Download ANHB1102 Basic Principles of the Nervous System • The nervous

ANHB1102
Basic Principles of the Nervous System













The nervous system is very complex. Nervous system is the foundation of our conscious experience,
personality and behaviour. Neurobiology combines behaviour and life sciences.
The Central Nervous system (CNS)
- Brain and spinal cord enclosed by cranium and vertebral column (bony boundaries) and protected by
meninges
- Tract – a bundle of axons in the brain and spinal cord
Peripheral nervous system (PNS)
- All the nervous system except the brain and spinal cord; composed of nervous and ganglia
- Nerve – a bundle of nerve fibers (axons) wrapped in fibrous connective tissue
- Ganglion – a knot-like swelling in a nerve where neuron cell bodies are concentrated
Both the peripheral and central nervous systems have specialised cells that carry ‘information’ or
‘commands’, called a neuron, with a cell body, axon and connections to other neurons.
Grey matter is neuron cells bodies and glia. White matter is myelinated axons (made up of lipids that appear
white) and glia.
CNS
- Spinal cord – grey matter on inside, white matter on outside
- Brain – white matter on inside, grey matter on outside
PNS
- Ganglia – when axons project outside of spinal cord, they connect with cell bodies, this large
grouping of cells bodies form the ganglia which project somewhere else on the body
- e.g. deficit of feeling in fingers, inflammatory diseases etc. – GP tries to find injury to PNS through
MRI
Control systems
- Molecular messengers – circulate with blood, must pass freely through circulatory system, variable
speed but generally slow
- Many aspects of control – different mechanisms, remembering earlier ‘experiences’
- e.g. paralysed adult doesn’t forget how to walk but the neurons have been damaged
Nervous system carries out its tasks in three basic steps
1. Sense organs receive information about changes in the body and external environment, and transmit
coded messages to the brain and spinal cord (CNS)
2. CNS processes information, relates it to past experiences, and determines appropriate response
3. CNS issues commands to muscles and gland cells to carry out such a response
- e.g. hand on a hot plate – temperature receptors in hand receive information of an increase in
temperature, if it gets too hot, CNS determines that hand must be moved, CNS tells muscles to
contract and move hand away.
PNS
- Peripheral nervous system contains sensory and motor divisions each with somatic (voluntary –
skeletal muscles) and visceral (involuntary – smooth muscles/cardiac tissue) subdivisions
Sensory (afferent) division carries signals from receptors to CNS.
- Somatic sensory division carries signals from receptors in the skin, muscles, bones and joints
- Visceral sensory division carries signals from the viscera (heart, lungs, stomach and bladder)
Motor (efferent) division carries signals from CNS to effectors (glands and muscles that carry out the body’s
response).
- Somatic motor division carries signals to skeletal muscles – output produces muscular contraction as
well as somatic reflexes – involuntary muscle contractions
- Visceral motor division (autonomic nervous system) carries signals to glands, cardiac and smooth
muscle – its involuntary responses are visceral reflexes
Visceral motor division (autonomic nervous system)
- Sympathetic division (Fight or flight) – tends to arouse body for action, accelerating heart beat and
respiration, while inhibiting digestive and urinary systems
- Parasympathetic division (Rest or digest) – tends to have calming effect, slows heart rate and
breathing and stimulates digestive and urinary systems






ANHB1102
Transmission of ‘information’
- Along a single axon of a neurons – electrical transmission
that is very fast
- Requires cell-to-cell communication – faster chemical
messengers across narrow gaps between cells
Universal properties of neurons
1. Excitability (irritability) – respond to environmental changes
called stimuli (can generate a signal – doesn’t always
produce something (e.g. inhibitory signal – turns something
off)
2. Conductivity – respond to stimuli by producing electrical signals that are quickly conducted to other
cells at distant locations
3. Secretion (neurotransmitter release) – when an electrical signal reaches the end of nerve fiber, the cell
secretes a chemical neurotransmitter that influences the next cell
Functional classes of neurons
1. Sensory (afferent) neurons – detect stimuli and transmit information about them toward the CNS
2. Interneurons – lie entirely within CNS connecting motor and sensory pathways and receive signals
from many neurons, carrying out integrative functions (make decisions on responses)
3. Motor (efferent) neurons – send signals out to muscles and gland cells (the effectors)
- e.g. can move arm to touch a table but cannot feel the touch of the table – sensory neurons affected,
motor neurons not affected
- e.g. hot plate – sensory neurons detect, interneurons decide to move hand away, motor neurons send
signals to move hand
Structure of a neuron
- Soma (cell body) is control centre of neuron. It has a single, centrally located nucleus with large
nucleolus, and cytoplasm containing organelles.
- Dendrites are branches that come off the soma. Primary site for receiving signals from other neurons.
The more dendrites the neuron has, the more information it can receive.
- Axon (nerve fiber) originates from a mound on the soma called the axon hillock. Axon is cylindrical,
relatively unbranched for most of its length. It takes information away from the soma. Myelin sheath
may enclose axon.
- Terminal part is a little swelling that forms a synapse (junction) with the next cell. It contains synaptic
vesicles full of neurotransmitter.
Nervous impulse fundamentals
- Action potential – momentary reversal of membrane potential. This change causes electrical
signalling within neurons
Resting Membrane Potential (RMP)
- Inner part of the cell contains large number of ions, outside has same ions
- In excitable cell, has more negatively charged inner membrane compared to outer membrane (RMP
approximately -70mV)
- Depolarisation – inner membrane becomes more positive through huge influx of sodium which elicits
action potential
-
Three different neurons giving signals
to one single
neuron (as one single neuron may not be able to elicit a response, however the three together will
reach the threshold to cause action potential to be fired).











