Development

Development doesn't start at birth — it starts with a single sheet of embryonic cells folding itself into a brain and spinal cord. This subtopic follows that process from the earliest stages of the nervous system's formation, through the disruptions (genetic, infectious, chemical) that can interfere with it, to the reflexes, motor skills, language, and social behavior that unfold across infancy and early childhood.

Key Takeaways

• Neurulation begins with gastrulation forming three germ layers; the notochord (from the mesoderm) signals the ectoderm above it to form the neural plate, which folds into the neural groove/folds and fuses into the neural tube (the CNS precursor), while neural crest cells form alongside it and migrate to become PNS components, skin pigment cells, facial structures, and part of the adrenal medulla.

• Neural tube defects like spina bifida result from incomplete neural tube closure; folic acid lowers this risk. Teratogens — including infectious agents (e.g., TORCH), alcohol (fetal alcohol spectrum disorders), and medications (thalidomide, some antiepileptic drugs) — can disrupt development, with effects that depend heavily on the timing of exposure.

• Primitive reflexes (Babinski, Moro, rooting, sucking, grasp, tonic neck, stepping) reflect an immature nervous system reliant on lower brain/spinal cord circuits; they disappear on a predictable timeline as the cortex matures, and their presence, absence, or persistence carries clinical meaning — most notably the Babinski reflex, normal in infants but pathological in adults.

• Motor development splits into gross motor skills (large muscles, whole-body movement) and fine motor skills (small, precise hand/finger movement); developmental milestones for both, along with receptive/expressive language development and the shift from caregiver-centered to independent social interaction, are general benchmarks with normal individual variation, not fixed deadlines.

Neurulation: How the Nervous System Forms

Neurulation is the process that lays the foundation for the brain and spinal cord, and it begins very early in embryonic development.

Gastrulation and the Three Germ Layers

Before neurulation can happen, the embryo goes through an earlier stage called gastrulation, during which it organizes itself into three primary germ layers: the ectoderm, the mesoderm, and the endoderm. As development continues, each of these layers gives rise to different tissues and organs throughout the body. The nervous system develops from the ectoderm — but its development doesn't happen in isolation.

The Notochord: A Signaling Center, Not a Precursor

The notochord forms within the mesoderm and plays a central role in directing the early stages of nervous system development. It's worth being precise about what the notochord actually is: it does not eventually become the brain or spinal cord. Instead, it functions as a signaling center, releasing molecular cues that tell the surrounding tissue how to develop.

One of the notochord's most important jobs is sending signals to the region of ectoderm directly above it. In response, that ectoderm begins to thicken, forming a specialized structure called the neural plate.

MCAT Callout — Notochord vs. Neural Tube: The notochord and the neural tube are easy to mix up because both are transient embryonic structures involved in nervous system development. The notochord signals — it never becomes brain or spinal cord tissue. The neural tube becomes the central nervous system. Keep the two roles separate: one directs, the other develops.

From Neural Plate to Neural Tube

As development continues, the neural plate begins folding inward. A groove forms down the middle, called the neural groove, while the tissue on either side rises up to form the neural folds. Those neural folds gradually move toward one another until they fuse. Once fused, they pinch off from the overlying ectoderm and form a hollow structure called the neural tube — the structure that will eventually develop into the central nervous system: the brain and spinal cord.

Neural Crest Cells

While the neural tube is forming, another group of cells forms along the edges of the neural folds: the neural crest cells. Unlike most cells at this stage, neural crest cells don't stay in one place — they migrate throughout the developing embryo and eventually become many different types of cells. They're often described as multipotent, meaning they have the ability to develop into multiple different cell types depending on the signals they receive.

As they migrate through the body, neural crest cells contribute to a wide variety of structures, including parts of the peripheral nervous system, pigment-producing cells in the skin, portions of the face, and part of the adrenal medulla.

When Development Goes Wrong: Neural Tube Defects and Teratogens

Embryonic development is an extraordinarily precise process — cells have to divide at the right time, migrate to the right location, and communicate with one another in very specific ways. When something interferes with that process, the consequences can be significant.

Spina Bifida and Folic Acid

Spina bifida occurs when part of the neural tube doesn't close properly during development. Depending on the severity of the defect, it can affect movement and sensation and can also interfere with bladder or bowel function.

This is also why folic acid is so important during pregnancy. Folate plays an essential role in cell division and in the formation of the neural tube, so getting enough folic acid significantly lowers the risk of neural tube defects.

