Ch.11 A&P

What are the basic functions of the nervous system?

Sensory input, Integration, Motor output

Sensory input

Information gathered by sensory receptors about internal and external changes

Integration

Processing and interpretation of sensory input

Motor output

Activation of effector organs (muscles and glands) produces a response

What are the two divisions of the nervous system

central nervous system (CNS) and peripheral nervous system (PNS)

Central Nervous System (CNS)

Brain and spinal cord, integrative, and control centers

periperal nervous system (PNS)

cranial nerves and spinal cord, communication lines between the CNS and the rest of the body

Motor Divisions of the nervous system

Sensory (afferent), Motor (efferent), Somatic system, Autonomic system, Sympathetic, Parasympathetic

Sensory (afferent) division

somatic and visceral sensory nerve fibers, conducts impulses from the receptors to the CNS

Afferent

sensory

Motor (efferent) division

motor nerve fibers, conducts impulses from the CNS to effectors (muscles and glands)

efferent

motor

Somatic nervous system

somatic motor (voluntary) conducts impulses from the CNS to the skeletal muscles

Autonomic nervous system

Visceral motor (involuntary), Conducts impulses from the cns to the cardiac muscles, smooth muscles, and glands

Sympathetic division

mobolizes body systems during activity

Parasympathetic division

Conserves energy, promotes house keeping functions during rest

Neuroglia

small cells that surround and wrap delicate neurons

Types of Neuroglia

Astrocytes (CNS), Microglial cells (CNS), Ependymal cells (CNS), Oligodendrocytes (CNS), Satellite cells (PNS), Schwann cells (PNS)

Astrocytes

CNS, Most abundant, versatile, and highly branched glial cells

glial cells

cells in the nervous system that support, nourish, and protect neurons

Astrocyte functions

Support and brace neurons, Play role in exchanges between capillaries and neurons, Guide migration of young neurons

Astrocyte functions (continued)

Control chemical environment around neurons, Respond to nerve impulses and neurotransmitters

Microglial cells

CNS, Small, ovoid cells with thorny processes that touch and monitor and migrate toward injured neurons • Can transform to phagocytize microorganisms and neuronal debris

Microglial functions

Can transform to phagocytize microorganisms and neuronal debris, regulate brain development, maintenance of neuronal networks, and injury repair

phagocytize microorganisms

Phagocytosis is a major mechanism used to remove pathogens and cell debris,

Bacteria, dead tissue cells, and small mineral particles are all examples of objects that may be phagocytized

Ependymal cells

Range in shape from squamous to columnar • May be ciliated (beat to circulate CSF), Line the central cavities of the brain and spinal column

Ependymal cell function

play a critical role in cerebrospinal fluid (CSF) homeostasis, brain metabolism, and the clearance of waste from the brain

Oligodendrocytes

Branched cells, Processes wrap CNS nerve fibers, forming insulating myelin sheaths thicker nerve fiber

Oligodendrocytes function

to myelinate central nervous system axons

Satellite cells

Surround neuron cell bodies in PNS

Satellite cells function

responsible for muscle regeneration throughout the lifespan, play a crucial role in muscle fiber maintenance, repair, and remodeling, and Function similar to astrocytes of CNS

Schwann cells

Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers

Schwann cell function

Similar function as oligodendrocytes, Vital to regeneration of damaged peripheral nerve fibers, produce the myelin sheath

Schwann Cells vs Oligodendrocytes

Oligodendrocytes are the cells that create myelin sheath around the axons of the central nervous system. Schwann Cells are the cells that create myelin sheath around the axons of the peripheral nervous system

Neurons

Structural units of the nervous system that conduct impulses, Can not divide (amitotic), High metabolic rate—requires a continuous supply of oxygen and glucose

Neuron Structional components

Neuron cell body, Neuron processes, Dendrites, The axon

Neuron cell body

Biosynthetic center of neuron and receptive region, synthesizes proteins, membranes, and other chemicals

Neuron Processes

Tracts and nerves, Dendrites and axon

Dendrites

Receptive (input) region of neuron • Convey incoming messages toward cell body as graded potentials not action potentials

Action potentials

An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls.

The axon (structure)

One axon per cell arising from axon hillock, Length varies,

Axon length

In some axon short or absent, in others up to 1 meter long

Nerve fibers

Long axons are called nerve fibers

axon terminus (bouton)

Profuse branches at the end of the axon, responsible for providing synapses between neurons, can be up to 10,000

The Axon (functions)

Conducting region of neuron • Generates nerve impulses • Transmits them along axolemma (neuron cell membrane) to axon terminal

Axon functions continued

Neurotransmitters released into extracellular space • Either excite or inhibit neurons with which axons in close contact • Carries on many conversations with different neurons at same time • Lacks rough ER and Golgi apparatus

Secretory region

Axon terminals

Tracts

Bundles of neuron processes in CNS

Nerves

Bundles of neuron processes in PNS

Nuclei

clusters of neuron cell bodies in CNS

Ganglion

clusters of neuron cell bodies that lie along nerves in PNS

Importance of the myelin sheath

This myelin sheath allows electrical impulses to transmit quickly and efficiently along the nerve cells

Myelin Sheath

Composed of myelin (Whitish, protein-lipoid substance), Segmented sheath around most long or large-diameter axons, Myelinated fibers

Function of myelin

Protects and electrically insulates axon - Increases speed of nerve impulse transmission

How myelin is formed in PNS

Formed by Schwann cells, the Myelin sheath wraps around the axon in a jelly-roll fashion, One cell forms one segment of the myelin sheath

how myelin is formed in the CNS

Formed by multiple, flat processes of oligodendrocytes, not whole cells, Can wrap up to 60 axons at once, Nodes of Ranvier are present, and No outer collar of perinuclear cytoplasm, Thinnest fibers are unmyelinated

Nodes of Ranvier

Gaps in the myelin sheath to which voltage-gated sodium channels are confined.

