Basic Pattern of the Central Nervous System
- Spinal Cord
- Central cavity surrounded by a gray matter core
- External to which is white matter composed of myelinated fiber tracts
- Brain
- Similar to spinal cord but with additional areas of gray matter
- Cerebellum has gray matter in nuclei
- Cerebrum has nuclei and additional gray matter in the cortex
- The ventricles of the brain are continuous with one another, and with the central canal of the spinal cord.
- They are lined with ependymal cells, and are filled with cerebrospinal fluid.
- The paired lateral ventricles lie deep within each cerebral hemisphere, and are separated by the septum pellucidum
- The third ventricle lies within the diencephalon, and communicates with the lateral ventricles via two interventricular foramina.
- The fourth ventricle lies in the hindbrain and communicates with the third ventricle via the cerebral aqueduct.
- Form the superior part of the brain and make up 83% of its mass
- Contain ridges (gyri) and shallow grooves (sulci)
- Deep sulci divide the hemispheres into five lobes: Frontal, parietal, temporal, occipital, and insula
- Central sulcus - separates the frontal and parietal lobes
- Lateral sulcus - separates the parietal and temporal lobes
- The precentral and postcentral gyri border the central sulcus
- Contain deep grooves called fissures
- Cerebral hemispheres are separated by the longitudinal fissure
- Transverse cerebral fissure separates the cerebellum from the cerebrum
- Separated from the cerebellum by transverse fissure
- Have three basic regions: cortex, white matter, and basal nuclei
- 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
- The three types of functional areas are:
- Motor areas - control voluntary movement
- 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
- Associative Voluntary motor - frontal lobe
- The premotor cortex is the region controlling learned motor skills.
- Broca's area is a motor speech area that controls muscles involved in speech production.
- The frontal eye field controls eye movement.
- Sensory areas - conscious awareness of sensation
- PrImary Somatosensory Cortex
- Located in the postcentral gyrus, this area:
- Receives information from skin and skeletal muscles
- Exhibits spatial discrimination
- Somatosensory homunculus - caricature of relative amounts of cortical tissue devoted to each sensory function
- Sensory associative areas of cerebral cortex occur in parietal, temporal, and occipital lobes
- The somatosensory association cortex integrates sensory information and produces an understanding of the stimulus being felt.
- The primary visual cortex and visual association area allow reception and interpretation of visual stimuli.
- The primary auditory cortex and auditory association area allow detection of the properties and contextual recognition of sound.
- The olfactory cortex allows detection of odors.
- The gustatory cortex allows perception of taste stimuli.
- The vestibular cortex is responsible for conscious awareness of balance.
- Association areas - integrate diverse information
- Prefrontal cortex is involved with intellect, cognition, recall, and personality, and is closely linked to the limbic system.
- Language areas involved in comprehension and articulation.
- General interpretation area receives input from all sensory areas, integrating signals into a single thought.
White Matter
- Types include:
- Commissures - connect corresponding gray areas of the two hemispheres
- Association fibers - connect different parts of the same hemisphere
- Projection fibers - enter hemispheres from lower brain or cord centers
Basal Nuclei
- Masses of gray matter found deep within the cortical white matter
- The corpus striatum is composed of three parts
- Caudate nucleus
- Lentiform nucleus - composed of the putamen and the globus pallidus
- Fibers of internal capsule running between and through caudate and lentiform nuclei
- Functions -Though somewhat elusive, the following are thought to be functions of basal nuclei
- Influence muscular activity
- Regulate attention and cognition
- Regulate intensity of slow or stereotyped movements
- Inhibit antagonistic and unnecessary movement
- Central core of the forebrain
- Consists of three paired structures - thalamus, hypothalamus, & epithalamus
- Encloses the third ventricle
- Thalamus
- Paired, egg-shaped masses that form the walls of the third ventricle
- Connected at the midline by the intermediate mass
- Functions
- Afferent impulses from all senses (except olfactory) converge and synapse in the thalamus
- Impulses of similar function are sorted out, edited, and relayed as a group
- All inputs ascending to the cerebral cortex pass through the thalamus
- Plays a key role in mediating sensation, motor activities, cortical arousal, learning, and memory
- Epithalamus
- Most dorsal portion of the diencephalon
- Pineal gland - extends from the posterior border and secretes melatonin
- Melatonin - a hormone involved with sleep regulation, sleep-wake cycles, and mood
- Hypothalamus
- Located below the thalamus,
- Contains mammillary bodies - small, paired nuclei bulging anteriorly from the hypothalamus
- Relay station for olfactory pathways
- Infundibulum - stalk of the hypothalamus; connects to the pituitary gland (main visceral control center of the body)
- Functions
- Regulates blood pressure, rate and force of heartbeat, digestive tract motility, rate and depth of breathing, and many other visceral activities
- Is involved with perception of pleasure, fear, and rage
- Controls mechanisms needed to maintain normal body temperature
- Regulates feelings of hunger and satiety
- Regulates Endocrine Functions
- Releasing hormones control secretion of hormones by the anterior pituitary
Brain Stem
- Consists of three regions - midbrain, pons, and medulla oblongata
- Controls automatic behaviors necessary for survival
- 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 peduncles - two bulging structures that contain descending pyramidal motor tracts
- Cerebral aqueduct - hollow tube that connects third & fourth ventricles
- Corpora quadrigemina - four protrusions (nuclei) of dorsal midbrain
- Superior colliculi - visual reflex centers
- Inferior colliculi - auditory relay centers
- Pons
- Bulging brainstem region between the midbrain and the medulla oblongata
- Fibers of the pons:
- Connect higher brain centers and the spinal cord
- Relay impulses between the motor cortex and the cerebellum
- Part of the Respiratory Center
- Medulla Oblongata
- Most inferior part of the brain stem
- Contains a choroid plexus on the ventral wall of the fourth ventricle
- Pyramids - two longitudinal ridges formed by corticospinal tracts (voluntary motor)
- Cardiovascular control center - adjusts force and rate of heart contraction
- Respiratory centers - control rate and depth of breathing
- Vasomotor Center
- Makes up 11% of the brain's mass
- Cerebellar activity occurs subconsciously
- Arbor vitae - pattern of white and gray matter
- Vermis connects hemispheres
- Cerebellar cortex - gray matter
- Cerebellar Peduncles -communicate between the cerebellum, and the brain stem.
- Coordinates voluntary muscle movements
- inferior peduncle receives proprioception
- middle peduncle receives desired motion from cerebrum
- cerebellum integrates a and b
- superior peduncle sends out coordinated information to cerebral voluntary motor cortex
- Provides precise timing & appropriate patterns of skeletal muscle contraction
- 1. Monitors intentions for movements - input from cerebral cortex
2. Monitors actual movements with feedback from proprioceptors
3. Compares intentions with actual movements
4. Sends out corrective signals to motor cortex
Functional Brain Systems
- Networks of neurons working together and spanning wide areas of the brain
- Limbic system - Includes the amygdala, hypothalamus, and anterior nucleus of the thalamus
- Amygdala - deals with anger, danger, and fear responses
- Cingulate gyrus - plays a role in expressing emotions via gestures, and resolves mental conflict
- Puts emotional responses to odors - e.g., skunks smell bad
- Reticular Formation
- Sends sensory impulses to the cerebral cortex to keep it conscious and alert
- Filters out repetitive and weak stimuli
- Suppressed by seratonin, darkness, alcohol
- Injury causes coma
- Functions Review
- The brain is protected by bone, meninges, and cerebrospinal fluid
- Harmful substances are shielded from the brain by the blood-brain barrier
- Meninges - three connective tissue membranes lie external to the CNS - dura mater, arachnoid mater, and pia mater
- Functions of the meninges
- Cover and protect the CNS
- Protect blood vessels and enclose venous sinuses
- Contain cerebrospinal fluid (CSF)
- Form partitions within the skull
- Dura Mater
- Leathery, strong meninx composed of two fibrous connective tissue layers
- The two layers separate in certain areas and form dural sinuses
- Three dural septa extend inward and limit excessive movement of the brain
- Falx cerebri - fold that dips into the longitudinal fissure
- Falx cerebelli - runs along the vermis of the cerebellum
- Tentorium cerebelli - horizontal dural fold extends into transverse fissure
- Arachnoid Mater
- The middle meninx, which forms a loose brain covering
- It is separated from the dura mater by the subdural space
- Beneath the arachnoid is a wide subarachnoid space filled with CSF and large blood vessels
- Arachnoid villi protrude superiorly and permit CSF to be absorbed into venous blood
- Pia Mater
- Deep meninx composed of delicate connective tissue that clings tightly to the brain
- Cerebrospinal Fluid (CSF)
- Watery solution similar in composition to blood plasma
- total volume = 150ml; 500 ml secreted daily to replace the circulating 150 ml.
- Made and circulated by ependymal cells
- Contains less protein and different ion concentrations than plasma
- Forms a liquid cushion that gives buoyancy to the CNS organs
- Prevents the brain from crushing under its own weight
- Protects the CNS from blows and other trauma
- Nourishes the brain and carries chemical signals throughout it
- Choriod Plexuses
- 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
- Help cleanse CSF by removing wastes
- Blood-Brain Barrier
- Protective mechanism that helps maintain a stable environment for the brain
- Bloodborne substances are separated from neurons by:
- Continuous endothelium of capillary walls
- Relatively thick basal lamina
- Bulbous feet of astrocytes
- Blood-Brain Barrier Functions
- Selective barrier that allows nutrients to pass freely
- Is ineffective against substances that can diffuse through plasma membranes
- Absent in some areas (vomiting center and the hypothalamus), allowing these areas to monitor the chemical composition of the blood
- Stress increases the ability of chemicals to pass through the blood-brain barrier
Cerebrovascular Accidents (Strokes)
- Caused when blood circulation to the brain is blocked and brain tissue dies
- Most commonly caused by blockage of a cerebral artery
- Other causes include compression of the brain by hemorrhage or edema, and atherosclerosis
- Transient ischemic attacks (TIAs) - temporary episodes of reversible cerebral ischemia
- Tissue plasminogen activator (TPA) is the only approved treatment for stroke
Degenerative Brain Disorders
- Alzheimer's disease - a progressive degenerative disease of the brain that results in dementia
- Parkinson's disease - degeneration of the dopamine-releasing neurons of the substantia nigra
- Huntington's disease - a fatal hereditary disorder caused by accumulation of protein huntingtin that leads to degeneration of basal nuclei
Spinal Cord
- CNS tissue is enclosed within the vertebral column from the foramen magnum to L1
- Structure - Provides two-way communication to & from brain
- Protected by bone, meninges, and CSF
- Conus medullaris - spinal cord terminal portion
- Filum terminale - fibrous extension of the pia mater; anchors the spinal cord to the coccyx
- Denticulate ligaments - delicate shelves of pia mater; attach the spinal cord to the vertebrae
- Cervical and lumbar enlargements - sites where nerves serving the upper and lower limbs emerge
- Cauda equina - collection of nerve roots at the inferior end of the vertebral canal
- Spinal nerves - 31 pairs attach to the cord by paired roots
- Cross-Sectional Anatomy of the Spinal Cord
- Anterior median fissure - separates anterior funiculi
- Posterior median sulcus - divides posterior funiculi
- Epidural space - space between vertebrae & dural sheath (dura mater) filled with fat & vein network
- Gray Matter
- Gray matter consists of soma, unmyelinated processes, and neuroglia
- Gray commissure - connects gray matter; encloses central canal
- Posterior (dorsal) horns - interneurons
- Anterior (ventral) horns - interneurons and somatic motor neurons
- Lateral horns - contain sympathetic nerve fibers
- Spinal Roots - Extensions
- ventral root - motor nerve axons
- dorsal root - sensory nerve axons
- ganglion- a bundle of cell bodies outside CNS
- dorsal root ganglia - contains cell bodies of sensory neurons
- Spinal nerve -fusion of dorsal root & ventral root
- White Matter Tracts (Columns)
- Fibers run in three directions - ascending, descending, and transversely
- Divided into three funiculi (columns) - posterior, lateral, and anterior
- Each funiculus contains several fiber tracks
- Fiber tract names reveal their origin and destination
- Fiber tracts are composed of axons with similar functions
- White Matter: Pathway Generalizations
- Pathways decussate
- Most consist of two or three neurons
- Most exhibit somatotopy (precise spatial relationships)
- Pathways are paired (one on each side of the spinal cord or brain)
- Ascending Sensory Spinal Pathways (Tracts or Columns)
- Located posteriorly (sensory)
- Spinothalamic
- Spinocerebellar
- Fasciculus gracilis and cuneatus
- Descending Motor Pathways (Tracts or Columns)
- Descending tracts deliver efferent impulses from the brain to the spinal cord, and are divided into two groups
- Direct pathways equivalent to the pyramidal tracts
- Indirect pathways (Extrapyramidal), essentially all others
- rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts
- Axial muscles that maintain balance and posture
- Head, neck, and eye movement
- Motor pathways involve two neurons (upper and lower)
- Paralysis - loss of motor function
- Flaccid paralysis - severe damage to the ventral root or anterior horn cells
- Lower motor neurons are damaged and impulses do not reach muscles
- no voluntary movement on same side as damage
- no reflex actions, muscle limp & flaccid, decreased muscle tone
- Spastic paralysis - only upper motor neurons of the primary motor cortex are damaged
- Spinal neurons remain intact and muscles are stimulated irregularly
- There is no voluntary control of muscles
- Paralysis on opposite side from injury
- Increased muscle tone, exaggerated reflexes
- Transection
- Cross sectioning of the spinal cord at any level results in total motor and sensory loss in regions inferior to the cut
- Paraplegia - transection between T1 and L1
- Quadriplegia - transection in the cervical region
- Poliomyelitis
- Destruction of the anterior horn motor neurons by the poliovirus
- Early symptoms - fever, headache, muscle pain and weakness, and loss of somatic reflexes
- Vaccines are available and can prevent infection
- Amyotrophic Lateral Sclerosis (ALS)
- Lou Gehrig's disease - neuromuscular condition involving destruction of anterior horn motor neurons and fibers of the pyramidal tract
- Symptoms - loss of the ability to speak, swallow, and breathe
- Death occurs within five years
- Linked to malfunctioning genes for glutamate transporter and/or superoxide dismutase
Developmental Aspects of the CNS
- CNS is established during the first month of development
- Gender-specific areas appear in response to testosterone (or lack thereof)
- Maternal exposure to radiation, drugs (e.g., alcohol and opiates), or infection can harm the fetus' developing CNS
- Smoking decreases oxygen in the blood, which can lead to neuron death and fetal brain damage
- The hypothalamus is one of the last areas of the CNS to develop
- Visual cortex develops slowly over the first 11 weeks
- Growth and maturation of the nervous system occurs throughout childhood and reflects progressive myelination
- Age brings some cognitive declines, but these are not significant in healthy individuals until they reach their 80s
- Excessive use of alcohol causes signs of senility unrelated to the aging process
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