CHAPTER 48: NERVOUS SYSTEM

ESSENTIAL KNOWLEDGE 2.e.2 – Timing and coordination of physiological

events are regulated by multiple mechanisms (11.1). 3.b.2 – A variety of intercellular and intracellular

signal transmissions mediate gene expression (11.1 & 11.4).

3.d.1 – Cell communication processes share common features that reflect a shared evolutionary history (11.2 & 11.2).

3.d.2 – Cells communicate with each other through direct contact with other cells or from a distance via chemical signaling (11.1 & 11.2).

ESSENTIAL KNOWLEDGE 3.d.3 – Signal transduction pathways

link signal reception with cellular response (11.3).

3.d.4 – Changes in signal transduction pathways can alter cellular response (11.4).

INTRODUCTION Two types of cells:

Glia (supporting) Neurons

Nervous system is comprised of two parts: Central (spinal cord, brain) Peripheral (outlying nerves)

Nervous system is a system of circuits of neurons and supporting cells that work together to communicate with rest of the body

DIVERSITY OF NERVOUS SYSTEMS Cnidarians:

Ex: hydra Nerve net (simplistic concentration of nerves)

Echinoderms: Ex: Seastar Radial nerves and central nerve ring

Flatworms: Cephalization (concentration of nervous

system in anterior/head region) Central nervous system: simple brain with 2

nerve cords

DIVERSITY OF NERVOUS SYSTEMS Annelids/Arthropods:

Ex: insects, crayfish Cephalization with complicated brain with

ventral nerve cord Also contain clusters of neurons called

ganglia Vertebrates:

Brain, dorsal spinal cord make up CNS Nerves and ganglia make up PNS

NERVOUS SYSTEM ORGANIZATION High degree of cephalization in

vertebrates Spinal cord: integrates simple responses

to stimuli and transports info to and from brain

Cerebrospinal fluid: fluid cushions brain and carries out circulatory functions

White matter: Axon in bundles (named for color of their myelin sheaths)

Gray matter: Neuron cell bodies, dendrites, and unmyelinated axons

NEURON STRUCTURE Cell body:

Contains nucleus and organelles Extensions:

Dendrites: received signals from other neurons, highly branched

Axon: transmits signals to other cells, longer Contains terminal branches called synaptic

terminals which release neurotransmitters (relay of signals across synapse)

Myelin sheath: Many axons wrapped in this insulating layer

NEURON COMMUNICATION Neurons

communicate with other cells at synapses Electrical synapses:

allow electrical current to flow directly from cell to cell (via gap junctions)

Chemical synapses: involves release of neurotransmitters

SUPPORTING CELLS (GLIA) Very numerous Give structural integrity and physiological

support to nervous system Astrocytes:

in CNS, facilitate info transfer at synapse (learning/memory), induce formation of blood-brain barrier, can act as stem cells

Radial glia: Guide embryonic growth of neurons, act as stem

cells Oligodendrocytes (CNS) and Schwann

cells (PNS): Insulate axons in mylein sheath by wrapping

around them

PROCESSING INFORMATION Three steps:

1) Sensory input Detection of external stimuli or internal

conditions Sensory neurons transmit this info to CNS

2) Integration Completed by interneurons Send output through motor neurons to effector

cells (muscle and endocrine cells) 3) Motor output

Response to signal/output Ex: reflex, hormone production and secretion

MEMBRANE POTENTIAL Membrane potential:

Electrical potential difference

Exists across the plasma membrane of all cells

Dependent upon concentration of certain ions on either side of the cell membrane

Outside cell: Na+ and Cl- Inside cell: K+ and a number

of negatively charged amino acids and other molecules

Sodium-potassium pumps maintain the concentration gradient/difference

GATED ION CHANNELS In addition to the ungated

K+ and Na+ ion channels, neurons also have gated ion channels Open and closed in response to

stimuli Stretch-gated ion channels:

found in stretch sensors, open in response to mechanical stimuli

Ligand/Chemically-gated ion channels: found in synapses, respond to chemical stimuli

Voltage-gated ion channels: found in axons, respond to change in membrane potential

RESTING POTENTIAL Resting potential:

Nontransmitting neuron

Ions continually diffuse (without energy use) through channels down their concentration gradient until balanced

Equilibrium potential:

The membrane voltage when the concentrations are balanced

Neurons at rest have more K+ channels open than Na+

ACTION POTENTIAL Action potential: when a neuron is

transmitting a signal due to the reception of a stimuli

Stimuli that open/close gated ion channels may increase or decrease membrane potential Graded potential: the stronger the stimuli =

more channels opens (and vice versa) Hyperpolarization: the result of a stimuli that

OPENS K+ channels (K+ flows OUT and membrane potential shifts)

Depolarization: the result of a stimuli that OPENS Na+ channels (Na+ flows OUT and membrane potential shifts)

