survey of neural basis: lecture 9: Brain development & evolution; Spinal cord and brainstem

lecture 9: Brain development & evolution; Spinal cord and brainstem

DRAW IT: Can you draw and label the brain parts present in early development?

The 3 pockets, front to back: PROSENCEPHALON (forebrain, most forward), MESENCEPHALON (midbrain, middle), RHOMBENCEPHALON (hindbrain, back). Memory hook: P-M-R.

DRAW IT: Can you draw and label the 5-part developmental brain?

Prosencephalon splits into TELENCEPHALON (1A) + DIENCEPHALON (1B). MESENCEPHALON (2) does not split. Rhombencephalon splits into METENCEPHALON (3A) + MYELENCEPHALON (3B). Bottom two are alphabetical going down: met, then myel.

DRAW IT: Can you sketch out the divisions of the peripheral nervous system?

PNS splits into SOMATIC (goes to the body) and AUTONOMIC (automatic, deep organs). Somatic splits into sensory/afferent (in) and motor/efferent (out). Autonomic splits into SYMPATHETIC and PARASYMPATHETIC, and each of those also has a sensory and a motor side.

DRAW IT: Can you draw an animal and label it with all the directional terms?

Dorsal (top/back), ventral (belly/bottom), rostral (nose/front), caudal (tail), medial (midline), lateral (side), proximal (closer to CNS/body core), distal (farther from CNS/body core).

DRAW IT: Can you draw and label a cross-section of the spinal cord?

Butterfly-shaped GRAY matter inside (cell bodies), WHITE matter outside (myelinated axons). Two DORSAL horns on top = sensory/afferent in, with the dorsal root and dorsal root ganglion. Two VENTRAL horns on bottom = motor/efferent out, with the ventral root. Left and right kept separate.

Neural plate to neural crest: the four developmental stages

NEURAL PLATE (region of the embryo thickens and separates from other cell types) to NEURAL GROOVE (plate sinks and invaginates, cells rise over it) to NEURAL TUBE (groove closes, retains a hole in the center) to NEURAL CREST (tube enlarges, line runs the longitudinal axis where the backbone will be).

Which developing structure has radial glia lining its interior?

The neural tube. Radial glia line the hollow center and act as scaffolding that migrating neurons climb along. That hole becomes the ventricles and central canal.

Differentiation

Cells becoming different from the cells around them and more specialized. A hallmark feature of nervous system development.

Which embryonic brain section does NOT subdivide?

The mesencephalon (midbrain). The prosencephalon and rhombencephalon are the two that split.

What ends up in each of the 5 developmental divisions?

Telencephalon = cerebral hemispheres/cortex. Diencephalon = thalamus, hypothalamus, pituitary. Mesencephalon = tectum and tegmentum. Metencephalon = pons and cerebellum. Myelencephalon = medulla oblongata.

Endocast (and why it's needed)

A plaster cast of the inside of a skull where the brain would have been, combined with serial sectioning. Needed because the brain is gelatinous, doesn't fossilize, and is one of the first things to go when an animal dies.

What did bony fish brains reveal?

Early vertebrate brains have the same five structures in the same order as ours (myel, met, mes, di, tel), matching what a human brain looks like during early development. All vertebrates share the same basic organization.

The two big evolution takeaways

1) We can learn about our brains by studying ancient species, since all vertebrates share the five-pouch plan. 2) Evolution works off the same basic blueprint. Nothing gets added.

Adaptation in brain evolution

Animals ELABORATE one or more parts of the same basic structure to cope with their niche. Example: an elephant nose fish in murky water devotes a large brain region to processing electrical information.

CNS vs PNS

CNS = brain and spinal cord, that's it. PNS = everything else.

Afferent vs efferent

AFFERENT = sensory, coming IN to the CNS (Afferent = Arrives). EFFERENT = motor, going OUT; movement and your ability to respond (Efferent = Exits). Motor always out, sensory always in.

Somatic vs autonomic nervous system

SOMATIC serves the body ("soma"), handling voluntary sensory in and motor out. AUTONOMIC is automatic, not under conscious control, innervating deep organs, gut, and viscera.

Sympathetic nervous system: what it does and the teeter-totter idea

"Fight or flight." Responds to the external environment by increasing arousal and quick reactivity. Balances against the parasympathetic like a teeter-totter. You can't have either one on all the time.

Who coined "fight-or-flight response"?

Walter Cannon, an American physiologist, describing the sympathetic nervous system preparing the body for strenuous physical responses to threats and challenges.

