Biology Chapter 49: Nervous Systems

• 21 questions on the exam are from Chapter 49

Nervous Systems

• Billions of neurons are organized to perform complex tasks through regional specialization and memory formation

Regional Specialization

• Comprehending spoken words occurs in the Parietal lobe
• Formulating a response (speaking or other action) occurs in the Frontal lobe
• Processing sensory input from the ear occurs in the Occipital lobe

Memory formation

• Synapses that are active in synchrony (A and B) are strengthened
• Synapses that are not part of an active circuit (C) are weakened or lost

CONCEPT 49.1: Nervous systems consist of circuits of neurons and supporting cells

• More than 500 million years ago, specialized nervous systems enabled animals to sense their environments/respond rapidly
• The human brain has 100 billion neurons, which make 100 trillion connections
• Cnidarians are one of the simplest animals with nervous systems, they have interconnected neurons arranged in nerve nets
• More complex animals have nerves, where the axons of multiple neurons are bundled together
• Nerves channel information flow along specific routes through the nervous system
• Bilaterally symmetrical animals exhibit cephalization: the clustering of sensory organs at the front end of the body
• Flatworms are the simplest such animals, having a central nervous system (CNS)
• The CNS consists of a brain and longitudinal nerve cords
• The peripheral nervous system (PNS) consists of neurons carrying information into and out of the CNS
• Annelids and arthropods have segmentally arranged clusters of neurons called ganglia
• Nervous system organization usually correlates with lifestyle
• In vertebrates, the CNS is composed of the brain and spinal cord
• In vertebrates, the PNS is composed of nerves and ganglia
• Region specialization is a hallmark of both systems in vertebrates

Glia

• The nervous systems of vertebrates and most invertebrates include glial cells in addition to neurons.
• Glial cells, or glia, have numerous functions to nourish, support, and regulate neurons:
• Embryonic radial glia form tracks along which newly formed neurons migrate
• Astrocytes participate in the formation of the blood-brain barrier, which restricts the entry of most substances into the brain
• Oligodendrocytes and Schwann cells form the myelin sheath
• Microglia provide immune and phagocytic functions
• Ependymal cells line ventricles and make/regulate CSF
• The radial glial cells and astrocytes can both act as stem cells
• They undergo unlimited cell divisions to self-renew, and also form more specialized cells

Organization of the Vertebrate Nervous System

• The CNS develops from the hollow dorsal nerve cord
• The cavity of the nerve cord gives rise to the narrow central canal of the spinal cord and the ventricles of the brain
• The canal and ventricles fill with cerebrospinal fluid, which supplies the CNS with nutrients and hormones and carries away wastes

Meninges

• Dura mater is tough and flexible (below the skull)
• Arachnoid mater is thinner (on top of CSF and blood vessels)
• Pia mater is the most tender
• Cerebrospinal fluid is located in the subarachnoid space between the arachnoid mater and the pia mater
• The primary function of the meninges is to protect the central nervous system.
• The brain and spinal cord contain gray and white matter
• Gray matter consists of neuron cell bodies, dendrites, and unmyelinated axons
• White matter consists of bundles of myelinated axons.
• The spinal cord conveys information to and from the brain and generates basic patterns of locomotion

Spinal Cord Anatomy

• Consists of cervical, pyramidal decussation, thoracic, lumbar, sacral, and coccygeal regions
• The vertebral column consists of vertebral bone, meninges, spinal cord and the dorsal/ventral roots
• Important spinal cord components include the dorsal root ganglion, the posterior median sulcus and nerve impulses
• Spinal cord anatomy includes epidural space, subdural space and subarachnoid space
• The spinal cord also produces reflexes independently of the brain
• A reflex is the body's automatic response to a stimulus

