The Central Nervous System

Brain – superiorly

Spinal Cord - inferiorly

Figure 12.3

4 Protective Structures of the Brain

• Cranium – bony helmet composed of the 8 cranial bones – frontal, parietal(paired), temporal(paired), occipital, sphenoid and the ethmoid bones

• Meninges = 3 connective membranes surrounding the brain:

Dura mater – outermost meninx; double-layered – outer periosteal layer ining the internal surface of the cranium and the inner meningeal layer separated from the underlying arachnoid mater by the SUDURAL space

Arachnoid mater – middle meninx separated from the underlying pia mater by the SUBARACHNOID space. Weblike extensions from the archnoid mater to the subarachnoid space gives this meninx its name ( Arachnida = spider family)

The suabarchnoid space contains CSF

Pia mater – innermost meninx that clings to the surface of the brain

• Cerebrospinal fluid ( CSF) – filtered from blood; located in the ventricles and also in the subarachnoid space hence, CSF is found inside and outside of the brain acting as a “liquid” cushion; provides buoyancy to the brain; provides nutrients; removes metabolic wastes

• Blood-Brain barrier – a selective barrier that prevents harmful substance in blood from crossing to the brain.

Figure 12.24

Figure 12.24b

Figure 12.5

Figure 12.26

Figure 12.27

Figure 12.12

The Brain• Weight: 3.5Ib ( 1600 g)

• 4 major regions:

• Cerebrum

• Diencephalon

• Brainstem

• Cerebellum

The Cerebrum• Superior region of the brain – accounts for 83% of total brain mass • Surface marked by elevated ridges called GYRI and shallow

grooves called SULCI; deeper grooves are called FISSURES• A median fissure called the LONGITUDINAL FISSURE divides the

cerebrum into right and left cerebral hemispheres.• 2 cerebral hemispheres are held together medially by the CORPUS

CALLOSUM• Each cerebral hemisphere is divided into 5 lobes:

_ Frontal lobe

– Temporal lobe– Parietal lobe– Occipital lobe– Insula

The frontal, temporal, parietal and occipital lobes can be viewed externally and are named for the overlying cranial bones

The insula can not be viewed externally and located deep to the lateral sulcus, covered by the frontal, temporal and parietal lobes

Figure 12.6c, d

Figure 12.6a, b

Figure 12.7

The Sulci in the cerebral hemispheres

• Central Sulcus – separates the frontal lobe from the parietal lobe. The gyrus in the frontal lobe located immediately in front of the central sulcus is called the PRECENTRAL GYRUS; the gyrus in the parietal lobe immediately behind the central sulcus is called the POSTCENTRAL GYRUS

• Lateral Sulcus – separates the temporal lobe from the parietal and frontal lobes

• Parieto-occipital Sulcus – separates the parietal lobe from the occipital lobe

Figure 12.6a

Figure 12.8a

3 regions in each Cerebral hemisphere

• Cerebral Cortex – highly convoluted and 2-4 mm thick; accounts for 40% of total brain mass; composed of gray matter = cell bodies, dendrites; location of our conscious mind

• Cerebral White Matter – deep to the cerebral cortex; composed of tracts with myelinated axons which have a “whitish” appearance

• Basal Nuclei – islands of nuclei(clusters of neuron cell bodies) within the cerebral White Matter

Figure 12.11b

3 Functional areas in the Cerebral Cortex

• Motor areas – control voluntary movements.

consist of the primary motor cortex, Premotor cortex, Broca’s area,

frontal eye field. All located in the frontal lobes

• Sensory areas – for the conscious awareness of sensation; consist of the Primary somatosensory cortex, Primary visual cortex, Primary auditory cortex, Primary olfactory cortex,

Primary gustatory cortex

• Association areas – integrate and interpret sensory inputs from the sensory areas hence, each primary sensory area above has an associated area

3 Functional areas in the Cerebral Cortex

The Cerebral Gray Cortex – Motor areas• Consists of 4 functional areas:• Primary motor cortex – Controls the voluntary movements of skeletal

muscles; located in the precentral gyrus of the frontal lobe in each cerebral hemisphere where the large neurons called PYRAMIDAL CELLS are located. The axons of the pyramidal cells bundle to form the PYRAMIDAL or CORTICOSPINAL TRACTS which cross over on the ventral side of the medulla oblongata (DECUSSATION OF THE PYRAMIDS) – this explains the contralateral control of voluntary movements of skeletal muscles by the cerebral hemispheres: voluntary movements on the left side of the body are controlled by the right cerebral hemisphere and voluntary movements on the right side are controlled by the left hemisphere

• Premotor Cortex – controls learned motor skills that are patterned or repetitious such as typing

