary bladder, etc.).

• Motor (Efferent) Division: Transmits motor commands from the CNS to effectors.

  • Somatic motor: Controls voluntary muscles.

  • Autonomic motor: Controls involuntary muscles, glands, and smooth/cardiac muscle.

Module 1.2 – Neural Tissue: Highly Cellular

Neuron Structure and Function

Neurons are the basic functional units of the nervous system, specialized for communication.

• Cell Body (Soma): Contains the nucleus and organelles; responsible for metabolic activities.

• Dendrites: Receive incoming signals from other neurons.

• Axon: Conducts electrical impulses away from the cell body to target cells.

• Axon Terminal: Forms synapses with target cells and releases neurotransmitters.

Functional Regions of a Neuron

• Receptive Region: Dendrites and cell body; receives signals.

• Conducting Region: Axon; transmits signals away from the cell body.

• Secretory Region: Axon terminal; releases neurotransmitters to target cells.

Classification of Neurons

• Multipolar: Most common; many dendrites, one axon. Found in CNS and motor neurons.

• Bipolar: One dendrite, one axon; found in special senses (e.g., retina).

• Pseudounipolar: Single process splits into two branches; found in sensory neurons of the PNS.

Functional Types of Neurons

• Sensory (Afferent): Transmits information into the CNS.

• Interneurons: Connect neurons within the CNS; process and integrate information.

• Motor (Efferent): Transmits information from the CNS to effectors (muscles/glands).

Neuroglia – Supporting Cells

Neuroglia are non-neuronal cells that support, protect, and nourish neurons.

Cell Type	CNS or PNS	Function
Astrocytes	CNS	Support neurons, maintain blood-brain barrier
Oligodendrocytes	CNS	Form myelin sheath in CNS
Microglia	CNS	Phagocytosis; immune defense
Ependymal cells	CNS	Produce and circulate cerebrospinal fluid
Schwann cells	PNS	Form myelin sheath in PNS
Satellite cells	PNS	Support neuron cell bodies in ganglia

Module 1.3 – Electrophysiology

Resting Membrane Potential

Neurons maintain a resting membrane potential, typically around -70 mV, due to differences in ion concentrations inside and outside the cell.

• ECF (Extracellular Fluid): High in Na+ and Cl-; positive charge.

• ICF (Intracellular Fluid): High in K+ and negatively charged proteins; negative charge.

• Cell Membranes: Selectively permeable to ions, maintaining the potential.

Ion Channels

• Leak Channels: Always open; allow ions to move along concentration gradients.

• Gated Channels: Open/close in response to stimuli.

  • Ligand-gated: Open in response to chemical signals.

  • Voltage-gated: Open/close in response to changes in membrane potential.

Changes in Membrane Potential

• Depolarization: Membrane potential becomes less negative (more positive).

• Repolarization: Membrane potential returns to resting value.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Local Potentials

• Graded changes in membrane potential; can summate to trigger action potentials if threshold is reached.

Action Potentials

Action potentials are rapid, all-or-none electrical impulses that propagate along the axon.

• Threshold: Minimum depolarization required to trigger an action potential.

• Phases:

  • Depolarization: Na+ channels open, Na+ enters cell.

  • Repolarization: K+ channels open, K+ exits cell.

  • Hyperpolarization: K+ channels remain open briefly.

• Refractory Periods:

  • Absolute: No new action potential can be initiated.

  • Relative: Stronger stimulus required to initiate another action potential.

Propagation and Conduction Speed

• Saltatory Conduction: Action potentials jump between nodes of Ranvier in myelinated axons, increasing speed.

• Continuous Conduction: Occurs in unmyelinated axons; slower.

• Axon Diameter: Larger diameter = faster conduction.

Module 1.4 – Neuronal Synapses

Types of Synapses

• Electrical Synapses: Direct physical connection via gap junctions; rapid transmission.

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

Synaptic Transmission

1. Action potential arrives at axon terminal.

2. Neurotransmitter released into synaptic cleft.

3. Neurotransmitter binds to receptors on postsynaptic cell.

4. Postsynaptic cell responds (local potential or action potential).

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes postsynaptic membrane, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes postsynaptic membrane, decreasing likelihood of action potential.

Summation

• Multiple EPSPs/IPSPs can summate to reach threshold and trigger action potentials.

Termination of Transmission

• Neurotransmitter diffusion, enzymatic degradation, or reuptake into presynaptic neuron.

Module 1.5 – Neurotransmitters

Neurotransmitter Receptors

• Ionotropic Receptors: Neurotransmitters bind to ion channels, causing direct ion flow.

• Metabotropic Receptors: Neurotransmitters bind to receptors that activate second messenger systems; slower, more specific effects.

Major Neurotransmitters

Neurotransmitter	Function
Acetylcholine	Stimulates muscle contraction, learning, memory
Norepinephrine	Alertness, attention, stress response
Epinephrine	Fight-or-flight response
Dopamine	Reward, motivation, motor control, repeat behavior
Serotonin	Mood regulation, sleep, appetite
GABA	motor control
Histamine	immune protection
Substance P	Pain transmission
Opioids	Pain relief, mood regulation
Neuropeptide Y	feeding behaviors/ feel full

Neurotransmitter Regulation

• Neurotransmitters can be upregulated or downregulated, affecting synaptic transmission and sensitivity.

• Neuromodulators act on or around the synapse to control effects.