Central Nervous System
The nervous system consists of two major parts, the central
nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists
of the brain and spinal cord. The PNS consists of the peripheral nerves (
nerves which are outside the CNS).
Gross structure of the CNS:
The brain: consists of 3 major parts-
- Cerebrum and diencephalon
- Brainstem (midbrain, pons, medulla)
- Cerebellum
The brain receives 15% of the output of blood from the
heart, is highly sensitive to O2 lack, can autoregulate its blood flow, is 1/50th
of the body weight and is located in the cranial cavity. CNS covered by 3
meninges (dura, arachnoid and pia) with CSF between the last 2 membranes; 4
ventricles; Blood brain barrier.
Cerebrum : is the largest part, is divided by a deep clept
(gap) into right and left cerebral
hemispheres. The hemispheres are connected by a mass of nerve fibres called the
Corpus Callosum. The outer layers consist of the nerve cell bodies (grey
matter)- cerebral cortex which shows lots of folds separated by gaps (sulci or
fissures) which give it a large surface area. Each hemisphere is divided into 4
lobes (eg frontal, parietal, temporal and occipital) which are connected by
masses of nerve fibres or tracts (white matter). Functions are many, include,
memory, thinking, intelligence, sensory perception, control of muscle action.
Frontal lobe: Located in front of the central fissure, motor
function (area 4, premotor area, supplementary motor areas- planning and
execution of voluntary movements. Also motor speech areas (Broca’s area); also
personality, emotional behavior
Temporal lobe: sensory areas (hearing and smelling); also
helps in control of emotional behaviour in limbic system and functions of the
autonomic nervous system; hippocampus involved in learning and memory;
Wernicke’s area involved in language comprehension.
Parietal lobe: has somatosensory areas which are involved in
sensory judgement and shapes of objects, spatial awareness
Occipital lobe: behind the parietal and temporal lobe- sight
and perception
pain, temperature, pressure and touch from postcentral
area).
The motor area from the right side of the brain controls the
left side of the body, similarly the right side of the brain receives sensory
info from the left side of the body.i.e each hemisphere controls the
contralateral side of the body. Left hemisphere
controls the expressed language, the Right hemisphere is dominant for
language intonation and body language and
mathematics functions
Within the cerebral hemispheres there are other areas
important in a variety of functions. These areas are groups of cell bodies or
nuclei. 1. Basal Ganglia- involved
in muscle tone and coordination. The basal ganglia starts and stops movements
it also contributes to cognitive (intelligence, knowledge, motor learning) and
emotional functions.
Diencephalon consists of the thalamus and the hypothalamus. 2.
Thalamus- receives sensory input
from skin, gut and sense organs and relays signals to the cerebral cortex and
also receives info from the cerebellum and basal ganglia
3. Hypothalamus-
located below and in front of Thalamus and above the pituitary gland (major
endocrine gland of the body). The functions of the hypothalamus include,
control of the autonomic nervous system (ANS), thirst and water balance, body
temperature, stress response, sleeping and waking cycles, secretion of hormones
and apetite.
Limbic System: group of structures (cingulated gyrus,
hippocampus, septum, amygdaloid and anterior nucleui) – imp in the regulation
of behaviour and memory
Brainstem:
consists of the midbrain, pons and medulla
Midbrain- consists of cell bodies and nerve fibres which act
as relay stations for nerve fibres and connect cerebrum with lower part of
brain.
Pons- located in fron of the cerebellum, mainly nerve fibres
which are on the surface and cell bodies which are deeper; this area acts as
relay stations
Medulla- 2.5cm long, grey matter deeper and white matter on
outer surface, contains vital centres for controlling the cardiac, respiratory,
blood vessels and reflexes for vomiting, coughing, sneezing and swallowing.
Lots of nerve fibres cross over (decussate) eg from right side of brain to left
side of body.
The brainstem also contains a collection of neurones called
the Reticular Formation, which conduct ascending and descending nerve impulses
and thus help in voluntary movement and sensory information getting to brain.
Cerebellum: Is
located behind the pons and below the cerebrum, has 2 hemispheres, grey matter
on surface and white matter deeper. Main function is to coordinate the
voluntary muscle movement, posture and balance.
SPINAL CORD
Is the elongated cylindrical part of the CNS, surrounded by
the membranes and cerebrospinal fluid. It is approx. 45cm long in an adult and
15-18mm thick. The spinal cord is the nervous link between the brain and the
rest of the body. In cross section there is grey matter (cell bodies) in the
centre surrounded by white matter (nerve fibres).
The grey matter is shaped like and ‘H’ and has cell bodies
of sensory cells, lower motor neurones and connector neurones. The white matter
is arranged in 3 columns or tracts (sensory nerve fibres, motor nerves,
connector neurones). The sensory nerves carry info from skin, tendons, muscles and
joints, whilst the motor nerves carry efferent info to the skeletal, smooth,
cardiac and secretory glands.
Fine Structure of the CNS
The CNS: Contains excitable (neurones- which don’t
replicate) and non-excitable cells (cells which do replicate). Three membranes
(Dura, Arachnoid and Pia mater) cover the CNS. The brain contains cavaties (ventricles),
containing cerebrospinal fluid (CSF).
Nerve cells or neurones:
The neurones consist
of a cell body, one axon and dendrites. The cell bodies make up the grey
matter, the axons and dendrites make up the white matter. Neurones can be
sensory (afferent) or motor (efferent) and communicate between nerves by a
synapse and chemical neurotransmitter.