Perinuclear cytoplasm

Cytoplasm situated near, or occurring around, the nucleus, none is involved in myelination of CNS

3 structural types of neuron classification

Grouped by number of processes, multipolar, bipolar, unipolar

multipolar neuron

3 or more processes, 1 axon, others dendrites, Most common; major neuron in CNS

bipolar neurons

2 processes, 1 axon and 1 dendrite, Rare, ex. Retina and olfactory mucosa

Unipolar neurons

1 short process, Divides T-like - both branches now considered axon

Unipolar distal process

(peripheral) process associated with sensory receptor

Unipolar Proximal process

(central) process enters CNS

3 functional classifications of neurons

sensory, motor, interneurons

Sensory functional neuron

Nerve cells that are activated by sensory input from the environment, Transmit impulses toward CNS, Almost all are Unipolar, Cell bodies in ganglia, in PNS

Motor Functional Neurons

Carry impulses from CNS to effectors, Multipolar, Most cell bodies in CNS (except some autonomic neurons)

Interneuron functional neuron

(association neurons) Lie between motor and sensory neurons, Shuttle signals through CNS pathways; most are entirely within CNS, 99% of body's neurons, Most confined in CNS

resting membrane potential

A resting (non-signaling) neuron has a voltage across its membrane called the resting membrane potential. The resting potential is determined by concentration gradients of ions across the membrane and by membrane permeability to each type of ion.

Graded potentials

Incoming signals operating over short distance

Action potentials

Long-distance signals of axons, Principle way neurons send signals, Occur only in muscle cells and axons of neurons

How action potential is generated

When different ions cross the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron.

How action potential is propogated

occurs as the local currents of an area undergoing depolarization causes depolarization of the forward adjacent area

absolute refractory period

When voltage-gated Na+ channels open neuron cannot respond to another stimulus, Ensures AP is an all-or-none event, Enforces one-way transmission of nerve impulses

Relative Refractory Period

The threshold for AP generation is elevated, and Inside of membrane is more negative than the resting state, Only an exceptionally strong stimulus could stimulate an AP

saltatory conduction

The propagation of action potentials along myelinated axons, from one node of Ranvier to the next node, increases the conduction velocity of action potentials. 30 times faster than continuous conduction

Continuous conduction

Continuous conduction is in unmyelinated axons and is slower than saltatory conduction in myelinated axons

Synapse

Nervous system works because information flows from neuron to neuron, Neurons functionally connected by synapses

postsynaptic neuron

In PNS may be a neuron, muscle cell, or gland cell, Receives the information, transmits impulses away from the synapse

presynaptic neuron

Neuron conducting impulses toward synapse - Sends the information

Less common types of synapses

Dendrodendritic, Somatodendritic

Dendrodendritic synapse

dendrite to dendrite

Somatodendritic synapse

dendrite to soma

Electrical Synapse

Abundant in embryonic nervous tissue, Neurons electrically coupled by gap junctions that connect cytoplasm of adjacent neurons
Communication very rapid • May be unidirectional or bidirectional • Synchronize activity

Chemical Synapse

More common then electrical synapses, Specialized for release and reception of chemical neurotransmitters, Typically composed of two parts

2 parts of chemical synapse

Axon terminal of presynaptic neuron containing synaptic vesicles filled with nuerotransmitters, Neurotransmitter receptor region on postsynaptic neuron's membrane

excitatory postsynaptic potential (epsp)

Specialized for release and reception of chemical neurotransmitters, Na+ influx greater than K+ efflux → net depolarization called EPSP (not AP), Depolarizes the cell

What does excitatory potential do for action potential

Increases postsynaptic neuron's ability to produce an action potential

inhibitory postsynaptic potential (IPSP)

Makes membrane more permeable to K+ or Cl, Hyperpolarizes the cell

What does inhibitory postsynaptic potential do for action potential

Reduces postsynaptic neuron's ability to produce an action potential

Synaptic integration

EPSPs AND IPSPs can summate to influence postsynaptic neurons, Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons, Only if EPSP's are predominate and bring to threshold AP

Summate

2 types temporal and spatial summation

Temporal summation

One or more presynaptic neurons transmit impulses in rapid-fire order

Spatial summation

Postsynaptic neuron stimulated simultaneously by large number of terminals at same time

neurotransmitter

Language of nervous system, Most neurons make two or more neurotransmitters, Neurons can exert several influences • Usually released at different stimulation frequencies

What are neurotransmitters classified by

by chemical structure and by function

Acetycholine (ACh)

First identified; best understood - Released at neuromuscular junctions, by some ANS neurons, by some CNS neurons, enables muscle action, learning and memory,

biogenic amines functions

Play roles in emotional behaviors and the biological clock, Imbalances associated with mental illness

Biogenic amines major neurotransmitters

Synthesized from amino acids, Catecholamines, Indolamines

Catecholamines

dopamine, norepinephrine, epinephrine