Biology 2108 Exam Three (Part Two)

What is the nervous system and what is its core purpose

It is the body’s electrical communication network, and detects what’s happening inside and outside the body, processes that information, and coordinates a response.

Three core functions of the nervous system

• Sensory Input: Detecting stimuli from inside or outside the body

• Integration: Processing that information in the CNS

• Motor Output: Generating a response

CNS v. PNS

CNS: brain and spinal cord

• the integration center: receives signals, processes them, and decides what to do.

  • protected by done and cerebrospinal fluid

PNS: all nervous tissue outside of the CNS

• Carries signals to and from the CNS

What are the two divisions of the PNS motor system

Somatic Nervous System: controls voluntary skeletal muscle movement

• Consciously controlled

Automatic Nervous System: controls involuntary functions (heart rate, breathing, etc.)

• splits into sympathetic (fight or flight) and parasympathetic (rest and digest)

Axon Hillock

The junction between the cell body and the axon

• This is where the decision to fire is made. If the signal reaches the threshold here, an action potential and triggered.

  • The most electrically excitable part of the neuron

Myelin Sheath

The fatty insulating layer wrapped around the axon via glial cells.

• It speeds up signal conduction by forcing the signal to jump between gaps called Nodes of Ranvier (called Saltatory Conduction), rather than traveling continuously.

Nodes of Ranvier

Gaps in the myelin sheath where the axon is exposed.

• The action potential regenerates at each node and jumps from node to node, much faster than continuous conduction.

Dendrites

Branched extensions that receive incoming signals from other neurons or sensory receptors.

• Signal travels towards the cell body

  • More dendrites means more inputs the neuron can recieve

Axon terminals

Branched endings of the axon that contains synaptic vessels filled with neurotransmitters

• When an electrical signal arrives, they trigger neurotransmitter release into the synapse to communicate with the next cell

Three types of Neurons

Sensory Neurons (afferent): carries signals from receptors TO the CNS

Interneurons: entirely within the CNS and performs integration (thought, memory processing)

Motor Neurons (efferent): carries signals FROM the CNS TO muscles and glands

Afferent V. Efferent

Afferent: Arrives at the CNS

Efferent: Exits the CNS

Neuroglia: Astrocytes

Forms the blood-brain barrier: controls what enters the brain via the blood

• Regulates the chemical environment around neurons, recycle neurotransmitters, and provides structural support

  • The most abundant glial cell in the brain

Neuroglia

Support cells of the Nervous System

Neuroglia: Oligodendrocytes

Produces the myelin sheath in the CNS

• One can myelinate segments of multiple axons simultaneously

  • Destroyed by Multiple Sclerosis

Neuroglia: Schwann Cells

Produces myelin in the PNS

• each Schwann cells myelinates only one segment of one axon, and these cells enable peripheral nerve regeneration.

Neuroglia: Microglia

The immune cells of the CNS

• they patrol for damage and infection, engulf debris and pathogens.

  • The brain’s built in defense system

Neuron At Rest

When a neuron is in this state, it is maintaining a state of readiness, like a compressed spring.

• actively maintaining a charge difference across its membrane using ion pumps and leak channels, storing potential energy ready to be released when triggered.

Membrane Potential

The electrical voltage difference across the cell membrane.

• It exists because ions are distributed unequally on each side of the membrane

  • Inside of neuron is approximately -70 mV and is maintained by ion channels and the Na/K pump

The Two Forces that Drive Ion movement across the Membrane

• Concentration gradient: Ions move from high to low concentration (like diffusion)

• Electrical Gradient: opposite charges attract, and like charges repel.

These two are called the Electrochemical Gradient

Most Important Ion at Rest in a Neuron

Potassium (K)

• Higher concentration inside the cell, and membrane is relatively permeable to K at rest.

  • K drifts out, making the neuron more negative.

