The Anatomy of Brain Function

The Brain

  • ~2% of body weight

  • receives ~20% of blood pumped from the heart

  • consumes ~20% of body’s energy

  • 100 billion neurons

  • 1,000,000 billion synapses

  • 10^1,000,000 possible circuits

Major Parts of the Brain

  • Cerebrum

    • cerebral hemispheres forebrain

    • two hemispheres, divided by longitudinal fissures or inter-hemispheric fissure

    • Cerebral cortex is the outermost surface layer of the cerebrum

  • Cortext = grey matter

    • surface of the brain 2-4mm thick

    • contains the cells bodies of the brain’s neurons

    • highly folded to maximise surface area (amount of cortex that can fit inside skulls)

    • white matter underneath the grey matter is all the wiring (axons of the neurons, connecting to the spinal cord and to other areas of the cortex

  • Cerebellum = hind brain

  • Brain stem

Frontal Lobe

  • Executive functions

    • reasoning, planning, problem solving

    • inhibitory control

    • working memory

  • Motor functions

    • premotor cortex — motor planning

    • primary motor cortex — execution

  • speech production (broca’s area)

Parietal Lobe

  • Primary somatosensory cortex

    • perception of touch

  • Sense of space and locations

    • gives sense of stable world around us relative to our position

  • Spatial Attention

    • directing attention and eye-movements to explore visual world

  • Linking vision to action

    • represents spatial location of objects around us for guiding actions

Occipital Lobe

  • Posterior part of the brain, inferior to parietal lobe

  • Primary visual cortex

    • all visual perceptions

  • Higher visual areas

    • different regions process shapes, colours, orientation, motion

Temporal Lobe

  • Primary auditory cortex

    • perception of sound

  • Language comprehension (wernicke’s area)

Limbic System, Medial Temporal Lobe

  • Amygdala

    • fear and arousal, responds to threat/danger

    • fear & learning phobias

  • Hippocampus

    • learning and memory, forming new episodic memories

    • damage causes anterograde amnesia (can’t form new memories)

Corpus Callosum

  • Neuron Connections between the left and right hemispheres

  • Allows brain communication between hemispheres

  • Split-brain patients—left & right hemispheres disconnected. The two hemispheres cannot communicate with each other

Phineas Gage

  • Railway worker

  • A iron rod, 1m in length, went through his head in 1848, yet he remained conscious during and after accident

  • Damaged frontal lobes

  • Died 12 years later

  • Suffered a profound change in personality — fitful, irreverent, indulging in profanity, no restraint

Broca’s Area — Speech Production

  • In 1861, Paul Broca described a patient who was unable to speak after damage to the left frontal lobe (Broca’s area)

Broca’s Aphasia

  • Speech is slow and non-fluent

  • Difficulty finding appropriate words (anomia)

  • speech still carries meaning

  • comprehension is (mostly) unaffected

Wernicke’s Area — Language Comprehension

  • In 1874, Carl Wernickle suggested that lessions to the left posterior temporal lobe led to deficits in language comprehension

Wernicke’s Aphasia

  • Unable to understand language — deficit in comprehension

  • Speech is fluent with normal prosody (rhythm, intonation)

  • Speech has no meaning, nonsense speech

Wilder Penfield

  • Stimulated the brain with electrical probes while the patients were conscious, during surgery for epilepsy

  • Published maps of motor and sensory cortices of the human brain

Homunculus

  • Primary sensory cortex and primary motor cortex

  • Brain function mapped by electrical stimulation

  • Brain stimulation leads to sensation or movement (muscle twitch)

  • Size of area on cortex determines sensitivity or fine motor control

Autonomic Nervous System

  • Central Nervous System = brain and spinal cord

  • Peripheral Nervous System

    • Somatic nervous system = voluntary, motor, and sensory

  • Autonomic Nervous System

    • Involuntary

    • Heart-rate, respiration, sweating

    • Stress, arousal, fight-or-flight

  • 2 divisions

    • Sympathetic Nervous System

      • emotional arousal, stress, fear

      • fight or flight response

      • increases heart-rate, respiration, perspiration, pupils dilate

    • Parasympathetic Nervous System

      • Rest and digest

      • lowers heart rate, respiration

      • Increases stomach, intestine activity (digestion)

      • opposes the sympathetic nervous system

Brainstem

Medulla

  • Autonomic nervous system functions

  • Controls heart rate, respiration, regulation of blood pressure, body temperature

  • Reflex centres for coughing, sneezing, swallowing, vomiting

Disorders of Consciousness

Persistent Vegetative State

  • Severe damage to upper brain (hemispheres and cortex)

  • If brainstem is not damaged, autonomic nervous system functions can remain

  • Sometimes normal respiration, control of heart rate, some face and eye movements remain

  • Patients have NO conscious awareness

“Locked-in” Syndrome

  • Amyotrophic Lateral Sclerosis (ALS) or Motor Neuron Disease

    • loss of motor neurons to spinal cord

  • or brain injury (following accident)

  • Intact cerebrum and brainstem, but ‘disconnected’ from spinal cord

  • Normal cognitive function, vision, and hearing, but patients cannot move

  • May be fully conscious and aware, but totally unresponsive

Cerebellum

  • Hind brain

  • Sense of balance and co-ordination of complex movement

  • Motor learning — fine adjustment of movement based on feedbacl

Primary Motor and Sensory Areas

  • Primary motor cortex activity leads to movement (muscle contraction)

  • Primary sensory cortex activity leads to sensation

  • Different parts of motor and sensory cortex map to different parts of the body (homunculus)

“Motor Programs” for movement

  • Movements planned and ‘programmed’ in the brain before initiation, like a computer program (theory from 1960s)

  • Brain creates program just before movement OR retrieves program for learnt skilled actions

Sense of Agency

  • Brain automatically links sensory events and own actions to infer causality

  • Sense that my action caused that event