ANHB1102
Temporal summation
1. Intense stimulation by one presynaptic neuron
2. EPSPs spread from one synapse to trigger zone
3. Postsynaptic neuron fires
Spatial summation
1. Simultaneous stimulation by several presynaptic neurons
2. EPSPs spread from several synapses to trigger zone
3. Postsynaptic neuron fires
Postsynaptic changes can be ‘summated’
- e.g. +15 (-55), +15 (-55), -5 (-75), -5 (-75)
- Net change is +20 and resting membrane potential at -50
- If threshold is -50mV then none of these synapses individually could cause an action potential
Myelin sheath is insulation around a nerve fiber, formed by
oligodendrocytes in CNS and Schwann cells in PNS. One
oligodendrocyte may myelinate multiple different segments
of multiple axons, whereas one Schwann cell myelinates
only one segment of an axon. If a Schwann cell was to die,
only one segment would be lost, whereas if an
oligodendrocyte dies, multiple segments would be lost.
Myelin sheath is segmented
- Node of Ranvier is a gap between segments
- Internodes – myelin-covered segments from one gap to the next
Signal conduction in nerve fibers
- Myelin creates resistance, electrical impulses therefore jump from node
to node (where there is less resistance), called salutatory propagation. This makes the signal faster.
- Myelin prevents sodium ions from leaking out. Sodium repels positive ions already in the axon which
forces them down the axon, eliciting depolarisation which elicits action potential at every node of
Ranvier.
- Sodium flow at node generates action potential. The positive charge flows rapidly along axon and
depolarises membrane – signal grows weaker with distance. Depolarisation of membrane at next node
opens Na+ channels, triggering new action potential.
Unmyelinated nerve fibers
- Many CNS and PNS fibers are unmyelinated. In PNS, Schwann cells hold 1 to 12 small nerve fibers
in surface grooves. Membrane folds once around each fiber.
Speed at which a nerve signal travels along surface of nerve fiber depends on two factors
1. Diameter of fiber – larger fibers have more surface area and conduct signals more rapidly
2. Presence or absence of myelin – myelin further speeds signal conduction
Conduction speed
- Small, unmyelinated fibers – 0.5 to 2.0 m/s
- Small, myelinated fibers – 3 to 15.0 m/s
- Large, myelinated fibers – up to 120 m/s
- Slow signals sent to the gastrointestinal tract where speed is less of an issue. Fast signals sent to
skeletal muscles where speed improves balance and coordinated body movement.
Four types of glia occur in CNS
- Oligodendrocytes form myelin sheaths in CNS that speed signal conduction
- Ependymal cells line internal cavities of the brain; secrete and circulate cerebrospinal fluid (CSF)
- Microglia wander through CNS looking for debris and damage
- Astrocytes are most abundant glial cells in CNS, covering brain surface and most non-synaptic
regions of neurons in the grey matter, with diverse functions
Two types of neuroglia occur only in PNS
- Schwann cells wind repeatedly around a nerve fiber and produce a myelin sheath
- Satellite cells cover the surface of nerve cell bodies, provide electrical insulation around the soma and
regulate the chemical environment of the neurons
ANHB1102
Organisation and Development of the Nervous System
 Axons and dendrites are both extensions of the soma (cell body).
 Key differences between axons and dendrites
Axons
Dendrites





