The Central Nervous System

Central nervous system (CNS)

includes brain and spinal cord; involved in movement, interpreting sensory information, maintaining homeostasis, and functions relating to mind

1. Motor functions 

1. include stimulation of a muscle cell contraction or a gland secretion; function of peripheral nervous system (PNS)

1. Sensory functions 

1. detection of sensations within and outside body; also is a function of PNS

1. Integrative functions 

1. include decision-making processes; exclusive function of CNS; includes a wide variety of functions:

Interpretation of sensory information

Planning and monitoring movement

Maintenance of homeostasis

Higher mental functions such as language and learning

Brain 

soft, whitish-gray organ, anatomically continuous with spinal cord; resides in cranial cavity and directly or indirectly controls most of body’s functions

Weighs between 1250 and 1450 grams; made of mostly nervous tissue; contains epithelial and connective tissues as well Internal cavities called ventricles; filled with cerebrospinal fluid (CSF)

Receives about 20% of total blood flow during periods of rest; reflects its requirements for huge amounts of oxygen, glucose, and nutrients

Cerebrum

enlarged superior portion of brain; divided into left and right cerebral hemispheres Each cerebral hemisphere is further divided into five lobes containing groups of neurons that perform specific tasks

Responsible for higher mental function such as learning, memory, personality, cognition (thinking), language, and conscience

Performs major roles in sensation and movement as well

Diencephalon

deep underneath cerebral hemispheres; central core of brain

Consists of four distinct structural and functional parts

Responsible for processing, integrating, and relaying information to different parts of brain, homeostatic functions, regulation of movement, and biological rhythms

Cerebellum

posterior and inferior portion of brain

Divided into left and right hemispheres 

Heavily involved in planning and coordination of movement, especially complex activities such as playing a sport or an instrument

Brainstem

connects brain to spinal cord

Involved in basic involuntary homeostatic functions

Control of certain reflexes

Monitoring movement

Integrating and relaying information to other parts of nervous system

Spinal cord

 long tubular organ enclosed within protective vertebral cavity; blends with inferior portion of brainstem; ends between first and second lumbar vertebrae

43–46 cm (17–18 inches) in length and only ranges from 0.65–1.25 cm (0.25–0.5 inches) in diameter

Central canal – an internal cavity within spinal cord that is continuous with brain’s ventricles; filled with cerebrospinal fluid (CSF)

White matter

 found in both brain and spinal cord; consists of myelinated axons 

Each lobe of cerebrum contains bundles of white matter called tracts; receives input from and sends output to clusters of cell bodies and dendrites in cerebral gray matter called nuclei 

Spinal cord contains white matter tracts that shuttle information processed by nuclei in spinal gray matter

Gray matter

 found in both brain and spinal cord; consists of neuron cell bodies, dendrites, and unmyelinated axons

Outer few millimeters of cerebrum is gray matter; deeper portions of brain are mostly white matter with some gray matter scattered throughout

Spinal cord is mostly gray matter that processes information (in cord center); surrounded by tracts of white matter (outside); relays information to and from brain

Gray matter

Communication between gray and white matter connects different regions of brain and spinal cord with one another; myelinated axons enable near instantaneous communication between locations

Make note – organization of gray and white matter in brain and spinal cord is reversed; spinal white matter is superficial while it is deep in brain

Cerebrum (Sulci)

shallow grooves on surface of cerebrum; 

Gyri (Cerebrum)

elevated ridges found between sulci; 

together increase surface area of brain; maximizing limited space within confines of skull

Fissures (Cerebrum)

deep grooves found on surface of cerebrum

Longitudinal fissure (Cerebrum) 

long deep groove that separates left and right cerebral hemispheres

A cavity is found deep within each cerebral hemisphere; right hemisphere surrounds right lateral ventricle; left hemisphere surrounds left lateral ventricle

Frontal lobes

most anterior lobes

Posterior border – called central sulcus; sits just behind precentral gyrus 

Neurons in these lobes are responsible for planning and executing movement and complex mental functions such as behavior, conscience, and personality

Parietal lobes

 just posterior to frontal lobes

Contains postcentral gyrus posterior to central sulcus

Neurons in these lobes are responsible for processing and integrating sensory information and function in attention

