Lecture 7 - Part 1 - Structure of the Nervous System Flashcards
What are the two original theories for composition of nervous system
Reticular theory
Cell theory
What’s the reticular theory
Composed of a network of structures - joined together tubes ( continuous ) - vascular - blood vessels
- reticulum = a fine network of net like structure
What’s the cell theory
Composed of discrete cells - everything else in body
What’s nervous system made of
Millions of individual cells called neurons ( nerve cells )
Role of nervous system
Sending (electrical) signals very rapidly throughout the body
Transmits info quickly into little electrical signals
Nervous system organisation ( reflex arc )
Stimulus - Receptor - Sensory neuron ( afferent ) - CNS - Motor neuron ( efferent ) - Effector - Response
What is the effector
Muscle or Gland
Muscle contracts
What is transduction
Converting stimulus into electrical signals ( receptor does this )
What is part of PNS
- Sensory neuron
- Motor neuron
What is part of CNS
- Brain
- Spinal cord
Summary of reflex arc
Stimulus - electrical signal ( transduction ) - done by receptor
Receptor sends signals to sensory nervous system via sensory neurones
CNS interprets electrical signal - another electrical signal along motor neuron = response ( muscle contracts )
Example of reflex arc
Myotactic reflex - knee jerk reflex
Modified muscle cell - wrapped with sensory nerves
What is stimulus in knee jerk reflex
Hammer in knee
What is response in knee jerk reflex
Leg in air
How does knee jerk reflex work
Stretching a muscle (by hitting the patellar tendon with a hammer), activates a muscle stretch receptor (the muscle spindle) and cause nervous activity in a sensory motor neuron
The sensory neuron synapses with an alpha motor neuron in the spinal cord, which causes the quadriceps muscle to contract (& causing the leg to ‘jerk’.
(At the same time the opposing muscle is inhibited)
What are the 3 types of muscle
- Skeletal
- Cardiac
- Smooth
Skeletal muscle
- Attached to skeleton
- Striated
- Forceful
- Voluntary
Cardiac muscle
- Found in heart
- Striated
- Forceful
- Involuntary
Smooth muscle
- Hollow organs/glands
- Unstriated
- Weak
- Involuntary
Somatic nervous system
Innervates skeletal muscle
Voulantary
Autonomic nervous system
Innervates smooth and cardiac muscle, + glands
Invoulantary
What is Autonomic system divided into
- Symphathetic
- Parasympathetic
Symphathetic nervous system
- Flight, fright + fight
- Increases heart rate
- Inhibits bladder contraction
- Dilates pupil
Parasympathetic nervous system
- Resting and digesting
- Decreases heart rate
- Contracts bladder
How many pairs of nerves connect CNS to rest of body
43
How many cranial nerves exit brain
12
How many exit at various levels of spinal cord
31
What are the cells of the nervous system
NEURONS or NERVE CELLS
Generate & transmit electrical signals
SUPPORTING CELLS
Supporting roles in Nerve Cell functions
Nerve cells and other cells comparison
Nerve cells are similar to other cells - plasma membrane, nucleus, organelles, obey same rules BUT…
Function is unique - transmit and store info
What is structure of neurone
- Several dendrites (input to the cell)
- Cell body (soma)
- A long axon
- A synaptic region (axon terminals) – (output)
How are neurones polar strcutures
Information comes one end and goes out other end - 2 ends to different things ( left and right )
How many directions is information flow and where
- One direction
- Dendrites to synapses
- A single neuron can receive input from many others
What do neurons form
Complex networks - when neurons connect together information comes into dendrites - axon - leaves synapse into 2nd neuron
Dendrites
- Receive incoming electrical information
- Their branching increases surface area = gather info from lots of areas
-Spines on dendrites increase surface area
further - Most input is on to dendrites (axondendritic), some incoming neurons connect to the cell body (axosomatic) or even the axon (axoaxonic)
- Each cell connecting to dendrite will produce electrical signal - conducted along dendrites and cell body
What is the cell body also called
Soma
Role of cell body ( soma )
- ‘Sums’ electrical signals coming from dendrites
- Contains the usual organelles including;
prominent nucleus and nucleolus, rough ER (known as Nissl substance) - Makes proteins
What is the rough ER called in cell body
Nissl substance
What does Nissl substance look like under microscope
Grainy
Role of Axon
- Takes electrical signals away from the cell body
- Movement of substances via ‘axonal transport’ and
‘axoplasmic flow’ - Transfers electrical signal along neurone
- Gets proteins to every part of cell
Size of axon
Varies in length from «1 mm to >1 m
Structure of axon
- Usually unbranched, but may have collaterals
- The axoplasm is surrounded by a ‘normal’ phospholipid bilayer membrane embedded with proteins (which act as ion channels)
- Joins soma at ‘axon hillock’ (where signal is generated)
- Ends in synaptic terminals (where it contacts other neurons)
- Possession of a myelin sheath
- Contains microfilaments (actin), microtubules & intermediate filaments
What is Axon Hillock
Where signal is generated
Where axon and soma join
Receive electrical signals from dendrites - added together at axon hillic = bigger electrical signals - travels along axon
What is myelin sheath
Fatty substance wrapped around
How are substances transported within axon
- Axoplasmic flow
- Axonal transport
What is axoplasmic flow
- Anterograde transport from cell body to synapse. -Involves intermediate filaments - transports proteins
- Slow (1-5 mm day)
- Repairs and maintains cells
What are axon terminals
Where axons terminate in series of synapses
Where electrical signals are transferred to other neurons or muscles.
