Introduction to the Nervous System and Nervous Tissue – Study Notes

Introduction to the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is responsible for controlling perception, voluntary movement, consciousness, personality, learning, and memory. It works closely with the endocrine system to maintain homeostasis of variables such as respiratory rate, blood pressure, body temperature, and the sleep/wake cycle.

• Central Nervous System (CNS): Consists of the brain and spinal cord. The brain contains about 100 billion neurons, while the spinal cord contains about 100 million neurons. The CNS integrates sensory information and coordinates responses.

• Peripheral Nervous System (PNS): Composed of cranial nerves (12 pairs) and spinal nerves (31 pairs) and their branches. The PNS carries signals to and from the CNS.

Functional Divisions of the Nervous System

The nervous system is divided functionally into sensory, integrative, and motor divisions:

• Sensory (Afferent) Division: Gathers information from internal and external environments via sensory receptors. Includes the somatic sensory division (signals from skin, muscles, bones, joints, and special senses) and the visceral sensory division (signals from organs such as the heart, lungs, and digestive tract).

• Integrative Functions: The CNS analyzes and interprets sensory input, determining appropriate responses. Most sensory input is disregarded unless a response is necessary.

• Motor (Efferent) Division: Carries out responses by transmitting signals to effectors (muscles and glands). Includes the somatic motor division (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of smooth muscle, cardiac muscle, and glands).

Nervous Tissue

Cellular Components of Nervous Tissue

Nervous tissue is composed of neurons and neuroglial cells, with about 80% of its volume being cells and 20% extracellular matrix.

• Neurons: Excitable cells that send and receive signals in the form of action potentials. Responsible for sensory, integrative, and motor functions. Most neurons are long-lived and amitotic.

• Neuroglial Cells: Smaller, more prevalent cells that support, protect, and assist neurons. They do not transmit signals but are essential for neuron function.

Structure of Neurons

Neurons have three main parts: the cell body (soma), dendrites, and axon.

• Cell Body (Soma): Contains the nucleus, organelles, and is the metabolic center of the neuron. Contains Nissl bodies (clusters of rough ER and ribosomes).

• Dendrites: Short, branched processes that receive input from other neurons and transmit electrical impulses toward the cell body.

• Axon (Nerve Fiber): A single process that carries electrical signals away from the cell body. Includes the axon hillock, axon collaterals, telodendria, and axon terminals (synaptic knobs).

Functional Regions of Neurons

Each neuron has three main functional regions:

• Receptive Region: Dendrites and cell body receive signals.

• Conducting Region: Axon transmits the signal.

• Secretory Region: Axon terminals secrete neurotransmitters to communicate with target cells.

Classification of Neurons

Neurons are classified structurally and functionally:

• Multipolar Neurons: One axon with multiple dendrites; most common type.

• Bipolar Neurons: One axon and one dendrite; found in the retina and olfactory epithelium.

• Pseudounipolar Neurons: Single axon with two branches; sensory neurons for touch, pressure, and pain.

Structural Class	Multipolar Neurons	Bipolar Neurons	Pseudounipolar Neurons
Features	One axon, multiple dendrites	One axon, one dendrite	Single short process splits into two axons
Function	Motor neurons, interneurons	Sensory (special senses)	Sensory (touch, pain, vibration)

Neuroglia (Neuroglial Cells)

Neuroglia support and protect neurons, maintain the environment, and assist in their functions. They can divide and fill in gaps when neurons die. There are six types: four in the CNS and two in the PNS.

• Astrocytes (CNS): Anchor neurons and blood vessels, regulate the extracellular environment, form the blood-brain barrier, and repair damaged tissue.

• Oligodendrocytes (CNS): Form myelin sheaths around axons, increasing conduction speed.

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

• Ependymal Cells (CNS): Circulate and produce cerebrospinal fluid.

• Neurolemmocytes (Schwann Cells, PNS): Myelinate axons and assist in axon repair.

• Satellite Cells (PNS): Support and regulate the environment around neuron cell bodies.

The Myelin Sheath

Structure and Function of Myelin

The myelin sheath is formed by neurolemmocytes in the PNS and oligodendrocytes in the CNS. It consists of multiple layers of plasma membrane and acts as an insulator, increasing the speed of action potential conduction.

• Internodes: Segments of axon covered by myelin.

• Nodes of Ranvier: Gaps between internodes where action potentials are regenerated.

• White Matter: Myelinated regions in the CNS.

• Gray Matter: Unmyelinated regions in the CNS.

Regeneration of Nervous Tissue

Regeneration is limited in the CNS but possible in the PNS if the cell body remains intact. The process involves degeneration of the distal axon, phagocytosis of debris, formation of a regeneration tube, and reconnection with the target cell.

Electrophysiology of Neurons

Resting Membrane Potential

The resting membrane potential is the electrical gradient across the plasma membrane of a neuron at rest, typically about -70 mV. It is maintained by the movement of potassium and sodium ions through leak channels and the sodium-potassium pump.

Ion Channels and Gradients

Ions cross the plasma membrane through leak channels, ligand-gated channels (open in response to chemicals), voltage-gated channels (open in response to voltage changes), and mechanically gated channels (open in response to mechanical stimuli).