Na Importance at Rest

At rest, Sodium wants to enter, and is pushed by both the concentration gradient (high outside) and electrical gradient (negative inside attracts positive Na), but the voltage-gates Na channels are closed at rest, so it mostly cannot get inside.

Large Anions inside the cell At Rest

Large negatively charged proteins and organic molecules trapped inside the cell — they cannot cross the membrane. Their negative charge contributes significantly to the negative interior of the resting neuron.

Ion channels vs Ion pumps

Channels: protein pores that allow ions to pass DOWN into the electrochemical gradient

• passive transport

Pumps: moves ions AGAINST their electrochemical gradient

• Active transport

Three types of Ion Channels

Leak Channels: always open; K leak channels establish the Resting Membrane Potential

Voltage-Gates Channels: open/close based on voltage; located mainly on the axon and critical for action potential

Ligand-gated Channels: open when a neurotransmitter binds; located mainly on dendrites and cell body

Na/K Pump

For every 1 ATP consumed,

• 3 Na are pushed OUT of the cell

• 2 K are pushed INTO the cell

this restores ion concentrations after signaling and maintains resting potential for the long term. It runs constantly.

Equilibrium Potential

The specific membrane voltage at which the electrical force and concentration force for one ion exactly cancel each other out — no net movement of that ion occurs.

K⁺ equilibrium ≈ −90 mV
Na⁺ equilibrium ≈ +60 mV

Why is RMP -70 mV?

While the membrane is more permeable to K at rest, the small Na leak through open channels nudges the RMP slightly less negative from -90 until -70 mV

Action Potential

A rapid, dramatic reversal and restoration of the membrane potential that travels down the axon like a wave. It is the neuron's signal — the electrical impulse that carries information from one end of the neuron to the other. Triggered at threshold, it runs to completion automatically.

Threshold

Critical membrane voltage (-55mV) at which an action potential is triggered

• enough voltage gated Na channels open that Na rushes in, making the inside more positive, opening more channels.

  • this is a self amplifying positive feedback loop that cannot be stopped once it has begun

All or None Principle

An action potential happens either completely or not at all.

• aa either full spike to (+40 mV) or none

A stronger stimulus doesn’t create a bigger spike, just a higher frequency of firing, which is how the nervous system makes a stimulus more intense.

Depolarization

During this, voltage gated Na channels are open, which allows it to rush INTO the cell. The membrane potential rises rapidly from -70 mV to +40 mV.

• This is the rising phase of the action potential strike

Repolarization

Voltage gates Na channels close, and Voltage gated K channels open. K rushes OUT of the cell. Membrane potential returns toward -70 mV

• This is the falling phase of the spike

Hyper-polarization

K channels are slow to close, meaning excess K leaves, lowering the membrane to dip to approximately -80 mV.

• The Na/K pump restores this, and membrane returns to rest.

Refractory Period

The period after an action potential when the neuron cannot fire again immediately.

Absolute refractory period — Na⁺ channels are inactivated; firing is physically impossible.

Relative refractory period — during after-hyperpolarization; firing requires a stronger-than-normal stimulus.

This ensures action potentials travel in one direction only.

When does Na enter the cell?

Depolarization.

• Na⁺ enters because both forces point inward simultaneously — the concentration gradient (high Na⁺ outside, low inside) AND the electrical gradient (negative inside attracts positive Na⁺). Voltage-gated Na⁺ channels open at threshold, allowing this flood.

When does K leave the cell?

Repolarization.

• K⁺ flows out because: its concentration gradient points outward (more K⁺ inside), AND because the inside is now positive (+40 mV), the electrical gradient also pushes K⁺ outward. Voltage-gated K⁺ channels open during this phase.

Why can’t electrical signals jump directly from neuron to neuron?

Neurons don’t physically touch.

• The synaptic cleft sits between them, so when it is time to cross, the electrical signal is converted into a chemical signal and crosses the gap itself, where it is turned back into an electrical signal.

Synaptic Cleft

The space between the pre- and postsynaptic neuron.

