The Nervous System

• Nervous System allows organisms to respond to external and internal stimuli - Consists of:

– Brain and spinal cord – Central Nervous System

– Peripheral Nerves – Peripheral Nervous System

– Neurons – functional unit of the nervous system, specialized cells for transmitting electrical and chemical signals

Anatomy of a Nerve cell:1. Cell body - contains nucleus, most of the cytoplasm and

most of the organelles• Dendrites and axon extend from the cell body

2. Dendrites - short and highly branched• receive stimulus and send to cell body

3. Axon - conducts impulses away from the cell body to another neuron or to a muscle or gland• microscopic in diameter but may extend a meter

or more in length• may divide forming branches – axon collaterals• divides at the end to form terminal branches that

end in synaptic terminals• synaptic terminals release neurotransmitters

(chemicals) that transmit impulse across the tiny gap between neurons – synapse

4. Myelin sheath – fatty mat’l surrounding the axons of neurons outside the CNS (sheath made of neuroglia in CNS) speed up transmission of impulse

– composed of Schwann cells that form insulation

– Nodes of Ranvier – gaps between Schwann cells (myelin sheath)

• Nerve – consists of hundreds or thousands of axons wrapped together in connective tissue

• in the CNS, bundles of axons are called tracts or pathways instead of nerves

• Ganglia – outside the CNS, cells bodies are usually grouped together in masses called ganglia

• inside CNS, collection of cell bodies called nuclei

Types of Neurons1. sensory (afferent) neurons – conduct impulses into CNS

from the periphery (sensory impulses)• Pick up stimulus from sensory receptors – mechanoreceptors,

chemoreceptors, thermoreceptors, photoreceptors

2. Interneurons (association neurons) – afferent neurons usually transmit impulses to interneurons

– located within CNS– neurons that integrate input and output– integration involves sorting and interpreting incoming sensory

information and determining the appropriate response– forms connecting lines b/w sensory and motor neurons

3. motor (efferent) neurons – transmit messages from CNS to effectors (musc. or gland)

4. sensory receptors, afferent and efferent neurons are part of the Peripheral Nervous System

Neuroglia (glial cells) – “nerve glue” – support cellsfor neurons in the CNS – three main types:1. Microglia – found near blood vessels,

phagocytes that migrate and remove foreign and degenerated material

2. Astrocytes – star shaped glial cells that have a variety of functions:

– some are phagocytic and remove invading microorganisms and debris from nervous tissue

– help regulate concentration of potassium ions in extracellular fluid of nervous tissue

– regulate concentration of neurotransmitters

3. Oligodendrocytes – glial cells that envelope neurons in CNS, forming insulating sheaths – speeds transmission of impulse

How Neurons WorkMembrane potential (resting potential)difference in electrical charge across theplasma membrane• Resting Potential (not conducting an impulse)

more negatively charged inside the cell compared to the interstitial fluid outside

membrane of neuron is polarized due to unequal distribution of ions – as a result the cell can produce an action potential (impulse)

Electrochemical potential -70mV slight excess of positive ions outside the membrane

and slight excess of negative ions inside the membrane

Na+ concentration is 10x greater outside the cell and K+ concentration 10x greater inside the cell

ion pumps, ion channels and gates cause a specific distribution of ions across the cell membrane

sodium-potassium pumps in the membrane pump Na+ out and K+ into cell – both are pumped against their concentration gradient (ATP) – for every 3 Na+ pumped out, 2 K+ are pumped in (more positive ions outside than in)

K+ tends to leak out by diffusion through ion channels causing further negative charge inside as compared to outside of cell

ion channels that allow the passage of Na+ are closed at resting potential

• Stimulation – all or none response (action potential)

• Threshold stimulus – minimum amount needed for depolarization to occur

• causes Na+ ion channels to open allowing Na+ to rush into interior of cell (depolarization)

• disturbs adjacent areas – Na+ channels open causing a depolarization wave – action potential

• polarity across membrane is momentarily reversed

• K+ channels also open but more slowly allowing repolarization

• Repolarization – after action potential passes, membrane begins to repolarize

• Na+ channels close and membrane become impermeable to Na+

• open K+ channels allow K+ leak out of the neuron repolarizing the membrane

• impulse is actually a series of depolarization and repolarization waves sweeping down the axon (takes place in less than 1 millisecond)

• resting conditions must be reestablished by sodium-potassium pumps

• Absolute Refractory Period – millisecond during which membrane is depolarized and cannot conduct an impulse

• Relative Refractory Period – enough Na+ channel gates have been reset that axon can carry impulse but threshold is higher

Impulse conduction video

Impulse conduction• impulse conduction is slower in unmyelinated

axons – continuous conduction – entire axon must depolarize

• diameter of axon affects speed of transmission• larger diameters transmit faster• vertebrate neurons are myelinated – speeds

up transmission• depolarization occurs only at the nodes of

Ranvier – action potential “jumps” from one node to the next – saltatory conduction

