Chapter 2: Basic Brain Anatomy Notes

Central Nervous System

• Composed of:

  • Brain

  • Spinal cord

• Encased in bone

• Descending motor tracts send efferent information (from brain to body)

• Nerve tracts that connect the rest of the body to the CNS

• Encased in soft tissue

• Ascending sensory tracts send afferent information (from body to brain)

Peripheral Nervous System

• Composed of:

  • Cranial nerves

  • Spinal nerves

  • Peripheral nerves

• Contains:

  • Relay neurons (interneurons)

  • Sensory neurons

  • Motor neurons

CNS vs PNS (overview)

• CNS = Brain and spinal cord

  • Encased in bone

  • Descending motor tracts carry efferent information (brain → body)

  • Ascending sensory tracts carry afferent information (body → brain)

• PNS = Neural elements outside CNS

  • Connects CNS to rest of body via nerves

The Brain

• Functions: Consciousness, intelligence, language

• 3 gross divisions: cerebrum, cerebellum, brain stem

Brain Facts and Fun Bits

• Brain Fact (typographic note):

  • Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn’t mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.

• Brain weight:

  • The weight of the human brain is about $3\ \text{lbs}.$

• Cerebrum weight share:

  • The cerebrum is the largest part of the brain and makes up $0.85\times W_{\text{brain}}$ of the brain’s weight.

• Water content:

  • The brain is made up of about $0.75$ of water (i.e., $75\%$).

• Brain cortex learning note:

  • Cerebral cortex grows thicker as you learn to use it.

Cerebrum

• Made up of ridges (gyri) and valleys (sulci) – folded tissues allow more neural tissue to be packed into a smaller space

• Rests on top of the brain stem

• Origin of our highest/most complex cognitive functions

• Cerebral cortex – surface tissue of the cerebrum

Cerebral Cortex Growth

• Brain Fact: Cerebral cortex grows thicker as you learn to use it.

Cerebral Meninges

• Dura mater – Dense protective tissue layer

• Arachnoid mater – Contains vascular supply to surface of brain

• Pia mater – Most delicate innermost meningeal layer

Cerebrospinal Fluid System

• CSF system – nutritive and protective

• CSF housed in ventricles within the brain

• Delivers nutrients, removes waste

• Produced in the choroid plexus

• The brain floats in a bath of cerebrospinal fluid which acts as a shock absorber to keep the brain from being crushed by its own weight

• The brain produces a half cup of fluid every day ($\frac{1}{2}\ \text{cup/day}$)

Grey Matter vs White Matter

• White matter – $60\%$

  • Axons of neurons covered in a white sheath of myelin (a protein and fatty substance)

  • Myelin insulates axons to allow electrical impulses to be conducted at about $100\ \text{m/s}$

  • Connects different areas/structures of the nervous system to one another and allows communication

• Grey matter – $40\%$

  • Unmyelinated neurons

  • Processes and regulates information in the CNS

  • Found in cortex, cerebellum, thalamus, basal ganglia, and spinal cord

Brain Development and Plasticity

• Brain Fact: bilingual brains – children who learn two languages before age five alter brain structure; adults have denser gray matter

White Matter Tracts (3 gross types)

• Association fibers – Connects different structures/areas within a single cerebral hemisphere

• Commissural fibers – Connects analogous areas between two cerebral hemispheres

• Projection fibers – Project from the brain to the spinal cord; transmit motor movements from CNS to PNS and sensory signals from PNS to CNS

The Cerebral Hemispheres

• Corpus callosum connects the right and left hemispheres through white matter

• Located at the base of the cerebral hemispheres

• Each hemisphere is responsible for different cognitive and motor functions

• Longitudinal fissure divides the brain into left and right halves (front to back)

Ambidexterity and Corpus Callosum

• Ambidexterity (left-handed or ambidextrous) associated with a corpus callosum about $11\%$ larger than in right-handed individuals

Right Cerebral Hemisphere

• Known as the hemisphere that was: silent, minor, unconscious, insubordinate

• Houses the ability to interpret non-linguistic signals: facial expressions, body language, gestures, and prosody

Right Hemisphere Deficits – Comprehension and Expression

• Comprehension deficits:

  • Prosody – concrete comprehension of language

  • Facial expressions – inability to recognize faces/facial expressions (prosopagnosia)

  • Melody and rhythm – recognize/interpret music (amusia)

• Expression deficits:

  • Prosody – monotone speech

  • Facial expressions – flat affect

  • Melody and rhythm – deficit in interpretation

• Perception of environmental sounds – unable to understand non-speech sounds (e.g., door slamming, birdsong)

• Macrostructure vs microstructure:

  • Macrostructure – inability to piece small details (microstructure) into the big picture (macrostructure)

• Visuospatial processing – difficulty perceiving depth, distance, shapes, etc.

• Sustained attention – difficulty attending to a single stimulus

• Selective attention – difficulty ignoring unimportant stimuli while attending to important ones

Left Hemisphere – Language Dominance

• Primarily responsible for expressive and receptive language

• Broca’s area – inferior-posterior region of frontal lobe; involved in finding words to express meaning

• Wernicke’s area – posteriorly located along the superior marginal gyrus of temporal lobe; interprets meaning of spoken words

• Left hemisphere deficits:

  • Damage to Broca’s area → Broca’s aphasia: know what they want to say but cannot find the right words; usually intact receptive language

  • Damage to Wernicke’s area → Wernicke’s aphasia: unable to comprehend speech; fluent speech often lacks meaningful content

Cerebral Lobes: Major Sulci and Lobes

• Central sulci – runs down the middle of the lateral surface; divides frontal and parietal lobes

• Lateral sulci – begins at the lower front of each hemisphere at an upward angle; passes the central sulcus; divides temporal lobe from frontal and parietal lobes

