Fundamentals of the Nervous System and Nervous Tissue: Study Notes

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Fundamentals of the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is the primary control and communication system of the body. It uses electrical and chemical signals to coordinate rapid, specific responses to internal and external stimuli.

Sensory input: Information is gathered by sensory receptors about changes inside and outside the body.
Integration: The nervous system processes and interprets sensory input and decides what should be done at each moment.
Motor output: The nervous system activates effector organs (muscles and glands) to cause a response.

Diagram of sensory input, integration, and motor output

Divisions of the Nervous System

The nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).

Central Nervous System (CNS): Consists of the brain and spinal cord. It is the integration and control center.
Peripheral Nervous System (PNS): Consists of nerves that extend from the brain and spinal cord (cranial and spinal nerves). It serves as communication lines between the CNS and the rest of the body.

Diagram showing CNS and PNS in the human body

Organization of the Nervous System

The PNS is further divided into sensory (afferent) and motor (efferent) divisions. The motor division is subdivided into the somatic nervous system and the autonomic nervous system (ANS), which itself has sympathetic and parasympathetic divisions.

Chart of nervous system organization

Cells of the Nervous System

Neurons

Neurons are the functional units of the nervous system. They are highly specialized cells that conduct electrical impulses. Key characteristics include:

Extreme longevity (can live a lifetime)
Amitotic (do not divide after maturity, with few exceptions)
High metabolic rate (require continuous oxygen and glucose)

Neuron cell body and dendritic spines

Structure of a Motor Neuron

Neurons have several processes extending from the cell body:

Dendrites: Receive incoming signals and convey them toward the cell body.
Axon: Conducts impulses away from the cell body to other neurons or effectors. The axon hillock is the cone-shaped area where the axon originates.

Myelin Sheath

The myelin sheath is a white, fatty covering that insulates axons and increases the speed of nerve impulse transmission. Myelinated axons conduct impulses much faster than nonmyelinated axons.

Cross-sectional view of a myelinated axon

Neuroglia (Glial Cells)

Neuroglia are supporting cells that protect, insulate, and support neurons. There are different types in the CNS and PNS:

Neuroglia in the CNS

Astrocytes: Most abundant; support neurons, anchor them to capillaries, and help maintain the blood-brain barrier.

Astrocyte supporting neurons and capillaries

Microglial cells: Act as the main defense in the CNS; can transform into phagocytes to remove debris and pathogens.

Microglial cell as a defensive cell in the CNS

Ependymal cells: Line cerebrospinal fluid-filled cavities and help circulate CSF with their cilia.

Ependymal cells lining CNS cavities

Oligodendrocytes: Form myelin sheaths around CNS nerve fibers.

Oligodendrocytes forming myelin sheaths in CNS

Neuroglia in the PNS

Satellite cells: Surround neuron cell bodies in the PNS; function similarly to astrocytes.
Schwann cells: Surround all peripheral nerve fibers and form myelin sheaths in the PNS; similar to oligodendrocytes in function.

Satellite and Schwann cells in the PNS

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); rare, found in special senses.
Unipolar neurons: Single process that divides into two branches; mainly in PNS as sensory neurons.

Multipolar neuron structure Bipolar neuron structure

Functional Classification

Sensory (afferent) neurons: Transmit impulses from sensory receptors toward the CNS.
Motor (efferent) neurons: Carry impulses from the CNS to effectors (muscles/glands).
Interneurons: Shuttle signals within the CNS; most abundant type.

Membrane Potentials and Nerve Impulses

Resting Membrane Potential

Neurons have a resting membrane potential of about -70 mV, generated by differences in ionic composition and membrane permeability. The sodium-potassium pump and selective ion channels maintain this potential.

Types of Signals

Graded potentials: Short-lived, localized changes in membrane potential; act as signals over short distances.
Action potentials: Long-distance signals of axons; do not decay with distance and follow the all-or-none principle.

Depolarization and Hyperpolarization

Depolarization: Membrane potential becomes less negative; increases likelihood of impulse generation.
Hyperpolarization: Membrane potential becomes more negative; decreases likelihood of impulse generation.

Synapses and Neurotransmitters

Synapses

A synapse is a junction where a neuron communicates with another cell. The presynaptic neuron sends the signal, and the postsynaptic neuron receives it. In the PNS, the postsynaptic cell may be a neuron, muscle, or gland cell.

Chemical Synapses

At chemical synapses, neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic cell, causing graded potentials.

Postsynaptic Potentials

Excitatory postsynaptic potentials (EPSPs): Depolarize the postsynaptic membrane, increasing the chance of an action potential.
Inhibitory postsynaptic potentials (IPSPs): Hyperpolarize the postsynaptic membrane, decreasing the chance of an action potential.

Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across synapses. Over 50 have been identified, and they can be classified by structure and function:

Acetylcholine (ACh): Released at neuromuscular junctions; can be excitatory or inhibitory depending on receptor type.
Biogenic amines: Include catecholamines (dopamine, norepinephrine, epinephrine) and indolamines (serotonin, histamine).
Amino acids: Glutamate (excitatory), GABA and glycine (inhibitory).
Peptides: Substance P (pain mediator), endorphins (natural opiates).
Purines: ATP and adenosine (inhibitory in the brain).
Gases and lipids: Nitric oxide, carbon monoxide, hydrogen sulfide (act via G protein–coupled receptors).

Neurotransmitter Receptors

Channel-linked receptors: Ligand-gated ion channels; immediate and brief action.
G protein–linked receptors: Indirect, complex, slow, and often prolonged responses; cause widespread metabolic changes.