The master controlling and communicating system of the body

Functions Sensory input – monitor

internal and external stimuli

Integration – interpretation of sensory input

Motor output – response to stimuli by activating effector organs

Central nervous system (CNS) Consists of Brain and spinal cordControls entire organism Integrates incoming information and responses

Peripheral nervous system (PNS)Link between CNS, body and environment Consists of Spinal and cranial nervesCarries messages to and from the spinal cord and brain

Sensory (afferent) divisionSensory afferent fibers –

carry impulses from skin, skeletal muscles, and joints (sensory receptors) to the brain

Visceral afferent fibers – transmit impulses from visceral organs to the brain

Motor (efferent) division Transmits impulses from the

CNS to effector organs (muscles, glands)

Two Divisions:

Somatic nervous system (Voluntary)Conscious control of skeletal

musclesConducts impulses from the

CNS to skeletal muscles

Autonomic nervous system (ANS)Regulates smooth muscle,

cardiac muscle, and glandsSubconscious or involuntary

control

Sympathetic Nervous System “Flight or fright system” Most active during physical activity

Parasympathetic nervous systemRegulates resting or vegetative functions such

as digesting food or emptying of the urinary bladder

The two principal cell types of the nervous system are:Neurons – excitable cells that transmit electrical signals

Supporting cells – cells that surround and wrap neurons

Structural units of the nervous system

Receive stimuli and transmit action potentials

Long-life

Amitotic

Have a high metabolic rate

Each neuron consists of:BodyAxondendrites

Cell Body (soma or perikaryon)

Contains the nucleus and a nucleolus & usual organelles

Has no centrioles (amitotic nature)

Has well-developed Nissl bodies (rough ER)

Nissl bodies -primary site of protein synthesis

Contains an axon hillock – cone-shaped area from which axons arise

Short, branched cytoplasmic extensions

They are the receptive, or input, regions of the neuron

Electrical signals are conveyed toward the cell body

Slender processes of uniform diameter arising from the axon hillock

Initial segment: beginning of axon

Axoplasm : Cytoplasm of axon

Axolemma : Plasma membrane of axon

Long axons are called nerve fibers

Usually there is only one unbranched axon per neuron

Rare branches, if present, are called axon collaterals

Presynaptic (Axon) terminal – branched terminus of an axon

Trigger zone: site where action potentials are generated; axon hillock and part of axon nearest to cell body

AP are conducted along the axons to axonal terminals and release neurotransmitters

AP conduction away from cell body

Neurons can be classified by structure: Multipolar Most common in both CNS & PNS Single axon, many dendrites (motor

neurons and interneurons of CNS) Bipolar two processes (one axon and one

dendrite) Are sensory neurons found in the

retina, olfactory nerve Unipolar single short process extending from

cell body Divides into two branches and

functions as both dendrite and axon (sensory neurons , dorsal root ganglia)

Neurons can be classified by function:Sensory (afferent) — transmit impulses from

receptors toward the CNS

Motor (efferent) — carry impulses from CNS to muscles and glands

Interneurons (association neurons) Link sensory and motor neurons within CNS Make up 99% of neurons in body

Neuroglia or glial cells: Supporting cells Surround neurons Mitotic Nonconducting 6 types

2 in PNS4 in CNS

1) Astrocytes In CNS only

Anchor neurons to capillaries

Regulate what substances reach the CNS from the blood (blood-brain barrier)

Regulate extracellular brain fluid composition

Pick up excess K+

Recapture released (recycle) neurotransmitters

2) Ependymal CellsCNS only

Line the cavities of the brain and spinal cord

Ciliated

Circulate the cerebrospinal fluid (CSF)

3) MicrogliaCNS only

Migrate toward injured neurons

Specialized macrophages

Phagocytize necrotic tissue, microorganisms, and foreign substances that invade the CNS

4) OligodendrocytesCNS only

Wrap extensions around neuron fibers (axons)

Form myelin sheath

1) Schwann Cells or NeurolemmocytesPNS only

Wrap around axons of neurons in the PNS

Forms myelin sheath

2) Satellite Cells PNS only

Surround neuron cell bodies

Provide support and nutrients to neuronal cell bodies

Protect neurons from heavy metal poisons (lead, mercury) by absorbing them

Myelinated Axons: Whitish, fatty (protein-lipid), segmented sheath around most long axons

Functions: Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse

transmission

Formed by Schwann cells in the PNS

In CNS formed by oligodendrocytes

Nodes of Ranvier : Gaps in the myelin sheath between adjacent Schwann cells

Unmyelinated Axons : Schwann cell surrounds nerve fibers but coiling does not take place

White matter – dense collections of myelinated fibers

Gray matter – mostly nerve cell bodies and unmyelinated fibers

In brain: gray is outer cortex as well as inner nuclei; white is deeper

In spinal cord: white is outer, gray is deeper

Synapse Junction between one neuron and

another

Where two cells communicate with each other

Presynaptic neuron – conducts impulses toward the synapse

Postsynaptic neuron – Cell that receive the impulse

Most are axodendritic or axosomatic

Electrical Synapses:Are gap junctions that allow

ion flow between adjacent cells by protein channels called Connexons

Not common in CNS

Found in cardiac muscle and many types of smooth muscle Action potential of one cell causes action potential in next cell

Chemical SynapsesMost are this type

Neurotransmitter released from synaptic vesicles of presynaptic neuron

Neurotransmitter binds to receptors on postsynaptic membrane

Binding of neurotransmitter to receptor permeability change in postsynaptic membrane

Released at chemical synapses

In response to AP Voltage-regulated calcium channels open

Ca2+ diffuse into presynaptic terminal

And causes synaptic vesicles to fuse with presynaptic membrane

This fusion releases neurotransmitter into the synaptic cleft via exocytosis

When bound to receptors on postsynaptic neuron, the neurotransmitter can either excite or inhibit the postsynaptic neuron

Resting neurons maintain a difference in electrical charge inside and outside cell membrane = RESTING MEMBRANE POTENTIAL (RMP)

The inside of the resting neuron is negatively charged, the outside is positively charged.