The Peripheral Nervous System

• The Peripheral Nervous System (PNS) transmits information to and from the CNS and regulates movement and the internal environment
• Afferent neurons transmit information to the CNS
• Efferent neurons transmit information away from the CNS
• The PNS has two efferent components: the motor system and the autonomic nervous system
• The motor system carries signals to skeletal muscles and can be voluntary or involuntary
• The autonomic nervous system regulates smooth and cardiac muscles and is generally involuntary
• The enteric nervous system, a part of the autonomic nervous system, exerts direct, partially independent control over the digestive tract, pancreas, and gallbladder
• The autonomic nervous system has sympathetic and parasympathetic divisions
• The sympathetic division regulates arousal and energy generation (fight-or-flight response)
• The parasympathetic division has antagonistic effects on target organs and promotes calming and a return to rest-and-digest functions, returning to homeostasis
• In both sympathetic and parasympathetic systems, the pathway for information flow involves a preganglionic and postganglionic neuron
• Preganglionic neurons have cell bodies in the CNS and release acetylcholine as a neurotransmitter
• Postganglionic neurons of the parasympathetic division release acetylcholine
• The sympathetic division releases norepinephrine

CONCEPT 49.2: The vertebrate brain is regionally specialized

• The vertebrate brain has three major regions: the forebrain, midbrain, and hindbrain
• Each region is specialized in function
• The forebrain handles olfactory input, regulation of sleep, learning, and any complex processing
• The midbrain coordinates routing of sensory input
• The hindbrain controls involuntary activities and coordinates motor activities
• Comparison of vertebrates shows that relative sizes of particular brain regions vary, reflecting the importance of particular brain functions
• Evolution has resulted in a close match between structure and function
• During embryonic development, the anterior neural tube gives rise to the forebrain, midbrain, and Hindbrain
• The brainstem, from the midbrain and hindbrain, joins with the spinal cord at the base of the brain
• The brainstem consits of the midbrain, pons, medulla oblongata
• The rest of the hindbrain gives rise to the cerebellum
• The forebrain divides into the diencephelon, which forms endocrine tissues in the brain
• The forebrain divides into the telencephalon, which becomes the cerebrum

The Cerebrum

• The cerebrum controls skeletal muscle contraction and is the center for learning, emotion, memory, and perception
• The outer layer of the cerebrum, or the cerebral cortex, is vital for perception, voluntary movement, and learning.
• The corpus callosum is a thick band of axons that enables the right and left cerebral cortices to communicate.

The Cerebellum

• This organ coordinates movement and balance and helps in learning and remembering motor skills.

The Diencephalon

• This gives rise to the thalamus, hypothalamus, and epithalamus (including pineal gland).
• The hypothalamus constitutes a control center that includes the body’s thermostat and central biological clock.
• The thalamus acts as a hub, relaying information between different subcortical areas and the cerebral cortex (main input center for sensory information going to cerebrum).

The Brainstem

• Consists of the midbrain, the pons, and the medulla oblongata (or medulla).
• The midbrain receives and integrates sensory information and sends it to specific regions of the brain.
• The pons and medulla are located to transfer information between the PNS and the midbrain and forebrain.
• The medulla is also in control of several automatic functions such as breathing, heart and blood vessel activity, swallowing, vomiting, and digestion

Arousal and Sleep

• The brainstem and cerebrum control arousal and sleep
• Arousal is a state of awareness of the external world
• Arousal and sleep are controlled in part by the reticular formation, a network formed mainly of neurons in the midbrain and pons
• The reticular formation controls the timing of sleep periods characterized by rapid eye movements (REMs) and by vivid dreams
• Sleep is also regulated by the biological clock and regions of the forebrain that regulate intensity and duration
• Sleep is a state in which external stimuli are received but not consciously perceived
• Sleep is an active state for the brain
• Some animals have adaptations that allow for substantial activity during sleep
• Sleep is essential for survival and may play a role in the consolidation of learning and memory

Biological Clock Regulation

• Cycles of sleep and wakefulness are examples of circadian rhythms, daily cycles of biological activity
• Circadian rhythms rely on a biological clock, a molecular mechanism that directs periodic gene expression and cellular activity
• Biological clocks are typically synchronized to light and dark cycles
• In mammals, circadian rhythms are coordinated by clustered neurons in the hypothalamus, called the suprachiasmatic nucleus (SCN)
• The SCN acts as a pacemaker, synchronizing the biological clock
• It maintains a roughly 24-hour cycle, even in the absence of environmental cues
• Is sensitive to stimulation by light via the retina
• Transmits info to the hypothalamus and pineal gland