• Approx. 20-40 nanometers wide

Neurotransmitters

Chemical messenger molecules released from the presynaptic axon terminal into the synaptic cleft

• they bind to specific receptor proteins on the postsynaptic neuron and change its membrane potential

  • GABA, Dopamine, Serotonin

Role of Calcium in Synaptic Transmission

The trigger linking electrical arrival to chemical release.

• When the action potential arrives at the axon terminal, it opens voltage-gated Ca²⁺ channels. Ca²⁺ floods in, triggering synaptic vesicles to fuse with the membrane (exocytosis) and release neurotransmitters.

Sequence of Events in a Chemical Synapse

1. Action potential arrives at axon terminal

2. Voltage-gated Ca channels open

3. Ca floods in

4. Synaptic vesicles fuse with membrane (Exocytosis)

5. Neurotransmitters are released into synaptic cleft

6. Neurotransmitters diffuse across cleft

7. They bind to the post synaptic receptors

8. Ligand-gated ion channels open

9. Ion flow changes post-synaptic potential

10. Neurotransmitter cleared!

Excitatory V. Inhibitory Synapse

Excitatory (EPSP): open channels allowing Na in. Membrane depolarizes (less negative), moving closer to the threshold

• Makes firing more likely

• Main excitatory neurotransmitter: Glutamate

Inhibitory (IPSP): open channels allowing Cl- in or K out. Membrane hyperpolarizes (more negative), moving further from the threshold

• Makes firing less likely

• Main inhibitory neurotransmitter: GABA

Summation

A neuron integrates all incoming EPSP and IPSP simeoultanerously. If excitatory dominates, the neuron fires, if inhibitory dominates, it doesn’t.

• this is how the nervous system makes decisions.

How is a neurotransmitter cleared from the synaptic cleft?

Three mechanisms:

Enzymatic breakdown: enzymes in the cleft degrade the neurotransmitter

Reuptake: the presynaptic terminal reabsorbs the neurotransmitter for reuse

Diffusion: It drifts away from the cleft.

Why are inhibitory synapses necessary?

Without inhibition, neural circuits would spiral into uncontrolled, excessive firing — similar to a seizure.

What is the fundamental three step model of nervous system function?

Sensory input (afferent) → Integration (CNS) → Motor Output (efferent)

Transduction

Conversion of a stimulus into an electrical signal via a sensory receptor.

• ex: photoreceptors converting light into electrical signals.

Sensation

The raw awareness of a stimulus

• This is what happens when a transduced signal reaches the CNS and is registered.

  • It is the basic detection of the signal before any interpretation

Perception

The interpretation and conscious experience of a sensation, giving it meaning based on context, memory, and attention. Happens in the cerebral cortex

Sensation V. Perception

Sensation = detection. The raw signal arriving at the CNS.
Perception = interpretation. The brain giving that signal meaning.

Phantom Limb Pain

The brain perceives pain from a limb that no longer exists — because the neural circuits representing that limb are still active.

• represents that perception is a construction of the brain, not a perfect recording of reality.

Reflex

A response that bypasses the brain. In a spinal reflex, the signal goes sensory receptor → spinal cord → motor neuron → muscle — without reaching the brain.

• The response happens before you are consciously aware of the stimulus.

Effectors

The organs that carry out a response: muscles and glands

Muscle responses: skeletal (movement), smooth (gut, blood vessels), cardiac (heart rate)

Gland responses: secretion of hormones, enzymes, sweat, saliva

Organization of the Human Nervous System

Sexual Reproduction V. Asexual Reproduction

Asexual reproduction — one parent produces genetically identical offspring (clones). No gametes needed. Fast and energy-efficient.

Sexual reproduction — two parents contribute gametes (egg + sperm) that fuse to form a genetically unique offspring. Requires meiosis and fertilization.

