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Week 12 · Chapter 11 + Review

The Muscular System & Exam 4 Synthesis

This week has two halves: first, Chapter 11 moves from the microscopic contraction mechanism (Ch. 10) to whole named muscles — how they're organized into pairs, how they're named, and where the major ones are. Then, since this closes Exam Block 4, the second half pulls Chapters 10–12 together — skeletal muscle tissue, the muscular system, and the neural tissue that controls it — reviews Chapter 12, connects everything to Lab 8–9, and builds your final study sheet before Exam 4.

By the end of this week, you will be able to:

1Define origin, insertion, agonist, antagonist, and synergist
2Explain how muscles work in functional pairs across a joint
3Decode muscle names using the five common naming conventions
4Locate several major skeletal muscles by region
5Review neurons, neuroglia, membrane potentials, and synaptic transmission (Ch. 12)
6Connect Chapters 10–12 into one coherent picture of muscle and the neural tissue that controls it for Exam 4
7Complete a comprehensive practice quiz and walk away with a one-page study sheet

Part 1 — Big Picture

Origin and insertion: how a muscle pulls a bone

Every skeletal muscle attaches to bone at two points. The origin is the fixed, typically proximal attachment point that stays relatively still. The insertion is the mobile, typically distal attachment point that gets pulled when the muscle contracts. Knowing which end is which is the foundation for understanding every muscle action covered this week.

Part 1 — Vocabulary & Word Parts

Muscular system roots

-flexorbends a joint

Flexor carpi radialis = a muscle that flexes the wrist on the radius side.

-extensorstraightens a joint

Extensor digitorum = a muscle that extends the fingers.

gluteusbuttock

Gluteus maximus = the largest muscle of the buttock region.

delt/o-triangular

Deltoid = the triangular shoulder muscle, named for its shape.

bi-, tri-, quad-two, three, four

Biceps = two heads of origin. Triceps = three. Quadriceps = four.

recti / rect/o-straight

Rectus abdominis = the "straight" abdominal muscle, running vertically.

Part 1 — Structure & Function

Muscles work in functional pairs

Muscles can only pull, never push — so producing opposite movements at a joint requires opposing muscle pairs. The agonist (prime mover) produces the main movement being described; the antagonist relaxes and stretches to allow that movement, then can reverse it. A synergist (not pictured) is a third muscle that assists the agonist or stabilizes the joint during the movement.

Part 1 — Structure & Function

Five common muscle naming conventions

Muscle names aren't random — they're built from a small set of naming conventions describing the muscle's location, shape, action, relative size, or number of origins. Once you recognize these patterns, an unfamiliar muscle name often tells you most of what you need to know about it without memorizing it directly.

Part 1 — Your Turn

Apply it before we move to review

A muscle called the "extensor digitorum longus" is being described in a lab practical.

Question 1 — Based on its name alone, what movement does this muscle most likely produce?

The name directly tells you: "extensor" indicates extension, and "digitorum" refers to the digits (fingers or toes). This is a direct application of the naming-by-action convention covered this week.

Question 2 — During this extension movement, what role does this muscle play?

Since this muscle is actively producing the extension movement being described, it's acting as the agonist (prime mover) for that specific action. It would only be called the antagonist if a different muscle (like a flexor) were the one actively contracting instead.

Part 1 — Clinical Correlation

Muscle strains and the agonist/antagonist balance

This connects the functional-pair concept directly to one of the most common sports injuries.

🩺 In practice

A muscle strain occurs when a muscle is stretched or torn beyond its normal range — often when the antagonist contracts unexpectedly or too forcefully while the agonist is still lengthening. Hamstring strains are a classic example: the hamstrings (antagonist to the quadriceps during knee extension) are frequently injured during activities requiring rapid, forceful direction changes, exactly where the agonist/antagonist balance is tested hardest.

Part 2 — Exam Blueprint

What Exam 4 covers

Exam 4 covers Chapters 10–12 — skeletal muscle tissue, the muscular system, and neural tissue — so plan on each chapter carrying roughly a third of the exam. As with every lecture exam this semester, this is one of several scores worth a share of your final grade, with the lowest lecture exam score dropped — so a strong showing here matters, but isn't the whole game.