Emotions

• Generation and experience of emotions the amygdala, hippocampus, and parts of the thalamus.
• These structures are grouped as the limbic system

Functional Imaging of the Brain

• Brain structures are studied with functional imaging techniques
• Positron-emission tomography (PET) enables a display of metabolic activity through injection of radioactive glucose
• Today, many studies rely on functional magnetic resonance imaging (fMRI), in which brain activity is detected through changes in local oxygen concentration
• The range of applications for fMRI include monitoring recovery from stroke, mapping abnormalities in migraine headaches, and increasing the effectiveness of brain surgery

CONCEPT 49.3: The cerebral cortex controls voluntary movement and cognitive functions

• The cerebrum, the largest structure in the human brain, is essential for language, cognition, memory, consciousness, and awareness of our surroundings.
• Each lobe of the cerebral cortex (frontal, temporal, occipital, and parietal) is the focus of specific brain activities. These brain activities include:
  • Sensory areas
  • Association areas
  • Motor areas
• The cerebral cortex receives input from sensory organs and somatosensory receptors
• Somatosensory receptors provide information about touch, pain, pressure, temperature, and the position of muscles and limbs
• Within sensory pathways, some carry sharp acute pain, others for slower throbbing pain

Focus on the CNS - Functions

• Afferent sensory pathway
• The sensory pathway enters the spinal cord
• Sometimes across midline
• The pathway travels up the white matter tract
• Transfers into the brainstem nuclei
• Sent to the thalamus nucleus
• The thalamus distributes the sensory motor information to the somatosensory for central processing.

Information Processing

• There is specificity to how types of inputs are distributed within the brain, for instance, specific distribution in thalamus
• Information received at the primary sensory areas is passed to nearby association areas that process particular features in the input.
• Integrated sensory information passes to the prefrontal cortex, which helps plan actions and movements.
• In the somatosensory cortex and motor cortex, neurons are arranged according to the part of the body that generates input or receives commands.

Language and Speech

• Studies of brain activity have mapped areas responsible for language and speech.
• Broca’s area is speech formation
• Damage to it results in can understand language but cannot speak.
• Wernicke’s area is speech comprehension
• Damage to this area may result in inability to understand language even though the person can still speak.
• Lateralization of Cortical Function
• The two hemispheres make distinct contributions to brain function.
  • The left hemisphere is more adept at language, math and logical operations.
  • The right hemisphere is stronger at facial and pattern recognition, spatial relations, and nonverbal thinking.
• The difference in hemisphere function is called lateralization.
• The two hemispheres work together by communicating through the fibers of the corpus callosum.
• Severing the connection between hemispheres results in a “split-brain” effect

Frontal Lobe Function

• The frontal lobes have a substantial effect on "executive functions," such as decision making, problem solving, judgement, impulse control, social and sexual behavior, motor function, spontaneity, memory, language
• Damage may impair decision making and emotional responses but leave intellect and memory intact (Phineas Gage, frontal lobe tumors, and frontal lobotomy)
• Doesn't fully develop until mid-20's-30ish...

Evolution of Cognition in Vertebrates

• Previous ideas that a highly convoluted neocortex is required for advanced cognition may be incorrect.
• The anatomical basis for sophisticated information processing in birds (without a highly convoluted neocortex) appears to be the clustering of nuclei in the top or outer portion of the brain (pallium)

