The Central Nervous System Chapter 8 The Central Nervous System

CNS Consists of: Receives input from sensory neurons. Brain. Spinal cord. Receives input from sensory neurons. Directs activity of motor neurons. Association neurons maintain homeostasis in the internal environment.

Embryonic Development Groove appears in ectoderm to fuse to form neural tube by 20th day after conception. Neural tube eventually forms the CNS. During 5th week, modified: Forebrain: telencephalon and diencephalon. Midbrain: unchanged. Hindbrain: metencephalon and myelencephalon. Part of ectoderm where fusion occurs becomes neural crest. Neural crest forms ganglia of PNS.

Embryonic Development (continued)

Embryonic Development (continued) Telencephalon grows disproportionately forming 2 the hemispheres of the cerebrum. Ventricles and central canal become filled with cerebral spinal fluid (CSF). CNS composed of gray and white matter. Gray matter consists of neuron cell bodies and dendrites. White matter (myelin) consists of axon tracts.

Cerebrum Only structure of the telencephalon. Largest portion of brain (80% mass). Responsible for higher mental functions. Corpus callosum: Major tract of axons that functionally interconnects right and left cerebral hemispheres.

Cerebrum (continued)

Cerebral Cortex Characterized by numerous convolutions. Frontal lobe: Elevated folds: gyri. Depressed groves: sulci. Frontal lobe: Anterior portion of each cerebral hemisphere. Precentral gyri: Contains upper motor neurons. Involved in motor control. Body regions with the greatest number of motor innervation are represented by largest areas of motor cortex.

Cerebral Cortex (continued)

Cerebral Cortex (continued) Parietal lobe: Primary area responsible for perception of somatesthetic sensation. Body regions with highest densities of receptors are represented by largest areas of sensory cortex. Temporal lobe: Contain auditory centers that receive sensory fibers from cochlea. Interpretation and association of auditory and visual information.

Cerebral Cortex (continued) Occipital Lobe: Primary area responsible for vision and coordination of eye movements. Insula: Implicated in memory encoding. Integration of sensory information with visceral responses. Coordinated cardiovascular response to stress.

Visualizing the Brain X-ray computed tomography (CT): Complex computer manipulations of data obtained from x-ray absorption by tissues of different densities. Soft tissue. Positron-emission tomography (PET): Radioisotopes that emit positrons are injected into blood stream. Collision of positron and electron result in emission of gamma rays. Pinpoint brain cells that are most active. Brain metabolism, drug distribution. Magnetic resonance imaging (MRI): Protons (H+) respond to magnetic field, which align the protons. Emit a radio-wave signal when stimulated. Brain function.

Electroencephalogram (EEG) Measures synaptic potentials produced at cell bodies and dendrites. Create electrical currents. Used clinically do diagnose epilepsy and brain death.

EEG Patterns Theta: Alpha: Beta: Delta: Recorded from parietal and occipital regions. Person is awake, relaxed, with eyes closed. 10-12 cycles/sec. Beta: Strongest from frontal lobes near precentral gyrus. Produced by visual stimuli and mental activity. Evoked activity. 13-25 cycles/sec. Theta: Emitted from temporal and occipital lobes. Common in newborn. Adult indicates severe emotional stress. 5-8 cycles/sec. Delta: Emitted in a general pattern. Common during sleep and awake infant. In awake adult indicate brain damage. 1-5 cycles/sec.

EEG Sleep Patterns 2 types of EEG patterns during sleep: REM (rapid eye movement): Dreams occur. Low-amplitude, high-frequency oscillations. Similar to wakefulness (beta waves). Non-Rem (resting): High-amplitude, low-frequency waves (delta waves). Superimposed on these are sleep spindles: Waxing and waning bursts of 7-14 cycles/sec. Last for 1-3 sec.

Basal Nuclei (basal ganglia) Masses of gray matter composed of neuronal cell bodies located deep within white matter. Contain: Corpus striatum: Caudate nucleus. Lentiform nucleus: Putman and globus pallidus. Functions in the control of voluntary movements.

Cerebral Lateralization Cerebral dominance: Specialization of one hemisphere. Left hemisphere: More adept in language and analytical abilities. Damage: Severe speech problems. Right hemisphere: Most adept at visuospatial tasks. Difficulty finding way around house.

Language Arcuate fasciculus: Broca’s area: Involves articulation of speech. In damage, comprehension of speech in unimpaired. Wernicke’s area: Involves language comprehension. In damage, language comprehension is destroyed, but speech is rapid without any meaning. Angular gyrus: Center of integration of auditory, visual, and somatesthetic information. Damage produces aphasias. Arcuate fasciculus: To speak intelligibly, words originating in Wernicke’s area must be sent to Broca’s area. Broca’s area sends fibers to the motor cortex which directly controls the musculature of speech.

Emotion and Motivation Important in the neural basis of emotional states are hypothalamus and limbic system. Limbic system: Group of forebrain nuclei and fiber tracts that form a ring around the brain stem. Center for basic emotional drives. Closed circuit (Papez circuit): Fornix connects hippocampus to hypothalamus, which projects to the thalamus which sends fibers back to limbic system.

Emotion and Motivation (continued) Areas or the hypothalamus and limbic system are involved in feelings and behaviors. Aggression: Amygdala and hypothalamus. Fear: Feeding: Hypothalamus (feeding and satiety centers). Sexual drive and behavior: Hypothalamus and limbic system. Goal directed behavior (reward and punishment): Hypothalamus and frontal cortex.