Concentration of K+ higher inside than outside cell

Na+ higher outside than inside

RMPs vary from -40 to -90mV in different neuron types

When bound to receptors on the postsynpatic neuron membrane: Causes the opening of positive

ion channels

Sodium ions enter rapidly

RMP becomes more positive

This positive change in the RMP is called depolarization

This brings the neuron closer to firing

• A positive change in the RMP– Caused by influx of

positive ions

– Causes the inside of the cell membrane to become less negative

– Depolarization spreads to adjacent areas

When bound to receptors on the postsynaptic membrane: Make the membrane more permeable

to negative ions (usually Cl-)

As negative ions rush into the neuron, the RMP becomes more negative

The negative change in the RMP = hyperpolarization

Brings the neuron farther from firing

• A negative change in RMP

• Usually caused by influx of chloride ions

• Decreases the likelihood of the neuron firing

• Short changes in the RMP in small regions of the membrane

• Can be positive or negative (depolarize or hyperpolarize the membrane)

• Alone, not strong enough to initiate an impulse

• summate or add onto each other

• Together, can trigger a nerve impulse (action potential)

EPSP (Excitatory Postsynaptic Potential) When depolarization occurs, response is

stimulatory & graded potential is called EPSP

Binding of a neurotransmitter on the postsynaptic membrane more positive RMP, reaches threshold (depolarization occurs)

producing an action potential and cell response

IPSP (Inhibitory Postsynaptic Potential) When hyperpolarization occurs,

response is inhibitory & graded potential is called IPSP

Binding of the neurotransmitter on the postsynaptic membrane more negative RMP (hyperpolarization)

Decrease action potentials by moving membrane potential farther from threshold

40 to 50 Known NeurotransmittersAcetylcholine (ACh) Norepinephrine (NE)GABADopamineSerotonin

Action Potential = Nerve Impulse

Consists of: Depolarization Propagation Repolarization

If depolarization reaches threshold (usually a positive change of 15 to 20 mV or more), an action potential is triggered

The positive RMP change causes electrical gates in the axon hillock to open

Sudden large influx of sodium ions causes a reversal in the membrane potential (becomes approx. 100mV more positive)

Begins at the axon hillock and travels down the axon

Chemically gated channels – open with binding of a specific neurotransmitter

Voltage-gated channels – open and close in response to membrane potential

Chemically Gated(on dendrite or soma)

Voltage Gated(on axon hillock and axon)

Movement of the action potential down the axolemma

voltage-gated sodium channels open in response to positive RMP change

Restoration of the RMP back to it’s negative state

A repolarization wave follows the depolarization wave

3 factors contribute to restoring the negative RMP: Sodium (Na+) gates close (it no longer

enters) Potassium (K+) gates open, potassium

rushes out Sodium/potassium pump kicks in

An active process: requires cellular energy

Actively pumps 3 sodium (Na+)

ions out of the cell and 2 potassium (K+) ions in

Potassium leaks back out

Period of time when electrical sodium gates are open

From beginning of action potential until near end of repolarization

No matter how large the stimulus, a second action potential cannot be produced

The interval following the absolute refractory period when:

Sodium gates are closedPotassium gates are openRepolarization is occurring

A stronger-than-threshold stimulus can initiate another action potential

A single EPSP cannot induce an action potential

EPSP’s can add together or SUMMATE to initiate an action potential

Spatial Summation Large numbers of axon

terminals stimulate the postsynaptic neurons simultaneously

Temporal SummationOne or more presynaptic neurons transmit impulses in rapid fire succession

An action potential is an “all or none” phenomenon

When threshold is reached, the action potential will occur completely

If threshold is not reached, the action potential will not occur at all

Occurs only in myelinated axons

Depolarization wave jumps from one node of Ranvier to the next

Results in faster nerve impulse transmission

A nerve impulse in the presynaptic neruon causes release of neurotransmitter into synaptic cleft

Neurotransmitter binding to receptors on postsynaptic neuron dendrite or soma cause certain chemically gated ion channels to open

If Na+ channels open: Rapid influx of Na+ ions (depolarization) A small positive graded potential occurs (EPSP) If RMP changes in a positive direction by 20mV (or reaches the

threshold), voltage gated sodium channels in the axon hillock open Sodium rushes in at the axon hillock resulting in an action potential As the positive ions get pushed down the axon, more voltage gated

sodium channels open and the depolarization continues down the axon (propagation)

The process of restoring the negative RMP begins immediately following the depolarization wave (repolarization)

The larger the axon diameter, the faster the impulse travels

Myelinated axons conduct impulses more rapidly

Fiber Types: Type A fibers

Large diameter axon with thick myelin sheath

Impulse travels at 15 to 150 m/sec. Sensory and motor fibers serving skin,

muscles, joints Type B fibers

Intermediate diameter axon, lightly myelinated

Impulse travels at 3 to 15 m/sec, Part of ANS

Type C fibers Small axon diameter, unmyelinated Slow impulse conduction (1 m/sec. or less) Part of ANS

Organization of neurons in CNS varies in complexity Convergent pathways: many

converge and synapse with smaller number of neurons. E.g., synthesis of data in brain

Divergent pathways: small number of presynaptic neurons synapse with large number of postsynaptic neurons. E.g., important information can be transmitted to many parts of the brain

Oscillating circuit: outputs cause reciprocal activation