Anatomy of the Nervous System

STRUCTURE OF THE VERTEBRAE iERVOUS SYSTEM

Terminology to Describe the iervous System

CiS (central nervous system): brain and spinal cord
PiS (peripheral nervous system): connect brain and spinal cord to rest of body
Somatic iS: axons convey messages from sense organs to CNS and from CNS to muscles; axons to muscles are extensions of cell bodies in spinal cord; part of each cell in CNS, part in PNS
Autonomic iS: controls heart, intestines, other organs; some cell bodies within brain or spinal cord, some in clusters along sides of spinal cord

Anatomical Terms Referring to Direction

Dorsal: top of brain — Ventral: bottom of brain
Anterior: front — Posterior: rear
Superior: above another part — Inferior: below another part
Lateral: toward side — Medial: toward midline
Proximal: close to point of origin — Distal: distant from point of origin
Ipsilateral: same side of body — Contralateral: opposite side of body
Coronal plane: brain structure from front — Sagittal plane: side
Horizontal plane: seen from above (transverse plane)

Terms Referring to Parts of iS

Lamina: layer of cell bodies separated from other cell bodies by layer of axons and dendrites
Column: set of cells perpendicular to surface of cortex, similar properties
Tract: (projection) set of axons within CNS
ierve: set of axons in periphery, from CNS to muscle/gland or from sensory organ to CNS
iucleus: cluster of neuron cell bodies w/in CNS

Ganglion: cluster of neuron cell bodies, usually outside CNS

Gyrus: protuberance on surface of brain
Sulcus: fold that separates one gyrus from another
Fissure: long, deep sulcus

The Spinal Cord

Part of CNS within spinal column; communicates with all sense organs and muscles (except those of head); segmented; each segment has sensory and motor nerve on each side
Bell-Magendie law: 1^st discoveries about functions of NS; entering dorsal roots

(axon bundles) carry sensory info, and exiting ventral roots carry motor info

Axons to and from the skin and muscles are the PNS
Dorsal Root Ganglia: Cell bodies of sensory neurons in clusters of neurons outside spinal cord; cell bodies of motor neurons inside spinal cord
Gray matter: (H-shaped) center of cord; densely packed with cell bodies and dendrites; many neurons of spinal cord send axons from gray matter to brain or other parts of spinal cord through white matter (mostly myelinated axons)
Each segment of spinal cord sends sensory info to and receives motor commands from brain; all info passes through tracts of axons in spinal cord The Autonomic iervous System
Neurons that receive info from & sends commands to heart & intestines

Sympathetic iS:

Network of nerves prepare organs for “fight or flight”; chains of ganglia to left and right of spinal cord’s central regions (thoracic and lumbar).
Ganglia connected by axons to spinal cord; closely linked, often act as single system in sympathy with each other; various events activate some parts more than others
Sweat glands, adrenal gland, muscle that constrict blood vessels, muscles that erect hair have ONLY sympathetic input

2. Parasympathetic iS

Facilitates nonemergency responses; opposite of sympathetic activities
Craniosacral system: cranial nerves and nerves from sacral spinal cord
Unlike ganglia in sympathetic system, parasympathetic ganglia not arranged in chain near spinal cord
Long preganglionic axons extend from spinal cord to parasympathetic ganglia close to each internal organ; shorter postganglionic fibers extend from parasympathetic ganglia into organs (Ganglia aren’t attached, can act independently)
Postganglionic axons release Ach, most postganglionic synapses of sympathetic NS use norepinephrine; certain drugs excite/inhibit different systems

The Hindbrain (rhombencephalon)

Posterior part of brain: medulla, pons, cerebellum
Brain stem: medulla, pons, midbrain, some central structures of the forebrain
Medulla: above spinal cord, extension of spine into skull; controls vital reflexes

 breathing, heart rate, vomiting, salivation, coughing, through cranial nerves

Cranial nerves: control sensations from head, muscle movements in head, most of parasympathetic output to organs
Some include both sensory and motor, others just one; receptors and muscles of head and organs connect to brain by 12 pairs of cranial nerves (one on right, one on left); each cranial nerve originates in nucleus that integrates sensory info, regulates motor output or both;
Cranial nerve nuclei for nerves V through XII are in medulla/pons, I through IV in midbrain/forebrain
Pons: anterior and ventral to medulla; contains nuclei for several cranial nerves; axons from each half of brain cross to opposite side of spinal cord  left hemisphere controls muscles on the right side of body and vice versa
Medulla and pons contain
Reticular formation: descending (1 of several brain areas that control motor areas of spinal cord) portions and ascending (sends output to much of cerebral cortex, selectively increasing arousal/attention in certain areas) portions
Raphe System: sends axons to much of forebrain, modify brain’s readiness to respond to stimuli
Cerebellum: large, many deep folds; contributes to control of movement, balance, coordination; damage causes trouble with shifting attention from auditory and visual stimuli, sensory timing. The Midbrain (mesencephalon)
Middle of the brain, surrounded by forebrain (birds, reptiles, fish, amph.)  Tectum: roof of midbrain