Memory Short-term: Medial temporal lobe: Memory of recent events. Medial temporal lobe: Consolidates short term into long term memory. Hippocampus is critical component of memory. Acquisition of new information, facts and events requires both the medial temporal lobe and hippocampus.

Long-Term Memory Consolidation of short-term memory into long-term memory. Requires activation of genes, leading to protein synthesis and formation of new synaptic connections. Altered postsynaptic growth of dendritic spines in area of contact. Cerebral cortex stores factual information: Visual memories lateralized to left hemisphere. Visuospatial information lateralized to right hemisphere. Prefrontal lobes: Involved in performing exact mathematical calculations. Complex, problem-solving and planning activities.

Long-Term Potentiation Type of synaptic learning. Synapses that are 1st stimulated at high frequency will subsequently exhibit increased excitability. In hippocampus, glutamate is NT. Requires activation of the NMDA receptors for glutamate. Glutamate and glycine or D-serine binding and partial depolarization are required for opening of channels for Ca2+ and Na+. May also involve presynaptic changes: Binding of glutamate to NMDA receptors and simultaneous depolarization, open receptor channels for Ca2+. Ca2+ causes long-term potentiation in postsynaptic neuron, release of NO from postsynaptic neuron. NO acts as a retrograde messenger, causing release of NT.

Neuronal Stem Cells in Learning and Memory Neural stem cells: Cells that both renew themselves through mitosis and produce differentiated neurons and neuroglia. Hippocampus has been shown to contain stem cells (required for long-term memory). Neurogenesis: Production of new neurons. Indirect evidence that links neuogenesis in hippocampus with learning and memory.

Thalamus and Epithalamus Composes 4/5 of the diencephalon. Forms most of the walls of the 3rd ventricle. Acts as relay center through which all sensory information (except olfactory) passes to the cerebrum. Lateral geniculate nuclei: Relay visual information. Medial geniculate nuclei: Relay auditory information. Intralaminar nuclei: Activated by many sensory modalities. Projects to many areas. Promotes alertness and arousal from sleep. Epithalamus contains: Choroid plexus where CSF is formed. Pineal gland which secretes melatonin.

Hypothalamus Contains neural centers for hunger, thirst, and body temperature. Contributes to the regulation of sleep, wakefulness, emotions, sexual arousal, anger, fear, pain, and pleasure. Stimulates hormonal release from anterior pituitary. Produces ADH and oxytocin. Coordinates sympathetic and parasympathetic reflexes.

Pituitary Gland Posterior pituitary: Stores and releases ADH (vasopressin) and oxytocin. Hypothalamus produces releasing and inhibiting hormones that are transported to anterior pituitary. Regulate secretions of anterior hormones. Anterior pituitary: Regulates secretion of hormones of other endocrine glands.

Midbrain Contains: Corpora quadrigemina: Cerebral peduncles: Superior colliculi: Involved in visual reflexes. Inferior colliculi: Relay centers for auditory information. Cerebral peduncles: Composed of ascending and descending fiber tracts. Substantia nigra: Required for motor coordination. Red nucleus: Maintains connections with cerebrum and cerebellum. Involved in motor coordination.

Hindbrain Metencephalon: Pons: Cerebellum: Surface fibers connect to cerebellum, and deeper fibers are part of motor and sensory tracts. Contains several nuclei associated with cranial nerves V, VI, VII. Contains the apneustic and pneumotaxic respiratory centerss. Cerebellum: Receives input from proprioceptors. Participates in coordination of movement. Necessary for motor learning, coordinating different joints during movement, and limb movements.

Hindbrain (continued) Myelencephalon (medulla oblongata): All descending and ascending fiber tracts between spinal cord and brain must pass through the medulla. Nuclei contained within the medulla include VIII, IX, X, XI, XII. Pyramids: Fiber tracts cross to contralateral side. Vasomotor center: Controls autonomic innervation of blood vessels. Cardiac control center: Regulates autonomic nerve control of heart. Regulates respiration with the pons.

Reticular Formation Reticular Formation: Complex network of nuclei and nerve fibers within medulla, pons, midbrain, thalamus and hypothalamus. Functions as the reticular activating system (RAS). Non specific arousal of cerebral cortex to incoming sensory information.

Ascending Spinal Tracts Convey sensory information from cutaneous receptors, proprioceptors and visceral receptors to cerebral cortex. Sensory fiber tract decussation may occur in medulla or spinal cord.

Descending Spinal Tracts Pyramidal (corticospinal) tracts descend directly without synaptic interruption from cerebral cortex to spinal cord. Function in control of fine movements that require dexterity. Reticulospinal tracts (extrapyramidal): Influence movement indirectly. Gross motor movement.

Cranial and Spinal Nerves Cranial nerves: 2 pairs arise from neuron cell bodies in forebrain. 10 pairs arise from the midbrain and hindbrain. Roman numerals refer to the order in which the nerves are positioned from front of the brain to the back. Most are mixed nerves containing both sensory and motor fibers. Spinal nerves: 31 pairs grouped into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and l coccygeal. Mixed nerve that separates near the attachment of the nerve to spinal cord. Produces 2 roots to each nerve. Dorsal root composed of sensory fibers. Ventral root composed of motor fibers.

Reflex Arc Unconscious motor response to a sensory stimulus. Stimulation of sensory receptors evokes APs that are conducted into spinal cord. Synapses with association neuron, which synapses with somatic motor neuron. Conducts impulses to muscle and stimulates a reflex contraction. Brain is not directly involved.