Nervous system

Module 6

Central Nervous (CNS)

-> brain & spinal cord

Peripheral Nervous (PNS)

-> nerves that goes to muscles & organs

Basic Structure of the Brain

Left Hemisphere - Send signals to activate muscles on the right side of body

Right Hemisphere - Sensory information from right side goes to left hemisphere (vice versa)

Cerebellum - Responsible coordinated movement (above brain stem)

Brain Stem

-> controls basic functions like heart rate and respiration

-> made up of midbrain, pons & medula ablongata

Diencephalon - thalamus & hypothalamus

Neurons

1. Found in mammals divided in 3 types based on # of processes that emerge from cell body

1.Bipolar - two process from cell body

1. specialized & found in retina of eye

2.Unipolar --> one process

1. located in peripheral nerves → cell body middle axon
2. usually sensory & transmit to & from spinal cord

3.Multipolar --> contain many branching dendrites 2 one axon

--> most common in CNS

Glial cells

--> support cells of brain

--> maintain delicate internal environment of CNS

--> there are 5 times as many glial cells as neurons

--> regulate nutrients & specific interstitial fluids

--> several types astrocytes, microglia, oligodendrocytes

Language Nervous System / Neural Coding

1. action potentials are language of nervous system neural coding
2. weight of object "coded" into action potential

--> heavier object, more action potentials per second

Chemical Synapse

--> presynaptic nerve releases neurotransmitter --> affects postsynaptic

1) Axon Terminal Presynaptic Cell

1. voltage-gated Ca2+ channels
2. synaptic vesicles containing
3. neurotransmitters
4. mitochondria

2) Synaptic Cleft

3) Postsynaptic Cell

1. chemical receptors
2. chemically gated channels
3. chem = neurotransmitters

Event @ Chemical Synapse

--> neurotransmitters synthesized in presynaptic neuron → stored synaptic vesicles

--> action potential (AP) in presynaptic depolarizes membrane & activates Ca2+

--> Ca2+ cause synaptic vesicle to fuse to wall synaptic terminal

-> cause exocytosis & release neurotransmitter

--> neurotransmitter into cleft & acts on chemical receptors on postsynaptic

--> receptors cause opening chemically gated ion channels

--> postsynaptic membrane potential changes

-> causing depolarization/ hyperpolarization (depends on neurotransmitter)

--> depolarization increases probability of action potential on postsynaptic neuron

--> hyperpolarization decreases likelihood

Neurotransmitters

--> are chemicals released by neurons @ axon terminal

--> synthesized in neuron stored synaptic vesicles released response to action potential

--> diffuses through synaptic cleft → produces response to post synaptic neuron

--> type of neurotransmitter --> 2 outcomes

1) excitatory (on) --> leading to depolarization of postsynaptic cell

-> it strong enough, may fire action potential

2) inhibitory (off) --> leading to hyperpolarization of postsynaptic

-> harder to generate action potential

Four main types of Neurotransmitters

1) Acethylcholine

2) Biogenic Amines

--> catecholamines

--> dopamine

--> norepinephrine

--> epinephrine

3) Amino Acids

--> Excitatory

--> Glutamate/ Aspartate

--> Inhibitory

--> GABA / Glycine

4) Neuropeptides

-> endogenous opioids (endorphine)

-> Vasoactive Intestinal Peptide (VIP)

Most Common Excitatory : Glutamate

Most Common Inhibitory : GABA, gamma-amino-butyric acid

EPSPs

--> excitatory potential neurotransmitter causes opening of chemically gated channels

--> EPSP & IPSP occur on dendrites of neutrons on CNS

--> gates selective (+) ions, mostly Na(+) flow in

--> cause local depolarization called --> EPSP

-> very local event that diminishes w time

-> called graded potential

--> influx Na+ will depolarize --> no AP

-> no AP cause no voltage gated channels

--> in order to genereate AP EPSP must depolarize axon Hillock

--> (+) current EPSP must be strong enough to spread from synapse where it originated to axon hillock

-> now can have action potential

Strength EPSP increases in Two Ways

1) Spatial Summation EPSP

--> when sufficient number voltage channels reach threshold fire

--> additive effect produced by many EPSPs that have been generated @ diff synapses on same post synaptic neuron @ same time

2) Temporal Summation EPSP

--> sufficient number of voltage channels reach threshold fire action potential

--> additive effect produced by many EPSPs that have been generated @ same synapse by series of high-frequency action potentials on presynapse

IPSPs

--> neurotransmitters in this situation hyperpolarize

-> called inhibitory post synaptic potential (IPSP)

--> do so by opening diff chemically gated channels

-> let Cl- into (making more (-))

-> let K+ leave (making more (-))

--> moves it further from threshold (less likely Action Potential)

-> will shut off nerve cells

--> spatial & temporal summation occur with IPSPs & EPSPs

-> for IPSP produce strong hyperpolarizations

Synaptic Integration

--> single postsynaptic nerve cell can receive hundred & synapse

--> battle between EPSPs & IPSPs

Basic Structures and Organization

1. Motor system includes the
2. Supplementary motor area
3. Premotor area
4. Primary motor cortex area
5. Basal ganglia
6. Spinal pathways
7. Motor nerves going to the muscles
8. Muscle receptors

