lecture 10: survey of neural basis:brainstem

EXAM Q: Explain the roles of cerebrospinal fluid and the meninges

CSF cushions the brain, removes waste/toxins, and fills the ventricles and central canal (it's the leftover neural tube). The meninges are three membranes that encase the brain and hold the CSF around it. Together they protect the brain physically and chemically.

EXAM Q: Compare and contrast spinal nerves and cranial nerves

Spinal nerves = 31 pairs, serve the body, connect to the spinal cord, and are ALL mixed (sensory in + motor out). Cranial nerves = 12 pairs, serve the head, connect to the brainstem, and VARY: some pure sensory, some pure motor, some mixed.

EXAM Q: List the structures in the midbrain

Tectum (superior colliculus for vision, inferior colliculus for auditory) and tegmentum (reticular formation, periaqueductal gray, substantia nigra, red nucleus).

EXAM Q: Name the areas in the diencephalon

Hypothalamus (bottom), thalamus (middle), and pineal gland (top). Plus the pituitary gland hanging off the hypothalamus.

EXAM Q: Name the deep telencephalic nuclei and what they do

Olfactory bulb (smell), basal ganglia = caudate/putamen/globus pallidus (movement; Parkinson's/dopamine), hippocampus (learning and memory), amygdala (emotion and fear).

The bottom-up developmental ladder

Myelencephalon (medulla) to Metencephalon (pons + cerebellum) to Mesencephalon (tectum + tegmentum) to Diencephalon (hypothalamus, thalamus, pineal) to Telencephalon (deep nuclei + cortex).

Reflexes: why the spinal cord doesn't wait for the brain

Going up to the brain and back is too slow. Hardwired spinal circuits pull your hand off a hot teapot before you're consciously aware you got burned. Info still travels up so you eventually know, but the protective reflex fires first.

Monosynaptic vs bisynaptic reflexes

Monosynaptic = one synapse, in and out. Bisynaptic (disynaptic) = synapses twice. Neither involves the brain. Once thought simple, now known to be complex.

Why reflexes aren't actually simple

The crossed extensor: stepping on something sharp lifts that foot (flexion) while the other leg extends to take your weight, involving both sides and running up and down the cord. The reflex of expectation: you suck in a breath stepping into a hole before you realize it.

Central canal

The tiny hole in the middle of the spinal cord's butterfly, running its whole length, filled with cerebrospinal fluid. It's the remnant of the neural tube from development.

The ventricles

Four fluid-filled cavities continuous with the central canal: the 4th by the cerebellum, the 3rd above it, and two large lateral ventricles (1st and 2nd) higher up on each side. Large lateral ventricles on a scan are normal.

Choroid plexus

The pink frilly structures that produce cerebrospinal fluid, which is then constantly recirculated.

Roles of cerebrospinal fluid

Cushions the brain so it doesn't slam against the skull, removes toxins/waste, and fills the ventricles and central canal. A spinal tap samples it for diagnosis.

The meninges

Three membranes encasing the brain like a bag, holding CSF around it. Head trauma can injure both the impact side and the opposite side as the brain ricochets. Meningitis is inflammation of the meninges.

Pyramidal decussation

The midline point on the ventral brainstem where pyramidal tract fibers cross from one side of the body to the other. Decussation means crossing over. It's the arbitrary landmark for where the spinal cord ends and the brain begins.

Contralateral vs ipsilateral

Contralateral = crossed to the opposite side of the body. Ipsilateral = on the same side. The nervous system is mostly contralaterally organized, which the pyramidal decussation explains.

Medulla oblongata

The caudal-most part of the brain (the myelencephalon), just above the pyramidal decussation. Same sensory-dorsal/motor-ventral layout as the cord, but also runs survival functions like breathing and heartbeat.

The three section planes

Sagittal cuts down the midline (left vs right). Coronal cuts like a crown across the head (removes a front or back chunk). Horizontal cuts top from bottom.

Metencephalon structures

The pons and the cerebellum. The cerebellum holds the majority of the brain's neurons and handles motor movement, especially discrete unconscious movements and tracking where your body is in space.

Gray/white arrangement: cord vs cerebellum

In the spinal cord, cell bodies are inside (butterfly) and tracts outside. In the cerebellum and cortex it's reversed: gray cell bodies outside, white myelinated tracts inside.

Cranial nerves: the basics

12 pairs, one per side of the head. Lowest numbers are most rostral (near the nose), 12 is most caudal (by the medulla). They bring sensory and motor info in and out for the head, the way spinal nerves do for the body.

Cranial nerve types: sensory, motor, or mixed

Unlike spinal nerves (all mixed), cranial nerves vary. Some are pure sensory, some pure motor, some mixed. This lets doctors localize problems: facial paralysis with intact sensation points to a motor nerve; lost sensation with intact movement points to a sensory nerve.

Cranial nerves I through VI

I Olfactory (smell, to telencephalon), II Optic (vision, to diencephalon), III Oculomotor (eye movement), IV Trochlear (eye movement), V Trigeminal (face touch and temperature), VI Abducens (eye, external rectus).

Cranial nerves VII through XII

VII Facial (moves facial muscles), VIII Auditory/vestibular (hearing + balance), IX Glossopharyngeal (back of mouth/throat, some taste), X Vagus (throat + innervates the gut), XI Spinal accessory (throat, neck muscles), XII Hypoglossal (throat, neck muscles).

Which cranial nerves control eye movement together?

III, IV, and VI coordinate both eyes so they move in sync, like tracking a car across the street.

Cranial nerve mnemonic

"On Old Olympus's Towering Top, A Finn And German Viewed Some Hops": Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Auditory, Glossopharyngeal, Vagus, Accessory, Hypoglossal.

