Nervous Tissue Notes

Big Picture

The nervous system has two major divisions and three core jobs. Everything you'll learn about nervous tissue, individual cells, pathways, and complex behaviors fits into this framework. Start here.

Foundation

Two Major Divisions of the Nervous System

I. Central Nervous System (CNS)

Brain (~85 billion neurons)
Spinal Cord (~100 billion neurons)

CNS Functions:

Processes incoming sensory information
Source of thoughts, emotions, memories
Stimulates muscle contraction and glandular secretion

II. Peripheral Nervous System (PNS)

"All nervous tissue outside the CNS." Four components:

Nerves: bundles of hundreds to thousands of axons. 12 pairs of cranial nerves emerge from the brain, 31 pairs of spinal nerves from the cord. Each follows a defined path and serves a specific region.
Ganglia: small masses of nervous tissue, primarily nerve cell bodies. Located outside the brain and cord, closely associated with cranial and spinal nerves.
Enteric plexuses: extensive networks of neurons in the walls of GI tract organs that regulate digestion.
Sensory receptors: structures that monitor changes in the internal/external environment (touch receptors in skin, photoreceptors in eye, olfactory receptors in nose).

PNS Subdivisions

Three Divisions of the PNS

1. Somatic Nervous System

Sensory neurons: somatic receptors (head, body wall, limbs, special senses) → CNS
Motor neurons: CNS → skeletal muscle only (voluntary)

2. Autonomic Nervous System (ANS)

Sensory: visceral organ receptors (stomach, lungs) → CNS
Motor: CNS → smooth muscle, cardiac muscle, glands (involuntary)
Two divisions: Sympathetic ("fight or flight") and Parasympathetic ("rest and digest")

3. Enteric Nervous System

"Brain of the gut." Over 1 million neurons in enteric plexuses extending throughout the GI tract.
Functions largely independently of the ANS and CNS, but communicates with the CNS via sympathetic/parasympathetic neurons.
Governs GI motility, secretions, and hormone release.

Three Core Jobs

Functions of the Nervous System

Sensory Function. Detect internal stimuli (changes in BP) and external stimuli (rain on your skin). Sensory info travels via cranial and spinal nerves to the brain and cord.
Integrative Function. Process information by analyzing it and making decisions about appropriate responses.
Motor Function. Once sensory info is integrated, the NS may elicit a motor response by activating effectors (muscles or glands) through cranial or spinal nerves.

Sensory in. Integrate in the middle. Motor out. Every NS interaction follows this loop.

Neuron Anatomy

Neurons are the message carriers of the nervous system. Highly specialized, capable of stretching long distances, and unable to undergo cell division. Every neuron has the same basic three parts, plus a set of specialized components for sending and receiving signals.

Two Cell Types

Cells of Nervous Tissue

1. Neurons

Form the complex processing network within the brain and spinal cord.
Connect all regions of the body to the brain and spinal cord.
Highly specialized: sensing, thinking, remembering, controlling muscle activity, regulating glandular secretions.
Can reach great lengths.
Cannot undergo cell division (mitosis).

2. Neuroglia

Smaller than neurons, but greatly outnumber them.
Support, nourish, protect, and maintain the interstitial fluid that bathes neurons.
Can continue to divide throughout the lifetime.

The fact that neurons can't divide but neuroglia can is why most brain tumors are gliomas (from neuroglia) rather than from neurons themselves.

The Defining Trait

Neuron Characteristics

Excitability

The ability to respond to a stimulus and convert it to an action potential.

Stimulus: any change in the environment strong enough to initiate an action potential.
Action potential: an electrical signal that propagates along the surface of the membrane of a neuron. Once initiated, it travels at constant strength.
Speed: 1 to 290 mi/hr depending on the fiber type.

Three Main Parts

Anatomy of a Neuron

1. Cell Body (perikaryon or soma)

Contains a nucleus and cytoplasm with typical organelles.
Contains Nissl bodies: clusters of free ribosomes and rough ER. They give gray matter its color.
Neurofibrils: bundles of intermediate filaments that provide cell shape and support.
Microtubules: assist in moving materials between cell body and axon.
Lipofuscin: yellowish-brown pigment clumps found in aging neurons. A product of neuronal lysosomes that accumulates with age. Doesn't harm the neuron.

2. Dendrites (Input)

The receiving portions of a neuron.
Plasma membrane contains receptors for binding chemical messengers from other cells.
Typically tree-shaped: short, tapering, highly branched.

3. Axon (Output)

Propagates nerve impulses toward another neuron, muscle fiber, or gland cell.
Axon hillock: cone-shaped elevation where the cell body joins the axon.
Axoplasm: cytoplasm of the axon.
Axolemma: plasma membrane of the axon.
Axon terminals: branching of the axon at the distal end into fine processes.