CONCEPT 49.4: Changes in synaptic connections underlie memory and learning

• Formation of the nervous system occurs in steps:
• Regulated gene expression and signal transduction determine where neurons form
• Then neurons compete for growth-supporting factors in order to survive
• Only half the synapses that form during embryo development survive into adulthood
• In the final phase, synapse elimination occurs
• Neuronal plasticity describes the ability of the nervous system to be remodeled after birth
• Changes can strengthen or weaken signaling at a synapse
• A defect in neural plasticity may underlie autism spectrum disorder
• Children affected with autism display impaired communication and social interaction, as well as stereotyped, repetitive behaviors
• There is evidence that autism spectrum disorder involves a disruption of activity-dependent remodeling at synapses.
• There is a strong genetic contribution to this and related disorders
• Neuronal plasticity is essential to the formation of memories
• Short-term memory is accessed via temporary links formed in the hippocampus
• In long-term memory, the links in the hippocampus are replaced by connections in the cerebral cortex
• Some consolidation of memory is thought to occur during sleep
• Why two systems? It is thought that a delay may help integrate new memories into existing infrastructure – providing meaningful associations

Long-Term Potentiation

• LTP (Long-term potentiation) involves a lasting increase in the strength of synaptic transmission
• Involves vertebrate brain
• LTP involves two types of glutamate receptors on the receiving cell
• The receptors on the postsynaptic membrane change in response to a stimulus
• Involves presynaptic neuron that releases the neurotransmitter glutamate

CONCEPT 49.5: Many nervous system disorders can now be explained in molecular terms

• Disorders of the nervous system include schizophrenia, depression, drug addiction, Alzheimer's disease, and Parkinson's disease
• Genetic and environmental factors contribute to diseases of the nervous system
• To distinguish between genetic and environmental variables, scientists often carry out family studies

Schizophrenia

• About 1% of the world's population suffers from schizophrenia
• Schizophrenia typically is characterized by hallucinations and delusions
• Research suggests that schizophrenia affects neuronal pathways that use dopamine as a neurotransmitter.
• Many drugs that alleviate the symptoms of schizophrenia block dopamine receptors
• There is a very strong genetic component to schizophrenia; however, there is also a strongenvironmental influence.
• Two broad forms of depressive illness exist
  • Major depressive disorder involves persistent lack of interest or pleasure in most activities
  • Bipolar disorder is characterized by manic and depressive phases
• Treatments for these types of depression include drugs that increase the activity of biogenic amines in the brain
• The brain's reward system rewards motivation with pleasure
• The neurotransmitter dopamine is associated with the reward system
• Some drugs are addictive because they increase activity of the brain's reward system
• These drugs include cocaine, amphetamine, heroin, alcohol, and tobacco.
• Drug addiction is characterized by compulsive consumption and an inability to control intake
• Addictive drugs enhance the activity of the dopamine pathway
• Drug addiction leads to long-lasting changes in the reward circuitry that cause a craving for the drug
• It is hoped that insights gained by research in this field will lead to better prevention and treatment of drug addiction

Alzheimer's Disease

• Has been found as a mental deterioration (dementia)
• Characterized by confusion and memory loss
• The incidence of Alzheimer's disease increases with age
• The disease is associated with the formation of amyloid plaques and neurofibrillary tangles in the brain
• It results in massive shrinkage of brain tissue, reflecting the death of neurons in many areas of the brain
• The neurofibrillary tangles observed in Alzheimer's disease are also found as a component of tau protein
• There is no cure for this disease, though some drugs are effective at relieving symptoms.

Parkinson's Disease

• A motor disorder caused by the death of dopamine-secreting neurons in the midbrain
• The disease is characterized by muscle tremors, flexed posture, and a shuffling gait
• Most cases of Parkinson's disease lack an identifiable cause, although one form is known to have a genetic basis.
• Parkinson's disease can be treated but not cured
• A dopamine-related drug, called L-dopa, can reduce the severity of Parkinson's disease symptoms
• Huntington's Disease:
  • Loss of striatum ("stripey" basal ganglia structures)
  • Loss of motor modulation, “move more”
  • Autosomal dominant
  • CAG trinucleotide repeat expansion in huntingtin gene
  • Late onset (30-50)
  • Fatal usually within 20 years of diagnosis

Future Directions in Brain Research

• In 2014, the National Institutes of Health and other U.S. government agencies launched a 12-year project, called BRAIN (Brain Research through Advancing Innovative Neurotechnologies)
• The objective is to:
  • Map brain circuits
  • Measure activity within those circuits
  • Understand how this activity is translated into thought and behavior