The Nervous System: Structure, Function, and Neural Communication

Chapter 11: The Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

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

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; connects the CNS to limbs and organs.

Organization of the Nervous System

Branch	Components	Division	Direction of Signal	Type of Control	Effectors	Response at Effector
CNS	Brain and spinal cord	---	---	---	---	---
PNS	Receptors, nerves, ganglia, plexuses	Sensory (afferent)	Afferent	---	---	---
		Motor (efferent)	Efferent	Somatic (voluntary), Autonomic (involuntary)	Somatic: Skeletal muscle Autonomic: Cardiac muscle, smooth muscle, glands	Somatic: Conscious movement Autonomic: Unconscious regulation of body functions

Divisions of the Peripheral Nervous System

• Sensory Division: Transmits sensory information from receptors to the CNS (e.g., light, sound, touch, temperature, pain).

• Motor Division: Transmits commands from the CNS to effectors (muscles and glands). Subdivided into:

  • Somatic Nervous System: Controls voluntary movements of skeletal muscles.

  • Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion). Includes sympathetic and parasympathetic divisions.

Neurons: Structure and Types

Neurons are the functional units of the nervous system, specialized for the transmission of electrical signals.

• Cell Body (Soma): Contains the nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Conducts impulses away from the cell body.

• Myelin Sheath: Insulating layer that increases signal speed.

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

Types of Neurons:

• Multipolar: Many dendrites, one axon (most common in CNS).

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

• Pseudo-unipolar: Single process splits into two branches (sensory neurons).

• Anaxonic: Many dendrites, no axon (rare, in brain and retina).

Glial Cells: Types and Functions

Glial cells support, protect, and nourish neurons. They are essential for proper nervous system function.

Glial Cell	Location	Function
Astrocytes	CNS	Support neurons, maintain blood-brain barrier, regulate ion/nutrient environment
Oligodendrocytes	CNS	Form myelin sheath around CNS axons
Microglia	CNS	Immune defense, remove debris
Ependymal cells	CNS	Line ventricles, produce/circulate cerebrospinal fluid
Schwann cells	PNS	Form myelin sheath around PNS axons
Satellite cells	PNS	Support neuron cell bodies in ganglia

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the plasma membrane of a resting neuron, typically around -70 mV.

• Maintained by the sodium-potassium pump and differential permeability of the membrane to ions.

• More K+ inside, more Na+ outside the cell.

Ion	Intracellular (mEq/L)	Extracellular (mEq/L)
Potassium (K+)	141	4
Sodium (Na+)	14	142
Chloride (Cl-)	4	103
Calcium (Ca2+)	0.0001	5
Proteins (Anions)	100	0

Graded Potentials and Action Potentials

• Graded Potentials: Small, local changes in membrane potential; can summate to trigger an action potential.

• Action Potentials: Rapid, large changes in membrane potential that propagate along axons.

Phases of Action Potential:

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

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

3. Hyperpolarization: Membrane potential becomes more negative than resting.

Key Equations:

• Resting membrane potential is determined by the Nernst equation: $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

Propagation of Action Potentials

• Action potentials propagate in one direction along the axon.

• Saltatory conduction occurs in myelinated axons, where action potentials jump between nodes of Ranvier, increasing speed.

Synaptic Transmission

Neurons communicate at synapses, which can be electrical (gap junctions) or chemical (neurotransmitter release).

1. Action potential arrives at presynaptic terminal.

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

3. Neurotransmitter vesicles fuse with membrane, releasing neurotransmitter into synaptic cleft.

4. Neurotransmitter binds to receptors on postsynaptic cell, opening ion channels and generating a postsynaptic potential.

Neurotransmitters and Their Effects

Chemical Class	Example	Effect	Mechanism
Acetylcholine	Acetylcholine	Excitatory or inhibitory	Ionotropic/metabotropic
Biogenic Amines	Serotonin, Dopamine, Norepinephrine	Modulatory, excitatory, or inhibitory	Metabotropic
Amino Acids	GABA, Glutamate, Glycine	GABA: Inhibitory Glutamate: Excitatory	Ionotropic/metabotropic
Peptides	Substance P, Endorphins	Excitatory or inhibitory	Metabotropic

Clinical Modulation of Neurotransmitters

• SSRIs: Selective serotonin reuptake inhibitors increase serotonin in synaptic cleft, used to treat depression and anxiety.

• Barbiturates: Enhance GABA activity, used to treat epilepsy.

• ADHD: Linked to dopamine and norepinephrine imbalance.

• Parkinson Disease: Caused by loss of dopamine-producing neurons.

Neural Integration and Pathways

• Excitatory Postsynaptic Potential (EPSP): Local depolarization, increases likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Local hyperpolarization, decreases likelihood of action potential.

• Summation: Multiple EPSPs and IPSPs combine to determine if threshold is reached (spatial and temporal summation).

• Neural Circuits: Convergent, divergent, reverberating, and parallel after-discharge circuits integrate and process information.

Example:

When you touch a hot surface, sensory receptors send signals via sensory neurons to the CNS, which processes the information and sends a rapid motor response to withdraw your hand.

Additional info: This summary integrates and expands upon the provided diagrams and tables, ensuring a comprehensive overview suitable for college-level General Biology students.