Teratogens and the Importance of Timing

Development can also be disrupted by teratogens — any substance or environmental exposure that interferes with normal fetal development.

Some teratogens are infectious agents. Certain viral or bacterial infections that occur during pregnancy can interfere with normal fetal development and sometimes affect the nervous system or other organs.

MCAT Callout — TORCH Infections: The classic group of infectious teratogens is remembered by the acronym TORCH — Toxoplasmosis, Other agents (including syphilis, varicella, and parvovirus B19), Rubella, Cytomegalovirus, and Herpes simplex virus. Congenital TORCH infections are linked to central nervous system damage, microcephaly, and hearing loss, among other effects.

A determining factor in how severe an exposure's effects turn out to be is timing. Different organs develop at different stages of embryonic development, so the exact same exposure can have very different effects depending on when it occurs. An exposure that's especially harmful during one developmental stage might have a much smaller effect if it occurs at a different stage.

Alcohol and Fetal Alcohol Spectrum Disorders

Environmental toxins and substance exposure can also interfere with prenatal development. One important example is alcohol. Alcohol consumed during pregnancy can disrupt normal brain and body development and lead to a group of conditions known as fetal alcohol spectrum disorders (FASD). The effects vary from one child to another but may include problems with growth, learning, memory, attention, and other aspects of development. Alcohol is especially concerning because it can interfere with multiple developmental processes at the same time while the fetus is still developing.

Medications as Teratogens: Thalidomide and Antiepileptic Drugs

Some medications can also act as teratogens. A famous example is thalidomide. In the late 1950s and early 1960s, thalidomide was commonly prescribed to help treat nausea and morning sickness during pregnancy because doctors believed it was safe. It was later discovered that taking the drug during early pregnancy could severely interfere with normal limb development. The consequences were devastating — thousands of children were born with major birth defects, including severely shortened or absent limbs. That experience fundamentally changed how medications are evaluated for safety during pregnancy.

Thalidomide isn't the only medication that can affect fetal development. Some antiepileptic drugs can increase the risk of neural tube defects, including spina bifida — another reason why medication use during pregnancy always has to be considered carefully.

Broader environmental and physiological factors can influence prenatal development as well. Severe malnutrition, chronic stress, smoking, toxin exposure, and drug use can all interfere with normal development. And once again, timing matters: if something interferes with development while the limbs are forming, it may affect the normal formation of the arms or legs; if that same exposure happens later, after the limbs have already developed, it might have little effect on them and instead interfere with a different organ system, such as the brain.

Primitive Reflexes

Beyond prenatal development, this subtopic also covers how movement and behavior change during infancy and childhood. One of the earliest things physicians and developmental researchers examine in infants is the presence of primitive reflexes — automatic, involuntary responses that appear very early in life.

Newborns don't have the same level of voluntary control over their movements that older children and adults do, so many of their behaviors are driven by these built-in reflexes. That's because the nervous system is still developing: during infancy, many of the pathways connecting the brain and spinal cord haven't fully matured yet, so movement is influenced more heavily by lower brain regions and spinal cord circuits than by the cerebral cortex.

As development continues, that gradually changes. The cortex gains more control over movement, and many primitive reflexes begin to disappear, replaced by movements that are increasingly voluntary and coordinated. That's exactly why primitive reflexes are useful clinically — their presence, absence, or persistence provides information about whether the nervous system is developing normally.

The Babinski Reflex

If you stroke the sole of an infant's foot, the toes fan outward and the big toe extends upward. In a baby, that's a completely normal response, because the pathways involved in voluntary motor control — particularly the corticospinal tract — are still developing. In healthy adults, the response is different: stroking the sole of the foot causes the toes to curl downward instead. If an adult shows the infant-pattern response (toes fanning, big toe extending), it can signal damage to the corticospinal tract or another upper motor neuron pathway.

MCAT Callout — Babinski Reflex, Infant vs. Adult: The Babinski reflex is a favorite MCAT trap because the "normal" answer flips depending on age. Toes fan out and the big toe extends upward → normal in an infant, pathological in an adult. Toes curl downward → normal in an adult. Always check which population the question is describing before deciding whether a Babinski response is expected or concerning.