Arise from axon hillock
Arise from receiving surface
Long (up to several meters)
Very short
Branched at distal end
Very branched along length
Terminal branches form synaptic knobs
No knobs formed at ends of branches
Contains vesicles with neurotransmitter
Do not have vesicles containing neurotransmitter
Carries impulses away from soma
Transmits impulses towards soma
Junctions are not only formed from pre-synapse to dendrite (post-synapse). Synapses are formed throughout
the nervous system.
Different synapses throughout nervous system
1. Axosecretory – axon terminal secretes directly into bloodstream
2. Axoaxonic – axon terminal secretes into another axon
3. Axodendritic – axon terminal ends on a dendrite spine
4. Axoextracellular – axon with no connection secretes into extracellular fluid
5. Axosomatic – axon terminal ends on soma
6. Axosynaptic -axon terminal ends on another axon terminal
Nervous system development
- Neural crest cells give rise to the neural tube – formed from the ectoderm (the outer most layer of a
trilaminar embryo). Gives rise to the two inner meninges, most of PNS, and other structures of
skeletal, integumentary, and endocrine systems.
- By the fourth week, the neural tube exhibits three primary vesicles at the anterior end
o Forebrain, or prosencephalon
o Midbrain, or mesencephalon
o Hindbrain, or rhompbencephalon
- By the fifth week, it subdivides into five secondary divisions
- The forebrain divides into the telencephalon and diencephalon, which becomes the cerebral
hemispheres and the optical vesicles – which will form the retina
- The midbrain does not divide further, and remains the mesencephalon
- And the hindbrain divides into two vesicles, the metencepholon (pons and cerebellum) and the
myelencephalon (medulla oblongata)
- The primary vesicles form from the neural tube at four weeks. These then develop into the secondary
vesicles forming these structures. Which then go on to develop the major brain regions we see at birth,
continuing on to adulthood.
Major folds in the brain
- The central sulcus, separates the precentral gyrus, and the post central gyrus. This area is our primary
somatosensory cortex and is our main receptive area for the sense of touch. The cortical homunculus
shows huge devotion to hands, face and lips, genitals and the gut.
- The pre-central gyrus is the site for our primary motor cortex. The homunculus has large lips, fingers
and hands, and also tongue and eyelids. It shows how sensory and motor functions work hand in hand
– to have fine motor movements, you need very good sensory perception.
The cerebrum or forebrain (cerebral cortex)
- Made up of white and grey matter, with grey matter (the neuronal
cell bodies) on the outside. It is highly folded, increasing the
overall surface area. Folds are called gyri, and grooves are called
sulci.







ANHB1102
The cerebellum or hindbrain
- Grey matter on the outside and thin folds called folia.
The brain stem
- Forms as an extension of the spinal cord and therefore, white matter is on
the outside.
Thalamus
- Primary relay centre, receiving and sending signals to various localised
areas of the cortex and spinal cord.
Midbrain
- Helps in processing visual cues, auditory cues, motor control, regulating
our sleep/wake cycle, arousal and temperature regulation.
Pons
- Responsible for conducting signals from the cortex to cerebellum and medulla, and carrying sensory
signals to the thalamus.
Medulla oblongata
- Contains four major centres – cardiac, respiratory, reflex and vasomotor centres. The cardio vascular
centre regulates the rate in which our heart beats. Any changes in blood pH or pressure will result in
this centre being activated to increase the heart rate to ensure sufficiently oxygenated and a sufficient
amount of blood reaches the body. The respiratory centre controls ventilation and respiration –
moving oxygen into the lungs, and carbon dioxide out. The medulla controls reflexes of vomiting,
coughing, sneezing and swallowing – therefore getting rid of foreign bodies, large objects – and
getting the right things in like food (macronutrients). The vasomotor centre works with the cardiac
and respiratory centre and regulates blood pressure and other processes.
The most basic divisions of the brain are the frontal, parietal, occipital, temporal, cerebellum and the brain
stem.
-


The frontal lobe is the region of cognition, storing short term memory, problem solving and our
personality traits.
- The parietal lobe is vital for integration of all senses, spatial orientation, visual perception, pain and
touch.
- The occipital lobe is the primary visual cortex as most visual processing occurs here.
- Cerebellum is the major regulator of coordination and timing of movements (damage to this area
doesn’t stop movement, but movement becomes erratic and slow).
- The temporal lobe is responsible for auditory processing. It contains the hippocampus which is
responsible for storing long-term memories. The temporal lobe also helps in visual processing (things
like recognising a person’s face.
The brain stem is vital for conduction of signals travelling to and from the brain, and keeping us alive.
Cranial nerves
- There are twelve pairs, with one on each side of the brain. They are involved with sensation,
movement, or both. Two arise from the cortex – the olfactory nerve – which allows us to perceive
smells, and the optic nerve, which is responsible for transmitting visual information to the brain. The
cranial nerves 3-12, which arise from the brain stem.
- Eye movement is controlled by cranial nerves 3, 4 and 5. The trigeminal nerve (CN 5) provides
sensation to the skin of the face, and controls the muscles during mastication (chewing).
- Facial expression is controlled via the facial nerve (CN 7).