Neurons and Synapses

Neurons in the nervous system

• Integrates and coordinates many of the body’s activities

• Has two parts: 

  • central nervous system (CNS): brain and spinal cord

  • peripheral nervous system (PNS): outside of brain and spinal cord

Excitatory synapse

neurotransmitters when released bind to receptors on postsynaptic cell, making it more likely to generate a response from postsynaptic cell

Inhibitory synapse

neurotransmitters bind to receptors and reduces the likelihood of the postsynaptic cell responding to it

Neurons (nerve cells)

excitable cells that generate and transmit messages

Myelin sheath structure

*Composed of plasma membranes of specialized glial cells

• In PNS, Schwann cells(type of glial cell) form the myelin sheath

*Gaps between adjacent Schwann cells are called nodes of ranvier

*Nerves impulses travel faster as they jump from one Node of Ranvier to the next

Synapse

junction between a neuron and another cell

• Involves a presynaptic neuron, postsynaptic cell (often another neuron) receiving the signal, and a synaptic cleft (the gap between the two)

Multiple sclerosis

The myelin sheath in the brain and spinal cord are progressively destroyed (over a period of time)

• Slows down nerve impulses

• Can result in paralysis and loss of sensation including loss of vision

Cell body

contains nucleus and other organelles, maintains the neuron

Myelin sheath function

• surrounds axons of the PNS (outside the CNS) and some within the CNS

• Provides electrical insulation that increase rate of conduction of nerve impulses

Axon

carries info away from cell body to either another neuron or an effector cell

Nerve impulse

signal that occurs in neurons

• the info(nerve impulse) moves along the neuron from dendrites→ cell body→ to axon

• Can be called an action potential is caused by a change of charge that occurs across plasma membrane

• The change in charge is caused by movement of NA+ and K+ ions across plasma membrane (proteins allow them to cross)

A resting neuron: inside neuron is more negatively charged

A neuron with a nerve impulse: charge along the membrane flips (happens VERY fast)

Neuroglial cells

• outnumber neurons 10 to 1

• Several different types

• Provide support to neurons, structural support, growth factors, and insulating sheaths

Dentrites role

receive signals from other cells, carries info toward cell body of a neuron

Structure of neurons

specialized for communicating with other cells

Parts of neuron:

Dendrites: many short, branching projections 

Axon: a single long extension

Cell body: enlarged central region

Sensory neurons

1. carries info from sensory receptors to CNS

Interneurons neurons

(found in brain/spinal cord) between sensory & motor neurons, integrate and interpret sensory signals, account for more than 99% of body’s neurons

Motor neurons

carries info away from CNS to effectors (muscle or gland)

Neurotransmitters

communication between a neuron and an adjacent cell

1. Nerve impulse (action potential), reaches the synaptic cleft, vesicles containing neurotransmitters undergo exocytosis

2. diffuse across the synaptic cleft and bind to receptors on the postsynaptic cell

3. This binding generates a signal in the postsynaptic cell