The Central Nervous System

12 The Central Nervous System

The Nervous System Central Nervous System (CNS) Peripheral Nervous System (PNS) Brain Motor Neurons Sensory Neurons Spinal Cord Somatic Nervous System Autonomic Nervous System Sympathetic Parasympathetic

CNS protection

CSF – cerebrospinal fluid • Liquid cushion for brain and spinal cord • Nourishes brain • Removes waste • Conducts chemical signals between parts of CNS • Produced in Choroid Plexuses: group of capillaries surrounded by ependymal cells AP revealed – CSF flow

Choroid Plexuses http://cal.man.ac.uk/student_projects/2002/MNQJ9PP2/Webpages/structure&functionchoroid.htm • Clusters of capillaries that form tissue fluid filters, which hang from the roof of each ventricle • Have ion pumps that allow them to alter ion concentrations of the CSF

CSF production • CSF is formed by the choroid plexus present within the four cerebral ventricles and a small amount by the ependymal cells • the choroid plexus of the lateral ventricles producing the most. • The rate of formation is approximately 0.35 ml/min or 500 ml/day; a rate which replaces the total volume of CSF approximately 2-3 times over in 24 hours.

Blood-Brain Barrier • Helps maintain a stable environment for the brain • Separates neurons from some bloodborne substances • Composition • Continuous endothelium of capillary walls • Basal lamina • Feet of astrocytes • Provide signal to endothelium for the formation of tight junctions

Blood-Brain Barrier: Functions • Selective barrier that allows nutrients to pass freely • Is ineffective against substances that can diffuse through plasma membranes (ex. Ethanol, caffeine) • Absent in some areas: • Ex. - hormones generally do not penetrate the brain from the blood, so in order to control the rate of hormone secretion effectively, there are specialized sites where neurons can "sample" the composition of the circulating blood. At these sites, the blood-brain barrier is 'leaky‘ (pituitary gland) • Capillaries of the choroid plexus • The BBB can break down under certain conditions: • hypertension, radiation, infection and brain trauma

The Brain • 4 Parts • Cerebrum • Diencephalon • Brain Stem • Cerebellum pg 348

Cerebral Cortex • The cortex – superficial gray matter; accounts for 40% of the mass of the brain • It enables sensation, communication, memory, understanding, and voluntary movements • Each hemisphere acts contralaterally (controls the opposite side of the body) • Hemispheres are not equal in function • No functional area acts alone; conscious behavior involves the entire cortex

Functional Areas of the Cerebral Cortex • The three types of functional areas are: • Motor areas – control voluntary movement • Sensory areas – conscious awareness of sensation • Association areas – integrate diverse information

Cerebral Cortex: Motor Areas; Frontal Lobe Cortical Regions • Controls voluntary movement and found in the posterior part of the frontal lobe • Primary Motor cortex • Conscious motor control • Motor homonculus • Premotor Cortex • Learned motor skills

Cerebral Cortex: Motor Areas; Frontal Lobe Cortical Regions • Broca’s area • Motor for speech • Frontal Eye Field • Prefrontal Cortex • Personality • Intelligence • Language comprehension

Primary Motor Cortex • Located in the precentral gyrus in the frontal lobe in each hemisphere • Pyramidal cells that have long axon project to the spinal cord and form a voluntary motor tracts called pyramidal tracts/corticospinal tracts • A pyramidal cell (or pyramidal neuron, or projection neuron) is a multipolar neuron found in the cerebral cortex. • These cells have a triangularly shaped soma • Pyramidal neurons compose approximately 80% of the neurons of the cortex • Release glutamate as their neurotransmitters, making them the major excitatory component of the cortex • Allows conscious control of precise, skilled, voluntary movements

Posterior Motor Anterior Motor map in precentral gyrus Toes Jaw Primary motor cortex (precentral gyrus) Tongue Swallowing Figure 12.9 Primary Motor Cortex Homunculus • Somatotopy mapping • Body is represented upside down • Although simplified in the figure, one should remember that: • A given muscle is controlled by multiple spots on the cortex • Individual cortical neurons send impulses to more than one muscle • Neurons that control related movements will overlap • Neurons that control unrelated movements do not cooparate Figure 12.9.1

Premotor Cortex • Located anterior to the precentral gyrus in the frontal lobe • Controls learned, repetitious, or patterned motor skills (playing a musical instrument) • Coordinates the movements of muscle groups either for simultaneous or sequential actions • mainly by sending activating impulses to the primary motor cortex • also by controlling directly by supplying 15% of the pyramidal tract fibers • Involved in the planning of movements by receiving sensory information and process them (moving arm through maze to take hidden object)

Broca’s Area • Located anterior to the inferior region of the premotor area • Present in one hemisphere (usually the left) • A motor speech area that directs muscles of the tongue • Is active as one prepares to speak • Frontal eye field • Located anterior to the premotor cortex and superior to Broca’s area • Controls voluntary eye movement