Forms of Asexual Reproduction

Budding — a new organism grows out of the parent's body

• hydra, yeast
  Fragmentation — body breaks into pieces, each regenerates

• sea stars
  Fission — parent splits into two equal offspring

• sea anemones
  Parthenogenesis — egg develops without fertilization

• Komodo dragons, bees, sharks
  Vegetative propagation — plants producing runners or bulbs.

Parthenogenesis

form of asexual reproduction in which an unfertilized egg develops into a new individual

• no sperm needed

• offspring are often haploid or diploid depending on species

Advantages of Asexual Reproduction

Fast: no need to find a mate; one individual can populate the environment rapidly

Energy Efficient: no resources spent on finding mates or producing gametes

All offspring can reproduce: no males needed

Works well in stable environments: if the parent is well adapted, clones will be too

Disadvantages of Asexual Reproduction

No genetic diversity — all offspring are clones. A single disease or environmental change can wipe out the entire population.
No adaptation — cannot evolve quickly in response to new threats.
Accumulation of harmful mutations — bad mutations passed to all offspring with no way to shuffle them out.

Advantages of Sexual Reproduction

Genetic diversity — offspring are genetically unique, increasing variation in the population.
Adaptability — diverse populations can evolve faster in changing environments.
Harmful mutations can be masked — a good allele from one parent can compensate for a bad one from the other.
Shuffles alleles through crossing over and independent assortment.

Disadvantages of Sexual Reproduction

Costly — finding and attracting mates requires time, energy, and resources.
Only half the population reproduces directly — males don't bear offspring, reducing reproductive efficiency.
Risk of disease transmission — close physical contact spreads STIs.
Slower — reproduction is more complex and time-consuming than asexual.

External V. Internal Fertilization

External fertilization — egg and sperm are released into the environment (usually water) and fuse outside the body.

• Requires water

  • produces many eggs to compensate for low survival rates.

  • Ex: most fish and amphibians

• Internal fertilization — sperm deposited inside the female's reproductive tract; fertilization occurs inside the body.

• Better protection

  • fewer offspring needed

  • reptiles, birds, mammals.

R-strategies V. K-strategies

r-strategists — produce many small offspring, little parental care, short lifespan, boom-and-bust populations.

• Thrive in unstable environments.

• insects, mice, dandelions.

K-strategists — produce few large offspring, heavy parental investment, long lifespan, stable populations near carrying capacity (K).

• Thrive in stable environments

• elephants, humans, whales.

Monogamy v. Polygamy

Monogamy: one male with one female forms a long lasting pairing

Polygamy: one individual mates with multiple partners

• polygyny - one male with multiple women

• polyandry - one female with multiple men

Sexual Selection

Individuals with certain traits are more successful at attracting mates, even if those traits reduce survival.

• drives the evolution of elaborate features, like peacock tails

Intersexual V. Intrasexual Selection

Intrasexual selection — competition between members of the same sex (usually males) for access to mates.

• elk fighting with antlers, elephant seals competing for harems.
  
  Intersexual selection — one sex (usually females) chooses among potential mates based on traits that signal genetic quality or resource availability.

• female peacocks choosing males with the most elaborate tails.

Sex V. Gender V. Gender Identity

Sex — biological characteristics (chromosomes, hormones, anatomy) typically categorized as male or female. A biological concept.

Gender — social and cultural roles, behaviors, and expectations associated with being a man, woman, or another identity. A social construct.

Gender identity — a person's internal, personal sense of their own gender. May or may not align with sex assigned at birth.

Gametes

Haploid sex cells that fuse during fertilization to form a new organism.

• Sperm : made in testes of a man

• Eggs: made in ovaries of a woman

Spermatogenesis

The process of producing mature sperm from stem cells.
occurs continuously after puberty in seminiferous tubules.

• each primary spermatophyte → 4 equal, functional sperm via meiosis.

• takes approximately 64-74 days

  • Millions produced daily

Oogenesis

begins before birth, arrested at meiosis 1 until puberty

• each primary oocyte → 1 functional egg + 2-3 polar bodies (which degrade)

  • Only 400 eggs released in a lifetime

Why does oogenesis only produce 1 egg?

