PSYB20H3 Chap.8 Brain Development, Reflexes and Motor Skills
Brain Development, Reflexes and Motor Skills
Building the brain before and after birth
Lecture 4 of PSYB20H3 starts with the brain. The neural tube forms in the embryonic stage and becomes the brain and spinal cord, and most neurons are produced by the end of the second trimester. The cerebrum has two hemispheres joined by the corpus callosum, each covered by a cerebral cortex with frontal, occipital, temporal and parietal lobes.
Cerebral lateralization, the specialization of the two hemispheres, begins before birth but is incomplete at birth; handedness is established by about two years.
Neurons and glia
Neurons are the basic units of the nervous system and pass electrical and chemical signals across synapses.
Glia cells outnumber neurons, keep forming throughout life and support them by nourishing cells, facilitating transport and removing waste. Oligodendrocytes, a type of glia, wrap neurons in insulating myelin.
Questions often ask which cell type does which job, so pair each cell with its function.
The brain growth spurt
From about seven months of gestation to two years of age, brain weight rises from 25% to 75% of adult weight.
Three processes account for the spurt: myelination, which speeds transmission and continues in the frontal cortex into adolescence or early adulthood; arborization, the branching of dendrites; and synaptogenesis, the formation of synapses. Plasticity means experience shapes the brain, and synaptic pruning removes neurons and connections that are rarely used.
Not all areas mature at once: subcortical centres lead at birth, followed by the primary motor and sensory areas.
Reflexes and what their loss signals
Reflexes are built-in, involuntary reactions that govern a newborn's movements. Survival reflexes such as rooting and sucking support feeding; primitive reflexes such as grasping and the Babinski reflex eventually disappear.
By about six months the primitive reflexes are gone, which signals that higher cortical centres are taking control over subcortical areas.
Motor skills as dynamic systems
Dynamic systems theory explains how motor skills are assembled for perceiving and acting.
An infant is motivated by a goal, creates a motor behaviour that depends on the nervous system and physical maturation, and tunes the movement through practice, perception and action. Milestones such as crawling, reaching and walking are learned through this adaptation, which makes the process active and a blend of nature and nurture.
Short answers that link brain maturation to this account earn more than answers that treat them separately.
What this chapter covers
- 01
Neural tube and cerebral cortex
- 02
Four lobes of the brain
- 03
Cerebral lateralization
- 04
Neurons, glia and myelin
- 05
Brain growth spurt
- 06
Synaptic pruning and plasticity
- 07
Survival and primitive reflexes
- 08
Dynamic systems theory
Worked example · free
How much does the brain grow in its growth spurt?
- 1At birth the brain is 25% of adult weight; by age two it is 75%, a rise of 50 percentage points.
- 1The spurt runs from about seven months of gestation to two years of age.
- 1Myelination, arborization and synaptogenesis account for the growth.
Key terms
- Corpus Callosum
- The corpus callosum is the band of fibres connecting the two cerebral hemispheres.
- Myelination
- Myelination is the coating of neurons with a myelin sheath that speeds the transmission of neural impulses.
- Babinski Reflex
- The Babinski reflex is a primitive reflex in which a newborn's toes fan out when the sole of the foot is stroked.
- Dynamic Systems Theory
- Dynamic systems theory explains motor skills as assembled from motivation, maturation and practice in which perception and action are coupled.
Brain Development, Reflexes and Motor Skills FAQ
What happens during the brain growth spurt?
Between about seven months of gestation and two years of age, brain weight rises from 25% to 75% of adult weight. The growth comes from myelination, the branching of dendrites called arborization, and synaptogenesis, the formation of synaptic connections, while pruning removes rarely used connections.
Why do primitive reflexes disappear?
By about six months, primitive reflexes such as grasping and the Babinski reflex have disappeared. Their disappearance indicates that cortical centres are taking charge of movements once run by the more primitive subcortical areas, so movement shifts from automatic reflexes toward voluntary action.
How does dynamic systems theory explain learning to walk?
The infant is motivated by a goal, creates a motor behaviour that depends on the nervous system and physical maturation, and then tunes it through practice as perception and action inform each other. Walking is assembled through this cycle of adaptation rather than switched on by maturation alone.
Exam move
Sketch a neuron and label myelin, dendrites and the synapse, then write which growth-spurt process changes each part. Finish by explaining one motor milestone with the three dynamic systems steps and one sentence on the brain change that supports it.
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