Fundamentals of the Nervous System and Nervous Tissue – Study Notes

Fundamentals of the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is the master controlling and communicating system of the body. It uses electrical and chemical signals to coordinate rapid and specific responses to internal and external stimuli.

• Sensory input: Sensory receptors detect changes inside or outside the body and send information to the nervous system.

• Integration: The nervous system processes and interprets sensory input, deciding what action is needed.

• Motor output: The nervous system activates effectors (muscles or glands) to produce a response.

Divisions of the Nervous System

The nervous system is divided into two main parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the integration and control center.

• Peripheral Nervous System (PNS): Consists mainly of nerves that extend from the brain and spinal cord. It includes cranial nerves, spinal nerves, and ganglia.

Functional Divisions of the PNS

• Sensory (afferent) division: Carries impulses from sensory receptors to the CNS. Includes somatic sensory fibers (from skin, muscles, joints) and visceral sensory fibers (from organs).

• Motor (efferent) division: Transmits impulses from the CNS to effectors. Subdivided into:

  • Somatic nervous system: Voluntary control of skeletal muscles.

  • Autonomic nervous system (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands. Divided into sympathetic (mobilizes body systems) and parasympathetic (conserves energy) divisions.

Neuroglia (Glial Cells)

Neuroglia in the CNS

Neuroglia are supporting cells that protect, insulate, and support neurons. Four main types are found in the CNS:

• Astrocytes: Most abundant; support neurons, regulate the chemical environment, and help form the blood-brain barrier.

• Microglial cells: Defensive cells that act as phagocytes, removing debris and pathogens.

• Ependymal cells: Line cerebrospinal fluid-filled cavities; their cilia help circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths around CNS nerve fibers, increasing the speed of impulse transmission.

Neuroglia in the PNS

• Satellite cells: Surround neuron cell bodies in the PNS; similar function to astrocytes.

• Schwann cells: Form myelin sheaths around peripheral nerve fibers and are vital for nerve fiber regeneration.

Neurons (Nerve Cells)

Structure of a Neuron

Neurons are the functional units of the nervous system. They are specialized for conducting impulses and have the following main parts:

• Cell body (soma): Contains the nucleus and organelles; biosynthetic and metabolic center.

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

• Axon: Long process that transmits impulses away from the cell body to other neurons or effectors.

Neuron Processes

• Dendrites: Receptive regions; convey incoming messages as graded potentials.

• Axon: Conducting region; generates and transmits nerve impulses. Axon terminals are the secretory region, releasing neurotransmitters.

Myelin Sheath

The myelin sheath is a white, fatty covering that insulates axons and increases the speed of impulse transmission.

• In the PNS: Formed by Schwann cells wrapping around the axon.

• In the CNS: Formed by oligodendrocytes, which can myelinate multiple axons.

Classification of Neurons

Structural Classification

• Multipolar neurons: Many processes (1 axon, many dendrites); most common in CNS.

• Bipolar neurons: Two processes (1 axon, 1 dendrite); found in retina, ear, olfactory mucosa.

• Unipolar (pseudounipolar) neurons: One process that divides into peripheral and central branches; mainly in PNS ganglia as sensory neurons.

Functional Classification

• Sensory (afferent) neurons: Transmit impulses from sensory receptors toward the CNS; mostly unipolar.

• Motor (efferent) neurons: Carry impulses from the CNS to effectors; multipolar.

• Interneurons: Lie between sensory and motor neurons; most abundant, mainly in CNS.

Membrane Potentials and Electrical Signals

Basic Principles of Electricity

• Voltage (V): Measure of potential energy generated by separated charge; measured in volts (V) or millivolts (mV).

• Current (I): Flow of electrical charge (ions) between two points.

• Resistance (R): Hindrance to charge flow.

• Ohm’s Law: $I = \frac{V}{R}$

Ion Channels in Neuronal Membranes

• Leakage (nongated) channels: Always open.

• Gated channels: Open or close in response to specific signals (chemical, voltage, or mechanical).

Resting Membrane Potential

Neurons have a resting membrane potential of about -70 mV, with the inside of the cell more negative than the outside. This is due to differences in ion concentrations and membrane permeability.

• Na+/K+ ATPase pump: Maintains the resting potential by pumping 3 Na+ out and 2 K+ in.

Graded Potentials and Action Potentials

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential. They are essential for initiating action potentials but decay quickly with distance.

Action Potentials (APs)

An action potential is a brief reversal of membrane potential that travels along the axon. It is an all-or-none phenomenon and does not decay with distance.

• Phases of an AP:

  1. Resting state: All voltage-gated channels closed.

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

  3. Repolarization: Na+ channels inactivate, K+ channels open, K+ exits cell.

  4. Hyperpolarization: Some K+ channels remain open, Na+ channels reset.

Propagation and Coding of Action Potentials

• Propagation: APs are self-propagating and travel in one direction along the axon.

• Stimulus intensity: Coded by the frequency of APs, not their amplitude.

Refractory Periods

• Absolute refractory period: No new AP can be generated.

• Relative refractory period: Only a strong stimulus can generate another AP.

Conduction Velocity

• Larger diameter axons: Conduct impulses faster.

• Myelinated axons: Exhibit saltatory conduction, which is much faster than continuous conduction in unmyelinated axons.

Synapses and Neurotransmission

Synapses

Synapses are junctions where neurons communicate with other neurons or effectors. They can be electrical (via gap junctions) or chemical (via neurotransmitters).

Chemical Synapse Transmission

1. AP arrives at axon terminal.

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

3. Ca2+ triggers exocytosis of neurotransmitter vesicles.

4. Neurotransmitter diffuses across synaptic cleft and binds to receptors on postsynaptic membrane.

5. Binding opens ion channels, generating graded potentials.

6. Neurotransmitter effects are terminated by reuptake, degradation, or diffusion.

Postsynaptic Potentials

• Excitatory postsynaptic potentials (EPSPs): Depolarize the postsynaptic membrane, increasing the likelihood of an AP.

• Inhibitory postsynaptic potentials (IPSPs): Hyperpolarize the membrane, decreasing the likelihood of an AP.

Neurotransmitters

Classification by Chemical Structure

• Acetylcholine (ACh): First identified; released at neuromuscular junctions.

• Biogenic amines: Dopamine, norepinephrine, epinephrine, serotonin, histamine.

• Amino acids: Glutamate, aspartate, glycine, GABA.

• Peptides: Substance P, endorphins, somatostatin, CCK.

• Purines: ATP, adenosine.

• Gases and lipids: Nitric oxide, carbon monoxide, endocannabinoids.

Classification by Function

• Excitatory vs. inhibitory: Depends on receptor type (e.g., ACh is excitatory at skeletal muscle, inhibitory at cardiac muscle).

• Direct (fast) vs. indirect (slow) action: Direct neurotransmitters open ion channels; indirect act through second messengers (e.g., G protein-coupled receptors).

Neural Integration and Circuits

• Serial processing: Input travels along one pathway to a specific destination (e.g., reflex arc).

• Parallel processing: Input travels along several pathways, allowing complex responses.

• Neural circuits: Patterns of synaptic connections (diverging, converging, reverberating, parallel after-discharge).

Developmental Aspects of Neurons

• Nervous system originates from the neural tube and neural crest (ectoderm).

• Neurons are amitotic after birth, except for a few populations (e.g., olfactory neurons, hippocampus).

• Learning reinforces certain synapses and prunes others (neural plasticity).