Neuron Structure and Function

Neuron Structure

• A neuron is a specialized cell in the nervous system
• Neurons transmit electrical and chemical signals

Neuron Components

• Cell Body (Soma) contains the nucleus and organelles, which maintain cell function
• Dendrites are branch-like structures that receive signals from other neurons
• Dendrites transmit signals to the cell body
• The axon is a long, thin extension that carries electrical impulses (action potentials)

Myelin Sheath

• Axons may be covered with a myelin sheath
• Myelin speeds up signal transmission

Neuron Pathway (Signal/Synaptic Transmission)

• Signal Reception involves dendrites receiving chemical signals (neurotransmitters) from a preceding neuron at the synapse
• Action Potential Generation occurs when a strong enough signal leads to an electrical impulse in the axon hillock
• Impulse Transmission is when the action potential travels down the axon via saltatory conduction (if myelinated) or continuous conduction (if unmyelinated)
• Neurotransmitter Release happens as the impulse causes the release of neurotransmitters into the synaptic cleft at the axon terminals
• Signal Transfer involves neurotransmitters binding to receptors on the next neuron's dendrites

Neuronal Communication Objectives

• Understand neuron and synapse structure(soma, dendrites, axon, pre-synaptic terminals)
• Knowing the types of vertebrate neurons (multipolar, bipolar, pseudounipolar)
• Grasp how information is propagated along a neuron
• Understanding events that result in synaptic communication
• Grasp the role of a neurotransmitter
• Describe neuromuscular junctions

The Structure of a Neuron

• Soma
• Axon
• Dendrites
• Pre-synaptic terminals
• Myelin

Neuron Types

• Multipolar which is a typical motor neuron
• Bipolar which is a typical special sensory neuron
• Unipolar which is a typical somatosensory neuron

Grey and White matter

• Ganglia is the collections of cell bodies outside the CNS
• Nuclei is the collections of cell bodies within the CNS

Information Propagation

• Action potentials (APs) are electrical signals
• Action potentials propagate along axons
• An AP is a rapid, transient depolarisation of the cell membrane
• Sodium channels open, allowing sodium to flow into the cell
• This reduces the resting membrane potential
• An AP results if the reduction reaches the threshold

Myelin Structure and Function

• Many neuron axons are covered in myelin ("myelinated axons")
• Oligodendrocytes are located in the CNS
• Schwann cells are located in the PNS
• Myelin acts as an insulator
• It increases the speed of conduction along the axon

Synapse components

• Neural communication involves synaptic and extra synaptic transmission

Synaptic communication Events

• Neurotransmitters chemicals released from an axon terminal acting on the postsynaptic membrane
• Neurotransmitters, can be excitatory, causing depolarisation leading to an AP, or inhibitory, further polarising the membrane reducing AP likelihood

Neuron Interaction

• There's convergence
• There's divergence

Neuromuscular Junction

• It is where the neuromuscular junction and skeletal muscle meet

Conditions Affecting Neurotransmission

• CNS Demyelination includes Multiple Sclerosis (MS)
• PNS Demyelination includes Guillain-Barre Syndrome (GBS)
• Synaptic Dysfunction includes Myasthenia Gravis (MG)
• Botulinum Toxin (BTX) is a related condition

Prenatal stages of development

• Pre-Embryonic Stage which is fertilisation of ovum by sperm, lasting from day 0 to 14
• Embryonic Stage is where Zygote implants in the uterus, lasting from day 15 to the end of the 8th week
• Fetal Stage from the end of the 8th week until birth (38-42 weeks = full term)

Embryonic Development details

• It happens between weeks 3-8
• During this time all tissues/organs are formed

Embryonic Cell processes

• Cell proliferation which is division/increase in number
• Cell migration which is movement
• Cell differentiation which is becoming more specialized
• Embryonic disk (3 layers of cells)
• Neural tube formation from cephalic to caudal
• Primitive brain & spinal cord form
• Heart, muscles, ribs, backbone, and digestive tract begin to develop
• External structures & internal organs form
• Sense of touch develops, embryo can move

Embryonic Cells

• Endoderm forms the Gut, liver, pancreas, and respiratory system
• Mesoderm forms the Muscles, skeleton, and excretory and circulatory system
• Ectoderm which is responsible for Sensory organs, hair, skin, nails, and the entire nervous system (CNS + PNS)

Neural Tube

• the ectoderm layer, forms the neural tube
• Disruptions in early development can cause NTDs
• These have lifelong implications

Baby study

• Born at 33 weeks, weight 1880g
• APGAR scores of 0, 0, and 3
• CPR for 8 minutes after birth
• Severe perinatal asphyxia due to large fetal-maternal hemorrhage
• Neonatal seizures on day 2 of life
• MRI showed extensive hypoxic-ischemic changes

Fetal Development (Weeks 13-24)

• Neurogenesis – generation of new nerve cells
• Most neurons are present by 24 weeks
• Major brain regions develop
• Eyes sensitive to light reacting to sound

Fetal Development (Weeks 25-38/40)

• Size increases
• Lungs mature
• Rapid brain development

Teratogens

• Medications
• Maternal infections and disorders
• Chemicals
• Substance abuse
• Therapeutic radiation

Factors affecting impact

• Dose
• Heredity
• Combination of negative influences
• Timing (Sensitive Period Concept)

Sensitive Periods

• Limited timeframe for body parts/behaviors to develop rapidly
• Cells become specialized
• Highly sensitive to the surrounding environment

Neonates

• A full-term infant is born from 37 weeks
• Babies are neurologically immature but still developing Myelination and Synaptogenesis
• Pre-Term baby is born before 37 weeks

Gestational age-based categories

• Extremely pre-term from 23-28 weeks
• Very pre-term from 28-32 weeks
• Moderately pre-term from 32-36 weeks
• Late pre-term from 36-37 weeks
• Low birth weight is less than 2500g