Moro (Startle) Reflex

The Moro reflex, also called the startle reflex, occurs when an infant suddenly feels unsupported or hears a loud sound: the arms quickly extend outward and then come back in. Physicians sometimes test this reflex because an absent or uneven response can be a sign that something isn't developing normally in the nervous system.

Rooting and Sucking Reflexes

The rooting reflex is directly related to feeding: if you touch an infant's cheek or the corner of the mouth, the infant automatically turns toward the stimulus, making it easier to find the breast or bottle. Closely related is the sucking reflex. Together, these two reflexes help newborns feed long before they have voluntary control over their movements.

Grasp Reflex

The grasp reflex appears when a finger is placed in an infant's palm — the fingers automatically wrap around it.

Tonic Neck (Fencing) Reflex

The tonic neck reflex occurs when an infant turns the head to one side: the arm and leg on that same side tend to straighten, while the opposite arm and leg bend. Because that posture resembles a fencer's stance, it's sometimes called the fencing reflex.

Stepping Reflex

The stepping reflex appears when an infant is held upright with the feet resting on a surface — the legs often make automatic stepping motions. The infant obviously isn't walking yet, but the reflex shows that some of the neural circuits involved in walking are already present very early in development.

MCAT Callout — Primitive Reflex Disappearance Ages: Approximate ages when each primitive reflex typically disappears (independently sourced, since exact timing isn't something you need to memorize down to the week, but knowing the general order and range is useful).

As infancy progresses, these early reflexes gradually disappear and are replaced by movements that are much more controlled and voluntary.

Motor Development: Gross and Fine Motor Skills

When people talk about motor development, they often divide it into two broad categories: gross motor skills and fine motor skills.

Gross motor skills involve the large muscles of the body — think of milestones like sitting up, crawling, standing, or taking those first few steps. All of these movements require increasing control over posture, balance, and coordination.

Fine motor skills are different: they involve smaller, more precise movements, especially of the hands and fingers. As children develop, they become better at things like picking up tiny objects, holding a spoon, and eventually writing with a pencil — all examples of fine motor skills becoming more refined over time.

Developmental Milestones as Benchmarks, Not Deadlines

Developmental milestones are useful for tracking how a child is developing, but it's important to understand what they actually represent: they're not rigid deadlines every child has to meet at exactly the same age. Instead, they're general benchmarks that give physicians and researchers a way of monitoring how development is progressing over time.

As children grow, their movements become more coordinated and precise. By around 15 months of age, many toddlers are walking independently, and around that same time they also become much better at using their hands — pointing with a single finger and developing a pincer grasp, in which they pick up small objects between the thumb and index finger. That milestone matters because it signals that the nervous system is becoming more refined and that movements are becoming increasingly voluntary and precise instead of being driven primarily by reflexes.

Between about 18 and 24 months of age, children become much more capable of interacting with the world around them — climbing stairs, turning pages in books, stacking blocks, and manipulating toys with greater control than just a few months earlier. By around 2 years of age, many children can also kick a ball, scribble with crayons, and climb onto furniture.

The individual ages matter less than the overall trend: as the nervous system continues to mature, movements become more coordinated, more purposeful, and increasingly precise.

Language Development: Receptive and Expressive Language

Language develops rapidly during this same period, and one of the most important distinctions to know is the difference between receptive language and expressive language. Receptive language is the ability to understand language, while expressive language is the ability to produce it.

One interesting feature of early development is that children usually understand a lot more than they can actually say — a toddler might follow a simple instruction like "go get your shoes" even though they aren't speaking in complete sentences yet.

As children continue to develop, their language abilities become more sophisticated: they begin with simple sounds like cooing and babbling, then start saying individual words, and later begin putting words together into short phrases and full sentences. Vocabulary also grows quickly during early childhood — there can be a period where it seems like children are learning new words almost every day, sometimes referred to as a vocabulary explosion.

Social Development in Infancy and Childhood

Social development is changing during this same window as well. Early in life, infants rely heavily on their caregivers — not just for food and protection, but also for comfort, security, and emotional support — so many of a child's earliest social interactions center around those caregivers.

As children continue to grow, they also become more independent. Their world gradually expands beyond their caregivers, and they begin learning how to interact with other people, build friendships, cooperate, and understand that others may think or feel differently than they do.

Tracking developmental milestones can be helpful, since they sometimes provide early clues that a child may benefit from further evaluation. At the same time, it's important not to overinterpret any single milestone — development is highly individual, and healthy children can reach the same milestone at slightly different ages. A difference of a few weeks or even a few months is often completely normal. Development is a gradual process that unfolds over many years, constantly shaped by both biology and experience as the nervous system continues to mature.