Cell division is unequal, so one cell gets almost all of the cytoplasm, while the others become polar bodies that degrade.

• Conserves resources, giving the egg maximum cytoplasm and nutrients for early embryo development.

Hormones Regulating the Reproductive System

The hypothalamus-pituitary-gonad axis:

• GnRH (hypothalamus) → stimulates pituitary to release.

• FSH (follicle-stimulating hormone) → stimulates gamete production.

• LH (luteinizing hormone) → triggers ovulation (females) or testosterone production (males).
  
  Gonadal hormones (testosterone, estrogen, progesterone) feed back to regulate the axis.

How does cannabis affect male and female reproduction

Male: THC reduces testosterone production, decreases sperm count, impairs sperm mobility and morphology, and reduces libido

• may impair hypothalamus-pituitary axis signaling

Female: Disrupts the hormonal cycle, affects ovulation timing, and alters GnRH signaling.

• associated with irregular menstrual cycles and potential effects on fetal development if used during pregnancy

Why are testes held outside the body in the scrotum?

Sperm production requires a body temperature about 2-3 degrees Celsius lower than the core body temp. (37 C)

• scrotum is that exact temp (34-35 C)

  • cremaster muscle can also raise or lower testes to regulate temperature.

Semen

The fluid ejaculated from the penis.

Seminal vesicle fluid

In the semen.

has fructose (energy for sperm), prostaglandins, alkaline fluid

Prostate gland fluid

in the semen.

contains enzyes, citric acid, zinc.

• helps liquify semen after ejaculation

Bulbourethral Gland Fluid

In the semen.

Mucus that lubricates and neutralizes acid in the urethra before ejaculation

Path of Sperm from Production to Ejaculation

• Seminiferous tubules

• Epididymis (maturation and storage)

• Vas deferens (muscular tube carrying sperm up)

• Ejaculatory duct

• Urethra

• Exits through Penis

Remembered through the Mnemonic: SEVEN UP

Epididymis

A coiled tube sitting on the back of each testis where sperm mature and are stored.

• they spend 2-3 weeks in here gaining mobility and the ability to fertilize an egg.

Urethra

Serves both the reproductive and urinary systems.

• carries both urine and semen, but never at the same time.

Seminiferous Tubules

Highly coiled tubes inside the testes where spermatogenesis occurs. They are lined with two key cell types:

• Spermatogonia, Sertoli cells

If uncoiled, the tubules from one testis would be about 250 meters long.

Sertoli Cells (Sustentacular Cells)

Nurse cells lining the seminiferous tubules that support and nourish developing sperm.

• forms the blood-testis barrier and protects developing sperm from the immune system

• Responds to FSH

• Secretes inhibin (feeds back to reduce FSH)

Leydig cells

Interstitial cells located between the seminiferous tubules that produce testosterone in response to LH from the pituitary.

Hormones that regulate Spermatogenesis

GnRH (hypothalamus) → pituitary releases:
FSH → acts on Sertoli cells → stimulates sperm production and inhibin secretion.
LH → acts on Leydig cells → stimulates testosterone production.
Testosterone → feeds back negatively to hypothalamus and pituitary to reduce GnRH and LH.
Inhibin → feeds back to reduce FSH specifically.

Male sexual response

Erection: parasympathetic nervous system causes vasodilation of penile arteries, and blood fills up corpus cavernosum and corpus spongiosum.

• compresses veins to maintain engorgement

Ejaculation: sympathetic nervous system triggers peristaltic contractions of epididymis, vas deferens, and accessory glands. bulbocavernosus muscle expels semen.

Female Reproductive System differences from Male

Female reproductive and urinary systems are completely seperate from one another, and have distinct openings, unlike the male reproductive and urinary systems

• female reproductive system is also largely internal with ovaries, fallopian tubes, uterus, and cervix all within the pelvic cavity.