Prefrontal Cortex • Located in the anterior portion of the frontal lobe • Involved with intellect, cognition (complex learning activities), recall and personality • Necessary for judgment, reasoning, persistence, and conscience • Closely linked to the limbic system (emotional part of the brain)

Sensory Areas • Areas that are associated with conscious awareness of sensation • Found in the parietal, temporal, occipital and insular lobes • Primary somatosensory cortex • Somatosensory association cortex • Visual and auditory areas • Olfactory, gustatory, and vestibular cortices

Posterior Sensory Anterior Sensory map in postcentral gyrus Genitals Primary somatosensory cortex (postcentral gyrus) Intra- abdominal Figure 12.9 Primary Somatosensory Cortex • Located in the postcentral gyrus, this area: • Receives information from the skin and from proprioceptors (position) in the skeletal muscles, joints and tendons • Identify the area of the body from which the signal was sent - spatial discrimination

Somatosensory Association Cortex • Located posterior to the primary somatosensory cortex and has connection with it • Integrates sensory information like temperature and pressure coming from the primary somatosensory cortex. • Forms understanding of the stimulus like size, texture, and relationship of parts • Ex.: putting the hand in the pocket and feeling something. The center integrate previous information to identify objects without seeing them

Visual Areas • Primary visual (striate) cortex • Seen on the extreme posterior tip of the occipital lobe • Most of it is buried in the calcarine sulcus (medial aspect of the occipital lobe) • Receives visual information from the retinas • Visual association area • Surrounds the primary visual cortex and covers much of the occipital lobe. • Interprets visual stimuli (e.g., color, form, and movement) by processing previous experiences

Auditory Areas • Primary auditory cortex • Located at the superior margin of the temporal lobe • Receives information from mechanoreceptors in the inner ear related to pitch, rhythm, and loudness • Auditory association area • Located posterior to the primary auditory cortex • Stores memories of sounds and permits perception of sounds (is it scream, speech, singing etc.) • Wernicke’s area - on the left posterior section of the superior temporal lobe, encircling the auditory cortex. Function in language comprehension

multimodal association areas • Most of the cortex receives inputs from multiple senses and sends outputs to multiple areas • Theses areas are called – multimodal association areas • In general the route is : • Information accepted by sensory receptors • Signal is transported to the appropriate primary sensory cortex • To sensory association cortex • To multimodal association cortex • Multimodal association allows to give a meaning to received information, stores memory, connect signal to previous experience, decide what action to take

Language Areas as example of multimodal association • Located in a large area surrounding the left (or language-dominant) lateral sulcus • Major parts and functions: • Wernicke’s area –sounding out unfamiliar words (sensory) • Broca’s area – speech preparation and production (motor) • Lateral prefrontal cortex – language comprehension and word analysis • Lateral and ventral temporal lobe – coordinate auditory and visual aspects of language

http://www.colorado.edu/kines/Class/IPHY3730/image/figure5-21.jpghttp://www.colorado.edu/kines/Class/IPHY3730/image/figure5-21.jpg

Lateralization of Cortical Function • Lateralization – each hemisphere has abilities not shared with its partner • Cerebral dominance – designates the hemisphere dominant for language. For most people: • Left hemisphere – controls language, math, and logic • Right hemisphere – controls visual skills, emotion, and artistic skills • For 10% of people the roles of the hemispheres are reversed or the hemispheres share functions equally • The hemispheres have communication with one another

Cerebral White Matter • Consists of deep myelinated fibers bundled into tracts • The tracts are classified according to the direction in which they run: • Commissures – • connect corresponding gray areas of the two hemispheres allowing them to work as one unit • The largest is the corpus callosum • Anterior and posterior commisures • Association fibers – connect different parts of the same hemisphere • Projection fibers – enter the hemispheres from lower brain or cord centers

Basal Nuclei • Masses of gray matter found deep within the cortical white matter • its major components include the caudate, Putamen, globus pallidus • The putamen and the globus pallidus form the lentiform nucleus • The lentiform and the caudata are called the corpus striatum

lentiform

Basal Ganglia Function • Organization • Input from cortex • Project motor to cortex • Function • coordinating the initiation of movements • Monitor movement • Inhibit unnecessary movement

The picture represents the Somatotopy mapping/ homunculi of the cerebral cortex

What white matter structure connects the two cerebral hemispheres? A. longitudinal fissureB. lateral ventriclesC. corpus callosumD. diencephalons

Damage to the white matter of the brain would prevent transmission of information between nuclei.

