The Nervous System

Chapters 11-15

General Overview• Master controlling and communications system of

the body• Rapid & specific electrical impulses cause almost

immediate responses

Three overlapping functions:1. Gather sensory input by monitoring internal and

external stimuli (changes) using millions of sensory receptors

2. Integration - Processes and interprets sensory input and makes decisions about what should be done

3. Effects a motor output (response) by activating muscles or glands.

The nervous system works in conjuction with the endocrine system to maintain homeostasis. The electrical impulses of the N.S. cause a more rapid response than the chemical hormones of the endocrine system.

Structural Classification• Central Nervous System:– Brain and spinal cord– Integrating & command center:

• Interpret incoming sensory info• Issue instructions based on past experience and current

conditions

• Peripheral Nervous System:– Nerves that extend from the brain and spinal cord (spinal

& cranial nerves)

– Serve as communication lines• Carries sensory input to the to CNS• Carries motor output from CNS to appropriate effector

(gland/muscle)

Functional Classification• Only applies to peripheral nervous system• Two sub-divisions– Sensory (afferent): Conveys impulses from sensory

receptors to CNS• Somatic sensory fibers: from skin, skeletal muscles & joints• Visceral sensory fibers: from visceral organs

– Motor (efferent):Carries impulses from CNS to effector organs, muscles & glands & effect a motor response• Somatic (voluntary): allows conscious or voluntary control of

skeletal muscles; reflexes are initiated involuntarily by same fibers

• Autonomic (involuntary): regulates involuntary events of smooth muscle, cardiac muscle & glands– Sympathetic: Mobilizes body systems during emergency (speed up)– Parasympathetic: Conserves energy & promotes non-emergent function

Nervous Tissue: Supporting Cells• CNS supporting cells are categorized as

neurologia, (“nerve glue”) or simply glia– 4 main categories• Astrocytes• Microglial cells• Ependymal cells• Oligodendrocytes

– support, insulate & protect the neurons:

• PNS supporting cells are either Schwann cells or satellite cells

Neuroglia: Astrocytes• Account for nearly half of

the neural tissue• “star” shaped - numerous

projections with swollen ends that cling to neurons anchoring them to capillaries; help mediate exchange between the two

• Help control chemical environment in the brain– Pick up excess ions– Recapture released

neurotransmitters

Neuroglia: Microglial Cells

• Spiderlike phagocytes• Dispose of debris – Dead brain cells– Bacteria

Neuroglia: Ependymal Cells• Lines the cavities of

the brain and spinal cord

• Beating of their cilia helps circulate cerebrospinal fluid within the cavities to form a protective cushion

Neuroglia: Oligodendrocytes• Wrap flat extensions tightly around nerve fibers• Produce fatty insulating covers around nerve cells

called myelin sheath

Schwann CellsForm myelin sheaths of around nerve fibers of the PNS

Satellite CellsAct as protective, cushioning cells

Neuron Anatomy• Cell body contains nucleus & metabolic

center• Dendrites conduct impulses toward the

cell body• Axons transmit impulses away from cell

body• Axons branch into many axon

terminals at the end• When impulses reach end of axon

terminals they stimulate the release of neurotransmitters into the extracellular space

• Synaptic clefts separate axon terminals of one neuron from dendrites of the next

• Myelin protects & insulates nerve fibers and increases transmission rate

• Gaps in myelin, called nodes of Ranvier, exist at regular intervals b/c myelin is formed from individual Schwann cells

CNS Myelin v. PNS Myelin• PNS myelin is formed from Schwann cells and CNS myelin is

formed from oligodendrocytes• Oligodendrocytes can coil around 60 fibers simultaneously

and the sheaths they form lack a neurilemma (outer cytoplasmic layer of cells)

• The neurilemma remains mostly intact when neurons are damaged and plays a large role in fiber regeneration, which is essentially absent in the CNS.