• transmission is much faster than in unmyelinated axons

Transmission across Synapses• synapse – gap between axon of one neuron and

dendrites of the next or between a neuron and an effector

• synapse between neuron and muscle cell is called a neuromuscular junction or motor end plate

• neurotransmitters act as chemical messengers to conduct the signal across the synapse

• synaptic vesicles contain neurotransmitter• When action potential reaches axon terminal,

calcium ions begin to diffuse in – Ca+ influx• vesicle fuses to presynaptic membrane and

dumps transmitter into synaptic cleft – diffuses across synapse

• neurotransmitter binds to highly specific receptors on the postsynaptic membrane (specific to the type of neurotransmitter) – binding begins depolarization and impulse continues

• enzymes in cleft decompose neurotransmitter to free up receptor sites for next impulse or the neurotransmitter is actively transported back into presynaptic vesicles (reuptake)

Neurotransmitters each have a different function:• Excitatory – stimulate neurons

– acetylcholine (stimulate muscle contraction)– norepinephrine, dopamine, and serotonin (affect mood)

• Inhibitory – stop depolarization– Gamma-aminobutyric Acid (GABA) – inhibits neurons in

brain and spinal cord• excitatory postsynaptic potential (EPSP) – if a

neurotransmitter is excitatory, it results in an EPSP) – causes partial depolarization bringing neuron closer to

firing– one EPSP is probably too weak to trigger an action

potential – EPSPs can be added together (summation) – results in firing of neuron

• inhibitory postsynaptic potential (IPSP) – occur when neurotransmitter causes postsynaptic membrane to hyperpolarize – brings membrane potential farther away from threshold and a stronger stimulus would be necessary to fire it

Organization of Neurons• CNS contains millions of neurons organized into

neural networks• neurons are organized into neural circuits

(specific pathways) within each network• Neural Circuits in all of the networks share many

organizational features:– Convergence – a single neuron is controlled by

converging signals from two or more presynaptic neurons – important mechanism by which CNS can integrate information from various sources

• Divergence – a single presynaptic neuron stimulates many postsynaptic neurons

• Reverberating circuit – an axon colateral synapses with an interneuron in a sequence that can send new impulses through the circuit

• example of positive feedback• new impulses generated again and again until fatigue or stopped

by inhibition (impt in cyclic behaviors such as breathing)

Central Nervous System• Vertebrate Nervous System is divided into

the Central Nervous System (brain and spinal cord) and the Peripheral Nervous System (sensory receptors and the nerves which act as communication lines)

• Parts of the body are linked to the brain by cranial nerves and to the spinal cord by spinal nerves

• Afferent neurons “inform” the CNS of changing conditions, efferent nerves transmit the “decisions” of the CNS

• Evolution of the Vertebrate Brain

• all vertebrates have same basic brain structure

• different parts are specialized in different vertebrate classes

• evolutionary trend is toward increasing complexity

• all vert. brains start off as three bulges that swell at the anterior end of the neural tube – these eventually become the hindbrain, midbrain, and forebrain

Hindbrain develops into medulla, pons, and cerebellum

(brain stem)

1. medulla – continuous with spinal cord– serves as a vital center - regulates

respiration, heartbeat, blood pressure, swallowing, coughing, and vomiting

2. cerebellum – coordinates muscle activity, responsible for muscle tone, posture and equilibrium

3. pons – bridge connecting the spinal cord and medulla with upper parts of the brain

Midbrain • most prominent part of brain in fish and

amphibians• acts as main association area (receives sensory

info, integrates it, and sends decisions to motor nerves)

• contains optic lobes for visual interpretations• birds, reptiles, and mammals

– consists of superior colliculi (center for reflexes such as pupil constriction) and inferior colliculi (centers for some auditory reflexes)

– also contains a center to help maintain muscle tone and posture

Forebrain develops into thalamus, hypothalamus, cerebrum,and olfactory bulbs1. thalamus – acts as a relay center for motor and

sensory messages – in mammals, all sensory info passes through thalamus before sent to cerebrum

2. hypothalamus – contains olfactory centers and acts a principal integration center for regulation of internal organs

– controls body temp in reptiles, birds, and mammals– regulates appetite, water balance, and emotional

response– links the nervous system to the endocrine system and

produces certain hormones3. Olfactory bulbs – impt in chemical sense of smell

4. Cerebrum – divided into right and left hemisphere

– most of cerebrum made of white matter (consists mainly of myelinated axons that connect to various parts of the brain)

– Cerebral Cortex – layer of gray matter which makes up the outer portion of the cerebrum (contains cell bodies and dendrites)

– most prominent part of brain in mammals– surface area is increased by convolutions– functions:

• conscious interpretations of sensations• voluntary muscle movement• memory• reasoning and problem solving• intelligence and personality