Sympathetic effects on the body (full list)

Pupils dilate, salivation reduced, heart rate increases, bronchi dilate, gastric secretion and movement decrease, pancreas and adrenal gland inhibited, intestinal movement lessens, bladder relaxes, produces ejaculation. Also sweating, muscle tension, faster breathing.

Parasympathetic effects on the body (full list)

Pupils constrict, tear glands stimulated, salivation increases, heart rate decreases, bronchi constrict, gastric secretion and movement increase, pancreas becomes active, intestinal movement increases, bladder contracts, produces erection.

Anatomical differences: sympathetic vs parasympathetic

SYMPATHETIC exits from thoracic and lumbar areas (thoracolumbar), with SHORT preganglionic and LONG postganglionic fibers, and uses the sympathetic chain ganglia just outside the cord. PARASYMPATHETIC exits from cranial and sacral areas (craniosacral), with LONG preganglionic and SHORT postganglionic fibers.

The energy argument for the autonomic nervous system

Energy is a valued commodity, so it's spent as needed. During threat, non-critical operations like growth are scaled back. The brain consumes about 20% of the body's fuel, so it must micromanage. Parasympathetic directs energy to growth, reproduction, and immune function.

Neurogenesis

The adult mammalian brain creating new neurons in certain areas. Scientists originally assumed neuron number was fixed. In rodents, running increases it and stress suppresses it.

The social isolation and running study: design and main finding

A 2x2 (socially vs. individually housed, crossed with running vs. control), using BrdU to mark new dividing cells in the dentate gyrus. Socially housed runners had the most new neurons, but socially ISOLATED runners showed LESS neurogenesis than isolated non-runners.

How did the neurogenesis study prove its mechanism?

They ruled out running amount via video sampling (no difference). Restraint tube testing showed socially housed runners had no stress hormone spike while isolated runners did. Removing the adrenal gland in isolated runners restored their neurogenesis, proving stress hormones were the cause.

Conclusion of the neurogenesis and social context study

Harmful elevations in stress hormones are mitigated when the stressful experience happens in the context of affiliative contact (friends). Without those hormones, more energy goes toward building neurons.

Hormones

Chemicals that travel throughout the body with various effects.

Dorsal, ventral, rostral, caudal

Dorsal = top/back (like a dolphin's fin). Ventral = bottom/belly/chest. Rostral = toward the nose/front/mouth. Caudal = toward the tail/rear.

Medial, lateral, proximal, distal

Medial = toward the midline/center. Lateral = toward the side. Proximal = closer to the CNS or body core. Distal = farther from the CNS or body core.

How do directional terms change in humans?

Because we walk upright, the brain pivots about 90 degrees. Ventral becomes synonymous with ANTERIOR and dorsal with POSTERIOR. Rostral still means toward the nose, so you go up and then over. We don't tend to use "dorsal" much for humans.

True or false: dorsal and anterior refer to the same area in humans

FALSE. Dorsal pairs with posterior (back); ventral pairs with anterior (front).

Why are directional terms always relative?

They only make sense compared to another structure. Like asking where Los Angeles is: from Virginia it's west, from Japan it's east. You'd say the olfactory bulb is rostral TO the cerebellum.

The three section planes

SAGITTAL divides left/right. CORONAL divides front/back. HORIZONTAL divides top/bottom.

Vertebral column and foramen

The vertebral column is a stack of many bones rather than one, which lets you articulate (bend and move). A foramen is any space or hole. The spinal foramen in each vertebra lines up into the channel the cord runs through; the intervertebral foramen between vertebrae is where spinal nerves enter and exit.

How many pairs of spinal nerves, and how are they named?

31 pairs, one out the left and one out the right at each level. They're named for the areas of the body they serve.

Why do lower spinal nerves stretch far down before exiting?

The spinal cord stops growing around age 2 while the body keeps growing taller, so the cord effectively pulls up relative to the vertebral column. This creates pressure on the lower nerves, with implications for lower nerve pain.

Dermatome (and shingles)

The specific area of skin associated with one pair of spinal nerves. Shingles produces a painful rash in a distinct area corresponding to a given dermatome.

Dermatome counts and what each level serves

CERVICAL 8 (head, back of head, shoulders, parts of forearms). THORACIC 12 (torso and rib cage, in bands). LUMBAR 5. SACRAL 5. COCCYGEAL 1. Memory hook: when an older person eats meals, 8, 12, 5, then another 5, and 1.

Which body area do cervical nerves innervate?

Top and back of the head, shoulders, and parts of the forearms. Not stomach (thoracic), not legs or feet (lumbar and sacral).

How do doctors use dermatomes clinically?

After a spinal cord injury they work up from the feet with the equivalent of a pin, asking when the patient can feel it. The boundary between no feeling and feeling pinpoints where the damage is. This is functional neuroanatomy.