Ovaries

Serves two functions:

Gametogenesis: produces eggs (oocytes) through oogeneiss

Hormone production: secretes estrogen and progesterone, which regulate the menstrual cycle, maintain pregnancy, and develop secondary sex characteristics

Fallopian Tubes

Captures the egg after ovulation and transport it towards the uterus using cilia and peristaltic contractions.

• fertilization normally occurs here, not in the uterus.

  • the journey to the uterus normally takes 3-4 days

Uterus

Muscular, pear shaped organ that…

• receives and implants the fertilized egg

• nourished the developing embryo/fetus via endometrium and placenta

• expels the fetus during labor via powerful myometrium contractions

• Sheds its lining if no fertilization occurs (menstruation)

Ovarian Follicle

A structure in the ovary consisting of an immature egg (oocyte) surrounded by follicle cells, which develop in stages:

• Primordial follicle → primary → secondary → Graafian (mature) follicle → ovulation (egg released) → corpus luteum (what remains after ovulation).

Females are born with 1 or 2 million follicles, but only 400 will ever ovulate

Phases of the Ovarian Cycle

Follicular phase (days 1–13) — FSH stimulates follicle development; growing follicles secrete estrogen; one dominant follicle selected.

Ovulation (day 14) — LH surge triggers release of the mature egg from the Graafian follicle.

Luteal phase (days 15–28) — ruptured follicle becomes the corpus luteum; secretes progesterone and estrogen to maintain endometrium.

Phases of the Uterine (Menstrual) Cycle

Menstrual phase (days 1–5) — endometrial lining sheds; estrogen and progesterone are low.

Proliferative phase (days 6–13) — rising estrogen causes endometrium to thicken and rebuild; corresponds with follicular phase.

Secretory phase (days 15–28) — progesterone from corpus luteum causes endometrium to become secretory and vascular, preparing for implantation; corresponds with luteal phase.

How are the Ovarian and Uterine cycles interconnected?

They run in parallel and drive each other through hormones:

Follicle development (ovarian) → estrogen rise → uterus proliferates (uterine).

Ovulation → corpus luteum forms → progesterone rise → uterus becomes secretory.

If no fertilization → corpus luteum degenerates → progesterone and estrogen fall → endometrium sheds (menstruation) → FSH rises → new follicle develops. The cycle repeats.

Specific effects of FSH and LH on the ovarian cycle

FSH (follicle-stimulating hormone):
• Stimulates follicle growth and maturation
• Stimulates estrogen production by follicle cells

LH (luteinizing hormone):
• The LH surge (day 13–14) triggers ovulation — rupture of the mature follicle
• Stimulates formation of the corpus luteum after ovulation
• Stimulates corpus luteum to produce progesterone

Effects of Estrogen and Progesterone on the Uterus

Estrogen (rising during follicular phase):
• Causes the uterus to proliferate and thicken
• Rebuilds the functional layer shed during menstruation

Progesterone (dominant during luteal phase):
• Makes the uterus secretory and vascular — glands develop, blood supply increases
• Prepares the uterus for implantation of a fertilized egg

Negative feedback in Female Hormonal Cycle

Negative feedback (most of the cycle):

• Rising estrogen and progesterone suppress GnRH, FSH, and LH — prevents multiple ovulations and controls cycle length.

Positive Feedback in Female Hormonal Cycle

Positive feedback (just before ovulation):

• High estrogen (from the dominant follicle) stimulates the pituitary to release a massive LH surge — the one moment in the cycle where a rising hormone causes more of itself.

  • This LH surge triggers ovulation.

Corpus Luteum

The ruptured follicle that remains after ovulation, transformed into an endocrine structure

• secretes progesterone and estrogen to maintain the uterus during luteal phase.

Uterus if egg is NOT fertilized

Without fertilization, no embryo produces hCG, so the corpus luteum degenerates

• progesterone and estrogen levels drop sharply

• functional layer of the endometrium loses its blood supply, breaks down, and is shed as menstruation