Nerve fiber: general term for any neuronal process (extension) that emerges from the cell body.

Where Communication Happens

Synaptic End Bulbs & Synapse

Synapse: the site of communication between two neurons or between a neuron and an effector cell.
Synaptic end bulbs: swellings at the tips of axon terminals.
Synaptic vesicles: tiny membrane-enclosed sacs inside the end bulbs that store chemicals.
Neurotransmitters: chemicals released from synaptic vesicles that excite or inhibit another neuron, muscle fiber, or gland cell.

Cellular Logistics

Axonal Transport Systems

Materials made or recycled in the cell body are needed at the axon terminals (and vice versa). Two transport systems handle this.

Slow Axonal Transport

Rate: 1 to 5 mm/day
Direction: one direction only, cell body → axon terminals
Supplies new axoplasm to developing or regenerating axons. Replenishes axoplasm in growing or maturing neurons.

Fast Axonal Transport

Rate: 200 to 400 mm/day
Direction: both directions
- Anterograde (forward): cell body → axon terminals. Moves organelles and synaptic vesicles.
- Retrograde (backward): axon terminals → cell body. Moves membrane vesicles and other cellular materials.
Uses motor proteins traveling along microtubules.

Clinical Correlation: Retrograde Transport & Disease

Retrograde transport carries trophic chemicals (like nerve growth factor) but can also carry harmful agents back to the cell body, including tetanus toxin and viruses that cause rabies, herpes simplex, and polio.

Neuron Classifications

Neurons are classified two ways: by structure (how many processes come off the cell body) and by function (which direction they send signals relative to the CNS). Both classifications matter clinically and on the exam.

Based on Shape

Structural Classification

1. Multipolar Neurons

Several dendrites and one axon.
Location: mostly in the brain and cord.
All motor neurons are multipolar.

2. Bipolar Neurons

One main dendrite and one axon.
Location: retina of the eye, inner ear, olfactory area of the brain.

3. Unipolar Neurons (Pseudounipolar)

Dendrites and one axon are fused together to form a continuous process emerging from the cell body.
Begin as bipolar neurons in the embryo. During development, the dendrites and axon fuse into a single process.
Most act as sensory receptors that detect stimuli.

Famously Named Cells

Special Multipolar Neurons

Purkinje cells: found in the cerebellum. Highly branched dendritic trees.
Pyramidal cells: found in the cerebral cortex. Pyramid-shaped cell bodies.

Based on Direction of Signal

Functional Classification

Classified by the direction in which the action potential is conveyed relative to the CNS.

1. Sensory (Afferent) Neurons

Sensory receptors at distal ends, or located just after sensory receptors that are separate cells.
Convey impulses toward the CNS.
Most are unipolar.

2. Motor (Efferent) Neurons

Convey APs away from the CNS to effectors (muscles and glands) in the periphery, through cranial or spinal nerves.
Multipolar.

3. Interneurons (Association Neurons)

Mainly located within the CNS, between sensory and motor neurons.
Function: process incoming sensory input and elicit a motor response by activating the appropriate motor neurons.
Most are multipolar.

Memory cue: afferent = arriving (at CNS). Efferent = exiting (CNS).

Neuroglia

Neuroglia ("nerve glue") make up half the volume of the CNS. Six types: four in the CNS and two in the PNS. They support, protect, and feed neurons, and most can multiply throughout life. Knowing where each one lives and what each one does is foundational anatomy.

General Characteristics

Neuroglia Overview

Make up about half the volume of the CNS.
Smaller than neurons but 2 to 25 times more numerous.
Do not generate or propagate action potentials.
Can multiply and divide in the mature nervous system.

Six Types

CNS (4)	PNS (2)
Astrocytes	Schwann cells
Oligodendrocytes	Satellite cells
Microglia
Ependymal cells

Clinical Correlation: Gliomas

Brain tumors derived from neuroglia. Highly malignant and grow rapidly. Because neurons can't divide but neuroglia can, almost all primary brain tumors are gliomas.

CNS Glia (1 of 4)

Astrocytes

Star-shaped cells with many processes.

Two Types

Protoplasmic astrocytes: many short, branching processes. Found in gray matter.
Fibrous astrocytes: long, unbranched processes. Found in white matter.

Functions

Supporting cells for neurons.
Create the blood-brain barrier by restricting movement of substances between blood and interstitial fluid.
Secrete chemicals in the embryo to regulate growth, migration, and interconnection of neurons.
Maintain the appropriate chemical environment for nerve impulse generation.
Influence learning and memory by affecting the formation of neural synapses.

