Brain Connections for Movement and Sensations

Speaking, planning, moving, and dreaming: the activities that make us uniquely human originate in the brain. Billions of neurons send projections all over the brain, making trillions of neural connections.¹ But how exactly does the brain develop and when does it start functioning?

The brain begins functioning as it grows. Neurons start releasing chemical messengers called neurotransmitters soon after they form. This allows them to communicate with each other right away. By 7 ½ weeks weeks gestation, the first reflexive motions appear, showing that basic neural circuits are active this early.⁶ By 8 weeks and 3 days gestation, scientists have directly measured brain activity from an unborn child the size of a grape.⁷

The brain begins to take on its characteristic adult shape around 6 months of pregnancy.¹⁰ Early in development, the brain’s surface is mostly smooth. The first major grooves that separate the lobes of the cortex appear around 20 weeks.¹¹

As the baby enters the sixth month, the brain forms ridges and grooves called gyri and sulci, which give it its familiar wrinkled appearance.¹² These folds increase the brain’s surface area by as much as fivefold, allowing it to fit more cells and connections within the skull.¹³ Although small signs of folding can be detected as early as 10 weeks, most of this process occurs during the third trimester as billions of neurons grow and connect.^{14 15 16 17 18} Even after birth, the cortex continues refining its folding patterns through adolescence and into adulthood.¹⁹

Around six months gestation, the brain begins myelination, the process of wrapping neurons in a fatty sheath that speeds up communication between neurons. A single myelinating cell can create over 3 times its weight in fatty sheathing each day.²⁶ While neural circuits still function before myelination, this process makes connections faster and more efficient. Myelination follows a clear sequence: sensory systems first, then motor systems, and finally the association areas involved in complex thinking. These start to myelinate about 9 months after birth.²⁷ Most myelination occurs during infancy and childhood and continues into the mid-20s.²⁸

Throughout development, the brain follows a process called blooming and pruning. During pregnancy and early childhood, the brain grows more neurons and connections than it needs. The total number of neurons peaks around 28 weeks gestation, and the total number of synapses peaks around 3 years after birth.²⁹ Over time, frequently used connections are strengthened, while unused ones are eliminated, making brain circuits more efficient.

The brain undergoes lifelong changes. While virtually every neuron is formed before birth, those neurons may continue moving to their correct locations, and integrate into circuits after birth.³⁰ Importantly, brain development is never “finished” — the process of wrapping neurons in fatty sheaths to speed up their connections continues into an individual’s mid-20s,³¹ and the processes of creating new synapses and eliminating others to support learning and memory lasts for the entire lifetime.³²

Prenatal sensory experience enhances the survival of neurons involved in sensory brain circuits. As the unborn child experiences sounds, light, touches, and tastes, these sensations become neural signals in the developing ears, eyes, skin and tongue, that move to the thalamus—the brain’s sensory relay center. Beginning around 24 weeks of gestation, strong connections form between the thalamus and the cortex, completing the circuitry used for processing sensations such as pain.³⁵ However, the fetus has likely perceived pain starting between 12 and 15 weeks of pregnancy.^{36 37} Connections between the cortex and thalamus have been growing for many weeks; some research shows that connections functionally similar to those seen at 24 weeks may already be present as early as 12 weeks.^{38 39}

Before the cortex is fully developed, early brain activity is supported by a temporary structure called the subplate. Neurons destined for the cortex first move into the subplate between 12 and 17 weeks, where they form early connections with incoming signals from other parts of the brain.^{40 41} These connections are organized in ways that resemble mature brain circuits and function similarly as well. For example, studies in animals show that sounds of similar frequency are processed adjacently in the cortex of both mature and unborn animals.⁴²

The unborn baby begins responding to touch at 7 ½ weeks gestation,⁴⁶ and sensory receptors quickly spread across the body. Hearing develops next, with the earliest responses to sound appearing at 19 weeks gestation, likely because the womb provides constant exposure to sound.⁴⁷ In contrast, the response to light develops later, around 26 weeks gestation.⁴⁸ This delay is likely due to the fact that less than 10% of outside light reaches the womb and is further filtered by the fetal eyelid. Therefore, only a small amount of light reaches the fetal retina, which is already largely developed, having formed by about 10 weeks.⁴⁹

Sensory receptors are often in place before the entire brain circuit that governs the reaction to the sensation. For each sensory system, the first detection of receptors and evidence of their functional response is outlined in the table below: