EXSS 155. Lesson 8A, Part 2. Neurons and Synapses

neurons

functional unit of nervous system

form complex processing network in the brain

connect all regions of the body to the CNS

electrically excitable

responsible for

1. sensing

2. thinking

3. remembering

4. muscle control

5. gland regulation

neuroglia (glial cells)

smaller in size, greater in number

provide support to the neuron

-maintain homeostasis

-form myelin

motor neuron

main components of neurons

dendrites

cell body (soma)

axon (many axons make up a nerve

Neuron

functional classification of neurons

neurons can be classified based on the direction of nerve impulse propagation:

1. sensory/afferent neurons (PNS)

2. motor/efferent neurons (PNS)

3. inter/association neurons (CNS)

myelination

myelin sheath: multilayered lipid, insulates axon, increases nerve conduction velocity; nucleus = Schwann cells

Node of Ranvier

Myelin

lipid and protein covering

increases speed and efficiency of AP propagation

Schwann cells = act as insulator and prevent ion leakage

Nodes of Ranvier = gaps in myelin sheath

(Myelination of motor neurons to muscles controlling bladder not complete until age 3)

Multiple Sclerosis (MS)

deterioration of myelin sheath

-sensory or motor neurons

-neurons become ineffective

-muscle atrophy

-decreased coordination

currently no cure

synapses

junctions between 2 neurons or between a neuron and an effector cell

presynaptic terminal (send the impulse)

postsynaptic terminal (receives the impulse)

gap junctions

electrical synapses

ionic current spreads to next cell

faster, 2-way transmission and capable of synchronizing neurons

synaptic clefts

chemical synapses

1-way information transfer from presynaptic to postsynaptic neuron

neurons/cells involved do not touch but are separated by a synaptic cleft

Botulinum toxin

blocks release of neurotransmitter at NMJ so muscle contraction cannot occur

bacteria found in improperly canned food

death occurs from paralysis of diaphragm

Curare

plant poison from poison arrows

causes muscle paralysis by blocking the ACh receptors

used to relax muscle during surgery

neurotransmitters

released from axon terminal into synaptic cleft in response to a nerve impulse

effects can be modified via:

-synthesis

-release

-removal

-receptor site (blocked or activated)

agonist

enhances effect of transmitter

antagonist

blocks action of neurotransmitter

common small-molecule neurotransmitters

ACh

Amino Acids: glutamate, aspartate, GABA

Amines: norepinephrine, epinephrine, dopamine, serotonin

ATP

common neuropeptides

substance P

endorphins

hypothalamic hormones

CCK

Application of Neurotransmitters in Depression

anxiety disorders and depression often due to a GABA deficiency

drugs like diazepam (Valium) enhance the action of GABA

-can help slow down obsessive thinking

-can relieve muscle spasms

Application of Neurotransmitters in Parkinson’s Disease

degeneration of neurons that release dopamine

-plays a role in executive function, motor control, etc.

symptoms worsen as disease progresses

-shaking, stiffness, difficulty with balance and coordination