Motor system

Premotor cortex --> located in front lobe

-> develops appropriate strategy for movements

Supplementary Cortex --> located frontal lobe

-> program motor sequences, more complex more this cortex used

-> important for repetitive movements (typing)

-> code has now been written & sent to primary motor cortex

Primary Motor Cortex (PMC)

--> located on precentral gyrus in frontal lobe

Motor Homunculus --> topographical representation body on surface of cortex

-> specific area motor cortex activates particular muscle

--> signals from primary motor cortex travel down spinal cord through corticospinal tract

Corticospinal Tract

--> major motor pathway from PMC to motor neurons

--> made of millions axons, cell bodies lie in PMC

--> tract begins in motor cortex descends down to brain stem

--> in medulla

-> 80 % nerve fibres cross to other side body (contralateral)

-> 20 % nerve fibres remain on same side (ipsilateral)

--> from brain stem fibres enter spinal cord

--> once fibres reach level spinal cord where they synapse with motor neurons

-> fibres previously on ipsilateral side cross to contralateral side

--> neurons cortical spinal tract synapse with motor neurons

-> which directly innervate muscle

Muscle Receptors

Proprioception --> "muscle sense", brain being aware of positions of limbs & extent of muscle contraction

Two Receptor Types :

1) Muscles Spindles :

-> detect muscle stretch/length & rate of change of muscle

2) Golgi Tendon Organs

-> detect muscle tension

Muscle Spindles

--> sense length & stretch

Gamma Motor Neurons

->two of them

-> activate intrafusual fibres

--> when whole muscle cell stretches sensory region of spindles also does

--> sensory region sensitive to change

-> depolarizes and triggers action potential sensory nerve

--> sends to brain

-> increase stretch muscle, increase Action Potential to brain

-> since muscle attached to limb brain knows

Alpha - Gamma Coactivation

--> ensures muscles spindle continues send brain signals

--> signals to whole muscles travel through alpha, intra not active

--> during muscle contraction command sent through gamma too

Reflex Arc

--> most basic type integrated neural activity

1) begins with receptor & receptor potential produces action potential afferent neuron

2) action potential enters spinal cord, produces action potentials on interneurons & eventually on efferent neuron

3) Efferent neuron activates effector (Ex. muscle)

--> reflex arc doesn't require output from brain to cause muscle to contract

1. Sensory Receptor 2. Afferent neuron 3. Interneuron (spine) 4. Efferent Neuron 5. Efferent organ

Stretch Reflex

--> example of reflex arc

--> reflex found in every muscle

Stretch Reflex Quadriceps :

1) tap tendon makes small stretch quad muscle

2) muscle stretch = muscle spindle stretch

3) Muscle spindles trigger action potential in afferent neuron that enters spinal cord

4) Motor Nerve of quadriceps activated while muscles hamstring inhibited

5) Quadriceps contract & hamstring relaxes, lower leg kicks out

*brain not involved w contraction*

Cerebellum " little brain "

--> contains more neurons than rest of brain combined

Functions :

--> contributes generation accurate limb movements

--> correcting ongoing movements & modifying strength of some reflexes

--> involved with pavlovian conditioning

--> learning new muscle movements & vestibular <-- eye move occular reflex

How it Assists Accurate Limb movements

--> must recieve --> must receive same info from:

-> motor cortex <-- travelling out to muscles

-> proprioception <-- position muscle

--> cerebellum can make sure muscle doing right

--> if movement incorrect cerebellum modifies signals from the primary motor cortex

Limbic System

--> composed of hypothalamus, amygdala & hippocampus, cingulate cortex & septum

--> found deep in brain & form ring around brain stem

--> Key function: to link higher thought process with more primitive emotional responses (fear, rage, sex)

--> involved w feeding, drinking, pain, motivation, learning

--> allows respond changes in environment

Hypothalamus

(base of brain, anterior brain stem)

Major Functions:

--> temp control, H2O regulation, regulates food intake, cardiovascular regulation, circadian clock, emotion coordination

-> and controls release hormones from pituitary

--> does most functions through use of negative feedback

Ex. body @ 39°C instead 37°C

--> hypothalamus detects & starts sweating to cool body down

Effects of SYN and PSYN usually opposite, where one excites other will inhibit.

Automatic Nervous System (ans)

--> not under voluntary control "automatic" system

Controls:

--> heart rate, pupils in eye, smooth muscle in walls of arteries, glands, many other organs

Two divisions of ANS:

1) sympathetic (SYN)

--> excites/inhibits

--> fight or flight response nerves

--> exist @ spinal cord in thoracic & lumber (center)

--> preganglionic neurons synapse in ganglia onto 2nd postganglic nerve

--> that travels to effector/ target organ

2) parasympathetic (PSYN)

--> storage & conservation

--> nerves exist @ brain stem & spinal cord very low sacral region

--> preganglionic nerves will synapse onto postganglionic nerve near effector organ

--> nerve will synapse onto target organ

--> every organ has SYN & PSYN

-> except adrenal only SYN

Neurotransmitters of ANS

--> preganglionic neurons that leave spinal cord in SYN & PSYN release neurotransmitters acetylcholine (ACh)

--> axons of PSYN preganglionic longer cause synapse occurs closer to effector organ

--> ACh will stimulate 2nd postganglionic neuron

--> sympathetic --> usually releases norepinephrine, sometimes ACh

--> parasynpathetic --> always ACh released