Temporal lobes 

form lateral surfaces of each cerebral hemisphere 

Separated from parietal and frontal lobes by lateral fissure

Neurons in these lobes are involved in hearing, language, memory, and emotions 

Occipital lobes

make up posterior aspect of each cerebral hemisphere 

Separated from parietal lobe by parieto-occipital sulcus

Neurons in these lobes process all information related to vision

Insulas 

deep underneath lateral fissures; neurons in these lobes are currently thought to be involved in functions related to taste and viscera (internal organs)

Gray Matter: Cerebral Cortex 

functionally most complex part of cortex; covers underlying cerebral hemispheres 

Most of cerebral cortex is neocortex (most recently evolved region of brain); has a huge surface area

Composed of 6 layers (of neurons and neuroglia) of variable widths 

All neurons in cortex are interneurons

Functions of neocortex revolve around conscious processes such as planning movement, interpreting incoming sensory information, and complex higher functions

Neocortex is divided into three areas: primary motor cortex, primary sensory cortices, and association areas

Primary motor cortex

 plans and executes movement

Primary sensory cortices

 first regions to receive and process sensory input

Association areas integrate different types of information: 

Unimodal areas 

 integrate one specific type of information 

Multimodal areas

 integrate information from multiple different sources and carry out many higher mental functions

Motor areas

most are located in frontal lobe; contain upper motor neurons which are interneurons that connect to other neurons (not skeletal muscle)

Primary motor cortex

 involved in conscious planning of movement; located in precentral gyrus of frontal lobe

Upper motor neurons of each cerebral hemisphere control motor activity of opposite side of body via PNS neurons called lower motor neurons; execute order to move

premotor cortex

Movement requires input from many motor association areas such as large … located anterior to primary motor cortex

Motor association areas

are unimodal areas involved in planning, guidance, coordination, and execution of movement

Frontal eye fields

paired motor association areas; one on each side of brain anterior to premotor cortex; involved in back and forth eye movements as in reading

Primary somatosensory area (S1), Somatosensory association cortex (S2) 

Two main somatosensory areas in cerebral cortex; deal with somatic senses; information about temperature, touch, vibration, pressure, stretch, and joint position 

Primary somatosensory area (S1)

in postcentral gyrus of parietal lobe 

Somatosensory association cortex (S2)

 posterior to S1

Classic senses

 touch, vision, hearing, smell, and taste each have a primary and a unimodal association area as does sense of equilibrium (balance); found in all lobes of cortex except frontal lobe

Primary visual cortex

 at posterior end of occipital lobe; first area to receive visual input; transferred to visual association area which processes color, object movement, and depth

Primary auditory cortex 

 in superior temporal lobe; first to receive auditory information; input is transferred to nearby auditory association cortex and other multimodal association areas for further processing

Gustatory cortex

taste information processing; scattered throughout both insula and parietal lobes

Vestibular areas 

deal with equilibrium and positional sensations; located in parietal and temporal lobes

Olfactory cortex

 processes sense of smell; in evolutionarily older regions of brain; consists of several areas in limbic and medial temporal lobes

Language processed in two areas of cortex: 

Broca’s area, Wernicke’s area (integrative speech area)

Broca’s area

 in anterolateral frontal lobe; premotor area responsible for ability to produce speech sounds

Wernicke’s area (integrative speech area)

 in temporal and parietal lobes; responsible for ability to understand language

Prefrontal cortex

 occupies most of frontal lobe; communicates with diencephalon, other regions of cerebral gray matter, and association areas located in other lobes; many functions including modulating behavior, personality, learning, memory, and an individual’s personality state

Parietal and temporal association areas 

occupy most of their respective lobes; perform multiple functions including integration of sensory information, language, maintaining attention, recognition, and spatial awareness

Basal nuclei

found deep within each cerebral hemisphere; cluster of neuron cell bodies, involved in movement; separated from diencephalon by a region of white matter called internal capsule; includes:

Caudate nuclei 

Putamen

Globus pallidus 

Caudate nuclei

 C-shaped rings of gray matter; lateral to lateral ventricle of each hemisphere with anteriorly oriented tail

Putamen

 posterior and inferior to caudate nucleus; connected to caudate nucleus by small bridges of gray matter; combination of putamen and caudate are sometimes called corpus striatum

Globus pallidus

sits medial to putamen; contains more myelinated fibers than other regions

Commissural fibers

connect right and left hemispheres and are horizontally running; corpus callosum, largest of four groups in this category, lies in middle of brain at base of longitudinal fissure

Projection fibers

Vertically running fibers that connect cerebral cortex of one hemisphere with other areas of same hemisphere (ascending/descending), other parts of brain, and spinal cord. 

