Lecture 14: Structure and development of the nervous system 1

Cerebrum

2 Hemispheres. Each hemisphere receives input from, and controls movement of, the opposite (Contralateral) side

Cerebellum

Contains as many cells as cerebrum. Has many connections to the cerebrum and spinal cord

Each side receives input from, and controls movement of, the same (ipsilateral) side

Brain Step

Relay center. Regulates body temperature, breathing, and consciousness

Spinal Cord

Encased in vertebral column. Spinal nerves are part of the peripheral nervous system (PNS)

Pyramidal Decussation

Point at the junction of the medulla and spinal cord where the motor fibers from the medullary pyramids cross the midline. The finbers then continue into the spinal cord primariliy as the corticospinal tract.

Anterior (Rostral)

Front

Posterior (Caudal)

Rear

Lateral

Side

Medial

Middle

Dorsal

Top (back)

Ventral

Bottom (belly)

Superior

Located above another structure

Inferior

Located below another structure

Superficial

Close to the surface

Deep

Further from the surface

Sagittal plane

Cut the brain in half vertically down the long side

Transverse (Coronal)

Cut the brain in half vertically down the short side

Horizontal plane

Cut the brain in half horizontally

Gyri

The ridges

Sulci

Valleys between the ridges (Central Sulcus, Lateral Sulcus/Sylvian Fissure, Parieto-Occipital Sulcus)

Fissure

Deeper than a Sulcus; demarcates major divisionsL

Longitudinal Fissure

Separates left and right hemispheres

Transverse Fissure

Separates Cerebrum from cerebellum

Parietal Lobe

Top right of the sagittal plane

Limbo Lobe

Top left of the sagittal plane

Temporal lobe

Bottom left of the sagittal plane

Occipital Lobe

Bottom right of the sagittal plane

Cortex (grey matter)

Made up of cell bodies of neurons

White matter

made up of myelinated axons

Nucleus

A mass of neurons, usually deep in the brain

Dorsal and ventral roots of the CNS

Myelinated by oligodedrocytes and protected by meninges

Dorsal and ventral roots of the PNS

Myelinated and protected by Schwann cells

Dorsal roots

Sensory

Ventral Roots

Motor

Somatic Nervous System

Voluntary Behaviors. Cell body of motor neurons in the CNS, axons in the PNS

Autonomic (Visceral) Nervous System

• Involuntary behaviors

• Portion of the NS that supplies motor innervation to structures that are not under voluntary control

• The ‘motor’ function is mostly smooth muscle, heart muscle, but also glands

Ganglion

A collection of neuron cell bodies in the PNS. An example is the dorsal root ganglia, which contains the cell bodies of sensory neurons entering the spinal cord via the dorsal roots.

Preganglionic neurons of the sympathetic system

Emnate from the thoracic and anterior lumbar regions of the spinal cord

Preganglionic neurons of the parasympathetic system

Emnate from various cranial nerves at the anterior end and sacral regions in the posterior end

Sympathetic nervous system

Relies on norepinephrine “fight or flight”. Dilates pupil, inhibits salivation, relaxes bronchi, accelerates heart, inhibits digestion, stimulates glucose release, secretes adrenaline, relaxes bladder, contracts rectum

Parasympathetic nervous system

Relies on ACh “rest and digest”. Constricts pupil, stimulates salvation, inhibits heart, constricts bronchi, stimulates digestive activity, stimulates galbladder, contracts bladder, relaxes rectum.

Cranial Nerves

12 pairs of nerves (numbered by galen) that originate mostly from the brain stem (3-12) and also from the cerebrum (1-2) and mainly innervate the head. Some are part of the CNS, the rest are part of the PNS or may have somas in the CNS and axons in the PNS. Some are only sensory, some are motor, and some are mixed. Relays information from the brain to regions of the head, neck, and GI track

Days 2-10 of neurodevelopment

Cleavage to blastocyst stage

Days 11-15 of neurodevelopment

Gastrulation and neural induction and formation of three primary “germ” layers

Days 16-25 of neurodevelopment

Neurulation and formation of neural tube

Days 26+ of neurodevelopment

Organogenesis, and brain “patterning”

Neural Induction

Happens during gastrulation (11-15).

• Step before this is blastulation, which is the source of ES cells

• gastrulation is when mesoderm is formed, and mesoderm induces the overlying neurectoderm to become neural fate; neural inducer produced by mesoderm is called noggin, an inhibitor of BMP4

Neural tube formation

called neurulation (days 16-25) neural tube becomes brain and spinal cord; neural crest derived as an offshoot of closure of the neural tube — become sensory and autonomic neurons, neuroendocrine cells, glia, and melanocytes

Regionalization/Patterning

Day 26+

• Anterior-Posterior (AP) patterning - forebrain, midbrain, hindbrain, spinal cord

• Dorsal-Ventral (DV) patterning - determines ventral and dorsal cell types (like tegmentum vs tectum)

Neurogenesis

Proliferation/Migration/Differentiation (lecture 15 stuff)

Axonal pathfinding/Synatogenesis

Step 5 of neurodevelopment

Target Dependent cell death/Synaptic pruning

Step 6 of neurodevelopment

Ectoderm

Primary germ layer for nervous system and skin

Endoderm

Primary germ layer for internal organs that forms before gastrulation

Mesoderm

Primary germ layer for muscle and skeleton which forms during gastrulation

Main neural inducers

Noggin, chordin, and follistatin.

• In there absence neurectoderm would become epidermis (skin)

• Are used in the modern day to help convert ES or iPS (induced pluripotent stem cell) stem cells into neurons in culture

• Neural inducer molecules are called morphogens

Noggin

Produced by the mesoderm and induces the overlying ectoderm to take on a neural fate

Morphogen

a substance whose non-uniform distribution governs the pattern of tissue development in the process of pattern formation, establishing positions of the various cell types within a tissue. More specifically, it is a signaling molecule htat acts directly on cells to produce specific cellular responses depending on its local concentration

Prosencephalon, Mesencephalon, Rhombencephalon

Three primary brain vesicles (day 28). Become forebrain, midbrain, and hindbrain (medulla, brainstem), respectively. Formation is direction by expression of patterning genes - includes transcription factors, morphogens, and cell signaling genes and molecules

Anterior-Posterior patterning

Controlled by morphogen called retinoic acid

Dorsal-Ventral patterning

Controlled by morphogen called sonic hedgehog

Sonic hedgehog

Initiates the formation of the nervous system and floor plate. Floor plate expresses Shh, and is secreted as a gradient (high ventral, low dorsal). Shh is also a ventralizer, induces motor neuron cell fate in the spinal cord and monoamine fates in midbrain. Used in modern era to induce neurogenesis of ES and iPS cells.

Day 36

Forebrain expands and adds telencephalic vesicles, Eyes begin to form, hindbrain develops into the metencephalon and myelecephalon, cranial nerves begin to form.

Day 49-90

Forebrain develops into the dienvephalon and telecephalon

6-9 months

gyri and sulci form. Cerebellum develops folia