Superior (vision)/inferior (hearing) colliculus: swellings on each side of tectum; important for sensory processing

Tegementum: under tectum; intermediate level of midbrain; covers other midbrain structures
Includes nuclei for 3^rd and 4^th cranial nerves, parts of reticular formation, extension of pathways between forebrain and spinal cord/hindbrain
Substantia nigra: dopamine-containing pathway; readiness for movement

The Forebrain (prosencephalon)

Most prominent part of mammalian brain
2 cerebral hemispheres, each organized to receive sensory info; mostly from opposite side of body, control muscles on contralateral side, by way of axons to spinal cord and cranial nerve nuclei
Cerebral cortex: outer portion
Limbic system: interlinked structures, form border around brainstem; important for motivations and emotions (ex: eating, drinking, sexual activity, anxiety, aggression
Includes: olfactory bulb, hypothalamus, hippocampus, amygdala, cingulate gyrus

Thalamus

Thalamus and hypothalamus form the diencephalon
Pair of structures in center of forebrain, one in each of the hemispheres
Most sensory info goes here 1^st, which processes it, sends output to cerebral cortex
Exception: olfactory info  progresses from olfactory receptors to olfactory bulbs, then directly to cerebral cortex
Many nuclei of thalamus receive input from a sensory system, transmit info to single area in cerebral cortex; cerebral cortex sends info back to thalamus, prolong certain kinds of input and expense of others  focus attention on stimuli

Hypothalamus

Small area near base of brain; ventral to thalamus, connections with rest of forebrain and midbrain (contains many distinct nuclei)
Through nerves and hypothalamic hormones, conveys messages to pituitary gland, alerting its release of hormones
Damage to hypothalamic nuclei  abnormalities in motivated behaviours Pituitary Gland

Endocrine gland attached to base of hypothalamus by a stalk that contains neurons, blood vessels and connective tissue

In response to messages from hypothalamus, synthesizes hormones that blood carries to organs throughout the body Basal Ganglia
Group of subcortical structures lateral to thalamus, includes:
Caudate nucleus, putamen and globus pallidus
Subdivisions that exchange info with different parts of cerebral cortex
Set of structures important for certain aspects of movement
Involved in learning, remembering how to do something, attention, language, planning, other cognitive functions Basal Forebrain
Lies on ventral surface of forebrain
iucleus basalis: receives input from hypothalamus and basal ganglia  sends axons that release Ach to areas in cerebral cortex; part of brain’s system for arousal, wakefulness and attention

Hippocampus

Large structure between thalamus and cerebral cortex, posterior of forebrain
Critical for storing memories, especially of individual events
Damage: can’t store new memories, don’t lose memories before damage The Ventricles
NS begins development as a tube surrounding a fluid canal  canal persists into adulthood as central canal (fluid-filled channel in center of the spinal cord) and as ventricles (4 fluid-filled cavities in the brain)
Each hemisphere contains 1 of 2 large lateral ventricles; towards posterior, connect with 3^rd ventricle (positioned at midline), separating right and left thalamus; 3^rd ventricle connects to 4^th in center of medulla
Cells called choroid plexus inside 4 vesicles produce cerebrospinal fluid (CSF) (clear fluid similar to blood plasma); CSF fills ventricles, flowing from lateral ventricles to 3^rd and 4^th  from 4^th ventricle, some flows into central canal of spinal cord, more goes into narrow spaces between brain and thin meninges

(membrane that surrounds the brain and spinal cord)

In one of narrow spaces, subarachnoid space, blood gradually reabsorbs CSF; brain has no receptors, but meninges do (inflammation is painful)

Cerebrospinal fluid cushions brain against mechanical shock when head moves; provides buoyancy; supports weight of brain; reservoir of hormones and nutrition for brain and spinal cord

If flow of CSF obstructed  accumulates within ventricles or in subarachnoid space, increasing pressure on brain

THE CEREBRAL CORTEX

Consists of cellular layers on outer surface of cerebral hemisphere; cells are grey matter, and axons extending inward are white matter
Neurons in each hemisphere communicate with neurons in other part of brain, through 2 bundles of axons: corpus callosum and anterior commissure; other commissures (path across midline), link subcortical structures
Cerebral cortex forms higher % in brain of primates, than other species of same size; as size devoted to forebrain increases, relative size of midbrain/medulla decrease.

