exsc 224 unit 1

four neural pathways

spinocerebellar, dorsal column-medial lemniscal, spinothalamic, lateral corticospinal

3 things the nervous system does

sensory input, integration, motor output

sensory input

Information gathered by sensory receptors about internal and external changes

integration

interpretation of sensory input

motor output

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

central nervous system

brain and spinal cord

peripheral nervous system

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

PNS divisions

sensory (afferent) and motor (efferent)

sensory (afferent) divison

-somatic and visceral sensory nerve fibers

-conducts impulses from receptors to the CNS

motor (efferent) division

somatic (voluntary)

and autonomic (involuntary)

autonomic nervous system

involuntary movements

(sympathetic and parasympathetic nervous systems)

sympathetic nervous system

fight or flight, mobilizes body systems during activity

parasympathetic nervous system

rest and digest, conserves energy

glial cells

cells in the nervous system that support, nourish, and protect neurons, 6 types

6 types of glial (neuroglia) cells

astrocytes, microglia, ependymal, oligodendrocytes, satellite, schwann cells

astrocytes

CNS, help protect brain, hold things together in CNS, clean up extracellular environment

microglia

CNS, small, ovoid cells with spiny processes

Phagocytes that monitor the health of neurons

ependymal cells

CNS; produce and circulate cerebrospinal fluid

Oligodendrocytes

CNS, produces myelin sheath

satellite cells

PNS; surround neuron cell bodies, make sure cells have nutrient access

Schwann cells

PNS, produce myelin sheath

neurons

nerve cells, long lives, high metabolic rate

three neuron classes

multipolar, bipolar, unipolar

multipolar neuron

many processes extend from the cell body; all are dendrites except for a single axon; most abundant, major neuron type in CNS, most are interneurons

bipolar neuron

two processes off of the cell body; one axon and one fused dendrite; many do not generate action potentials; rare; found in some special sensory organs (eye, ear)

Unipolar (pseudounipolar)

one process extends from the cell body and forms central and peripheral processes, which together comprise an axon; found mainly in PNS; common only in dorsal root of ganglia of spinal cord and sensory ganglia of cranial nerves

parts of the cell body of a neuron

Nissl bodies (rough ER), golgi, mitochondria, cytoskeleton

clusters of cell bodies in the CNS are called ____, whereas in the PNS are called ____

nuclei, ganglia

axon hillock

the cone-shaped area on the cell body from which the axon originates

dendrites

receptive/input region of a neuron, large surface area, convey input toward cell body -> graded potentials

axon

one per cell, numerous terminal branches -> buttons, generates and transmits action potentials

myelin sheath

protects and insulates axon, increase impulse transmission speed, in PNS: layers (concentric) of schwann cell membrane, in CNS: formed by oligodendrocytes, nodes of ranvier

white matter

dense collections of myelinated fibers, bundles of axons; forms tracts that connect parts of brain, ascending and descending tracts in the spinal cord

grey matter

mostly neuron cell bodies and unmyelinated fibers; cortex of cerebrum and cerebellum and central portion of spinal cord, forms nuclei deep within brain

neuron function

Neurons are highly irritable

Respond to adequate stimulus by generating an action potential (nerve impulse)

AP is always the same regardless of stimulus

principles of electricity

-More positive charges outside cell than inside cell

-Energy is required to separate opposite charges across a membrane

-If opposite charges are separated, the system has potential energy

-Energy is liberated when the charges move toward one another

voltage

measure of potential energy generated by separated charge

potential difference

voltage measured between two points

resistance

hindrance to charge flow

conductor

substance with low electrical resistance (water with electrolytes)

insulator

substance with high electrical resistance (myelin)

main types of ion channels

leakage, gated

Leakage (nongated) channels

always open

gated channels

chemically gated, voltage gated, mechanically gated

chemically gated channels

open when the appropriate chemical (neurotransmitter) binds to receptor

resting membrane potential

not static; generated by differences in ionic makeup of ICF and ECF and differential permeability of the plasma membrane; more Na+ outside, more K+ inside

two types of membrane potentials that act as signals

graded potentials, action potentials

graded potentials

incoming short distance signals; dendrites, cell bodies; short-lived and localized, depolarizations, hyperpolarizations, occurs when gated ion channels open, magnitude varies with stimulus strength, decrease in magnitude with distance, decay of potential with distance

action potentials

long distance signals of axons; axons only

changes in membrane potential

depolarization, repolarization, hyperpolarization

depolarization

membrane potential moves toward 0mV inside, becoming less negative; local currents open Na+ VGC, Na+ influx causes more depolarization, at threshold (-55mV) positive feedback leads to opening of all Na+ channels, and a reversal membrane polarity to +30mV

hyperpolarization

the membrane potential increases, the inside becoming more negative; some K+ channels remain open, allowing excessive K+ efflux, happens after hyperpolariztion of the membrane