Comparing Neurons: CNS v. PNS• In the CNS:

– cell bodies are found in clusters called nuclei within bony skull or vertebral column

– Bundles of nerve fibers (neuron processes) are tracts

– White matter refers to myelinated regions (carry messages) and gray matter refers to unmyelinated regions (carry nutrients & convert glucose to energy)

• In the PNS:– small collections of cell bodies

called ganglia may be found– Bundles of nerve fibers are called

tracts

Functional Classification of Neurons• Neurons are grouped according to the direction the

impulse is traveling relative to the CNS• Sensory (afferent) neurons:– Always found in ganglia outside CNS– Dendrite endings usually associated with specialized

receptors

• Interneurons: – Connect motor & sensory neurons– Cell bodies are always in CNS

• Motor (efferent) neurons:– Carry impulses from CNS to viscera, muscles, glands– Cell bodies always in CNS

Structural Classification of Neurons• Based on number of processes extending from the cell

body• Multipolar: several processes– Most common structural type– Includes all moter and association neurons

• Bipolar: two processes (axon and dendrite)– Rare in adults– Act as sensory receptors in special sense organs (eye & ear)

• Unipolar: single process– Very short process; divides almost immediately into

proximal (central) and distal (peripheral) fibers– Only small branches at the end of peripheral fibers are

dendrites, the rest function as axons and therefore carry impulese both toward and away from cell body

Structure Differences in Neurons

Functional Properties of Neurons• Irritability: Ability to respond to stimulus and convert

it to a nerve impulse• Conductivity: Ability to transmit impulses to other

neurons, muscles or glands

The Nerve Impulse: Polarization • Resting neurons have a polarized membrane– Fewer positive ions inside the plasma membrane then in

the surrounding tissue fluid– Major internal ion is K+, major external ion is Na+

– As long as internal environment is relatively negative, neuron will stay inactive

The Nerve Impulse: Depolarization• Neural response to stimuli is always the same– Permeability of plasma membrane changes briefly– Normally it is virtually impermeable to sodium– Adequate stimulus causes “sodium gates” to open– Sodium will rush into the neuron along its

concentration gradient

• Polarity of cell membrane is temporarily changed; inside is now more positive and the cell is in a depolarized state

• This activates the neuron to transmit an action potential or nerve impulse

• Nerve impulses are an “all or none” response

The Nerve Impulse: Repolarization• Membrane almost immediately becomes impermeable to

sodium again• Potassium ions rapidly diffuse out of the neuron into the the

tissue space, restoring the electrical conditions of the resting state

• The neuron is now repolarized and initial sodium and potassium concentrations are restored by the Na/K pump

Propagation of Nerve Impulse

Animation

Nerve Impulses Along Myelinated Fibers• Fibers that have myelin sheaths conduct impulses much faster

because the nerve impulse literally jumps from node to node along the length of the fiber; “saltatory conduction”

• No current can flow along the axonal membrane where there is no fatty insulation

• While an action potential is occurring, the sodium channels are open or recovering and another action potential definitely cannot occur. This is often referred to as absolute refractory period.

• During hyperpolarization, the postassium channels are also open. Action potentials are more difficult to generate than at resting potential thus making this the relative refractory period.

Rate of Nerve Impulses• Myelin: myelin increases rate of impulse– Continuous conduction: action potentials are

generated continuously along the axon– Saltatory conduction: APs jump from node to node

• Axon diameter: larger diameter = faster impulses– Type A: Largest diameter & thick myelin (150m/s)– Type B: lightly myelinated, intermediate diameter (15 m/s) – Type C: smallest diameter and unmyelinated ((1m/s)

Synapses

Neural Communication at Synapses• When the action potential reaches the axonal endings, the

axon terminals release chemicals called neurotransmitters• These neurotransmitters diffuses across the synapse and

bind to receptors on the membrane of the next neuron• If enough neurotransmitter is released a nerve impulse will

occur.

Post-Synaptic Potentials • Neurotransmitter synapses are

categorized according to how they affect the post-synaptic neuron, excitatory or) inhibitory

• EPSPs (excitatory post-synaptic potentials) open both Na+ and K channels resulting in a net depolarization. This doesn’t generate an action potential but helps trigger one at the axonal hillok.

• IPSPs induce hyperpolarization by making the membrane more permeable to K+ or Cl-. This makes the inside more negative and threshold harder to reach.

Factors that Impair Nerve Impulses

• Alcohol, sedatives and anesthetics block nerve impulses by reducing membrane permeability to sodium ions – If sodium cannot enter an action potential cannot

occur

• Cold or continuous pressure interrupt blood circulation therefore reducing oxygen and nutrient delivery to neurons– Warming or removal of pressure results in a surge

of impulses and the uncomfortable ‘prickly’ feeling.