Organization of the Cerebral Cortex

Microscopic structure of cells of cerebral cortex vary from one cortical area to next, correlate with differences in function
In humans and other mammals, cerebral cortex contains up to 6 distinct laminae (layers of cell bodies parallel to surface of cortex, separated by layers of fibre)
Vary in thickness and prominence between brain areas, can be absent
Lamina V: sends long axons to spinal cord and other distant areas; thickest in motor cortex (greatest control of muscles)
Lamina IV: receives axons from various sensory nuclei of thalamus is prominent in all primary sensory areas, absent in motor cortex
Cells of cortex organized into columns of cells perpendicular to the laminae; cells within a column have similar properties The Occipital Lobe
Posterior (caudal) end of cortex, main target for visual info.
Posterior pole of occipital lobe  primary visual cortex (striate cortex)  destruction causes cortical blindness in that part of visual field (ex: normal eyes and pupillary reflexes, but no conscious visual perception or visual imagery – not even dreams)
People with eye damage become blind, an intact occipital cortex and previous visual experience  imagine visual scenes
Eyes provide stimulus, and visual cortex provides the experience

The Parietal Lobe

Lies between occipital lobe and central sulcus (deepest grooves in surface of cortex); area just posterior to central sulcus; postcentral gyrus (primary somatosensory cortex)  receives sensations from touch, muscle-stretch, joint receptors
Postcentral gyrus includes 4 bands of cells parallel to central sulcus
Separate areas along each band receive simultaneous info from different parts of body; 2 receive mostly light-touch info; one receives deep pressure info; one receives a combo of both
Postcentral gyrus represents the body 4 times
Info about touch and body location important for interpreting visual and auditory info; parietal lobe monitors all info about eye, head, and body positions and passes it on to brain areas that control movement
Lobe not only essential for spatial info, also essential for numerical info The Temporal Lobe
Lateral portion of each hemisphere, near the temples
Primary cortical target for auditory info; left temporal lobe (human temporal lobe) essential to understand spoken language
Lobe contributes to complex aspects of vision  perception of movement, recognition of faces.
Tumour here can give rise to hallucinations (auditory/visual); tumour in occipital lobe involve simple sensations (flashes of light)
Important for emotional and motivational behaviour
Damage  Kluver-Bucy Syndrome (behavioural disorder; don’t experience normal fears and anxiety; possibly due to emotional/cognitive change) The Frontal Lobe
Primary motor cortex and prefrontal cortex; extends from central sulcus to anterior limit of brain
Posterior portion of frontal lobe just anterior to central sulcus is precentral sulcus (specialized for control of fine movements – moving one finger at a time)
Separate areas responsible for different parts of body, mostly on contralateral, sometimes slight control of ipsilateral side)
Prefrontal cortex: most anterior portion of frontal lobes; integrates much info

Modern View of the Prefrontal Cortex

Different parts perform different functions
Major function is working memory; ability to remember recent events

Damage to prefrontal cortex  trouble on delayed-response task (see/hear something, then have to respond after delaying)

Making decisions and planning movements; behaviors depending on context
Prefrontal cortex damage  fail to adjust context (behave inappropriately)

Rise and Fall of Prefrontal Lobotomies

Prefrontal lobotomy: surgical disconnection of prefrontal cortex to the rest of the brain  damaging prefrontal cortex/cutting connection with rest of cortex
Make patients tamer without impairing sensation or coordination
Consequences: apathy, loss of ability to plan, memory disorders, loss of emotion
Discontinued with the invention of psychotropic drugs How do the parts work together?
Binding problem: (large-scale integration) how various brain areas produce perception of a single object; association areas perform advanced processing on certain sensory system; few cells combine senses
Binding: occurs when perceive 2 sensations as happening at same time, in same place
Simultaneity of lights and sounds causes you to bind them and perceive an illusion
Binding depends on perceiving 2 or more aspects of stimulus as coming from approximately same location

RESEARCH METHODS Effects of Brain damage

Broca: damage to left frontal cortex; lose ability to speak
Reports of behavioural impairments after brain damage; inability to recognize faces or perceive motion, shift in attention to right side of body and world, increased/decreased hunger, change in emotional responses, memory impairments, etc.
Human studies have limitations: few people have damages confined to one area, and no 2 people have exact same damage
Produce localized damage in animals
Ablation: remove brain area (surgical knife)
Lesion: damage to inner brain area, with a stereotaxic instrument (device for precise placement of electrodes in brain, to damage a specific area)
Gene-knockout: biochemical methods to direct a mutation to particular gene that is important for certain types of cells, transmitters or receptors