• Cerebral Lobes divisions (per lateral surface features)

  • Frontal lobe

  • Parietal lobe

  • Temporal lobe

  • Occipital lobe

Frontal Lobe – Features and Functions

• Expressive language – Broca’s area (inferior-posterior left frontal lobe)

• Personality and some memory – Prefrontal cortex (very front of frontal lobes)

• Motor movements – Primary motor cortex (motor strip)

  • Located in the posterior gyrus of frontal lobe just anterior to the central sulcus

  • Left primary motor cortex plans motor speech movements

  • Damage near base of motor strip may cause apraxia of speech

• Motor Homunculus – representation of surface area within the motor strip dedicated to control of each body part; more surface area for parts with fine motor movement

Temporal Lobe – Functions

• Most memory created and housed here – Hippocampi ("seahorse")

• Located in the inferior and medial sections of temporal lobes where cortex folds on itself

• Moves experiences from short-term memory into long-term memory

• Primary Auditory Cortex – located in the superior temporal gyrus, in front of Wernicke’s area

  • Left primary auditory cortex – auditory comprehension of verbal language

  • Right primary auditory cortex – comprehension of environmental sounds and music

Language Circuit and Visual Pathways

• Arcuate Fasciculus – a bundle of fibers connecting expressive (Broca’s area) and receptive (Wernicke’s area) language areas to one another; plays a vital role in repetition

• Vision:

  • Primary visual cortex – located on the posterior section of the occipital lobes; receives visual information from the eyes

  • Each visual cortex processes information from the contralateral visual field

  • Information transmitted to visual association areas (parieto-occipital) for processing

• Visual Association Areas – process and interpret visual information; enable visual perception; lesions may cause visual agnosia

Parietal Lobe – Functions

• Primarily sensory in nature – Primary sensory cortex (sensory strip)

• First gyrus of parietal lobes; receives tactile/proprioceptive (somatic) information

• Left sensory cortex receives sensory info from the right side of the body

• Right sensory cortex receives sensory info from the left side of the body

• Sensory homunculus – representation of cortical surface area dedicated to each body part; more area for regions with more sensory receptors

Subcortex and Brain Organization

• Subcortex – functions usually beneath awareness

  • Refines motor plans

  • Regulates heartbeat, breathing, arousal, and sleep/wake cycle

  • Coordinates viscera and digestive system

• Primary structures: brainstem, cerebellum, thalamus, basal ganglia

Brainstem

• Connects spinal cord to brain

• Midbrain – houses substantia nigra

• Pons – attaches cerebellum to rest of CNS

• Medulla – many motor fibers decussate here to the other side of the body

• Lesions above the medulla → contralateral hemiparesis/hemiplegia; lesions below the medulla in the spinal cord → ipsilateral hemiparesis/hemiplegia

Cerebellum – The "little brain"

• Divided into two hemispheres

  • Each hemisphere receives information from contralateral hemisphere for processing

  • Vermis – receives body information via projections through the pons

  • Attached to the pons via peduncles – superior, middle, and inferior peduncles

• Functions:

  • Error control device for motor plans – detects and corrects errors in movement

  • Ensures movements are coordinated and free of errors

  • Monitors motor plan intent and compares to actual body execution; if error detected, cerebellum adjusts force, timing, and sequencing of muscle contractions

• The more rapid and precise a movement, the greater the likelihood of error; the cerebellum works harder to prevent it

Thalamus

• Sensory relay station of the brain

• Receives afferent sensory information from the body and directs it to the appropriate processing area

• Receives motor plans that the cerebellum has checked for errors and sends refined plans for motor execution

Basal Ganglia

• Group of subcortical structures located within the cerebral hemispheres on either side of the thalamus

• Includes caudate nucleus, putamen, globus pallidus

• Roles:

  • Initiation of movement

  • Maintenance of muscle tone

  • Inhibition of extraneous movements

Spinal Cord and Reflexes

Spinal Cord

• Bundle of white and grey matter in the spinal column

• Transmits sensory (afferent) information from the body to the brain and motor (efferent) information from the brain to the body

Spinal Cord Anatomy and Pathways

• Begins at the medulla

• Narrows at the conus medullaris

• Ends at the conus medullaris

• Spinal cord breaks up into loose strands of nerves called the cauda equina

Stretch Reflex (Spinal Reflex)

• A spinal reflex – originates at the spinal cord below the level of awareness

• Occurs when a muscle is stretched passively (e.g., doctor taps knee)

• The tap stretches the muscle and generates an afferent signal from the muscle spindle fibers

• The afferent signal is sent to the spinal cord; the spinal cord recognizes that this sensory signal contradicts efferent signals from the brain (the muscle was moved by an external tap, not by the brain)

• To correct this, the spinal cord generates an efferent signal to bring the body part back to where the brain’s efferent signals want it to be; this all occurs without input from the brain and below the level of awareness

Spasticity

• Stretch reflex is normal and beneficial unless hyperactive due to pathology

• Hyperactive stretch reflex contributes to spasticity (hypertonia + resistance to movement)

• Common clinical manifestation: spastic cerebral palsy – stretch reflex may affect one limb or the entire body, impairing speech production

Spinal Nerves

• Control trunk, arms, and legs

• Connect the spinal cord to a muscle, organ, or gland

• There are 31 pairs of spinal nerves originating within the grey matter of the spinal cord and exiting through the vertebral column into soft tissues

• Not a direct role in speech except for the phrenic nerve, which innervates the diaphragm, the primary muscle for inspiration needed for phonation

Cranial Nerves

• Innervate muscles of the head, face, and neck

• Cranium encases the brain

• Connect muscles and structures of head, face, and neck to the CNS

• Twelve paired nerves

• All are motor, sensory, or mixed sensory-motor