• Broca’s Area – controls skeletal muscles involved in speech production hence, referred to as the “motor speech area”. Present in only the frontal lobe in the left cerebral hemisphere. Arcuate fasciculate connects the Broca’s area to the Wernicke’s area located in the left temporal lobe to produce language

• Frontal Eye Field – controls voluntary movements of the skeletal muscles that position the eyes

Decussation of the Pyramids

Cerebral White Matter

Cerebral White Matter• Second region in the cerebrum deep to the cerebral

cortex; consists of myelinated tracts• 3 types of tracts based on direction:• Commissural tracts = Commissures – connect corresponding

areas in the two cerebral hemispheres. Corpus callosum is a commissure that connects the right and left cerebral hemispheres.

• Projection tracts – connect the cerebrum to lower brain areas and the spinal cord. Pyramidal or corticospinal tracts are projection tracts

• Association tracts – connect areas within the same cerebral hemisphere. Arcuate fasciculate is an association tract

Basal Nuclei

Projection tracts

Basal Nuclei• Islands of gray matter in the cerebral white matter• 3 major Basal Nuclei:• Caudate nucleus• Putamen• Globus pallidus

– Putamen + globus pallidus = LENTIFORM NUCLEUS– Lentiform nucleus + caudate = CORPUS STRIATUM

• Basal nuclei are associated with nuclei in the brain stem and are involved in initiating and stopping movements; they inhibit unnecessary movements

Ventricles

Ventricles• Cavities in the brain that contain CSF• Lateral ventricles – each cerebral hemisphere contains a lateral

ventricle; the 2 lateral ventricles are connected by a median membrane called the septum pellucidum; 2 lateral ventricles are connected to the third ventricle below by a channel called the interventricular foramen.

• Third ventricle – located in the diencephalon; connected to the fourth ventricle below via the cerebral aqueduct

• Fourth ventricle – located in the brain stem

Diencephalon

Diencephalon• Consists of 3 paired structures:• Thalamus – the relay station for sensory inputs to the cerebral cortex

hence, the thalamus is referred to as the “Gateway to the cerebral cortex”. Visual relay center in the thalamus is the lateral geniculate nucleus (LGN); auditory relay center is the medial geniculate nucleus (MGN). Thalamus contains the third ventricle

• Hypothalamus – located below the thalamus; controls– Activities of the Autonomic Nervous System

– Emotional response

– Core body temperature as it contains the body’s thermostat

– Food intake as it contains the satiety center

– Water intake as it contains the thirst center

– Sleep-wake cycles

– Endocrine function as it produces 9 hormones

• Epithalamus – forms the roof of the third ventricle; contains the pineal gland, an endocrine gland that secretes the hormone, melatonin, the sleep-inducing chemical.

Brain Stem

Figure 12.15c

Table 12.1.1

Figure 12.14

Brain Stem• Composed of an outer white matter and an inner gray matter • Consists of 3 regions:• Midbrain – contains the cerebral aqueduct; corpora quadrigemina = 4 nulei called

colliculi – 2 superior colliculi act as the visual reflex center and the 2 inferior colliclui act as auditory reflex center; CN III, IV issue from nuclei in the midbrain.

Midbrain contains 2 pigmented nuclei – red nuclei and the substantia nigra

Red nuclei regulate limb flexion; substantia nigra contain dopamine-releasing neurons (=dopaminergic hormone) which project and modulate activities of the basal nuclei – degeneration of these dopaminergic neurons to the basal nulei causes Parkinson’s disease

• Pons – located between the midbrain and the medulla oblongata; 3 pairs of cranial nerves issue from nuclei in the pons ( CN V, VI, VII); connect the motor cortex and the cerebellum hence, involved in the control of skeletal muscle movements. Contains respiratory centers

• Medulla oblongata – most inferior region that blends in with the spinal cord at the level of the foramen magnum. Decussation of the pyramids on its ventral surface,; CN VII, VIII, IX, X, XI, XII issue from the medulla.

Contains autonomic reflex centers: Cardiovascular center, respiratory center, emetic center, swallowing center, coughing reflex center

Cranial Nerves III - XII

Figure 12.15b

2 pigmented nuclei in the Midbrain

CEREBELLUM

The Arbor Vitae

Cerebellum• Composed of outer gray matter and an inner white matter; the

cerebellar inner white matter resembles a branching tree referred to as the “Arbor Vitae”.

