Neurons
Introduction
Neurons
masses of nerve cells that transmit information to other nerves, tissues or cells (nerve impulses)
1. Cell Body - contains the nucleus and two extensions
2. Dendrites - shorter, more numerous, receive information
3. Axons - single, long fiver which conducts impulses away from the cell body
The nervous system is divided into two parts:
1. Central Nervous System (CNS) - brain and spinal cord
2. Peripheral Nervous System (PNS) - peripheral nerves through the body
- includes 31 pairs of spinal nerves
- includes 12 pairs of cranial nerves
Three Basic Functions
1. Sensory Function - gathers info about changes occurring within and around the body - Sensory receptors, at ends of peripheral nerves send signals (nerve impulses) to the CNS. Sensory Neuron2. Integrative Function - information is "brought together," interpreted, to create sensations, create thoughts, add to memory, make decisions, etc. Association neuron or interneuron3. Motor Function - responses to signals (impulses). Signals sent from the CNS to effectors (muscles or glands). The goal is usually to maintain stable conditions (especially internal) - Homeostasis. Motor neurons.- Somatic Nervous System (skeletal muscles)- Autonomic Nervous System (smooth muscles, glands)Neurons: Nerve cells.
Unique structure - cell body with many extensions or processes (nerve "fibers") which conduct impulses.
There are two types of processes:
1. Dendrites - shorter, more numerous. These, along with the cell body, form the receptive surfaces of neurons.
2. Axons - single, long "fiber" which conducts impulse away from the cell body. Sometimes it is "branched" (collaterals). End has many fine branches.
Neuroglial Cells (neuroglia)
- supportive tissue of the nervous system (more numerous than neurons). Five types
1. Microglial Cells 2. Oligodendrocytes
3. Astrocytes
4. Ependymal Cells
5. Schwann cells
*Myelin Sheaths
9.4 Neurons
Axon
Dendrite
Neurofibril
Chromatophilic substance
Myelin
Nodes of Ranvier
Myelinated (white matter) vs Unmyelinated (grey matter)
Classification of Neurons
1. Functional (sensory neurons, motor neurons, interneurons)2. Structural (multipolar, bipolar, unipolar)9.5 Cell Membrane PotentialResting Potential / Threshold Potential / Action Potential1. Neuron membrane maintains resting potential2. Threshold stimulus is received3. Sodium channels open4. Sodium ions diffuse inward, depolarizing the membrane5. Potassium channels open6. Potassium ions diffuse outward, repolarizing the membrane7. The resulting action potential causes a local bioelectric current that stimulates adjacent portions of the membrane.8. Wave of action potentials travel the length of the axon as a nerve impulse9.6 Nerve Impulse*Propagation of action potentials along a nerve axonImpulse Conduction – speed of an impulse proportionate to diameter of axon (greater diameter = faster impulse)
Myelinated axons conduct faster than unmyelinated axons
Example: Motor neuron associated with skeletal muscle 120 m/2
Sensory neuron on skin (unmyelinated) travels at .5 m/s
A: Neuron (Presynaptic)
B: Neuron (Postsynaptic)
Mitochondria
Synaptic vesicle full of neurotransmitter
Autoreceptor
Synaptic cleft
Neurotransmitter receptor
Calcium Channel
Fused vesicle releasing neurotransmitter
Neurotransmitter re-uptake pump
The Synapse
Synapse – junction between two communicating neuronsNerve pathway – nerve impulse travels from neuron to neuronSynaptic TransmissionDendrite cell body along axon synapse (gap)To complete the signal, a NEUROTRANSMITTER is released at the gap to signal the next neuronExcitatory – increase membrane permeability, increases chance for threshold to be achievedInhibitory – decrease membrane permeability, decrease chance for threshold to be achievedTypes of NeurotransmittersAcetylcholine – stimulates muscle contractionMonoamines – Norepinephrine & Dopamine (sense of feeling good, low levels = depression)Serotonin (sleepiness)
Endorphins (reduce pain, inhibit receptors)
Synapses are highly susceptible to drugs and fatigue
Curare (poison used by S. American Indians) and atropine stops Acetylcholine from depolarising the post-synaptic membrane, i.e. become paralysed.