CNS Glia (2 of 4)

Oligodendrocytes

Resemble astrocytes but smaller, with fewer processes.

Function

Form and maintain the myelin sheath around CNS axons.

Myelin sheath: a multilayered lipid and protein covering around some axons. It insulates them and increases the speed of nerve impulse conduction.

CNS Glia (3 of 4)

Microglia

Small cells with slender processes that give off spine-like projections.

Function: Phagocytes

Remove cellular debris formed during normal development of the nervous system.
Phagocytize microbes and damaged nervous tissue.

Microglia are the immune cells of the CNS. They're macrophages of the brain.

CNS Glia (4 of 4)

Ependymal Cells

Cuboidal to columnar cells arranged in a single layer, with microvilli and cilia.

Location

Line the ventricles of the brain and the central canal of the spinal cord.

Functions

Produce, monitor, and assist in the circulation of cerebrospinal fluid.
Form the blood-CSF barrier.

PNS Glia (1 of 2)

Schwann Cells

Encircle PNS axons.

Functions

Form the myelin sheath around PNS axons. One single Schwann cell can enclose as many as 20 unmyelinated axons, or wrap a single axon.
Participate in axon regeneration. Regeneration is more easily accomplished in the PNS than the CNS, largely because of Schwann cells.

PNS Glia (2 of 2)

Satellite Cells

Surround the cell bodies of neurons in PNS ganglia.

Functions

Provide structural support.
Regulate the exchange of materials between neuronal cell bodies and interstitial fluid.

Myelination & Tissue Organization

Myelin is what makes fast nerve conduction possible. Where neuronal cell bodies cluster and where axons bundle determines whether you call something a ganglion, nucleus, nerve, or tract. The same cells, just organized differently.

Insulation for Speed

Myelination Overview

Myelin: a multi-layered protein and lipid covering.

Myelinated neurons: axons with a myelin sheath.
Unmyelinated neurons: axons lacking a myelin sheath.

Function

Electrically insulates the axon to increase the speed of nerve impulse conduction.

Nodes of Ranvier

Gaps in the myelin sheath that appear at regular intervals along the axon. Action potentials "jump" from node to node (saltatory conduction), which is why myelinated axons conduct so much faster.

Two Types of Glia Make Myelin

Schwann cells (PNS)
Oligodendrocytes (CNS)

Amount of myelin increases from birth to maturity. An infant can't coordinate movements as quickly because they have less myelin and slower nerve conduction.

PNS Myelination

Schwann Cell Myelination

Each Schwann cell wraps one axon segment between two nodes of Ranvier.

Two Layers

Inner layer (myelin sheath): approximately 100 layers of Schwann cell membrane.
Outer layer (neurolemma): encloses the myelin sheath. Only found around PNS axons (not in the CNS).

Why Neurolemma Matters

When a PNS axon is injured, the neurolemma aids regeneration by forming a regeneration tube that guides and stimulates the regrowth of the axon. This is the main reason PNS injuries can heal but CNS injuries usually can't.

CNS Myelination

Oligodendrocyte Myelination

One oligodendrocyte myelinates parts of several axons. A single oligodendrocyte sends out about 15 broad, flat processes that spiral around CNS axons.

No neurolemma, because the oligodendrocyte cell body and nucleus do not envelop the axon.
Nodes of Ranvier are present but less numerous than in the PNS.
Little to no regrowth after injury, likely because of the lack of a neurolemma.

Clinical Correlation: Multiple Sclerosis (MS)

Cause: autoimmune disease leading to progressive destruction of myelin sheaths in the CNS. The myelin deteriorates into hardened scars or plaques (scleroses).

Who: females 2 to 1, whites ages 20 to 40.

Diagnosis: MRI shows numerous plaques in white matter of the brain and spinal cord.

Most common form: relapsing-remitting. Attacks followed by periods of remission. Typical attack every year or two.

Symptoms: heaviness or weakness in muscles, abnormal sensations, double vision.

Treatment: beta-interferon (lengthens time between relapses, decreases severity).

Clinical Correlation: Guillain-Barre Syndrome (GBS)

Definition: An acute demyelinating disorder in which macrophages strip myelin from axons in the peripheral nervous system.

Manifestations: ascending paralysis. The most common cause of acute paralysis.

Cause: likely the result of the immune system's response to a bacterial infection.

Naming the Same Cells in Different Places

Collections of Nervous Tissue

The same kinds of cells get different names depending on where they're found.

