Embryonic Development of the Brain

• 3rd week – ectoderm thickens to form neural plate, which is later flanked by neural folds

• This neural groove deepens, forming a neural tube by 4th week—differentiates into the CNS = brain development begins

Embryonic Development of the Brain

• Between ectoderm and neural tube a neural crest forms, and the 3 primary brain vesicles appear:1. FOREBRAIN, or prosencephalon [pros-en-sef-uh-lon]

2. MIDBRAIN, or mesencephalon [mes-en-sef-uh-lon]

3. HINDBRAIN, or rhombencephalon [rom-ben-sef-uh-lon]

• The rest of the neural tube becomes the spinal cord

Embryonic Development of the Brain—figure 12.3

• Week 5 – Secondary brain vesicles arise• Forebrain divides: telencephalon [tel-en-sef-uh-lon, -luhn]

(“endbrain”) and diencephalon [dahy-en-sef-uh-lon] (“interbrain”)

• Hindbrain constricts: metencephalon [met-en-sef-uh-lon] (“afterbrain”) and myelencephalon [mahy-uh-len-sef-uh-lon] (“spinal brain”)

• Eventually the endbrain sprouts two lateral swellings – like Mickey’s ears– This eventually becomes the cerebrum

• The other brain structures form the midbrain, pons, cerebellum and the medulla oblongata [ob-lawng-gah-tuh]

• All but the cerebellum form the brain stem

• Brain continues to grow rapidly; positions change– Midbrain & cervical flexures develop– Surfaces crease & fold = convolutions• Increase surface area = more neurons

Brain--major parts

• Brain stem- continuous with spinal cord– Medulla oblongata, pons, midbrain

• Diencephalon [dahy-en-sef-uh-lon] - above brain stem– Thalamus, hypothalamus & pineal gland

• Cerebrum- at top and largest part– Surface covered with gray matter- cortex– Beneath is cerebral white matter

• Cerebellum- back of brain stem– Means “little brain”

• Cranial meninges [mi-nin-jeez]- dura mater, arachnoid [uh-rak-noid] mater & pia mater

Figure 10.6b

Ventricles of the brain-filled with cerebrospinal fluid

• Lateral ventricles, one deep within each hemisphere are large C-shaped chambers

• They are separated by a thin membrane, the septum pellucidum

• Interventricular foramen allows comminucation between the lateral ventricle and the narrow third ventricle

• The third and fourth ventricles are connected by the canal-like cerebral aqueduct

The Cerebral Hemisphere

• 3 basic regions:1. Cerebral cortex2. Internal white mater3. Basal nuclei (gray matter within white matter)

• Surface folds = gyri• Shallow grooves = sulci• Deeper grooves that separate brain= fissures• Longitudinal Fissure- divides it into left & right

hemispheres• Connected by corpus collosum• Transverse fissure- separates cerebral hemispheres from

cerebellum

Cerebrum- Structure (cont)

• Each hemisphere has 4 lobes– Frontal, parietal, temporal, occipital

• Central sulcus separates frontal & parietal– Precentral gyrus anterior to sulcus= primary motor area– Postcentral gyrus = primary sensory area

• Each hemisphere is concerned with the sensory and motor functions of the opposite side fo the body.

• The two hemispheres are not equal in function.

Figure 10.11a

Figure 10.10

Figure 10.13

Function areas of Cortex

• Specialized areas anatomically located• Sensory areas receive input and responsible

for perception• Motor areas- initiate movements• Associative areas- complex integration: e.g.

memory, emotion, reasoning, etc.

Motor Areas

• Mainly from anterior part of hemisphere• Primary motor area- precentral gyrus• Broca’s speech area- – interacts with premotor area & primary motor

area to regulate breathing and speech muscles

Sensory Areas

• Primary somatosensory area- postcentral gyrus.– input includes: touch, proprioception, pain, itching,

tickle, temperature • Primary visual area- occipital lobe• Primary auditory area- temporal lobe• Primary gustatory area – base of postcentral gyrus• Primary olfactory area- medial aspect of temporal

lobe

Sensory Pathways

• Relay information from periphery to cerebral cortex• 3 neurons in each pathway.• Posterior column- medial lemniscus [lem-nis-kuhs]

pathway– Fine touch- body location, texture, size– Proprioception- [proh-pree-uh-sep-shuhn]

position & motion of body parts– Vibratory sensations- fluctuating touch stimuli

Association Areas• Adjacent to sensory & motor areas• connected with tracts- interpret information• E.g. somatosensory [suh-mat-uh-sen-suh-ree] association

area– Posterior to primary somatosensory area– Integrates sensation- exact shape & texture of object

compares with stored memories• Wernike’s area- left temporal & parietal [puh-rahy-i-tl]

lobes– Interprets meaning of speech– Right hemisphere adds emotional content

Lateralization

• Left gets input from & sends output to right side of body and vice versa

• Left important for spoken & written language, numerical & scientific skills & reasoning