Part 2 — Ch. 12 Review

The neuron: parts and classes

Chapter 12 is the third chapter on Exam 4. Week 13 teaches neural tissue in full alongside the spinal cord; this review condenses the Ch. 12 material you need for this exam, using the same terms.

Structure: receive, integrate, conduct, transmit

Dendrites — branching processes that receive incoming signals.
Cell body (soma) — holds the nucleus and organelles; integrates incoming signals.
Axon hillock — the cone-shaped region where the axon leaves the cell body; action potentials typically begin at the start of the axon here (the initial segment).
Axon — the single long process that conducts action potentials away from the cell body; it may branch into collaterals and is often wrapped in myelin with gaps called Nodes of Ranvier.
Axon terminals — the axon's endings, which release neurotransmitter at a synapse.

Structural classes (by number of processes)

Class	Shape	Where you find it
Multipolar	Two or more dendrites and one axon	Most common neuron in the CNS; includes the motor neurons that control skeletal muscle
Unipolar	Also called pseudounipolar; dendrites and axon are continuous, and the cell body sits off to one side	Most sensory neurons of the PNS
Bipolar	One dendrite and one axon, cell body between them	Rare; special senses (sight, smell, hearing)
Anaxonic	Many processes, no distinguishable axon	Brain and special sense organs

Functional classes (by direction of signal flow)

Sensory (afferent) — carry signals from receptors toward the CNS.
Motor (efferent) — carry signals from the CNS out to effectors (muscles and glands).
Interneurons — connect sensory and motor neurons, mostly within the brain and spinal cord; by far the most numerous class.

Part 2 — Ch. 12 Review

Neuroglia: four in the CNS, two in the PNS

Cell	Main job
Central nervous system (CNS)
Astrocytes	Largest and most numerous glia; maintain the blood–brain barrier, provide a structural framework, repair damaged tissue, and regulate the fluid around neurons
Oligodendrocytes	Myelinate CNS axons; one cell can myelinate segments of several axons
Microglia	Phagocytic defense cells; remove debris and pathogens
Ependymal cells	Line the brain's ventricles and the spinal cord's central canal; produce, monitor, and help circulate cerebrospinal fluid (CSF)
Peripheral nervous system (PNS)
Satellite cells	Surround neuron cell bodies in ganglia and regulate their environment
Schwann cells	Myelinate PNS axons (each cell covers one segment of a single axon); also enclose unmyelinated PNS axons and aid repair

Exam trap: both oligodendrocytes and Schwann cells make myelin. Oligodendrocytes work in the CNS and can wrap several axons; Schwann cells work in the PNS and wrap one segment of one axon. Week 13's diagram shows the five most-tested glia; add satellite cells to complete the set of six.

Part 2 — Ch. 12 Review

Membrane potentials: resting, graded, and action

Resting membrane potential: about −70 mV. At rest, the inside of the membrane is negative relative to the outside. Three things keep it there:

The sodium–potassium exchange pump uses ATP to move 3 Na⁺ out for every 2 K⁺ in, keeping Na⁺ high outside the cell and K⁺ high inside.
At rest the membrane is far more permeable to K⁺ than to Na⁺ (leak channels), so K⁺ diffuses out and leaves the inside negative.
Large negatively charged proteins stay trapped inside the cell.

Three words for changes in membrane potential

Depolarization — the inside becomes less negative (moves toward 0 mV and beyond).
Repolarization — the membrane returns toward the resting potential after depolarizing.
Hyperpolarization — the inside becomes more negative than the resting potential.

Graded potentials vs. action potentials

Feature	Graded potential	Action potential
Where	Dendrites and cell body	Axon, starting at the initial segment
Size	Varies with stimulus strength	All-or-none: the same size every time threshold is reached
Direction	Depolarizing or hyperpolarizing	Always depolarization, then repolarization
Distance	Local; fades with distance	Propagates the full length of the axon without weakening
Summation	Yes, can add together	No
Refractory period	None	Yes
Channels	Chemically or mechanically gated	Voltage-gated Na⁺ and K⁺ channels

Part 2 — Ch. 12 Review

The action potential, step by step

One action potential at a single patch of axon membrane. A graded depolarization must carry the membrane from −70 mV to threshold; from there the voltage-gated channels take over and the full spike follows automatically.