• Cerebellum consists of two cerebellar hemispheres connected medially by the VERMIS

• Each cerebellar hemisphere is composed of three lobes:

– Anterior lobe– Posterior lobe– Flocculonodular lobe – cannot be viewed on the external surface

of the cerebellum• Function: For posture, balance(equilibrium) and smooth,

coordinated skeletal muscle movements

• Cerebellar function is affected by alcohol intoxication hence tandem gait which indicates proper balance, is lost

Table 12.1.2

The Spinal Cord

The Spinal Cord

The Spinal Cord• Second part of the CNS inferior to the brain• Extends from the foramen magnum to the level of the first lumbar

vertebra where it terminates in a tapering cone-shaped structure called CONUS MEDULLARIS

• Contains two enlargements – cervical and lumbar enlargements

• The length of the spinal cord = 42cm(17in) long

• Composed of an outer white matter and an inner gray matter which appears like the letter “H” or a butterfly in cross-section; the CENTRAL CANAL passes through the center of the gray matter

• 31 pairs of spinal nerves exit the spinal cord.

• Collection of nerve roots inferior to the conus medullaris is called the CAUDA EQUINA

3 Protective structures of the Spinal Cord

3 protective structures of the Spinal Cord• Vertebral column – forms a bony encasement

• Spinal Meninges – 3 types: Spinal dura mater = spinal dural sheath –outer meninx; single-layered and does not line the internal surface of the vertebrae; space between the vertebral column and the spinal dural sheath is called the EPIDURAL SPACE filled with fat and veins; anesthesia to block pain is injected into the epidural space

Arachnoid mater – middle meninx; separated from the spinal dural sheath by the subdural space; separated from the pia mater by the subarachnoid space which contains CSF

Pia mater – inner meninx; forms the DENTICULATE LIGAMENTS which secure the spinal cord laterally to spinal dural sheath; covers the FILUM TERMINALE - a fibrous extension of the conus medullaris - which anchors the spinal cord to the coccyx.

• Cerebrospinal fluid(CSF) – located in the central canal and the subarachnoid space acting as a liquid cushion, provides nutrients and removes metabolic wastes

Figure 12.29c

Figure 12.29d

Cervical and Lumbar enlargements

• Cervical enlargement – located in the cervical region; spinal cervical nerves from the cervical enlargement control voluntary movements of the upper limbs.

• Transection of the spinal cord in/above the cervical enlargement leads to QUADRIPLEGIA – loss of voluntary movements of all four limbs.

• Lumbar enlargement – located around lumbar region; nerves from the lumbar enlargement control voluntary movements of the lower limbs.

• Transection of the spinal cord below the cervical enlargement but above the lumbar enlargement leads to PARAPLEGIA – loss of voluntary movements of the lower limbs.

Lumbar Tap

Lumbar Tap• CSF is taken from the subarachnoid space

for analysis, if meningitis is suspected.

• The lumbar tap is performed around the fifth lumbar vertebra, definitely below the spinal cord to avoid potential damage to the spinal cord.

Gray Matter and White Matter

The Gray Matter and the White Matter of the Spinal Cord

• The inner GRAY MATTER is “H”-shaped or butterfly-shaped; the two masses of gray matter are connected by the gray commissure, which surrounds the central canal.

• Consists of the DORSAL HORNS, VENTRAL HORNS and the LATERAL HORNS

• Dorsal Horns – consist of interneurons

• Ventral Horns – house the cell bodies of somatic motor neurons

• Lateral Horns – house sympathetic neurons that innervate organs

• The outer WHITE MATTER – composed of mostly myelinated axons that allow communication between different parts of the spinal cord and between the spinal cord and the brain

• 3 types of fiber tracts:

– Ascending tracts – run vertically; sensory inputs from sensory receptors to the spinal cord and then to the brain; the axons decussate and the left cerebral hemisphere receives sensory inputs from the right side of the body and vice versa

– Descending tracts – run vertically; motor output coming down from the brain or within the cord to lower levels

– Transverse tracts = commissural tracts – run horizontally from one side of the spinal cord to the other

Figure 12.32

Ascending Tracts - Sensory inputs

Table 12.2.2

Descending Tracts – Motor output

Table 12.3.1

Terminology

• Paresthesia – sensory loss

• Paralysis – loss of motor function

• Flaccid paralysis – indicates damage to the ventral horns in the spinal cord

• Spastic paralysis – damage to the precentral gyri which results in hemiplegia, paralysis on one side of the body; damage to the left precentral gyrus results in right side paralysis and vice versa

Amyotrophic Lateral Sclerosis (ALS) = Lou Gehrig’s disease

• Involves progressive destruction of ventral horn motor neurons and the pyramidal tracts – loss of skeletal muscle movements = loses the ability to walk or move the arms( quadriplegia), to speak, to swallow and to breathe

(due to paralysis of the diaphragm)

Lumbar Myelomeningocele – Spina Bifida cystica – incomplete formation of the vertebral arches