• Right more involved with spatial and pattern recognition and emotional content

Basal Nuclei

• Deep gray = basal nuclei (basal ganglia)–Globus palladus, putamen [pyoo-tey-min],

caudate nucleus

Diencephalon

• Thalamus- critical relay for sensory input– Transmits motor information from cerebellum & basal nuclei to

cerebrum• Hypothalamus- important for homeostasis

– Control of ANS-regulation of many activities– Control of pituitary and hormone production– Regulation of emotional & behavior patterns– Regulation of eating & drinking– Control of body temperature– Regulation of circadian rhythms & states of consciousness

• Pineal gland- secretes melatonin

Figure 10.6a

Brain Stem- Midbrain

• Connects pons to Diencephalon– Large tracts = cerebral peduncles (motor)

• Nuclei = substantia nigra, red nuclei, cranial nerves III & IV

• Superior colliculi – nuclei involved in tracking visual stimuli

• Inferior colliculi – auditory input & startle reflex

Brain Stem- Pons

• Pons (bridge)- nuclei & tracts– Connect left & right of cerebellum

• Ascending & descending tracts• Nuclei – motor relays from cerebrum to

cerebellum , respiration & cranial nerves V, VI, VII, VIII

Brain Stem- Medulla

• Medulla Oblongata- inferior part of brainstem– white matter extending between spinal cord & other parts of

brain• several nuclei: cardiovascular center

– (heart rate)• Medullary rhythmicity area

– (respiratory rhythm)• Other sensory & reflex motor areas

– Some related to cranial nerves

Cerebellum

• Two cerebellar hemispheres– Posterior to medulla and pons, below cerebrum

• Cerebellar cortex –gray matter• Tree like white matter & nuclei• Attached to brain stem via cerebellar

peduncles

Cerebellar function

• Gets wide range of sensory input• Compares with programmed motor activity

from cerebral cortex• Smoothes & coordinates complex activities• Regulates posture & balance• Required for skilled motor activities

Limbic System

• Ring of structures on inner border of cerebrum and floor of diencephalon

• “emotional brain” – pain , pleasure, anger, affection, docility

• Involuntary activity related to survival• Important in memory development

Reticular formation

• Netlike arrangement of gray and white mater• Ascending part = Reticular Activating System

(RAS)• Projects to cerebral cortex & helps maintain

consciousness• Inactivation => sleep

Cerebrospinal Fluid (CSF)

• Circulates through ventricles of brain and the subarachnoid space.

• 4 ventricles: 2 lateral, third & fourth• Formed in choroid plexuses

– = Specialized capillary networks in wall of ventricles covered by ependymal cells

• Flows through ventricles then from 4th to central canal of spinal cord & subarachnoid cells– Reabsorbed through arachnoid villi into superior saggital sinus

Brain blood supply

• Requires ~20% body’s oxygen supply– 4 min lack => permanent damage

• Requires continuous glucose supply• Protected by Blood-brain barrier

– Allows lipid soluble materials: O2, CO2, alcohol, anesthetic agents but controls entry of other materials

• Created by tight capillaries and glial cells

Figure 10.7

Figure 10.8

Figure 10.9

Figure 10.11b

Figure 10.12

Figure 10.14a

Sensory Pathways (cont)

• Spinothalamic pathways-• anterior & lateral spinothalamic tracts• Relay impulses for pain, tickle, itch & thermal

sensations.

Somatic Motor Pathways

• Signals converge on lower motor neurons• Lower motor neurons stimulate muscles directly• Input comes from:

– Local interneurons- e.g. reflexes– Upper motor neurons- corticospinal tracts– Basal ganglia- help with muscle tone– Cerebellum- coordination

Figure 10.15

Memory

• Process for storing & retrieving information• Involves structural & functional changes• Involves association areas, parts of limbic

system & diencephalon• Skill memory also involves cerebellum & basal

ganglia

Central Nervous System, Spinal Nerves, And Cranial Nerves

Cranial Nerves (table 9.6)

• I Olfactory- sensory (smell)• II Optic- sensory (vision)• III Oculomotor- motor (eye)• IV Trochlear- motor (eye)• V Trigeminal- Mixed

– Sensory around eyes & upper mouth motor to chewing• VI Abducens- motor (eye)• VII Facial- mixed

