The Central Nervous System Prepared by I Gede

The Central Nervous System Prepared by I Gede Purnawinadi, S. Kep. , M. Kes.

Organization of the Nervous System • Central nervous system (CNS) • Brain and spinal cord • Integration and command center • Peripheral nervous system (PNS) • Paired spinal and cranial nerves • Carries messages to and from the spinal cord and brain

Organization of the Nervous System The two principal cell types of the nervous system are: Neurons – excitable cells that transmit electrical signals Neuroglia - supporting cells

Neuron Classification • Functional: • Sensory (afferent) — transmit impulses toward the CNS • Motor (efferent) — carry impulses away from the CNS • Interneurons (association neurons) — shuttle signals through CNS pathways

Neuroglia

Central Nervous System: • Brain • Spinal cord

CNS Protection • Hair, skin, cranium • Meninges • Cerebrospinal fluid • Blood brain barrier

Meningeal Layers • Meningeal layer of the brain cushion and protect delicate neural tissue

Cerebrospinal Fluid • Shock absorbing medium • Provides a optimum and stable environment for generating nerve impulses • Provides a medium for the exchange of nutrients and wastes between blood and nervous tissue • Formed by selective transport across ependymal cells • Volume 125 -150 ml and is replaced > 3 times/day, flow maintained by 10 mm. Hg paressure gradient • Path: ventricles subarachnoid space, reabsorbed into blood in dural sinuses through arachnoid villi

Blood Brain Barrier Extensive capillaries & sinuses Tight junctions promoted by astrocyte Limits permeability for most molecules except O 2, CO 2, alcohol, steroids, H 2 O Protects brain: hormones & circulating chemicals • Protects CNS from chemical fluctuations • Prevents entry of harmful substances • Prevents entry of molecules that could act as neurotransmitters • Brain receives 15% of blood pumped by heart • Brain responsible for about half of body’s glucose consumption • Membrane transporters move glucose from plasma into the brain interstitial fluid • • Figure 9 -6: The blood-brain barrier

The Central Nervous System (CNS) • The central nervous system (CNS) consists of the brain and spinal cord, which occupy the dorsal body cavity. • The CNS is the integrating and command center of the nervous system. • It interprets sensory input and dictates motor responses based on past experience, reflexes, and current conditions. • The Brain: the average adult man’s brain has a mass of about 1600 g (3. 5 lb); that of a woman averages 1450 g (3. 2 lb).

Brain Development (a) Formation of two major flexures by week 5 of development causes the telencephalon and diencephalon to angle toward the brain stem. Development of the cerebral hemispheres at (b) 13 weeks, (c) 26 weeks, and (d) birth.

Brain Organization • Trillion interneurons fill the brain • Up to 200, 000 synapses each • Levels of complexity üCerebral cortex üBasal nuclei üThalamus üHypothalamus üCerebellum üBrain stem

Brain component Cerebral cortex Basal nuclei (lateral to thalamus) Basal nuclei Thalamus (medial) Thalamus Hypothalamus Cerebellum Midbrain Brain stem (midbrain, pons, and medulla) Pons Medulla Spinal cord

Brain component Cerebral cortex Basal nuclei Thalamus Hypothalamus Cerebellum Brain stem (midbrain, pons, and medulla) Major Functions 1. Sensory perception 2. Voluntary control of movement 3. Language 4. Personality traits 5. Sophisticated mental events, such as thinking memory, decision making, creativity, and self-consciousness 1. Inhibition of muscle tone 2. Coordination of slow, sustained movements 3. Suppression of useless patterns of movements 1. Relay station for all synaptic input 2. Crude awareness of sensation 3. Some degree of consciousness 4. Role in motor control 1. Regulation of many homeostatic functions, such as temperature control, thirst, urine output, and food intake 2. Important link between nervous and endocrine systems 3. Extensive involvement with emotion and basic behavioral patterns 1. Maintenance of balance 2. Enhancement of muscle tone 3. Coordination and planning of skilled voluntary muscle activity 1. Origin of majority of peripheral cranial nerves 2. Cardiovascular, respiratory, and digestive control centers 3. Regulation of muscle reflexes involved with equilibrium and posture 4. Reception and integration of all synaptic input from spinal cord; arousal and activation of cerebral cortex 5. Role in sleep-wake cycle

Cerebral Hemispheres • The cerebral hemispheres form the superior part of the brain. • Together they account for about 83% of total brain mass and are the most conspicuous parts of an intact brain.

