Ch. 10-11 Nervous System

Nervous System Classification

• Central Nervous System (CNS)

• Peripheral Nervous System (PNS)

• Systems provide sensory, integrative, motor functions

Central Nervous System

• Brain and spinal cord (tracts)

Peripheral Nervous System

• all nerves leaving brain/spinal cord

• somatic nervous system

• autonomic nervous system

Somatic Nervous System

• Effector: skeletal muscles

• One required neuron to message effector

• communicates VOLUNTARY (conscious) instructions to muscle

Autonomic Nervous System

• Effector: smooth + cardiac muscle, glands

• 2 required neurons to message effector

• communicates instructions to viscera (involuntary subconscious actions)

• Ex: gland secretion, adrenaline, goosebumps, etc

Effector

• muscle that does the action

Neurons

• structural and functional part of the nervous system that react to physical and chemical changes

Nerve impulses

• action potential

• bioelectrical changes, a form of info, that neurons transmit

Nerve Fibers

• Dendrites and axons

• where impulse is sent along

Nerves

• bundle of fibers (axons)

Nervous Tissue

• neuroglial cells (glial)

• supports neurons

Neuron Structure

• common features

• cell body (soma)

• cytoplasm filled tubes that conduct impulses

• cell membrane

• cellular organelles

• CHROMATOPHILIC SUBSTANCE (Nissl Bodies) - think of rough endoplasmic reticulum

• large nucleus —> visible nucleolus

• MATURE NEURONS DO NOT DIVIDE

Dendrites

• nerve fiber

• usually short + highly branched

• main receptive surfaces

• conducts impulses into cell body

• cell body will have many dendrites

Axons

• nerve fiber

• arise from “axonial hillock”

• begins as single fiber → will branch at end

• conducts impulses AWAY from cell body

• axon terminal ends at synaptic knob

Schwann Cells

• Neuroglial cell

• part of PNS

• speed up nerve cell signal —> jumps node to node

Myelin sheath

• covering around an axon

• part of schwann cell

Neurilemma

• membrane portion of schwann cell

Nodes of Ranvier

• narrow gaps between schwann cells

“white matter”

• fibers within CNS that are myelinated

Types of Neurons and Neuroglial Cells

neurons differ in size/shape, length of axon/dendrite, function

• bipolar

• unipolar

• multipolar

Bipolar Neuron

2 fibers:

• one arising from each end

• emerge from cell body

• Ex: sensory neurons in eyes, nose, ear

Unipolar

• sensory neuron

• single nerve fiber: extends from body and branches off

• one end connects to peripheral body part (dendrite) and other to brain/spinal cord (axon)

• ex: skin receptors, joints, muscles, internal organs

Multipolar

• many nerve fibers extend from cell body

• only one nerve fiber acts as axon

• Ex: most lie in brain/spinal cord

Neuron Classification

• Sensory

• Interneuron

• Motor Neuron

Sensory Neuron

• (from stimulus)

• afferent

• carries nerve impulses from peripheral body (skin) INTO brain/spinal cord

• most unipolar

Interneuron

• association

• lie WITHIN brain/spinal cord

• link to other neurons OR transmit impulses from one part of brain to other

• most multipolar

• (integration, process info)

Motor Neuron

• efferent

• carry impulses OUT of brain/spinal cord to effectors

• most multipolar neurons (always communicate w/ effectors)

Ischemic cell change

• eventually disintegrates the cells

• neurons deprived of oxygen irreversibily change (alter shapes + shrink nuclei)

Ischemia

• Oxygen deficiency can result from lack of blood flow

Hypoxemia

• abnormally low blood oxygen level

Neuroglial Cell Function

• fill spaces

• support neurons

• provide structural framework

• produce myelin

• carry on phagocytosis

Neuroglial Cells

• Schwann

• Microglial

• Oligodendrocytes

• Astrocytes

• Ependymal

Microglial Cell

• Found through out CNS

• support neurons and phagocytize bacterial cells + other debris

• know as maids of nervous system —> they clean

Oligodendrocytes

• adjacent to nerve fibers

• forms myelin WITHIN CNS

• myelinates multiple axions at a time

Astrocytes

• commonly between neurons and blood vessels

• provides support

• helps regulate nutrients (what can pass through)

• constrict and dilate blood vessels

• known as bouncer of nervous system

• forms blood brain barrier

Ependymal

• produce, secrete, circulate vertebral spinal cord

snapping your fingers

• bioelectrical signal travels legnth of motor neuron to muscle in hand

• release acetyleboline at neuromuscular junction

• msucle contracts

• brain stopped action, impulse ceases due to breakdown of acetyidoline by enzymes

• calcium ions return to cisternae (storage) of muscle

• hand relaxes

Nerve impulse

• signal transmitted along axon (nerve fiber)

• consists of wave of electrical depolarization that reverses potential difference across nerve cell membrane

Impulse conduction steps

• resting potential

• stimulation

• threshold potential

• depolarization

• repolarization

• refractory period

Surface of a resting neuron

• polarized (electrically charged) with respect to inside

• Potential difference

• -70 mV

• polarization of resting neuron is due the unequal concentration of negative and positive neurons

Resting potential

• greater concentration of K+ on inside & Na+ ions on outside of membrane

Why does the inside stay negative in a resting neuron?