– Sensory to front of tongue & motor to facial expression, lacrimal and some salivary glands

Cranial Nerves

• VIII Vestibulocochlear- sensory (ear)• IX Glossopharyngeal- mixed

– Sensory for rest of tongue, pharynx & palate, blood pressure– Motor to pharyngeal muscles, parotid salivary gland

• X Vagus- mixed (major visceral nerve)– Sensory from pharynx, ear, diaphragm, visceral organs in ventral

cavity– Motor to palatal & pharyngeal muscles & organs in ventral cavity

• XI Accessory- Motor to voluntary muscles including sternocleidomastoid and trapezius

• XII Hypoglossal- motor to tongue

Aging

• Rapid growth during first few years– Size of neurons & proliferation of neuroglia

increases– Increases development of dendritic branches &

synaptic contacts• Decline in brain mass from early adulthood on

Spinal Cord StructureProtection and Coverings

• Spinal cord in vertebral cavity-–Surrounded by bone

• Wrapped in meninges- – 3 layers of connective tissue

• Spinal cord meninges are continuous with brain meninges

Spinal Meninges• Epidural space lined with fat• Dura mater- tough ,dense connective tissue

– Extends to 2nd sacral vertebra – Well beyond spinal cord

• Arachnoid mater- collagen and elastic fibers• Subarachnoid space-

– cerebral spinal fluid circulates in this space• Pia mater- transparent layer

– adheres to surface of brain & spinal cord– Contains blood vessels

Figure 10.1

Gross Anatomy Of Spinal Cord

• Runs to 2nd lumbar vertebra• Roots of spinal nerves for lumbar, sacral &

coccygeal nerves in vertebral cavity before leaving = Cauda Equina

• Enlargements: cervical & lumbar– Include nerves for upper & lower limbs

• Each spinal segment gives rise to a spinal nerve – 31 pairs

Figure 10.2

Internal Structure Of Spinal Cord

• Two grooves- left & right halves– Anterior median fissure & posterior median

sulcus• Gray matter- 3 horns on each side– Anterior, posterior, lateral

• Anterior- somatic motor neurons• Posterior- sensory neurons• Lateral- autonomic motor neurons

Internal Structure Of Spinal Cord (cont)

• White matter- organized into columns– Anterior, posterior & lateral white columns

• Each column contains one or more tracts having a common destination

• Sensory = ascending tracts– Carry information toward brain

• Motor = descending tracts– Carry information down spinal cord

Figure 10.3

Spinal Nerves

• Serve particular area of body• Contain 2 bundles of axons = roots• Dorsal root- only sensory axons• Swelling called dorsal root ganglion– Contains Cell bodies of sensory neurons

• Ventral root- axons of somatic & autonomic motor neurons

Spinal Nerves (cont)

• Named and numbered according to level of vertebra they emerge from

• C1-8, T1-12, L1-5, S1-5 & 1 coccygeal• C1 from above atlas• Rest through intervertebral foramina

Spinal Nerve Composition

• Roots unite to form nerve at foramina• Mixed sensory & motor axons• Each axon wrapped in endoneurium• Axons grouped in fascicles wrapped in

perineurium• Outer covering = epineurium

Figure 10.4

Distribution Of Spinal Nerves

• After leaving vertebra nerves branch• Some join with axons from neighboring nerves to

form plexuses• Names then relate to area they are in or region

innervated• Spinal nerves T2-T11 do not form plexuses=

intercostal nerves– Supply abdominal muscles, skin of chest & back and

muscles between robs.

Plexuses

• Cervical plexus- posterior head, neck, shoulders & diaphragm

• Brachial plexus-upper limbs & some neck & shoulder muscles

• Lumbar plexus- abdominal wall, external genitals & part of lower limbs– e.g. ilioinguinal, femoral, obdurator nerves

• Sacral plexus- buttocks, perineum & lower limbs– e.g. Gluteal, sciatic & pudendal nerves

Figure 10.2

Spinal Cord Functions

• Routes signals along pathways• Gray matter integrates signals • Reflex = fast involuntary sequence of actions in

response to a stimulus• Inborn reflex e.g. withdrawal reflex• Can also have learned reflexes,– e.g. driving skills

• Can be spinal or cranial integration

Reflex arc (patellar reflex)

1. Sensory receptor- responds to stimulus• Tap below patella

2. Sensory neuron- to dorsal horn & brain3. Integrating center- e.g. single synapse

• Sensory to motor neurons

4. Motor neuron- from center to effector• Via ventral horn

5. Effector- responder (muscle or gland)• Patellar reflex- rectus femoris contracts

Figure 10.5