Transcranial magnetic stimulation: apply intense magnetic field to a portion of the scalp, temporarily inactivates neurons below the magnet

After any damage/inactivation, need to specify exact behavioural deficit

Effects of Brain Stimulation

If brain damage impairs some behaviour, stimulation should increase it; insert electrodes to stimulate brain areas
Optogenetics: turn on activity in targeted neurons by device that shines laser light within brain
For humans, insert electrodes into already exposed brain OR apply magnetic field to scalp to stimulate brain areas beneath it (less invasive)
Limitation  complex behaviors/experiences depend on temporal pattern of activity across many brain areas not just general increase of activity in 1  Artificial stimulation  artificial response.

Recording Brain Activity

Damage to some brain area impairs behavior and stimulation of area increases behaviour
In animals, insert electrodes to record brain activity or record activity of many individual neurons at same time
EEG (electroencephalograph): records electrical activity of brain through electrodes attached to scalp that measure average activity at any moment of population of cells under it; output amplified and recorded;
Evoked potentials/responses: results of device recording spontaneous brain activity/activity response to
MEG (magnetoencephalograph): measures faint magnetic fields generated by brain activity; identifies approximate location of activity to close range; identify times at which various brain areas respond, trace a wave of brain activity to point of origin to all areas that process it
PET (positron-emission tomography): high-resolution image of activity in living brain by recording emission of radioactivity from injected chemicals
Receive injection of chemical containing radioactive atoms; when radioactive atom decays  releases positron that collides with nearby electron, emitting 2 gamma rays in opposite directions;
Head surrounded by gamma ray detectors; when 2 detectors record gamma rays at same time, identify a spot half-way between as point of origin; computer uses info to determine how much radioactive chemical is located in each area; ones with most radioactivity  most blood flow  most brain activity
fMRI (functional magnetic resonance imaging): PET scans replaced by fMRI’s (less expensive/risky); regular MRIs record energy released by H2O molecules after removal of magnetic field; fMRI based on haemoglobin (with O2 reacts to magnetic field differently then without O2)
When brain area active; blood vessels dilate to allow more blood flow; as brain area uses O2, % of haemoglobin without oxygen increases
Complex to interpret; given area can have many functions; if we think we know what an fMRI pattern means, use pattern to identify what someone is doing or thinking

Correlating Brain Anatomy with Behaviour

Early ways to study brain function was to find someone with unusual behavior and look for unusual features in brain
Phrenology: relating skull anatomy to behaviour; problem  lack of sample size, skull shape doesn’t relate to brain anatomy today, examine detailed brain anatomy
CAT (computerized axial tomography): inject dye to blood, place head in CT scanner; X-rays passed through head  recorded by detectors on opposite side; scanner rotated slowly until measurement taken at each angle over 180 degrees. From measurements, construct image of brain
MRI (magnetic resonance imaging): atom with odd atomic weight has axis of rotation; MRI applies powerful magnetic field to align all axes of rotation  tilts with brief radio frequency field; when radio turned off, atomic nuclei release electromagnetic energy as they relax and return to original axis; by measuring energy, MRI forms image of brain; shows small anatomical details; problem  need to lie still (motionless)

Brain Size and Intelligence

 Brain size isn’t strongly related to intelligence

Comparisons Across Species

All mammal brains have same organization, but differ in size
Humans don’t have largest brains; intelligence may rely on brain-to-body ratio

(ex: humans have largest brains in proportion to our body  most intelligent) Problem: 1) brain-to-body ratio not accurate 2) lack meaning of animal intelligence

Comparisons among Humans

Studies of brain size/intelligence  correlations barely above 0, based on skull size, not measured well; today, correlation ~0.3 between brain size/intelligence
Certain brain areas more important than others for intelligence
Problems with this research  if record all brain areas during task, evidence may confirm some by chance and task may activate different parts in different people because they approach task differently Comparisons of Men and Women
Men have on average bigger brains that women, but equal IQs
Although male and female brains differ, behavioral differences small
In countries where men/women have equal opportunities, perform same on tests
Women have deeper sulci on the surface of cortex, especially in frontal and parietal area; surface area of cortex equal for men and women; because surface lined with neurons, sexes have same number of neurons despite differences in brain volume but intelligence also depends on white matter
Conclusion: men and women’s brain structurally different, accomplish same thing
Human NS requires a lot of assembly, “axons and dendrites here, keep connections that work, discard others, make new connections, keep only the successful ones”
Brain’s anatomy is plastic: changes rapidly in early development, continues changing throughout life