Corona radiata

are fibers that spread out in a radiating pattern; condense around diencephalon to form two V-shaped bands called internal capsules

Association fibers

restricted to a single hemisphere; connect gray matter of cortical gyri with one another.  Horizontally running, long association fibers are bundled into tracts and connect different cortical lobes.

Possible pathway

1. Action potential originates in gray matter

2. Action potential is sent to another area of gray matter by projection fibers

3. Second (new) action potential is generated by gray matter; spreads to neighboring gray matter by association fibers

4. Lastly, a third action potential is generated; can be sent to other cerebral hemisphere by commissural fibers

Limbic system 

important functional brain system, includes limbic lobe (region of medial cerebrum), hippocampus, amygdala, and pathways; connect each of these regions of gray matter with rest of brain

Found only within mammalian brains 

Involved in memory, learning, emotion, and behavior

Limbic lobe

 and associated structures form a ring on medial side of cerebral hemisphere; contain two main gyri: cingulate gyrus and parahippocampal gyrus

Hippocampus 

 in temporal lobe; connected to a prominent C-shaped ring of white matter (fornix) which is its main output tract; involved in memory and learning

Amygdala

 anterior to hippocampus; involved in behavior and expression of emotion, especially fear

Diencephalon 

at physical center of brain; each with its own nuclei that receive specific input and send output to other brain regions:

1. Thalamus

2. Hypothalamus

3. Epithalamus

1. Thalamus

1. main entry route of sensory data into cerebral cortex

Consists of two egg-shaped regions of gray matter; make up about 80% of diencephalon

Third ventricle is found between these two regions 

Thalamic nuclei receive afferent fibers from many other regions of nervous system excluding information about the sense of smell

1. Hypothalamus

1. collection of nuclei anterior and inferior to larger thalamus

Neurons perform several vital functions critical to survival; include regulation of autonomic nervous system, sleep/wake cycle, thirst and hunger, and body temperature

Inferior hypothalamus 

anatomically and functionally linked to pituitary gland by an extension called infundibulum; hypothalamic tissue makes up posterior portion of this endocrine gland

Hypothalamus secretes a number of different releasing and inhibiting hormones; affect function of pituitary gland; in turn, pituitary gland secretes hormones that affect activities of other endocrine glands throughout body

Mammillary bodies 

connect hypothalamus with limbic system; receive input from hippocampus; involved in memory regulation and behavior

Input from outside nervous system; endocrine system (among others) provides information from receptors that detect changes in body temperature and receptors that detect changes in osmotic concentration of blood

1. Epithalamus

1. superior to thalamus; most of its posterosuperior bulk is an endocrine gland called pineal gland; secretes melatonin; hormone involved in sleep/wake cycle

Cerebellum 

makes up posterior and inferior portion of brain; functionally connected with cerebral cortex, basal nuclei, brainstem, and spinal cord; interactions between these regions together coordinate movement

Divided into three lobes:

1. Anterior

2. Posterior

3. flocculonodular lobes

Cerebellar cortex 

outer layer of gray matter

Cortex is extremely folded and branching white matter is called arbor vitae because it resembles tree branches

cerebellar peduncles

Inner white matter contains clusters of gray matter (deep cerebellar nuclei) scattered throughout

White matter converges into three large tracts called … only connection between cerebellum and brainstem

Brainstem

one of oldest components of brain; vital to our immediate survival as its nuclei control many basic homeostatic functions such as heart rate and breathing rhythms

Controls many reflexes (programmed, automatic responses to stimuli); functions in movement, sensation, and maintaining alertness

Superior and inferior colliculi

protrude from posterior surface of brainstem; two paired projections that form roof of midbrain (tectum); involved in visual and auditory functions respectively; project to thalamus

Descending tracts 

white matter tracts that originate in cerebrum and form anteriormost portion of midbrain; called crus cerebri

Pons

 inferior to midbrain; has a prominent anterior surface that contains descending motor tracts from crus cerebri, some of which pass through pons en route to spinal cord

Other tracts enter cerebellum by way of middle cerebellar peduncle

Reticular formation and cranial nerve nuclei are located posterior to these tracts

Medulla oblongata

 most inferior structure of brainstem; continuous with spinal cord at foramen magnum

Pyramids

on anterior surface of medulla, contain upper motor neuron fibers of corticospinal tract (also called the pyramidal tract) as they travel from cerebral cortex to spinal cord