Strychnine and some nerve gases inhibit or destroy acetylcholinesterase formation. Prolongs and enhances any stimulus, i.e. leads to convulsions, contraction of muscles upon the slightest stimulus.
Cocaine, morphine, alcohol, ether and chloroform anaesthetise nerve fibers. (inhibitory)
Mescaline and LSD produce their hallucinatory effect by interfering with nor-adrenaline & serotonin
9.8 Impulse Processing
Neuronal pool – groups of neurons that make hundreds of synaptic connections and work together to perform a common function
9.9 Types of Nerves
Sensory Nerves – conduct impulses into the brain or spinal cord
Motor Nerves – carry impulses to muscles of glands
Mixed Nerves - contain both sensory and motor nerve
9.10 Nerve Pathways
Reflex arc – simple pathway, includes only a few neurons (reflexes)
Reflex Behavior – automatic, subconscious responses to stimulu
Knee-jerk reflex (patellar tendon reflex
stimulus knee sensory nerve spinal cord motor nerve
Withdrawal reflex – occurs when you touch something painful
Neuron Label
Nerve Cells Coloring
Oligodendrocytes (purple) Astrocyte (green) Ependymal Cells (orange) Body of Neuron (blue)
For each of the cells above, color the nucleus a darker shade of purple, green, blue, orange
Myelin sheaths (pink) Capillary (red)
Microglial cells (yellow) Nodes or Ranvier and the Axon (brown)
What is the function of:
1) Oligodendrocytes ______________________________________________________________
2) Astrocytes ____________________________________________________________________
3) Microglial cells ________________________________________________________________
4) Myelin sheaths _______________________________________________________________
5) Trace the path of a nerve impulse in a neuron: _________________________________________
1. The skeletal muscles are controlled by the _______________________________nervous system.
2. The smooth muscles and glands are controlled by the __________________________ nervous system.
3. Neurons are composed of a network of fine threads called _________________________________
4. The nervous system consists of two parts, the brain and spinal cord make up the ______________ nervous system, and the nerves throughout the body make up the ___________ nervous system.
5. ______________________________are cells found between neurons and blood vessels.
6. Neurons consist of a cell body, axons, and _________________________, which receive information.
7. Neurons that have a single process extending from the cell body are classified as _________, if they have two processes, they are classified as _____________
8. White matter is composed of axons that are sheathed in _________________________
9. Two ions necessary to create an electric current in a nerve fiber are potassium and ______________.
10. The ______________________ function of the nervous system refers to information being interpreted so that the brain can make decisions.
11. The junction between two communicating neurons is called the ____________________________
12. Acetylcholine, serotonin and endorphins are all forms of ___________________________
13. The _________________________arc refers to a simple nerve pathway that would be involved with involuntary actions (like knee-jerk, or withdrawal)
14. Groups of neurons that perform a common function, such as the storing of procedural memory (tying your shoe) are called neuronal __________.
15. __________________________________cells help destroy bacterial cells and cellular debris.
16. There are 31 pairs of ____________________________ nerves.
17. These types of neurotransmitters increase membrane permeability, thus increasing the chance that threshold will be achieved.
18. When a threshold is achieved, an event called the __________________ potential occurs.
19. Gaps in the myelin sheath are called Nodes of _________________________________
20. These support cells are responsible for secreting the myelin sheaths: _________________________
21. A nerve impulse is received by the dendrites and then travels down the ___________________
22. There are twelve pairs of ________ nerves.
23. This neurotransmitter stimulates the muscles to contract. __________________
24. These cells form a membrane that covers specialized brain parts: ___________
25. These cells have the same function as oligodendrocytes, but are packed within the myelin: _________________
*You will also need to be able to label the parts of the neuron, see neuron label handout..