ACTION POTENTIAL Once depolarization reaches a certain

membrane potential (called the threshold) an action potential is triggered Stronger stimuli = higher frequency of action

potentials Involves BOTH Na+ and K+ ion

channels Na+ channels open quickly in response to

depolarization K+ channels open more slowly

ACTION POTENTIAL Sequence of events:

1) Stimulus depolarizes membrane to threshold 2) Na+ gates open causing influx of Na+

(causing further depolarization) 3) More Na+ activation gates open, causing

membrane potential to be shifted towards Na+ concentration (rising phase)

4) Falling phase: when Na+ inactivation gates close and K+ activation gates open (bring membrane potential towards K+ concentration)

5) Undershoot: membrane’s permeability towards K+ is higher (than at rest), continual OUTFLOW of K+ temporarily hyperpolarizes membrane

PERIPHERAL NERVOUS SYSTEM Carries information to and from the CNS Regulates movement and homeostasis Made of:

Paired cranial nerves and spinal nerves Associated ganglia Contains BOTH sensory and motor neurons

Two parts: 1) Somatic nervous system

Carries signals to and from skeletal muscles 2) Autonomic nervous system

Maintains internal environment (by controlling smooth/cardiac muscles)

PERIPHERAL NERVOUS SYSTEM Autonomic NS (three divisions):

1) Sympathetic division Accelerates heart and metabolic rate Generates energy

2) Parasympathetic division Carries signals for self-maintenance activities

(digestion and slow heart rate) Conserves energy

3) Enteric division Networks of neurons that control secretions of

digestive tract, pancreas, gallbladder Control contractions of smooth muscles (peristalsis)

BRAIN Embryonic development

Forms three portions (midbrain, hindbrain, forebrain) – called cephalons

As fetus develops, these three portions specialized further into 5 regions

Forebrain: becomes cerebrum (outer portion of which becomes cerebral cortex)

Cerebral cortex: extends out and around brain Mid/Hindbrain: become brainstem,

cerebellum Brainstem: consists of midbrain, pons, medulla

oblongata

BRAINSTEM Controls (in part):

Attention span Alertness Appetite Motivation Homeostasis

Medulla oblongata (medulla): Control center for homestatic functions (breathing,

swallowing, heart and blood vessel action, digestion) Pons:

Functions w/ medulla in above activities Conducts information between the rest of the brain and

spinal cord Midbrain:

Receives and integrates sensory information

CEREBELLUM Controls learning, remembering motor skills,

coordination, error-checking (during perception, cognitive functions)

Integrates information from auditory and visual systems together with input from joints and muscles

Provides automatic coordination of movement and balance

DIENCEPHALON Includes:

Epithalamus Includes pineal gland and choroid plexus Clusters of capillaries produces cerebrospinal fluid

Thalamus Major input and sorting center for sensory information Major output center for motor information from cerebrum Receives input from cerebrum and other brain parts to

regulate emotion and arousal Hypothalamus

Major brain region for homeostatic regulation Produces posterior pituitary hormones and releases

hormones that control anterior pituitary Contains regulating centers for survival functions and

sexual/mating behaviors, alarm response and pleasure

CEREBRUM Functions:

Determines Intelligence Personality Interpretation of Sensory Impulses Motor Function Planning and Organization Touch Sensation

Divided into right and left hemispheres Communicate to each other via the corpus

callosum (thick band of axons) Each hemisphere:

Covered with gray matter (called cerebral cortex) Contains inner white matter (that includes group of neurons

important to planning and learning movements)

CEREBRUM Largest and most complex part of

mammalian brain Divided into lobes:

Frontal Lobe- associated with reasoning, planning, parts of speech, movement, emotions, and problem solving

Parietal Lobe- associated with movement, orientation, recognition, perception of stimuli

Occipital Lobe- associated with visual processing

Temporal Lobe- associated with perception and recognition of auditory stimuli, memory, and speech

CNS INJURIES AND DISEASES Schizophrenia:

Characterized by psychotic episodes involving hallucinations and delusions

Both genetic and environmental components Treatments: focus on drugs that blocking dopamine

receptors Bipolar disorder:

Involves swings of mood from high to low Includes major depression (a persistent low mood) Both bipolar and depression have genetic and

environmental components

CNS INJURIES AND DISEASES Alzheimer’s disease:

Dementia characterized by confusion, memory loss, personality changes

Age-related (more frequency with higher age) Progressive disease Involves death of neurons in large areas of the

brain

CNS INJURIES AND DISEASES Parkinson’s Disease:

Progressive, age-related motor disorder Characterized by difficulty in movements, rigidity,

muscle tremors Death of neurons lead to motor issues (from the

accumulation of a particular protein)

EXCLUSION STATEMENTS The types of nervous systems,

development of the human nervous system, details of the various structures and features of the brain parts, and details of specific neurologic processes are beyond the scope of the course and the AP Exam.