Depolarization to threshold

A graded depolarization brings the membrane to threshold, typically between −60 and −55 mV.

Na⁺ channels open — rapid depolarization

Voltage-gated Na⁺ channels open, Na⁺ rushes in, and the inside becomes positive, peaking at about +30 mV.

Na⁺ channels inactivate, K⁺ channels open — repolarization

Na⁺ channels close and inactivate while voltage-gated K⁺ channels open; K⁺ flows out and the membrane repolarizes.

Return to normal permeability — brief hyperpolarization

The K⁺ channels close slowly, so the membrane briefly dips below resting (toward about −90 mV) before settling back at −70 mV.

All-or-none. A stimulus that reaches threshold produces a full-size action potential; one that does not reach threshold produces none. A stronger stimulus does not make a bigger action potential — it makes them fire more often.

Refractory periods. During the absolute refractory period (from the moment Na⁺ channels open until they return to their resting state), no stimulus, however strong, can trigger another action potential. During the relative refractory period that follows (until the membrane is back at resting potential), a larger-than-normal stimulus can trigger one.

Part 2 — Ch. 12 Review

Propagation: continuous vs. saltatory

Once an action potential starts at the initial segment, it regenerates itself along the axon toward the axon terminals. It cannot travel backward, because the membrane it just left is still in its refractory period.

Feature	Continuous propagation	Saltatory propagation
Axon type	Unmyelinated	Myelinated
How it moves	Regenerated at each neighboring patch of membrane, step by step along the whole axon	"Jumps" from one Node of Ranvier to the next; only the nodes depolarize
Speed	Slower	Much faster
Energy cost	Higher: ions cross the membrane along its entire length, so the pumps work harder	Lower: ions cross only at the nodes

Two factors set conduction speed. Myelin (saltatory propagation is faster than continuous) and axon diameter (a larger axon offers less resistance, so the signal moves faster). The fastest axons are large-diameter and myelinated; the slowest are small-diameter and unmyelinated. This is why losing myelin, as in multiple sclerosis (covered in Week 13), slows or blocks conduction.

Part 2 — Ch. 12 Review

Signal transmission at a chemical synapse

At a chemical synapse, the presynaptic neuron's axon terminal releases a neurotransmitter that crosses the synaptic cleft and acts on the postsynaptic cell. Signals cross in one direction only. Using acetylcholine (ACh) as the example:

Action potential arrives

An action potential reaches the axon terminal and depolarizes it.

Ca²⁺ enters, neurotransmitter is released

Voltage-gated Ca²⁺ channels open; Ca²⁺ flows in and triggers exocytosis of synaptic vesicles, releasing ACh into the synaptic cleft.

Binding opens chemically gated channels

ACh diffuses across the cleft and binds receptors on the postsynaptic membrane, opening chemically gated ion channels and producing a graded potential.

Neurotransmitter is removed

Acetylcholinesterase (AChE) breaks ACh into acetate and choline; the presynaptic terminal reabsorbs the choline to make new ACh. Removal ends the signal so the synapse can respond again.

Three ways neurotransmitters are removed: diffusion away from the synapse, breakdown by enzymes (such as AChE), and reabsorption by the presynaptic neuron or neuroglia.

EPSP vs. IPSP. An excitatory postsynaptic potential (EPSP) is a graded depolarization that moves the postsynaptic membrane closer to threshold. An inhibitory postsynaptic potential (IPSP) is a graded hyperpolarization that moves it farther from threshold.

Summation. One EPSP is usually too small to reach threshold, so EPSPs must add together. Temporal summation: repeated stimuli arrive at the same synapse in rapid succession. Spatial summation: stimuli arrive at several synapses at the same time. The neuron adds all EPSPs and IPSPs together; if the net result brings the axon's initial segment to threshold, an action potential fires.