Changes in Membrane Potential

• Depolarization: Membrane potential becomes less negative (e.g., influx of sodium ions).

• Repolarization: Return to resting membrane potential.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Local and Action Potentials

• Local Potentials: Small, reversible, and decremental changes in membrane potential that travel short distances.

• Action Potentials: Large, uniform, and nondecremental changes that travel the length of the axon. Generated at the trigger zone and involve voltage-gated sodium and potassium channels.

Events of an Action Potential

1. Local potential depolarizes the axolemma to threshold (-55 mV).

2. Voltage-gated sodium channels open, sodium enters, and depolarization occurs to about +30 mV.

3. Sodium channels inactivate, potassium channels open, and repolarization begins.

4. Potassium channels close, and the membrane may hyperpolarize before returning to rest.

The Refractory Period

• Absolute Refractory Period: No new action potential can be generated.

• Relative Refractory Period: A stronger stimulus can generate another action potential.

Propagation of Action Potentials

Action potentials are self-propagating and travel in one direction due to the refractory period. Conduction speed depends on axon diameter and myelination:

• Continuous Conduction: Occurs in unmyelinated axons; each segment must be depolarized.

• Saltatory Conduction: Occurs in myelinated axons; action potentials "jump" from node to node, increasing speed.

Neuronal Synapses

Types of Synapses

• Electrical Synapses: Direct electrical connection via gap junctions; bidirectional and nearly instantaneous. Found in areas requiring synchronized activity.

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft; unidirectional and allow for varied signal strength and effects.

Events at a Chemical Synapse

1. An action potential triggers calcium channels to open in the presynaptic neuron.

2. Calcium influx causes synaptic vesicles to release neurotransmitters into the synaptic cleft.

3. Neurotransmitters bind to receptors on the postsynaptic neuron, opening ion channels and generating a local potential.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Local depolarization, brings membrane potential closer to threshold.

• Inhibitory Postsynaptic Potential (IPSP): Local hyperpolarization, moves membrane potential farther from threshold.

Summation and Neural Integration

• Temporal Summation: Rapid, repeated release of neurotransmitter from one presynaptic neuron.

• Spatial Summation: Simultaneous release from multiple presynaptic neurons.

Types of Channels and Pumps in Neurons

• Ligand-Gated Channels: Found on dendrites and cell body; bind neurotransmitters.

• Voltage-Gated Sodium and Potassium Channels: Found on axon; generate action potentials.

• Voltage-Gated Calcium Channels: Found at axon terminals; trigger neurotransmitter release.

• Leak Channels and Sodium-Potassium Pump: Found throughout the neuron; maintain resting potential.

Neurotransmitters and Receptors

Types of Neurotransmitter Receptors

• Ionotropic Receptors: Ligand-gated ion channels; rapid, short-lived effects.

• Metabotropic Receptors: G-protein coupled; slower, longer-lasting, and more varied effects.

Major Neurotransmitters

• Acetylcholine (ACh): Excitatory at neuromuscular junctions; broken down by acetylcholinesterase.

• Biogenic Amines (Monoamines): Include norepinephrine, epinephrine, dopamine, serotonin, and histamine; involved in mood, cognition, and homeostasis.

• Amino Acid Neurotransmitters: Glutamate (excitatory), GABA and glycine (inhibitory).

• Neuropeptides: Substance P (pain), opioids (pain relief), neuropeptide Y (feeding behavior).

Neuromodulation

Neuromodulators alter synaptic transmission by affecting neurotransmitter release, postsynaptic sensitivity, or gene transcription. Some hormones act as neurohormones to modulate neuronal activity.

Psychiatric Disorders and Treatments

• Schizophrenia: May involve excess dopamine; treated by blocking dopamine receptors.

• Depressive Disorders: May involve deficiencies in serotonin, norepinephrine, or dopamine; treated with SSRIs and other medications.

• Anxiety Disorders: May involve norepinephrine, serotonin, and GABA; treated with antidepressants and GABA enhancers.

• Bipolar Disorder: Treated by blocking sodium ion channels to decrease action potential generation.

Neuronal Pools and Circuits

Neuronal Pools

Groups of interneurons in the CNS that perform common functions. Input neurons initiate signals that branch to multiple neurons, generating EPSPs or IPSPs. The number of contacts influences action potential initiation.

Neuronal Circuits

• Diverging Circuit: One neuron branches to multiple postsynaptic neurons; allows communication with multiple brain/body regions.

• Converging Circuit: Multiple input neurons converge on a single postsynaptic neuron; important for spatial summation and sensory integration.

Inhibitory Mechanisms

• Negative Feedback: Neuromodulators inhibit overexcitation.

• Synaptic Fatigue: Prolonged excitation decreases neurotransmitter sensitivity and receptor numbers.

Epileptic Seizures

Epilepsy involves recurrent seizures due to abnormal, excessive electrical activity in neuronal pools. Loss of inhibitory mechanisms leads to widespread excitation. Treatments aim to restore inhibitory function and prevent seizures.