Chapter 12 Notes

Nervous Tissue

Anatomical Divisions of the Nervous System

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

• Central Nervous System (CNS): Consists of the brain and spinal cord. The CNS is responsible for integrating, processing, and coordinating sensory data and motor commands.

• Peripheral Nervous System (PNS): Includes cranial nerves and spinal nerves. The PNS connects the CNS to limbs and organs, serving as a communication relay.

Functional Divisions of the Nervous System

The nervous system is also divided functionally based on the direction and type of information flow.

• Sensory Division (Afferent): Carries sensory information to the CNS from receptors in peripheral tissues and organs.

• Motor Division (Efferent): Carries motor commands from the CNS to muscles or glands.

Functional Divisions of the Motor Division

• Somatic Nervous System (SNS): Sends impulses that lead to contraction of skeletal muscle (voluntary control).

• Autonomic Nervous System (ANS): Sends impulses that lead to contraction of smooth muscle, cardiac muscle, and activation of glands (involuntary control).

Cell Types in the Nervous System

The nervous system contains two main categories of cells: neurons and neuroglia.

• Neurons: Specialized for communication; they generate and transmit electrical impulses.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons; do not conduct impulses.

The Neuron

Neurons are the functional units of the nervous system, with unique characteristics and structures.

• Longevity: Neurons can live a lifetime.

• Amitotic: Most neurons do not divide after maturity.

Structure and Processes

• Cell Body (Soma): Contains the nucleus and organelles; site of metabolic activity.

• Nissl Bodies: Rough endoplasmic reticulum in neurons, important for protein synthesis.

• Dendrites: Short, branched projections that receive signals from other neurons.

• Axon: Single, long process that transmits impulses away from the cell body.

• Collaterals: Branches along the length of the axon at right angles.

• Telodendria: Series of extensions off the main axon trunk.

• Axon Terminals (Synaptic Boutons): Knoblike endings of axon; release neurotransmitters.

• Axon Hillock: Thick area where the cell body joins the axon; site of action potential initiation.

• Myelin: Insulating sheath around axons, increases speed of impulse conduction.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

Classification of Neurons

By Structure (Number of Processes from Cell Body)

• Bipolar Neurons: Have two processes (one axon, one dendrite); found in special sense organs.

• Unipolar (Pseudounipolar) Neurons: Have one process that splits into two branches; common in sensory neurons of the PNS.

• Multipolar Neurons: Have three or more processes (one axon, multiple dendrites); most common type in CNS.

By Function

• Sensory (Afferent) Neurons: Carry information from receptors to the CNS.

• Motor (Efferent) Neurons: Carry information from the CNS to effectors (muscles or glands).

• Interneurons: Integrate sensory and motor neurons; found mostly in the CNS.

Types of Neuroglia

• Ependymal Cells (CNS): Line ventricles of brain and central canal of spinal cord; produce and circulate cerebrospinal fluid (CSF).

• Astrocytes (CNS): Maintain blood-brain barrier, provide structural support, regulate ion and nutrient concentrations, and repair tissue after injury.

• Oligodendrocytes (CNS): Form myelin sheaths around CNS axons.

• Microglia (CNS): Act as phagocytes, removing debris and pathogens.

• Schwann Cells (PNS): Form myelin sheaths around PNS axons; assist in axon regeneration.

White Matter and Gray Matter in the CNS

• White Matter: Regions of the CNS with myelinated axons; responsible for rapid signal transmission.

• Gray Matter: Regions with neuron cell bodies, dendrites, and unmyelinated axons; involved in processing and integration.

Neurophysiology

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the neuron's plasma membrane at rest.

• Distribution of Ions: More Na+ outside, more K+ inside the cell.

• Average Resting Potential: $-70\ \mathrm{mV}$

• Polarized: The inside of the membrane is negatively charged relative to the outside.

• Maintenance: Maintained by leak channels (allow passive movement of ions) and the Na+-K+ pump (actively transports 3 Na+ out and 2 K+ in).

Equation for Na+-K+ Pump:

$3\ \mathrm{Na}^+_{\text{out}} : 2\ \mathrm{K}^+_{\text{in}}$ per ATP hydrolyzed

Graded Potentials

Graded potentials are small, localized changes in membrane potential that occur in the dendrites or cell body.