Cerebral Cortex • Each half of cortex divided into four major lobes • • Occipital lobe - carries out initial processing of visual input Temporal lobe - initial reception of sound sensation, taste, smell Parietal lobe - somatosensory processing Frontal lobe responsible for • Voluntary motor activity • Speaking ability • Elaboration of thought

Primary Motor Cortex • Located in the precentral gyrus • Composed of pyramidal cells whose axons make up the corticospinal tracts • Allows conscious control of precise, skilled, voluntary movements • Motor homunculus – caricature of relative amounts of cortical tissue devoted to each motor function

Primary Somatosensory Cortex • Located in the postcentral gyrus, this area: • Receives information from the skin and skeletal muscles • Exhibits spatial discrimination • Somatosensory homunculus – caricature of relative amounts of cortical tissue devoted to each sensory function

Motor and sensory areas of the cerebral cortex

Brain Function: Cerebral Lateralization • Each lobe has special functions

Basal Nuclei • Act by modifying ongoing activity in motor pathways • Primary functions • Regulates muscle tone throughout the body • Selecting and maintaining purposeful motor activity while suppressing useless or unwanted patterns of movement • Helping monitor and coordinate slow, sustained contractions, especially those related to posture and support • Controls large automatic movement

Thalamus • Final relay point for ascending sensory information • Coordinates the activities of the cerebral cortex and basal nuclei • Domain-specific information processing

Hypothalamus • Receives indirect sensory inputs from all sensory systems • Sends neural outputs to various motor control nuclei • Sends neural outputs to sympathetic and parasympathetic nervous systems • Sends both neural and hormonal outputs to pituitary

Hypothalamus • Controls somatic motor activities at the subconscious level • Controls autonomic function • Coordinates activities of the endocrine and nervous systems • Secretes hormones • Produces emotions and behavioral drives • Coordinates voluntary and autonomic functions • Regulates body temperature • Coordinates circadian cycles of activity • 4 Fs: feeding, fighting, fleeing, and reproductive behavior

Limbic System • Cingulated gyrus • Coordinates sensory input with emotions • Emotional responses to pain • Basic, inborn behavioral patterns related to survival and perpetuation of the species • Regulates aggressive behavior • Hippocampus - sends memories out to the appropriate part of the cerebral hemisphere for long-term storage and retrieving them when necessary, Plays important role in motivation and learning • Amygdala - involved in emotional responses, hormonal secretions, and memory,

Cerebellum • Basic functions: coordination, balance, motor learning, etc. • Vestibulocerebellum – balance and control of eye movement • Spinocerebellum – enhances muscle tone and coordinates skilled voluntary movement – important in synchronization and timing • Receives input concerning desired action from motor cortex • Receives feedback concerning actual action from proprioceptors, vestibular apparatus, eyes • Compares inputs and sends adjustments or corrective signals to motor tracts • Cerebrocerebellum – planning and initiation of voluntary activity by providing input to the cortical motor areas also involved in procedural memories

Brain Stem: Midbrain, Pons & Medulla • An important link between spinal cord and higher brain levels, relays motor and sensory impulses between other “higher” parts of the brain and spinal cord • Midbrain – eye movement control • Pons/Medulla • Signal relay • Involuntary functions • Many cranial nerves enter

Spinal Cord Regions • Cervical • Thoracic • Lumbar • Sacral

Spinal Cord Organization • Gray matter: mostly cell bodies • Dendrites & terminals • Spinal reflex integrating center • White matter • Bundles of myelinated axons • Ascending tracts – sensory • Descending tracts – motor • Dorsal roots • Ventral roots

Spinal Cord Organization

Spinal Cord: Integrating Center Neural Reflexes: Overview ü Stimulus ü Sensory receptor ü Sensory (afferent) neuron ü CNS integration ü Efferent (motor) neuron ü Effector (target tissue) ü Response (movement) ü Feedback to CNS

Neural Reflex

Patellar Tendon Reflex: Stretch & Reciprocal Inhibition

Cross Extensor Reflex/Withdrawal Reflex