Diencephalon • Consists of three paired structures – • thalamus • Hypothalamus • epithalamus • Encloses the third ventricle

Thalamic Function • The thalamus is the “gateway to the cerebral cortex” • Major relay station for mostsensory impulses that arrive to the primary sensory areas in the cerebral cortex: • taste, smell, hearing, equilibrium, vision, touch, pain, pressure, temperature • Contributes to motor functions by transmitting information from the cerebellum and basal ganglia to the cerebral primary motor area • Connects areas of the cerebrum • Impulses of similar function are sorted out, edited, and relayed as a group

Hypothalamus • Located below the thalamus and forms the inferolateral walls of the third ventricle • Infundibulum – stalk of the hypothalamus; connects to the pituitary gland • Mammillary bodies • Small, paired nuclei bulging anteriorly from the hypothalamus • Relay station for olfactory pathways

Hypothalamic Function • Function as the main visceral control in the body • Regulates blood pressure, rate and force of heartbeat, digestive tract motility, rate and depth of breathing, eye pupil size etc. • Perception of pleasure, fear, and rage • Maintains normal body temperature (contains body’s “thermostat”) by initiating cooling or heat-generating processes • Regulates food intake (hunger) • Regulation of water intake and thirst – produce ADH, contain thirst center • Regulates sleep and the sleep cycle

Endocrine Functions of the Hypothalamus • Releasing hormones control secretion of hormones by the anterior pituitary • The supraoptic and paraventricular nuclei produce ADH and oxytocin

Epithalamus • Most dorsal portion of the diencephalon; forms roof of the third ventricle • Pineal gland – extends from the posterior border and secretes melatonin • Melatonin – a hormone involved with sleep regulation, sleep-wake cycles, and mood • Choroid plexus – a structure that secretes cerebral spinal fluid (CSF)

Brain Stem • Consists of three regions – • midbrain, • Pons • medulla oblongata • Similar to spinal cord – deep gray matter surrounded by white tracts but contains embedded nuclei in the white matter • Controls automatic behaviors necessary for survival (breathing, digestion, heart rate, blood pressure) • Provides the pathway for tracts between higher and lower brain centers • Associated with 10 of the 12 pairs of cranial nerves

Midbrain • Located between the diencephalon and the pons • Midbrain structures include: • Cerebral pedunclesof the midbrain – two structures that contain descending pyramidal motor tracts (what areas do the pyramidal tracts connect?) • Cerebral aqueduct – hollow tube that connects the third and fourth ventricles • Various nuclei (what are nuclei)

Midbrain Nuclei • Nuclei that control cranial nerves III (oculomotor) and IV (trochlear) • Corpora quadrigemina • 2 Superior colliculi – visual reflex centers; coordinate head and eye movement when we follow a moving object (does not have to be a conscious decision) • 2 Inferior colliculi – auditory relay centers between hearing receptors to sensory cortex. • Reflexive responses to sound – turn head toward sound

Midbrain Nuclei • Substantia nigra – functionally linked to basal nuclei, contains melanin pigment (precursor of dopamine - NT) • Red nucleus – largest nucleus (rich blood supply) of the reticular formation; relay nuclei for some descending motor pathways

Pons • between the midbrain and the medulla oblongata • Forms part of the anterior wall of the fourth ventricle • Fibers of the pons: • Connect higher brain centers and the spinal cord • Relay impulses between the motor cortex and the cerebellum • Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial) • Contains nuclei of the reticular formation

Medulla Oblongata • Most inferior part of the brain stem • Along with the pons, forms the ventral wall of the fourth ventricle • Contains the choroid plexus of the fourth ventricle • Pyramids – two longitudinal ridges formed by corticospinal tracts descending from the motor cortex • Before entering the spinal cord the corticospinal tracts crossover. • The point in which the crossover occur is called decussation of the pyramids • Results in the controlling of each cerebral hemisphere in the movements of the opposite side

Medulla Nuclei • Inferior olivary nuclei – gray matter that relays sensory information regarding the stretch of muscle and joints • Cranial nervesIX (glosopharyngeal,X (vagus), XI (accessory; neck muscle), and XII (hypoglossal; tongue) are associated with the medulla • Vestibular nuclear complex – synapses that mediate and maintain equilibrium • Ascending sensory tract nuclei, including nucleus cuneatus and nucleus gracilis • Serve as relay station for general somatic sensation

Medulla Nuclei • The medulla has an important role in the autonomic reflex center that maintain homeostasis: • Cardiovascular control center – adjusts force and rate of heart contraction • Respiratory centers – control rate and depth of breathing • Additional centers – regulate vomiting, hiccuping, swallowing, coughing, and sneezing • There is an overlap between medulla and hypothalamus that uses medullary centers to carry out instructions

The Cerebellum • Located dorsal to the pons and medulla; the activity is subconscious • Two bilaterally symmetrical hemispheres connected medially by the vermis • Folia – transversely oriented gyri • Each hemisphere has three lobes – anterior, posterior, and flocculonodular (deep to the vermis and posterior lobe; can not be seen from the outside) • Neural arrangement – gray matter cortex, internal white matter, scattered nuclei • Arbor vitae – distinctive treelike pattern of the cerebellar white matter

Cerebellar Function • Adjust ongoing movements on the basis of comparison between arriving sensation to one previously experienced • Posture: • Balance • Equilibrium • Fine Tune Movements • Timing • Rate • Range • Force

The Central Nervous System