Central Nervous System (CNS)
consists of the brain and spinal cord
Peripheral Nervous System (PNS)
consists of nerves to face, ganglia, nerves to upper limb, nerves to lower limbs
The nervous system
- is a communication system that is involved in the coordination of almost all body functions
- it is designed to receive signals from the internal and external environment
- the signals are then interpreted to determine whether to stimulate or inhibit the activity of organs, tissues, and cells throughout the body
- one of the main objectives is the maintenance of homeostasis, or a relatively stable internal environment
input
- special senses - vision, hearing, taste, pain, body position
- homeostasis - blood pressure and carbon dioxide levels, ion levels (pH (H+))
integration
- a pain stimulus may trigger an immediate reflex, or it may be ignored
- a soccer player processes information about her own body position and speed, as well as that of the body, her teammate and opponents can produce the muscle contractions needed to send a perfect pass
output
- control of muscle contraction and glandular secretion of hormones
- maintenance of homeostasis - increased carbon dioxide in the blood stimulates breathing rate
- maintaining mental activity
neurons
are one of the main cell type within the nervous system,
- neuron’s have 3 structures facilitating the separate functions
1. input (dendrites)
2. processing (cell body)
3. output (axons)
Multipolar – many dendrites, large cell body – variety of inputs requiring processing. (most neurons are multipolar, e.g. motor neurons)
Bipolar – few dendrites, small cell body – limited input, focus on conduction of signal with minimal processing (sight and smell processing)
Pseudo-unipolar – dendrites not processed through cell body – simple relaying of signal with little processing (most other sensory neurons)
glial cells
are the other main cell type in the nervous system
- they perform structural support and protective roles by contributing to the blood-brain barrier (prevents movement of some molecules and compounds into the brain), as well as immune and nutrient provision functions
glial cells in the CNS
- Astrocytes - highly branched - provide structural support, regulate neuronal signalling, contribute to blood-brain barrier, and help with neural tissue repair
- Ependymal cells - epithelial-like - line ventricles of brain and central canal of the spinal cord, circulate cerebrospinal fluid (CSF), some form choroid plexuses which produce CSF
- Microglia - small, mobile cells - protect CNS from infection, become phagocytic in response to inflammation
- Oligodendrocytes - cells with processes that can surround several axons - cell processes form myelin sheaths around axons or enclose unmyelinated axons in the CNS
Glial cells of the PNS
- Schwann cells - single cells surrounding axons - form myelin sheaths around axons or enclose unmyelinated axons in the PNS
- Satellite cells - single cells surrounding cell bodies - support neurone, providing nutrients, protect neurone from heavy-metal poisons
myelin sheaths and nodes of raniver
specialised glial cells form coatings (myelin sheaths) around axons greatly increasing the velocity at which they can conduct action potentials (electrical signals)
- w/in the CNS these cells are called oligodendrocytes
- w/in the PNS these cells are called Schwann cells
Nodes of Ranvier: gaps between myelin sheaths, about every millimetre along axon
- allows for ion movement and faster conduction compared to unmyelinated axons
Stages of communication in a neuron
- generation of action potential in cell body
- action potential propagation along axon
- communication with target cell at synapse - chemical communication
Electrical communication
In cells that have electrical properties (e.g. nerve, muscle) there are different environments inside and outside the cell, with an uneven distribution of types of ions. The membrane becomes polarised, with the inside being more negatively charged than the outside. This creates an electrical potential across the cell membrane
- potential energy used in secondary active transport of glucose
- sodium down both chemical gradient and electrical gradient
Electrical gradient
Electrical gradient - Polarity – opposites attract each other, similar repel each other – positive ions attracted to negatively charged area
resting membrane potential
there are ion channels that are always open (leak ion channels) and ion channels that open when specific signals are present (gated ion channels)
Gated ion channels can be singled to open by chemicals (chemically gated) or electrical changes (voltage gated)
- sodium (Na+) concentration gradient into cell
- potassium (K+) concentration gradient out of cell
- leak channels allow some ion movement
- sodium-potassium pump - active transport of Na+ out and K+ in maintains resting membrane potential
- negative inside cell, positive outside cell
Potential energy – stored energy – polarity is a stored gradient that when opened will result in movement based on the chemical (concentration gradient) and electrical potential ( electrical gradient).
action potential
electrical signals conducted along a cell membrane (neuron, muscle cell)
- all or non - always the same magnitude one initiated
- AP initiated by local threshold potential being reached
membrane potential
depolarises during an action potential
resting membrane potential, depolarisation, and repolarisation
- resting membrane potential
- established by leak channels and Na+/K+ pump - depolarization
- open voltage gated Na+ channels
- open when local membrane potential reaches threshold
- positive sodium ions move in - repolarizaton
- Na+ channels close, voltage gated K+ channels open
- positive potassium ions move out
- Na/K+ pump works to re-establish resting membrane potential
saltatory conduction
jumping from one node of Ranvier to the next
Synapse
junction where the neuron interacts with another neuron or cell
- conversion of electrical message in axon into chemical message sent to next cell
Presynaptic terminal (before synapse)
1. action potential triggers voltage gated Calcium channels
2. Calcium moves in and stimulates vesicles to
3. release neurotransmitter into synaptic cleft (space between cells)
Postsynaptic membrane (after synapse)
4. neurotransmitters bind receptors on post synaptic membrane
- opening or closing chemically gated channels for: sodium, potassium, or chloride (Cl-)
Acetylcholine (ACh) - neurotransmitter
Site of release: CNS synapse, ANS synapse, and neuromuscular junctions
Effect: excitatory or inhibitory
Clinical Example: alzheimer disease is associated with a decrease in acetylcholine-secreting neutrons. Myasthenia gravis (weakness of skeletal muscles) results from a reduction in acetylcholine receptors
Norepinephrine (NE) - neurotransmitter
Site of release: selected CNS synapses and some ANS synapses
Effect: excitatory
Clinical example: cocaine and amphetamines increase the release and block the reuptake of norepinephrine, resulting in overstimulation of postsynaptic neurons
Cobra - venom is a neurotoxin
- venom blocks receptors for the neurotransmitter Acetylcholine (ACh) on the post synaptic membrane
- venom is similar in structure to ACh
- unable to stimulate muscle contraction
- numbness and paralysis
- breathing impacted as respiratory muscles paralyze
spinal cord
collection of tracts leading to (ascending) and from (descending) the brain
- spinal nerves exit between each vertebrae
Dorsal - back
Ventral - front