Tegmentum (midbrain floor)

Contains the reticular formation, responsible for wakefulness, attention, and arousal (state of consciousness). Think of it as a volume knob for arousal. Also holds the periaqueductal gray, substantia nigra, and red nucleus.

Tectum (midbrain roof)

Contains the superior colliculus (visual processing) and inferior colliculus (auditory processing). They stack incoming sensory information so you can orient to where an event happened.

Superior vs inferior colliculus

Superior colliculus = vision. Inferior colliculus = auditory. You use your inferior colliculus to locate where a voice is coming from.

Reticular formation

A cluster of nuclei in the brainstem/midbrain tegmentum controlling arousal, wakefulness, attention, and sleep/wake cycles. Acts like a volume knob for consciousness.

Hypothalamus

The geographical center of the brain and bottom of the diencephalon, with ~44 nuclear groups. Runs water balance, weight, and temperature. Where John Flynn's electrodes produced quiet biting vs affective attack in cats.

Pituitary gland

The master gland hanging off the hypothalamus that controls most other glands. It's how the brain controls the endocrine (hormone) system, the way spinal nerves control the body and cranial nerves control head muscles.

Thalamus

Looks like a giant potato on top of the midbrain. Almost all information going to the cerebral cortex is routed through it first. It's the interface between lower brain areas and the cortex.

Lateral geniculate nucleus (LGN)

The thalamic nucleus that receives visual information from the eye (optic tract to LGN). Mapped via degeneration: lesioning visual cortex made the matching LGN cells die because they lost their targets.

The four types of thalamic nuclei

1) Specific sensory (e.g. LGN, sends a sense to one cortical spot), 2) Motor (lesion causes movement deficits), 3) Specific association (ties sensory and motor together for a region), 4) Nonspecific (diffuse cortex-wide connections, don't degenerate when one cortical area is removed).

Pineal gland

Sits above the thalamus at the top of the diencephalon. Involved in circadian rhythms and body cycles.

Deep telencephalic nuclei

Buried structures below the cortex, near the thalamus but not the thalamus or hypothalamus: the olfactory bulb, basal ganglia, hippocampus, and amygdala.

Basal ganglia

Caudate, putamen (the caudate-putamen is one continuous structure), and globus pallidus. Regulate movement, coordinating with the cerebellum. Parkinson's results from too little dopamine reaching them; also tied to Huntington's.

Hippocampus

A deep telencephalic nucleus buried in the temporal lobe, responsible for learning and memory, especially short-term. Activated by stress hormones; smaller volume is linked to depression.

Amygdala

A deep telencephalic nucleus involved in emotion processing and fast processing of incoming sensory info. Closely tied to fear: lesions block fear learning and impair reading fearful faces.

Limbic system

A ring of structures around the thalamus involved in emotion: fornix, septum, amygdala, cingulum, hippocampus. Named by Paul MacLean, who linked it to play, parenting, and infant separation cries, behaviors new to mammals.

Cerebral cortex: gray and white layout

Gray on the outside (cell bodies), white on the inside (myelinated axon bundles). Not the same as the whole brain, just the outer sheet.

Gyri and sulci

Gyri are the ridges/bulges, sulci are the grooves/valleys. Folding packs more surface area (more cell bodies) into a small skull. Sheep have folds; mice don't.

The frontal lobe

Contains the motor cortex plus higher processing: judgment, reasoning, personality, and decision-making. This is where Fritsch and Hitzig got contralateral movement by stimulating.

The parietal lobe

Handles somatosensory information (touch and pain from the body), on the postcentral gyrus.

The temporal lobe

Handles language and hearing; sits near the ears. Also contains the hippocampus.

The occipital lobe

Handles vision, in the very back, receiving input from the lateral geniculate nucleus.

The limbic lobe

Seen only on a midsagittal cut, just above the corpus callosum. Also called the cingulate gyrus; tied to emotion.

Central sulcus and its gyri

The central sulcus divides the frontal from the parietal lobe. The precentral gyrus (frontal side, in front) is primary motor output. The postcentral gyrus (parietal side, behind) is primary somatosensory input.

Corpus callosum

A thick band of pure white myelinated axons, the main communication pathway between the left and right hemispheres. Severing it is split-brain surgery.

Brodmann areas

Brodmann, a German histologist, mapped the cortex by cytoarchitectonics using Nissl stain on serial sections, numbering regions by how thick each of the six layers is. Numbers are jumbled: area 17 = visual, 41 = auditory, 4 = motor.

Cerebral cortex facts

1 to 4 mm thick, six cellular layers, about 1.5 square meters of surface area. Cells here do higher processing and rely heavily on the thalamus to route information.

Phineas Gage

An 1848 railroad foreman whose frontal cortex was pierced by a tamping iron. He lost about 11% of his white matter, severing frontal links to the limbic system and basal ganglia, and his personality changed. First detailed case linking brain damage to behavior.

Brain lateralization

The two hemispheres look nearly identical but have different functions. Left = language, right = visual-spatial processing and facial recognition.

Split-brain surgery and who studied it

Severing the corpus callosum to reduce epileptic seizures. Gazzaniga and Sperry (Nobel Prize) studied these patients, including Vicki, whose hands initially fought over grocery items.

Split-brain visual field results

Contralateral control means the left hemisphere handles the right visual field and vice versa. A stimulus to the right visual field reaches the left hemisphere and can be named. A stimulus to the left visual field reaches the right hemisphere and can't be named but can be pointed to with the left hand.

Split-brain moral judgment finding

Split-brain patients couldn't tell accidentally poisoning a boss from attempting to, because the language-only left hemisphere saw both as administering rat poison. Suggests the right hemisphere (emotion) is needed for complex judgments about intent.