7/24: Brain Anatomy

central nervous system

CNS → a major division the nervous system

• Brain

• Spinal cord

• Contains sensory and motor neurons

peripheral nervous system

PNS → a major division the nervous system

• Nerves

  • Spinal

  • Cranial

• Contains sensory and motor neurons

oligodendrocytes

• Manufactures myelin in CNS

• 1 cell produces many myelin sheaths

schwann cells

• Manufactures myelin in PNS

• 1 cell produces a single myelin sheath

blood brain barrier

→ refers to the CNS’ lack of participation in the lymphatic system

• Dye injected into bloodstream will dye all tissue except for spinal cord + brain

• This is because the CNS doesn’t have open capillaries (vessels don’t have gaps or holes like other tissues, which permit free flow of substances in and out of blood)

• CNS produce their own solution (CSF) by deliberately selecting components from the blood (there’s no leakage)

extracellular fluid

→ fluid that flows around cells to provide nutrients and collect waste

• a.k.a interstitial fluid

• Liquid part of the blood (plasma) that leaks out of all vessels except nervous ones

• Collected into lymphatic system (part of immune system) = lymph vessels → lymph nodes → lymph organs

cerebrospinal fluid

CSF → extracellular solution (that isn’t plasma) deliberately made by the brain to surround / suspend organs of the CNS

• Made from blood by tissue called chloroid plexus

• Carries nutrients in and wastes out from the CNS’ organs

• Made continuously and is fully exchanged 4x daily

• Recycled from CNS into blood supply via holes in the dura mater

projection neurons

→ have axons that innervate distal areas of the brain (synapses far out from its origin)

• Part of CNS

• Releases glutamate (excitatory)

interneuron

→ have axons that innervate proximal areas of the brain (synapses locally about its origin)

• Part of CNS

• Releases GABA (inhibitatory)

motor neurons

→ nervous cells controlling muscle contraction and gland secretion

• Part of PNS & CNS

• Axons called efferent fibers

efferent fibers

→ axons of motor neurons directing information away from CNS; output

sensory neurons

→ nervous cells detecting changes in external and internal environments

• Part of PNS & CNS

• Axons called afferent fibers

afferent fibers

→ axons of sensory neurons directing information towards CNS; inputs

nerves

→ enclosed, cable-like bundles of axons in the PNS

• Spinal

• Cranial

• Most contain a mix of motor and sensory neurons

spinal nerves

→ nerves whose axons enter/leave spinal cord

• Significant part of PNS

• 31 pairs

  • Nearly 1 pair for each vertabrae in spine

cranial nerves

→ nerves whose axons enter/leave brain

• Significant part of PNS

• 12 pairs

  • Nearly all pairs (except 10th) process movements and sensory information around head and neck;

  • Vagus (X) nerve is the exception that branches extensively in the thoracic and abdominal cavities (upper half of body)

spinal cord

→ brings sensory information to the brain + connects the axons of motor neurons (efferent fibers) to effector organs throughout the body

• Significant part of CNS

• Maintains a certain degree of autonomy from its CNS counterpart (the brain) since it contains various reflexive control circuits

• Has a long, conical structure that is approximately as thick as an adult pinkie

anatomical directions

Transverse plane (Y -axis, up & down)

• Superior → above

  • Rostral → upper

• Inferior → below

  • Caudal → lower

Sagittal plane (Z - axis, front & back)

• Anterior → in front

  • Ventral → front-facing

• Posterior → behind

  • Dorsal → back-facing

Frontal plane (X - axis, moving along left and right)

• Medial → away midline

• Lateral → toward midline

neuraxis

→ imaginary line that runs along the length of the CNS

• Relevant for reference by anatomical directions

somatic nervous system

→ branch of PNS that interacts with external environment (processes sensory information)

• Afferent nerves’ sensory signals: Body surface → CNS

• Efferent nerves’ motor signals: CNS → skeletal muscles

autonomic nervous system

→ branch of PNS that regulates internal environment (senses and regulates smooth muscle, cardiac muscle, & glands)

• Afferent nerves’ sensory signals: Internal organs → CNS

• Efferent nerves’ motor signals: CNS → internal organs

• 2 divisions

  • Sympathetic

  • Parasympathetic

sympathetic division

→ division of ANS that primes the body for threatening situations

• “Fight-flight-freeze” response

• Always active to some extent (regulates many organs’ functions)