Part 2 — Cross-Chapter Connections

Three threads worth tracing before the exam

Thread 1 — Microscopic contraction powers macroscopic movement. Chapter 10's sliding filament mechanism (myosin pulling actin) is literally what's happening inside every fiber of the whole named muscles covered in Chapter 11 — the two chapters describe the exact same process at two different scales.

Thread 2 — Naming conventions connect to joint movements from Chapter 9. A muscle named "flexor carpi radialis" only makes sense once you already know what flexion means (Ch. 9) and where the wrist and radius are (Ch. 7–8) — Chapter 11's vocabulary is built directly on top of the last three chapters.

Thread 3 — Neural tissue controls muscle tissue. The neuromuscular junction from Chapter 10 is a chemical synapse running Chapter 12's exact sequence: a motor neuron's action potential reaches the axon terminal, Ca²⁺ enters, acetylcholine (ACh) is released, binds receptors on the sarcolemma, and acetylcholinesterase clears it. Watch the two different calcium jobs: in the axon terminal, Ca²⁺ triggers neurotransmitter release; inside the muscle fiber, Ca²⁺ released from the sarcoplasmic reticulum exposes the myosin-binding sites on actin.

Part 2 — Lab Crosswalk

Connecting lecture to Lab 8–9

Labs 8 and 9 test the same two skills from this block, applied hands-on: naming specific muscles on physical models (straight from Ch. 11 vocabulary and naming conventions), and identifying which muscle produces a given movement (straight from Ch. 9's movement terms plus Ch. 11's agonist/antagonist concept). Neural tissue (Ch. 12) has no lab in this block — its hands-on work comes later with Lab 10 and Lab Exam 3 — so on Exam 4 you'll meet it as lecture content.

Comprehensive Practice Quiz

Exam 4 dress rehearsal — 15 questions across Ch. 10–12

Question 1

Which muscle type is voluntary and striated?

Skeletal muscle is voluntary and striated. Cardiac muscle is also striated but involuntary; smooth muscle is neither voluntary nor striated.

Question 2

A sarcomere runs from one Z-disc to the:

A sarcomere is defined as the region between two Z-discs. Fascicles and a muscle's origin are much larger structures, unrelated to the sarcomere's specific boundaries.

Question 3

Which protein makes up the thick filaments?

Myosin makes up the thick filaments. Actin makes up the thin filaments instead, and collagen is unrelated to the sarcomere's contractile proteins.

Question 4

The fixed, typically proximal attachment point of a muscle is called the:

The origin is the fixed attachment point. The insertion is the mobile end that gets pulled, and a sarcomere is a microscopic contractile unit, unrelated to whole-muscle attachment points.

Question 5

During elbow flexion, the biceps brachii acts as the:

The biceps brachii is the agonist (prime mover) for elbow flexion — it's the muscle actively producing that movement. The triceps brachii would be the antagonist in this scenario.

Question 6

A muscle named "gluteus maximus" is named according to which convention?

"Maximus" indicates size — this is the largest of the three gluteal muscles. Shape-based names (like deltoid) and origin-count names (like biceps) follow different conventions.

Question 7

Calcium's role in muscle contraction is to:

Calcium exposes the myosin-binding sites on actin, allowing contraction to begin. ATP (not calcium) provides the energy for the power stroke, and neither is involved in anchoring a muscle to bone.

Question 8

A synergist muscle's role is to:

A synergist assists the agonist or stabilizes the joint during a movement. Producing the opposite movement describes the antagonist instead, and a synergist works alongside the agonist rather than replacing it.

Question 9

"Flexor carpi radialis" is named using which combination of conventions?

"Flexor" describes the action (flexion), and "carpi radialis" describes the location (wrist, radius side) — a combination of two naming conventions, not shape/size or origin count alone.

Question 10

Rigor mortis is a direct illustration of ATP's role in muscle:

Rigor mortis shows ATP's role in relaxation — without ATP, myosin can't release actin, leaving the muscle locked in contraction. It has nothing to do with muscle growth or the naming conventions covered in Chapter 11.

