The Central Nervous System

The Central Nervous System

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Objectives

Describe the functions of the different areas of the brain.

Distinguish between different types of memory and describe the roles of different brain regions in memory.

Describe the location of the hypothalamus and explain the significance of this region.

Explain the role of the medulla in the control of visceral functions.


Objectives (continued)

Explain the role of the basal nuclei and cerebellum in motor control.

Describe the structures involved in a reflex arc.


CNS

Consists of: Brain. Spinal cord.

Receives input from sensory neurons.

Directs activity of motor neurons.

Association neurons (interneurons) maintain homeostasis in the internal environment.

Figure 8-1


Cerebrum

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.

Figure 8-6


Cerebral Cortex

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.

Figure 8-7


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.

Figure 8-7


Cerebral Cortex (continued)

Temporal: Contain auditory centers that receive sensory

fibers from cochlea. Interpretation and association of auditory and

visual information. Occipital:

Primary area responsible for vision and coordination of eye movements.

Insula: Implicated in memory encoding. Integration of sensory information with visceral

responses. Coordinate cardiovascular response to stress.


Epilepsy

Jeremey is a 15 year old male high school football player who was tackled during practice. Jeremey became very angry and fell to the ground with sudden onset of unconsciousness. His body stiffened with arms and legs extended. He did not breathe for about 10 seconds, then began violent rhythmic muscular contractions accompanied by hyperventilation.

Jeremey awoke in a confused state in ER. About an hour later he experienced a second seizure.


Epilepsy

General term for the primary condition that causes seizures.

Is not a diagnosis but a symptom. Conditions in which no underlying correctable

cause for the seizure is found. Incidence: 20-70/100,000 Seizure activity demands 250% increase in

ATP. 02 consumption increases 60%. Cerebral blood flow increase 250%. Glucose and 02 already depleted.


Epilepsy

Epileptic seizures: Partial:

Seizures beginning locally, involving neurons unilaterally

Have a local focal onset, usually originating from cortical brain tissue.

Simple: Without impairment of consciousness.

Complex: Impaired consciousness. Involve neurons bilaterally, do not have a focal onset

and originate from subcortical or deeper brain focus. Generalized:

Seizures bilaterally symmetric, without local onset.


Epilepsy

Pathophysiology: Epileptic focus is a group of neurons that evidence a

paroxysmal depolarization shift and sudden changes in membrane potential.

Plasma membrane is more permeable. May be due to abnormalities in K+ or Ca++ conductance. Defects in GABA inhibitory system. Abnormality in N-methyl-D- aspartate receptor.

Neuronal discharge spreads to adjacent neuronal tissue. Tonic:

Muscle contraction with increased muscle tone. Loss of consciousness.

Clonic: Alternating contractions and relaxations of muscles.


Electroencephalogram (EEG)

Measures synaptic potentials produced at cell bodies and dendrites. Create electrical

currents. Used clinically do

diagnose epilepsy and brain death.

Figure 8-10


EEG Patterns Alpha:

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.

Figure 8-11


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.

Figure 8-12


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. Damage:

Difficulty finding way around house.

Figure 8-14


Language 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. 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

Hypothalamus and limbic systems.

Limbic system: 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.

Figure 8-16


Emotion and Motivation (continued)

Areas or the hypothalamus and limbic system are involved in feelings and behaviors.

Aggression: Amygdala and hypothalamus.

Fear: Amygdala and hypothalamus.

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: 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.

Postsynaptic changes: Glutamate binds to NMDA

and AMPA receptors. Opens Ca2+ and Na+

channels. Presynaptic changes:

Ca2+ causes, release of NO from postsynaptic neuron.

NO acts as a retrograde messenger, causing release of NT in bouton.

Figure 8-17


Thalamus

Thalamus: 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.


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.

Anterior pituitary: Regulates

secretion of hormones of other endocrine glands.

Figure 11-12


Midbrain

Contains: Corpora quadrigemina:

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:

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.

Figure 8-23


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.


Alzheimer’s Disease

Gertie is a 76 year-old female shopping for groceries with her daughter. Gertie became separated from her daughter. She was aimlessly wandering the aisles when an employee asked if she could help her. Gertie become angry and combative when the employee tried to assist her. Her daughter, hearing the commotion found her mother, and tried to calm her down. The two left the store without the groceries.

Vivian, her daughter immediately took her to ER. She tested positive for memory difficulty on the Geriatric Depression Scale.



Gradual onset and continuing decline in cognitive function.

Most common dementia for the elderly. Incidence: 4 million in US Risk factors:

Increasing age Female gender Genetics:

Presenilin gene on chromosome 1, 14 Epsilon 4 allele on chromosome 19

Head trauma



Pathophysiology: Abnormal amyloid beta proteins Oxygen radicals Excess glutamate and aspartate ICF excess [Ca++] cause cell protein

denaturiation Neurofibrilary tangles Neritic plaque Degeneration of cholinergic neurons



Clinical manifestations: Insidious onset Forgetfulness increasing over time Memory loss Deteriorating ability for problem

solving Judgment deteriorates Behavioral changes Labile


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.

Figure 8-28

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The Central Nervous System