• Stimulus: Causes gated ion channels to open.

• Chemically-Gated Ion Channel: Opens in response to binding of a specific chemical (e.g., neurotransmitter).

• Depolarization: Membrane potential becomes less negative (e.g., Na+ influx).

• Repolarization: Return to resting membrane potential after depolarization.

• Hyperpolarization: Membrane potential becomes more negative than resting (e.g., K+ efflux).

Characteristics of Graded Potentials

• Short-distance signals only

• Decremental: Effect decreases with distance from the site of stimulation.

• Graded: Stronger stimulus produces a greater change in membrane potential.

Action Potentials

Action potentials are long-distance electrical signals that travel along axons.

• Begin at: Axon hillock and continue down the axon.

• Voltage-Gated Ion Channels: Open or close in response to changes in membrane potential.

• Resting Membrane Potential: $-70\ \mathrm{mV}$

• Threshold: If depolarization reaches threshold (typically $-55\ \mathrm{mV}$) at the axon hillock, an action potential is triggered.

Phases of an Action Potential

1. Depolarization to Threshold: Stimulus causes membrane potential to reach threshold.

2. Depolarization: Voltage-gated Na+ channels open, Na+ enters, membrane potential rises rapidly.

3. Repolarization: Na+ channels inactivate, K+ channels open, K+ exits, membrane potential returns toward resting.

4. Hyperpolarization: K+ channels are slow to close, membrane potential becomes more negative than resting.

5. Restoration: K+ channels close, Na+-K+ pump restores ion concentrations.

• Absolute Refractory Period: Time during which another action potential cannot be triggered.

• "All-or-None" Principle: All stimuli above threshold produce identical action potentials.

Speed of Action Potentials

• Myelination: Myelinated axons conduct impulses faster.

• Continuous Propagation: Occurs on unmyelinated axons; depolarization occurs along the entire length.

• Saltatory Propagation: Occurs on myelinated axons; depolarization occurs only at nodes of Ranvier; much faster.

• Axon Diameter: Larger diameter = faster conduction.

Axon Types

Type	Diameter	Myelination	Speed
Type A	Large	Myelinated	Fastest
Type B	Medium	Myelinated	Intermediate
Type C	Small	Unmyelinated	Slowest

Example: Type A fibers conduct motor signals to skeletal muscles; Type C fibers carry slow pain.

Multiple Sclerosis: Demyelination of CNS axons leads to impaired signal conduction.

The Synapse

A synapse is the junction where information is transmitted from one neuron to another or to an effector cell.

• Presynaptic Neuron: Sends the signal.

• Postsynaptic Neuron: Receives the signal.

• Synaptic Cleft: Small gap between the two neurons.

• Neurotransmitter: Chemical released by presynaptic neuron into the synapse; binds to receptors on postsynaptic cell.

Events of a Cholinergic Synapse (using Acetylcholine)

1. Action potential reaches axon terminal of presynaptic neuron.

2. Voltage-gated Ca2+ channels open; Ca2+ influx triggers release of acetylcholine from vesicles.

3. Acetylcholine diffuses across synaptic cleft and binds to receptors on postsynaptic neuron.

4. Depolarization may occur on postsynaptic neuron, producing a graded potential.

Types of Neurotransmitters and Neuropeptides

• Acetylcholine (ACh): Excitatory at neuromuscular junctions; involved in muscle contraction.

• GABA (Gamma Aminobutyric Acid): Main inhibitory neurotransmitter in the brain.

• Dopamine: Involved in reward, motivation, and motor control; deficiency linked to Parkinson's disease.

• Serotonin: Regulates mood, appetite, and sleep.

• Endorphins: Act as natural painkillers; produce feelings of well-being.

Postsynaptic Potentials

After neurotransmitter binding, the postsynaptic neuron may experience changes in membrane potential.

• Excitatory Postsynaptic Potential (EPSP): Causes depolarization, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes postsynaptic neuron, making it less likely to reach action potential.

Summation

• Temporal Summation: One presynaptic neuron transmits multiple impulses in rapid succession.

• Spatial Summation: Multiple presynaptic neurons stimulate a postsynaptic neuron simultaneously.