How are electrical signals transferred to other neurons
Through the release of a chemical (neurotransmitter) - bought by axoplasmic flow + axonal transport to cell body
What do axon terminals contain
Vesicles - lots of tiny bags which contain proteins/chemicals - release neurotransmitter
How do neurons talk to each other
By releasing chemicals
What are neuroglial cells
Supporting cells - nerve glue
What are the neuroglial cells in CNS
Oligodendrocytes
Microglia
Ependymal cells
Astrocytes
What are the neuroglial cells in PNS
Schwann cells
Satellite cells
What are Schwann cells
- Produce myelin in the PNS (in the process of ‘myelinisation’)
- The Schwann cell membrane repeatedly wraps itself around an axon
- A single Schwann cell provides myelin for only one neuron but a single neuron receives its myelin from several Schwann cells
What is the membrane of Schwann cells and what does it do
Myelin
Surrounds axon with hundreds of layers
What are nodes of ranvier
Gaps in the myelin sheath formed between the bits of myelin provided by individual Schwann cells
Where is the electrical signal generated
Nodes of ranvier
Role of nodes on ranvier
- Regenerates electrical signal
- Insulator - makes sure electrical signals can be transported along axon
What happens if there’s no myelin
Nervous system slows down/ loses control = doesn’t conduct electrical activity
Some small axons
Remain umyelinated and rather than being wrapped in Schwann cell cytoplasm, simply lie in a groove on the surface of such a cell.
Oligodendrocytes
- Produce myelin in the CNS
- A single oligodendrocyte provides myelin for only several neurons
- Wrap around neurons in CNS
Astrocytes
- Star-shaped with ‘feet’ covering blood vessels
- Important for blood:brain barrier
- Also have a role in damage repair (proliferate & form scar tissue) - protects brain
- Store nutrients (glucose)
- Isolate synapses = mop up neurotransmitters
- Disease: main source of CNS tumours = Gliomas
What is main role of astrocytes
Blood brain barrier - controls what goes in/out
Replication of astrocytes
Causes cancer/ tumour - does not come from nerve cells, comes from astrocytes - as nerve cells in CNS don’t replicate
Can neurone in brain regenerate
No
What is main source of CNS tumours
Gliomas
Ependymal cells
- Line ventricles of brain and central canal of spinal cord
- Involved in secretion of CSF
- Surface cells of ventricles, brain = fluid filled space
Microglia
- Phagocytic cell
- Normally ‘dormant’, but involved in CNS injury/inflammation
- Move to damage site
- Phagocytose debris & pathogens
- Release cytokines to attract other immune cells, e.g. lymphocytes
Main role of microglia
Hoover of brain - eating u and digesting things around brain that need to be destroyed
What are the 4 basic morphological types of nerve cell
- Anaxonic
- Unipolar
- Bipolar
- Multipolar
Anaxonic nerve cell
(no axon - rare) – e.g. retinal amacrine cell
Unipolar nerve cell
only one neuron emanating from the cell body = one process coming out of cell body
e.g. some sensory neurons
Bipolar nerve cell
one dendritic process and one axon leave the cell body) - e.g. retina
Multipolar nerve cell
(many neurites emanating from the cell body)
- e.g. most CNS neurons (e.g. the typical neuron)
Which nerve cell is retina example of
Bipolar nerve cell
What is organisation of a nerve
- A nerve is a collection of neurons
- It can contain both afferent and efferent fibres
- Individual neurons are separated by a connective tissue sheath –endoneurium
- The perinurium splits nerves into fascicles
- The epineurium forms an outer coat
What forms outer coat in nerve
Epineurium
What does perinurium do
Splits nerves into fascicles
What is meant by reticulum
A fine network of net like structure
What is a nerve
Collection of neurons
What happens in a Myotactic reflex
Skeletal muscle contracts
How many neurons or nerve cells
10^11
How many supporting cells ( neuroglia )
10^12
What is function of neurons/ nerve cells
Generate and transmit electrical signals
What is function of supporting cells ( neuroglia )
Supporting roles in nerve cell functions
What do dendrites connect to
Cell body
What contains the nucleus
Cell body ( soma )
Where are electrical signals from dendrites added together
Axon Hilic
How many directions is axoplasmic flow
One - cell body to synapse
How many directions is axonal flow
Both - synapse to cell body and back
What is function of axonal flow
Relase waste products
What is function of axoplasmic flow
- Repair and maintain cell
- Transport proteins
What do axons terminate into
Synapses ( axon terminals )
What is final area of neurone
Axon terminal
How is a chemical released
Axoplasmic flow and axonal transport to cell body
What is stuck on side on Shcwann Cell
Remaining nucleus of Schwann Cell
What happens if neuron in brain dies
It stays dead forever - cannot regenerate
What makes the myelin
Oligodendrocytes
