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