Why Development Matters for the MCAT

This subtopic closes out the chapter by connecting the earlier biology of the nervous system to how that biology unfolds over time, from the earliest embryonic stages through early childhood. Watch for:

• Notochord vs. neural tube. The notochord signals; it never becomes the CNS. The neural tube becomes the brain and spinal cord.

• Neurulation sequence. Gastrulation → germ layers → notochord signaling → neural plate → neural groove/folds → neural tube (+ neural crest cells forming alongside it).

• Neural crest cell fates. Peripheral nervous system components, skin pigment cells, facial structures, and part of the adrenal medulla — not just "leftover" cells.

• Timing-dependence of teratogen effects. The same exposure can have very different consequences depending on which developmental stage it occurs during.

• Babinski reflex, infant vs. adult. Toe fanning/big toe extension is normal in infants, pathological in adults — a classic flipped-answer trap.

• Gross vs. fine motor skills. Large-muscle, whole-body movement vs. small, precise hand/finger movement.

• Receptive vs. expressive language. Understanding vs. producing language — understanding typically outpaces production early on.

Common MCAT Mistakes

• Treating the notochord as a precursor to the CNS. The notochord only signals — it never becomes brain or spinal cord tissue. The neural tube, not the notochord, becomes the CNS.

• Assuming a Babinski-positive response always means pathology. Toe fanning and upward big-toe extension is a normal finding in infants. The same response in an adult is what signals possible corticospinal tract damage.

• Confusing gross motor and fine motor skills. Gross motor skills involve large muscles and whole-body movement (sitting, crawling, walking); fine motor skills involve small, precise hand and finger movement (pincer grasp, writing). Don't lump milestones from one category into the other.

• Mixing up receptive and expressive language. Receptive language is understanding; expressive language is producing. Children's receptive abilities typically outpace their expressive abilities early on — a toddler can often follow instructions well before they can speak in full sentences.

MCAT-Style Concept Check

Question: A pediatrician strokes the sole of a healthy 2-week-old infant's foot and observes the toes fan outward with the big toe extending upward. Which of the following best explains this finding?

• A) It is abnormal and suggests damage to the infant's corticospinal tract.

• B) It is a normal finding in infants, reflecting a corticospinal tract that has not yet fully matured.

• C) It is abnormal and suggests cerebellar dysfunction affecting motor coordination.

• D) It is normal only if the same response also appears when the infant is an adult.

Answer: B

Explanation: Toe fanning with upward extension of the big toe is the expected Babinski response in infants because the corticospinal tract — the pathway responsible for mature, voluntary motor control — is still developing. This rules out (A) and (C), which both treat a normal infant finding as pathological. (D) is incorrect because the expected response actually flips with age: the same toe-fanning pattern that is normal in an infant becomes a sign of possible corticospinal tract or upper motor neuron damage if it appears in an adult, whose normal response is toes curling downward instead.

FAQ

What's the difference between the notochord and the neural tube?

The notochord is a mesoderm-derived structure that signals the ectoderm above it to begin forming the nervous system — it never becomes brain or spinal cord tissue itself. The neural tube forms afterward, from the folding and fusion of the neural plate, and it's the structure that actually becomes the central nervous system.

What does the acronym TORCH stand for?

TORCH stands for Toxoplasmosis, Other agents (including syphilis, varicella, and parvovirus B19), Rubella, Cytomegalovirus, and Herpes simplex virus — a group of infectious teratogens linked to effects such as central nervous system damage, microcephaly, and hearing loss when contracted during pregnancy.

Why is the Babinski reflex normal in infants but not in adults?

In infants, the corticospinal tract — the pathway responsible for mature, voluntary motor control — is still developing, so stroking the sole of the foot produces toe fanning and upward big-toe extension as a normal response. In adults, that same pathway has matured, and the expected response is toes curling downward. If an adult shows the infant-pattern response instead, it can signal damage to the corticospinal tract or another upper motor neuron pathway.

What's the difference between gross motor skills and fine motor skills?

Gross motor skills involve the large muscles of the body and whole-body movements, such as sitting up, crawling, standing, and walking. Fine motor skills involve smaller, more precise movements, especially of the hands and fingers, such as picking up small objects, developing a pincer grasp, and eventually writing.