• Strong stimulation increases blood flow to organs involved in intense physical activity / immediate survival + shunts blood flow from organs that are not ostensibly necessary

parasympathetic division

→ division of ANS supporting activities in a relaxed, neutral state

• “Rest and digest” response

• Also always active to some extent (also regulates many organs’ functions)

• Responsible for increasing the body’s energy stores

brain nuclei

→ refers to a collection of neurons that are clustered together that regulate a shared function

ipsilateral

→ refers to structures on the same side of the body

• Taste and smell are the only sensory systems that are ipsilaterally organized

contralateral

→ refers to structures on the opposite side of the body

anatomical directions for limbs

• Superficial → closer to surface

• Deep → further from surface

• Proximal → close to trunk of body

• Distal → away from trunk of body

hindbrain

→ a.k.a. brain stem

• Medulla oblongata

• Pons

• Cerebellum

medulla oblongata

→ part of the hindbrain; the most caudal part whose brain nuclei regulate autonomic functions

Notable nuclei:

• Area postrema → brain nuclei that initiates vomiting when poisons are detected

• Reticular formation → regulates sleep and arousal

pons

→ part of the hindbrain; bulge that relays information between the cerebrum and cerebellum

• Location of several cranial nerve nuclei central to perception

cerebellum

→ part of the hindbrain; facilitates sensory-motor integration (overall motor learning as the body grows and changes over time)

• a.k.a “Little brain”

• Has ↑ [ ] of neurons of any other part of the brain, but these neurons are small

• Damage leads to awkward movement and poor coordination (precision and accuracy); severe cases lead to paralyzation

superficial cerebellar cortex

→ a continuous thin layer of cells in the cerebellum tightly folded in the style of an accordion

• Locale where afferent fibers of the cerebellum synapse

• Neurons send axons inwards to deep cerebella nuclei, which eventually project to CNS

midbrain

→ collection of nuclei that orchestrate complex reflexive behaviours

• Tectum

• Tegmentum

tectum

→ part of the midbrain; appears as 2 pairs of bumps on the dorsal surface

• Superior colliculi → top 2 bumps involved in orienting to things in peripheral vision

• Inferior colliculi → bottom 2 bumps involved in orienting to unexpected sounds

tegmentum

→ part of the midbrain; that includes several structures that coordinate and motivate complex species-typical movements

• Some areas process pain + responses to threats

forebrain

• Cerebrum & cerebral cortex

• Basal ganglia

• Limbic system

• Thalamus

• Hypothalamus

hypothalamus

→ part of the forebrain; connects the nervous system to the endocrine system via the pituitary gland

• Considered to be the “master hormonal regulator” since the hormones it releases regulates the functions of other endocrine glands

• Bilateral structure made up of several nuclei that regulate ANS activity:

  • Notably involved with behaviours directly related to survival (feeding, fighting, fleeing, & mating)

  • Body temp., circadian rhythm, hunger, sex, etc.

hormone

→ chemical substance that is released into the blood by an endocrine gland

endocrine gland

→ secretes hormones into the blood

• Master hormonal regulator = hypothalamus

thalamus

→ part of the forebrain; structure sandwiching the hypothalamus

• Bilateral structure made up of several nuclei that relay ascending sensory information to different regions of the cerebrum

  • Lateral geniculate nuclei (LGN) → transmits visual info

  • Media geniculate nuclei (MGN) → transmits auditory info

cerebrum

→ part of the forebrain interchanged with cerebral cortex; where sensory information enters conscious awareness

Primary area → input/output area to cerebral cortex

Association cortex →

• Where our understanding of the world (perception) is formed

• Where we decide how to move purposefully

• Multi-layered structure

  • 6 layers in mammals

  • 3 layers in reptiles

• Embedded with cortical columns (neurons interconnected between layers) that are partially distinct functional units

• NOT made up of distinct nuclei (except for in birds)

lobes of cerebrum

• Frontal → controls movement

• Parietal → process tactile info (touch)

• Occipital → process visual info (vision)

• Temporal → process auditory info (hearing)

• Lateral fissure → junction between frontal, parietal, and temporal lobe that processes taste and smell

  • Insular cortex → processes gustatory info (taste)