Question 11

The motor neurons that control skeletal muscle are structurally classified as:

These motor neurons are multipolar — two or more dendrites and a single axon. Most sensory neurons are unipolar, and bipolar neurons are rare, found in special sense organs.

Question 12

Which neuroglial cell myelinates axons in the central nervous system?

Oligodendrocytes myelinate CNS axons. Schwann cells do the same job in the PNS, and microglia are phagocytic defense cells that make no myelin.

Question 13

During the absolute refractory period of an axon:

In the absolute refractory period the voltage-gated Na⁺ channels are already open or inactivated, so no second action potential is possible. A larger-than-normal stimulus can succeed only during the later relative refractory period.

Question 14

Saltatory propagation of an action potential:

Saltatory propagation happens in myelinated axons, where the action potential jumps between Nodes of Ranvier. It is faster and uses less energy than continuous propagation, which is the patch-by-patch pattern of unmyelinated axons.

Question 15

At a chemical synapse, what directly triggers the release of neurotransmitter from the axon terminal?

When the action potential depolarizes the axon terminal, voltage-gated channels let Ca²⁺ flow in, and that calcium triggers exocytosis of the synaptic vesicles. Binding to postsynaptic receptors happens after release, not before.

Score: 0 / 15

Final Study Sheet

Exam 4 One-Page Recap: Chapters 10–12

Your last stop before the real exam — everything from this block, condensed onto one page.

Ch. 10 — Muscle Types

Skeletal — voluntary, striated
Cardiac — involuntary, striated
Smooth — involuntary, non-striated

Ch. 10 — Organization

Muscle → Fascicle → Fiber → Myofibril → Sarcomere (Z-disc to Z-disc).

Ch. 10 — Contraction

Sliding filament theory: myosin pulls actin inward. Requires calcium (exposes binding sites) + ATP (powers release/re-cock).

Ch. 11 — Attachment Points

Origin — fixed, proximal
Insertion — mobile, distal

Ch. 11 — Functional Pairs

Agonist — prime mover
Antagonist — opposes/reverses
Synergist — assists/stabilizes

Ch. 11 — Naming Conventions

Location, Shape, Action, Size, Number of origins.

Ch. 12 — Neurons

Dendrites receive; cell body integrates; axon conducts; axon terminals transmit
Structural: multipolar, unipolar, bipolar, anaxonic
Functional: sensory (afferent), motor (efferent), interneurons

Ch. 12 — Neuroglia

CNS: astrocytes, oligodendrocytes (myelin), microglia, ependymal cells
PNS: satellite cells, Schwann cells (myelin)

Ch. 12 — Membrane Potentials

Resting about −70 mV (Na⁺/K⁺ pump + K⁺ leak). Graded = local, variable size, can summate. Action = all-or-none, propagates, has refractory periods.

Ch. 12 — Action Potential

Threshold (about −55 mV) → Na⁺ in (depolarization, to about +30 mV) → K⁺ out (repolarization) → brief hyperpolarization → rest. Absolute refractory: no new AP. Relative: only a stronger stimulus.

Ch. 12 — Propagation

Continuous (unmyelinated, slower) vs. saltatory (myelinated, node to node, faster). Larger diameter + myelin = fastest.

Ch. 12 — Chemical Synapse

AP arrives → Ca²⁺ enters → neurotransmitter released → binds receptors → removed (diffusion, enzymes such as AChE, reabsorption). EPSP = toward threshold; IPSP = away. Summation: temporal + spatial.

Key Terms This Week

Skeletal MuscleSarcomereSliding Filament TheoryCalciumATPOriginInsertionAgonistAntagonistSynergistMultipolar NeuronOligodendrocyteSchwann CellResting Membrane PotentialAction PotentialThresholdRefractory PeriodSaltatory PropagationEPSPIPSP

You've completed Exam Block 4. Chapters 10–12 are covered, cross-referenced, and drilled. Good luck on Exam 4 — and when you're ready, Week 13 opens Exam Block 5 by revisiting neural tissue and building on it with the spinal cord.

Next: Week 13 →