1. Cranial meninges

1. three layers of membranes that surround brain

1. Cerebrospinal fluid (CSF)

1. fluid that bathes brain and fills cavities

1. Blood-brain barrier 

1. prevents many substances from entering brain and its cells from blood

Cranial meninges

composed of three protective membrane layers of mostly dense irregular collagenous tissue

Structural arrangement from superficial to deep: epidural space, dura mater, subdural space, arachnoid mater, subarachnoid space, and pia mater

Epidural space

 between inner surface of cranial bones and outer surface of dura mater; only a potential space as dura is normally tightly bound to bone only allowing for passage of blood vessels

Dura mater (dura)

 outermost meninx; thickest and most durable of three meningeal layers; double-layered membrane composed mostly of collagen fibers with few elastic fibers

Subdural space

serous fluid-filled space; found deep to dura mater and superficial to arachnoid mater; houses veins that drain blood from brain

Arachnoid mater 

second meningeal layer deep to subdural space; thin weblike membrane composed of dense irregular collagenous tissue with some degree of elasticity

Subarachnoid space

found deep to arachnoid mater and superficial to pia mater; contains major blood vessels of brain; filled with CSF

Pia mater

 deepest meningeal layer; only meninx in physical contact with brain tissue

Follows contour of brain, covering delicate tissue of every sulcus and fissure

Permeable to substances in brain extracellular fluid and CSF; allows for substances to move between these two fluid compartments; helps to balance concentration of different solutes found in each fluid

ependymal cells, cerebrospinal fluid

Four ventricles within brain are linked cavities that are continuous with central canal of spinal cord 

Lined with …

Filled with …

Right and left lateral ventricles (first and second ventricles); within their respective cerebral hemisphere:

Resemble ram’s horns when observed in anterior view; horseshoe-shaped appearance in lateral view

Three regions: anterior horn, inferior horn, and posterior horn

Third ventricle

narrow cavity found between two lobes of diencephalon; connected to lateral ventricles by interventricular foramen

Fourth ventricle

 between pons and cerebellum; connected to third ventricle by cerebral aqueduct (small passageway through midbrain)

Continuous with central canal of spinal cord

Contains several posterior openings that allow CSF in ventricles to flow into subarachnoid space

Cerebrospinal fluid (CSF)

 clear, colorless liquid similar in composition to blood plasma; protects brain in following ways:

Cushions brain and maintains a constant temperature within cranial cavity

Removes wastes and increases buoyancy of brain; keeps brain from collapsing under its own weight

Choroid plexuses 

where majority of CSF is manufactured; found in each of four ventricles where blood vessels come into direct contact with ependymal cells (also produce some CSF themselves)

Blood-brain barrier 

 protective safeguard that separates CSF and brain ECF from chemicals and disease-causing organisms sometimes found in blood plasma

Consists mainly of simple squamous epithelial cells (endothelial cells) of blood capillaries, their basal laminae, and astrocytes

tight junctions;

Unique features of endothelial cells found in barrier:

Neighboring endothelial cells are bound together by many … prevent fluids and molecules from passing between them; influenced by activity of astrocytes on developing brain

Limited capacity for movement of molecules and substances into and out of cell by endocytosis and exocytosis

Spinal cord 

composed primarily of nervous tissue; responsible for both relaying and processing information; less anatomically complex than brain but still vitally important to normal nervous system function 

Spinal cord has two primary roles:

1. relay station, Processing station

1. relay station

1. Serves as a … and as an intermediate point between body and brain; only means by which brain can interact with body below head and neck

1. Processing station

1. … for some less complex activities such as spinal reflexes; do not require higher level processing

spinal meninges

Three … include dura mater, arachnoid, and pia mater; structurally similar to brain meninges except that spinal cord dura has only one layer and pia mater has some structural enhancements

Actual or potential spaces between spinal cord meninges are same as those found between cranial meninges with following features:

Epidural space

actual space due to absence of a periosteal dura; found between meningeal dura and walls of vertebral foramina; space is filled with veins and adipose tissue; cushions and protects spinal cord

Subdural space

only a potential space much like epidural space surrounding brain; dura and arachnoid are normally adhered to one another

Subarachnoid space

found between arachnoid and pia mater; filled with CSF; base of spinal cord contains a large volume of CSF; useful site for withdrawing samples for clinical laboratory testing