Clusters of Cell Bodies

Location	Term
PNS	Ganglion (cluster of cell bodies, associated with spinal/cranial nerves)
CNS	Nucleus (cluster of cell bodies)

Bundles of Axons

Location	Term
PNS	Nerve (cranial nerves connect brain to periphery; spinal nerves connect cord to periphery)
CNS	Tract (interconnections between brain and spinal cord)

Color-Coded by Anatomy

Gray Matter vs. White Matter

White Matter

Primarily myelinated axons.
White appearance comes from the lipid in the myelin.

Gray Matter

Primarily cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia.
Appears gray because of the Nissl bodies (grayish color) and the lack of myelin.

Both gray and white matter contain blood vessels.

Organizational Difference: Cord vs. Brain

Spinal cord: white matter outside, gray matter inside.
Brain: gray matter outside (cortex), white matter inside.

The brain wears its gray matter on the outside, the cord hides it on the inside. Same tissue types, opposite arrangement.

Synapses & Signaling

The synapse is where one neuron talks to another, or to a muscle or gland. Two ways this happens: directly (electrical) or via a chemical messenger (chemical). Most synapses are chemical.

Exam Scope: Anatomy vs. Physiology

For this anatomy exam, focus on the structures involved in synaptic communication: presynaptic and postsynaptic neurons, the synaptic cleft, synaptic end bulbs, vesicles, and the structural difference between electrical (gap junctions) and chemical synapses.

The detailed mechanisms of action potentials, graded potentials, EPSPs/IPSPs, ion channels, summation, and neurotransmitter pharmacology are physiology content and are not testable on the anatomy exam. They are included here for context and will be covered in detail in physiology.

Anatomy of the Synapse

Synapse Definition & Components

Synapse: a region where communication occurs between two neurons or between a neuron and an effector cell (muscle or gland cell).

Key Terms

Presynaptic neuron: nerve cell that carries the impulse toward the synapse.
Postsynaptic neuron: nerve cell that carries the impulse away from the synapse.
Postsynaptic cell: cell that receives the signal.
Effector cell: cell that responds to an impulse at the synapse.

Structural Types Based on Location

Axodendritic: synapse between axon and dendrite (most common).
Axosomatic: synapse between axon and cell body.

Communication Type 1

Electrical Synapses

Action potentials are conducted directly between plasma membranes of adjacent neurons via gap junctions.

Each gap junction contains 100+ connexons (tubes that connect the cytosol of two cells). Ions flow between cells.
Common in: visceral smooth muscle, cardiac muscle, developing embryo, parts of the brain.

Two Advantages

Faster than chemical synapses.
Synchronization: coordinates the activity of a group of neurons or muscle cells (coordinated contraction of the heart, propelling food through the GI tract).

Communication Type 2

Chemical Synapses

Plasma membranes of pre- and post-synaptic neurons do not touch. Communication is indirect through diffusion of a neurotransmitter across a small gap (the synaptic cleft).

The chemical signal is converted into an electrical signal in the postsynaptic cell.
There is a delay (synaptic delay) of about 0.5 msec.
Most chemical synapses transfer information in one direction only: presynaptic neuron → postsynaptic neuron, effector, muscle, or gland.

Chemical Messengers

Neurotransmitter Classes

About 100 substances are known or suspected to be neurotransmitters. Many are also hormones released into the bloodstream by endocrine cells. Two broad classes by size:

1. Small Molecule Neurotransmitters

Acetylcholine (ACh): PNS and some CNS neurons. Excitatory or inhibitory depending on the receptor.
Amino acids:
- Excitatory: glutamate, aspartate
- Inhibitory: GABA (about 1/3 of all brain synapses), glycine (about 1/2 of spinal cord synapses)
Biogenic amines (catecholamines):
- Norepinephrine (NE): arousal, mood regulation, dreaming. Also produced by adrenal medulla as a hormone.
- Epinephrine: hormone produced by adrenal medulla.
- Dopamine: emotional response, addictive behaviors, pleasurable experiences, regulation of skeletal muscle contraction.
- Serotonin: sensory perception, temperature regulation, mood, appetite, induction of sleep.
ATP and other purines: excitatory in CNS and PNS.
Nitric oxide (NO): excitatory free radical. Vasodilation of blood vessels.
Carbon monoxide (CO): excitatory NT produced in the brain.

2. Neuropeptides

3 to 40 amino acids linked by peptide bonds.
Found in CNS and PNS. Excitatory and inhibitory.
Enkephalins and endorphins: potent natural analgesics that bind to opiate receptors. Linked to memory, learning, feelings of pleasure, body temperature regulation, hormonal control of puberty, sex drive, and reproduction.
Substance P: released by neurons that transmit pain. Enhances perception of pain. Suppressed by enkephalin and endorphin.