• there are largely charged negative ions found inside that are too large to cross the cell membrane

• phosphate PO4^-3, sulfate SO4^-2, and proteins

What controls the distribution of ions on both sides of the membrane?

1. Facilitated diffusion (protein channels)

2. Active Transport (Sodium Potassium Pump)

Active transport

• sodium potassium pump

• ATP is used to move 3 Na+ ions out of the cell while pumping 2 K+ ions into the cell

Potassium ions

• tend to pass through cell membrane much more easily than sodium ions (na+)

Stimulus

• environment changes affect membrane permeability by opening gated ion channels thereby depolarizing the membrane

What does it mean when potential changes are graded?

• the degree of change in resting potential is directly proportional to the intensity of the simulation

Threshold potential

• level of potential change at which action potential is triggered in a neuron

• permeability of the membrane suddenly changes, and voltage gated ion channels begin to open

In order to trigger an action potential (nerve impulse)…

• the membrane potential must be reduced from resting potential (-70mV) to -55mV

• this is because Na+ ions are crossing into the cell interior

Threshold Stimulus

• a stimulus strong enough to create an action potential in a motor neuron

Subthreshold Stimulus

• a stimulus too small to create an action potential in a motor neuron

Changes that affect the resting potential

• Membrane depolarizes - as the membrane rp decreases (from -70 mV)

• meaning stimulation has occurred

• changes in resting potential of a membrane vary

• amount of change depends on intensity

• summation

summation

• additive phenomenon of intensity

• Concentration gradient

• aids the movement of positive ions into the cell

Action potential

• as sodium ions diffuse inwards - membrane depolarizes

• potassium diffuses out - membrane repolarizes (happens at the same time)

• whole process takes 1/1000 second

• action potential works to maintain original concentration

Local current

• triggered when an action potential occurs in one region of a nerve fiber membrane, causing a bioelectric current to flow to the adjacent downstream part of the membrane

• creates wave of action potentials (which move down the fiber)

• nerve impulse created through progression of action potentials

Refractory Period

• as local current progesses - the repolarized neuron has Na+ ions inside and K+ ions outside (not resting potential)

• sodium potassium pump must re-establish resting potential (proper ion distribution of NA+ outside and K+ inside) before another action potential can be stimulated

Summarized Events leading to conduction of a nerve impulse

• nerve cell membrane maintains resting potential (-70mV) by diffusion of Na+ and K+ down their concentration gradients as cell pumps the gradients

• neurons get stimulus —> local potentials, may sum to reach threshold (-55 mV)

• sodium channels in trigger zone (axonial hillock) of axon open

• sodium ions diffuse in —>. membrane depolarizes(+40mV)

• potassium channels in membrane open

• potassium ions diffuse out —> repolarizes membrane

• action potential occur sequentialy along length of axon

Unmyelinated nerve

• conducts an impulse over its entire surface (grey matter)

Myelinated Nerve

• an myelinated Nerve acts differently in that it jumps from node to node because myelin prevents the conduction of the impulse

• moves many times faster than an unmyelinated nerve

impulse conduction

• speed is proportional to the diameter of the fiber

• greater the diameter —> faster impulse (up to 120mps)

• thin unmyelinated nerve (like sensory fiber), may only travel 0.5 mps

Nerve conduction

• all or nothing response

• when nerve fiber responds, it responds completely

How is the electrical current from one neuron passed to the next neuron or effector?

• chemical conduction via neurotransmitters

Synapse

• junction between two neurons (or neuron and effector)

Synaptic cleft

• neurons are not in direct contact at the synapse, a gap

Presynaptic neuron

• end of the neuron, axon terminal

Postsynaptic neuron

• cell body or dendrite that receives the impulse

Synaptic transmission

• the process of crossing the synapse

Synaptic knobs

• axonal termini

• distal end of the axon (contains the neurotransmitter)

Neurotransmitter

• chemical substances made by the neuron specifically to transmit a message

Excitatory

• increase membrane permeability to Na+ ions and trigger an impulse

Excitatory Neurotransmitters

• acetylcholine - stimulates skeletal muscles

• Norepinephrine + epinephrine - brain and heart rate

Inhibitory

• decrease permeability and therefore make it less likely threshold will be reached

Inhibitory Neurotransmitters

• dopamine - sleep, memory, learning, mood, attention

• glycine - found in CNS and retina

neurotransmitters

• synthesized in the cytoplasm of the synaptic knobs and stored in the synaptic vesicles

Reflex behavior

• simplest, involuntary nerve pathway

• automatic subconscious responses to changes within or outside the body

• Ex: swallowing, sneezing, coughing, vomitting,heart rate, breathing rate, blood pressure

Reflex arc

• simplest form of a reflex

• consisting of a receptor, sensory neuron, interneuron, motor neuron, and effector.

• knee jerk

• withdrawal reflex

Knee Jerk

• patellar tendon reflex

• simplest form only employing two nerves(sensory and motor)

withdrawal reflex

• sensory, assosciation, motor nerve

Reflex centers of a reflex arc

• stimulus (receptor)

• Sensory neuron (afferent neuron)

• Interneuron (assocation neuron)

• Motor neuron (efferent neuron)

• Effector (muscle or gland or organ)