  • Piriform cortex → processes olfactory info (smell)

divisions between cerebrum lobes

• Longitudinal fissure → separates left and right hemispheres

  • Visible from dorsal view

• Lateral fissure → separates frontal and temporal lobe

  • Visible from lateral view

• Central sulcus → separates frontal lobe and parietal lobe

  • Visible from dorsal and lateral view

• Corpus callosum → large bundle of axons that connects corresponding parts of the left and right hemispheres + unifies perceptions and memories

somatosensory cortex

• Primary cortical area in parietal lobe

• Area where tactile info enters cerebrum

• Receives input from different parts of the body

primary motor cortex

• Primary cortical area in frontal lobe

• Motor neurons synapse in the spinal cord

• Controls different parts of the body

primary auditory cortex

• Primary cortical area in temporal lobe

• Area where auditory info enters cerebrum

primary visual cortex

• Primary cortical area in occipital lobe

• Area where visual info enters cerebrum

insular cortex

• Primary cortical area in lateral fissure

• Area where gustatory info enters cerebrum

cerebral cortex

→ surface covering the cerebrum

• Gray matter → outermost portion of with high concentration of cell bodies

• White matter → region beneath gray matter primarily composes of myelinated axons

convolutions

→ increase surface area of the cerebral cortex

• Sulci → small grooves

• Fissures → large or major grooves

• Gyri → ridges between sulci and fissures

cross sections of brain

• Horizontal cut → cut across y level (transverse plane)

• Sagittal cut → cut down axis of symmetry (sagittal plane)

• Coronal cut → cut across x level (frontal plane)

sensory association cortex

→ specific area where a sense’s perception takes and memories are stored, adjacent to every primary cortical (sensory) area

(ex. Premotor cortex is adjacent to primary motor cortex)

• 1 per sensory system

basal ganglia

→ subcortical structure of the forebrain; circuit of nuclei regulating voluntary movement, reinforcement learning, and habits

• Input: Forebrain (especially the frontal lobe) → for movement

• Output:

  • Ascension: Cerebrum (via thalamus) → for sensory processing and decision-making

  • Descension: Midbrain, hindbrain → for direct movement

• Used to be called the primitive “reptilian” brain

• Neurological “movement” disorders like Parkinson’s are related to the loss of signaling in this region

limbic system

→ subcortical structure of the forebrain; regulates emotions and the formation of episodic memories (personal experiences)

• Hippocampus → area in the temporal critical for explicit memory formation

• Amgydala → area in the temporal lobe critical for processing emotion (especially fear)

• Cingulate cortex → large area connecting limbic areas that overlies the corpus callosum

brain anatomy

• Forebrain

• Midbrain

• Hindbrain

meninges

→ 3 protective layers of tissue wrapping organs of the CNS

• Dura mater → outermost layer; thick, tough, unstretchable tissue

• Arachnoid membrane → middle layer; web-like extensions create a soft, spongey layer filled with CSF

  • Subarachnoid space → area where large blood vessels and capillaries delivering nutrients and oxygen course though and form arachnoid trabeculae

• Pia mater → innermost layer; saran-wrap like

ventricles of the brain

→ 4 interconnected chambers in the center of the brain filled with choroid plexus

• (2) Lateral ventricles → largest chambers right underneath cerebrum

• Third ventricle → lies between 2 thalamic nuclei at center of the brain

• Cerebral aqueduct → long, tube-like structure connecting the third and fourth ventricles

• Fourth ventricle → located in hindbrain (between pons and cerebellum)

  • Connects to central canal of spinal cord

brain development

IN THE WOMB

0-4 weeks: Embryonic membrane pinches off and forms neural progenitor cells, which begin to form a hollow neural tube (frame for the CNS & CSF) via symmetrical cell division (where 2 daughter cells are also neural progenitor cells). This is forming the brain’s ventricle space.

4-8 weeks: Symmetrical cell division halts, commencing asymmetrical cell division (where neural progenitor daughter cells migrate away from the neural tube and divide into either 2 neurons or 2 glial cells instead). This is gradually forming the rest of the cerebrum.

5 months: Fetus has produced roughly 85 billion neurons in the human brain (the most we could ever have). However, many neurons die (undergo apoptosis) before birth (ostensibly due to a lack of accommodation in the network at term).

neurogenesis

→ production of new neurons (via asymmetrical cell division of neural progenitor cells)

• Researchers are uncertain as to whether neurogensis occurs in adult mammals or even humans

apoptosis

→ process of programmed cell death that occurs in multicellular organisms

• This highly regulated and controlled form of death suicide ensures that a dying cell doesn’t extend its illness to neighbouring cells; apoptosis prevents cancer