How is information kept separate in the spinal cord?

In both directions. Left handles only the left side of the body and right only the right. Dorsal (sensory) and ventral (motor) also stay separate. Two halves stacked in two directions.

Gray matter vs white matter in the spinal cord

GRAY is the butterfly-shaped inside, containing cell bodies, which look gray with no stain needed. WHITE is the outer edge, containing myelinated axons; myelin is fatty and fat appears white. Made by oligodendrocytes, which provide insulation.

Why does it matter that axons run on the outside of the cord?

Axons carry information, so long projections traveling up and down the cord run through the outer white matter, while the cell bodies belonging to that segment sit inside in the gray matter.

Dorsal horn vs ventral horn

DORSAL horn (top) receives sensory/afferent information coming in. VENTRAL horn (bottom) sends motor/efferent information out and contains the motor neuron cell bodies.

How does a motor neuron produce movement?

Myelinated fibers descend in the white matter and synapse on a cell body in the ventral horn. An action potential travels out the spinal nerve to the neuromuscular junction, where acetylcholine is released and the muscle contracts.

Ganglion vs nucleus

A GANGLION is a collection of cell bodies OUTSIDE the CNS. A NUCLEUS is a group of neurons INSIDE the CNS (and also the DNA-containing part of any animal cell).

Dorsal root ganglion, and how sensory information gets in

A collection of sensory cell bodies hanging off the spinal nerve before it enters the cord, still in the periphery. Smashing your finger sends action potentials into that cell body; its axon then synapses in the dorsal horn, and those cells relay the information up.

Easy way to spot the dorsal side in a spinal cord diagram

Look for the dorsal root ganglia on the incoming spinal nerves. Their presence marks the dorsal side.

Axonal regeneration: why the PNS recovers and the CNS doesn't

PNS: macrophages clean up debris and no inhibitory molecules are present, so the axon regrows via a growth cone. CNS: no macrophages to clear debris and inhibitory molecules ARE present, so no regrowth.

Brainstem

The medulla oblongata, pons, and midbrain together.

Medulla oblongata

The most ventral (lower) area of the brain, part of the myelencephalon, where information from the spinal cord enters. Regulates respiration, blood pressure, arousal, and muscle tone. All vertebrates have a brainstem.

Pons

A bulge on the ventral side, part of the metencephalon. Latin for "bridge." The brainstem hub for information transfer, carrying information to more dorsal areas and from one side of the brain to the other.

Cerebellum

Literally "little brain," dangling off the dorsal brainstem, part of the metencephalon. Contains about 80% of all the brain's neurons and regulates motor coordination and balance.

Why are field sobriety tests cerebellum-dependent?

The cerebellum is one of the first areas affected by alcohol, so walking a straight line or touching your nose with eyes closed reveals impairment. The cerebellum is also involved in attention, learning, and executive control.

Cresyl violet stain

A common histological method for staining neuron cell bodies, turning them vibrant violet. Shows the cerebellum is more densely packed with neurons than other areas.

Reticular formation

A cluster of nuclei and fibers throughout the central brainstem. Maintains arousal levels so animals can direct attention toward relevant targets, and is involved in sleep/wake cycles.

Tectum and its contents

The "roof" of the midbrain. Contains the SUPERIOR COLLICULI (processing visual information) and the INFERIOR COLLICULI (processing and localizing auditory information).

Tegmentum and its contents

The "floor" of the midbrain. Contains the most dorsal reticular formation, the PERIAQUEDUCTAL GRAY (sensation of pain), the SUBSTANTIA NIGRA ("black substance," produces a neurochemical for smoother integrated movement), and the RED NUCLEUS (pinkish, evolved as limbs emerged, role in limb movement).

Corpus callosum vs anterior commissure

CORPUS CALLOSUM is a thick band of myelinated axons connecting the DORSAL sections of the hemispheres. ANTERIOR COMMISSURE provides more VENTRAL communication.

Thalamus

Two large egg-shaped structures, one per hemisphere, in the diencephalon. Filters and directs sensory information to the appropriate area of the cerebral cortex. Made of several nuclei, each specialized for a sensory type.

Which sense does NOT pass through the thalamus?

Olfaction (smell). It's an evolutionarily old system routed straight to relevant brain areas on entry, to facilitate survival.

Hypothalamus

Sits under the thalamus in the diencephalon. Prominent in motivational systems: sexual behavior, aggression, parenting, hunger, thirst, and stress.

Pituitary gland

A pea-sized structure dangling from the hypothalamus. Part of the endocrine (hormonal) system; on signal from the hypothalamus it triggers hormone release throughout the body.