4 phases of an action potential

resting state, depolarization, repolarization, hyperpolarization

resting state of an AP

only leakage channels for Na+ and K+ are open

all gated channels closed

each Na+ channel has 2 voltage-sensitive gates

repolarization

Na+ channel slow inactivation gates (open at rest, block channel once open) close, membrane permeability to Na+ declines to resting levels, slow voltage-sensitive K+ gates open, K+ exits the cell and internal negativity restored

threshold

at threshold, membrane is depolarized by 15 to 20mV, Na+ permeability increases, Na+ influx exceeds K+ efflux, begins positive feedback cycle

absolute refractory period

Time from opening of Na+ channels until resetting of the channels

Ensures that each AP is an all-or-none event

Enforces one-way transmission of nerve impulses

conduction velocity

• Conduction velocities of neurons vary widely

• Rate of AP propagation depends on

- Axon diameter

• Larger diameter fibers have less resistance to local current flow so faster impulse conduction

- effect of myelination

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

nerve fiber classification

according to:

diameter

degree of myelination

speed of conduction

group A nerve fibers

Large diameter, myelinated, fast conduction, transmit at 150 m/s; ex: somatic sensory, motor fibers

group B nerve fibers

Intermediate diameter, lightly myelinated fibers, transmit at 15 m/s; ex: ANS fibers

group C nerve fibers

Smallest diameter, unmyelinated, slow; ex: ANS fibers

the synapse

junction that mediates information transfer from one neuron to another neuron or an effector cell

presynaptic neuron

conducts impulses toward the synapse

postsynaptic neuron

transmits impulses away from the synapse

electrical synapse

very rapid, uni/bidirectional, important in embryonic nervous tissue, some brain regions, heart

chemical synapse

specialized for the release and reception of neurotransmitters; typically composed of 2 parts:

axon terminal - presynaptic neuron

receptor region - postsynaptic neuron

postsynaptic potentials

Graded potentials

Strength determined by:

Amount of neurotransmitter released

Time the neurotransmitter is in the area

types of postsynaptic potentials

EPSP—excitatory postsynaptic potentials

IPSP—inhibitory postsynaptic potentials

Integration: Summation

Temporal summation

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

Spatial summation

Postsynaptic neuron is stimulated by a large number of terminals at the same time

neurotransmitter receptors

two types:

channel linked receptors (direct) -> ACh and amino acids

G-protein-linked receptors (indirect) -> 2nd messenger systems; biogenic amines, neuropeptides, and dissolved gases

Channel-linked receptors

-ligand-gated ion channels

-action is immediate and brief

-excitatory receptors are channels for small cations

-inhibitory receptors allow Cl- influx that causes hyperpolarization

G-protein-linked receptors

1. neurotransmitter (1st messenger) binds + activated receptor

2. receptor activates G protein

3. G protein activates adenylate cyclase

4. adenylate cyclase converts ATP to cAMP (2nd messenger)

5. a. cAMP changes membrane permeability by opening/closing iono channels

b. cAMP activates enzymes

c. cAMP activates specific genes

neuronal pools

functional groups of interconnected neurons that integrate incoming information and forward the processed information to other places

simple neuronal pool

single presynaptic fiber branches and synapses with several neurons in the pool

4 Types of Circuits in Neuronal Pools

converging, diverging, reverberating, parallel-after-discharge

divergent circuit

one incoming fiber stimulates ever increasing number of fibers, often amplifying circuits; may affect a single pathway or several; common in both sensory and motor systems

converging circuit

Opposite of diverging circuits, resulting in either strong stimulation or inhibition

Also common in sensory and motor systems

reverberating circuit

chain of neurons containing collateral synapses with previous neurons in the chain

parallel after-discharge circuit

incoming fiber stimulates several neurons in parallel arrays to stimulate a common output cell

Patterns of Neural Processing

serial and parallel processing

serial processing

refers to processing one piece of information at a time, such as memorizing a list item by item; ex: reflexes - rapid, autonomic response to stimuli that always cause the same response; reflex arcs have 5 essential components: receptor, sensory neuron, integration center, motor neuron, effector

parallel processing

input travels along several pathways; one stimulus promotes numerous responses, important for higher-level mental functioning, ex: a smell may remind one of the odor and the associated experiences

brain vesicles

telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon

Telencephalon

cerebrum

Diencephalon

thalamus, hypothalamus, epithalamus

mesencephalon

midbrain

metencephalon

pons and cerebellum

myencephalon

medulla oblongata, spinal cord

ventricles of the brain

two lateral ventricles, third ventricle, fourth ventricle; connected to each other and central canal of spinal cord, lined by ependymal cells, contain CSF

cerebral hemisphere

one of the two major portions of the forebrain, covered by the cerebral cortex; frontal lobe, parietal lobe, occipital lobe, temporal lobe, gyri of insula

the frontal lobe contains the ____ gyrus and ____ sulcus

precentral, central

the parietal lobe contains the _____ sulcus, _____ sulcus, _____ sulcus, and _____ gyrus

parieto-occipital, lateral, central, postecentral

gyrus

ridge

sulci

shallow grooves that separate gyri

gyri of insula

underneath temporal lobe

the temporal lobe contains the _____ sulcus

lateral

longitudinal fissure

separates cerebral hemispheres

transverse cerebral fissure

separates cerebrum and cerebellum

functional areas of cerebral cortex

Motor areas, Sensory areas and association areas (conscious behavior involves the entire cortex)