Cell bodies: Groups of cell bodies are called nuclei in CNS,
and ganglia in PNS, they are also found in the spinal cord.
Axons: Are found in groups called tracts in the CNS; bundles
of axons are called nerves outside the CNS, which can be myelinated or
non-myelinated
Dendrites: short processes, like branching axons, can be
synapses or sensory receptors
Non- Excitable cells:
Four types are found in the CNS which make up to a ¼ to a ½
of the volume of brain tissue. These are Astrocytes, Oligodendrocytes,
Microglia and Ependymal cells.
The Astrocytes are star shaped with branching processes
which wrap around blood vessels, thus forming
the blood brain barrier. The blood brain barrier, separates the neurones
from the blood, protects the brain from toxic substances, but does allow lipid
soluble substances through (eg glucose, alcohol, O2, CO2).
Oligodendrocytes are found around nerve cell bodies and
axons and form and maintain the myelin sheath (same as schwann cells).
Microglia- found around blood vessels and involved in
phagocytic, inflammatory and cell destruction functions
Ependymal Cells- Form the epithelial linings of the
ventricles of the brain and the central canal of the spinal cord
Meninges (membranes)- 3 membranes cover the brain and spinal
cord. An outer Dura mater, a middle Arachnoid mater and an inner Pia mater.
Between the Arachnoid and the Pia is the subarachnoid space which contains CSF.
The Dura consists of 2 dense fibrous layers, an inner layer protects the brain
and and outer layer lines the inner surface of the skull. The inner layer
extends into the spinal cord and is
separated from the vertebrae by the epidural space. The Pia mater is fine
connective tissue with minute blood vessels adhering to the brain.
Cavaties (ventricles):- In the brain are 4 cavaties or
ventricles containing CSF. The Right, left, 3rd and 4th .
CSF- Is a clear, slightly alkaline fluid, which is secreted
continuously (0.5ml/min) into the ventricles by choroids plexuses or vascular
areas. Approx. volume around brain and spinal cord is 120mls. It is similar to
plasma, but has very little protein. Its functions include: supporting and
protecting brain and spinal cord, maintaining uniform pressure, cushions brain
and acts as shock absorber, keeps CNS moist and allows exchange between it and
nerves.
Peripheral Nervous System
The peripheral nervous system (PNS) consists of the nerves outside the CNS. Nerves consist of
cell bodies, axons and dendrites. The nerves can also be split into myelinated
and non-myelinated. Where the myelinated are mainly larger peripheral fibres
surrounded by a myelin sheath (formed by Schwann cells). Between these cells
are Nodes of Ranvier which help in rapid nerve transmission. These nerves can
be split into the motor division and sensory division. The motor division can
be further split into the voluntary nerves and the involuntary nerves (ANS).
The nerves of the PNS consists of 31 pairs of spinal nerves, 12 pairs
of cranial nerves and a large number of ANS nerves. The spinal
nerves are named according to the the vertebral location. They arise from both
sides of the spinal cord. There are 8 cervical, 12 thoracic, 5 lumber, 5 sacral
and 1 coccygeal.
There are 12 pairs of cranial nerves which can be sensory,
motor or mixed.
Sensory – egs I (smell),II (sight), VIII (hearing); Motor-
III, IV (move eye muscles), XII (swallowing, speech); Mixed- VII (taste,
expressions), X(wide functions- called the vagus).
Spinal Reflexes: Are reflexes which are independent
of the brain. A spinal reflex consists usually of 3 parts, a sensory neurone, a
connector neurone in the spinal cord and a lower motor neurone. An example of
this would be a withdrawal reflex due to a painful stimulus. So a toe or finger
which touched a painful stimulus, would stimulate a sensory receptor, sending signals
up a sensory afferent nerve, which would synapse with a connector interneurone
in the grey matter, which would then synapse with a motor efferent neurone
which would send signals to a neuromuscular or neuroeffector junction causing
the muscle to flex and thus withdraw the finger or toe.
But the simplest reflex which only involves 2 neurones i.e
without a connector neurone is called a monosynaptic reflex eg. Knee jerk. This
reflex protects against excessive joint movement. It can be demonstrated at any
point where a stretched tendon crosses a joint.
Nerve and Nerve transport
Structure:
Neurones = Nerve cell body+ axon + dendrites.
Bundles of axons make up a nerve fibre.
Bundles of nerve fibres are enclosed by a coat of connective
tissue called the Perineurium.
Nerve fibres can be split into Myelinated and Non-Myelinated
types. Both have an outer membrane called the Neurilemma (which is the outer
layer of the Schwann cells). Most nerves are myelinated. The myelinated nerve
has an axon covered by a lipid protein layer called Myelin (secreted by the
Schwann cells) which in turn is enclosed by the Neurilemma. At regular
intervals along the axon there are gaps or interuptions to the myelin sheath,
called the Nodes of Ranvier at which fast nerve transport occurs. The non-myelinated
nerve fibre has a Neurilemma and an axon. These are small fibres with diameter
less than 1 micrometers. Nerve fibres could also be classified based on size, 3
groups are used (A,B,C). Where ‘A’ are myelinated and large (15-20micrometres)
and have fast conduction (100 m/s), ‘B’ are preganglionic autonomic fibres,
intermediate in size and ‘C’ are Non-myelinated fibres with the smallest
diameter(less than 1.2 micrometres) and
low speed (1-3m/s).
Nerve Impulses:
A nerve fibre conducts nerve impulses (action potential or
wave of depolarization) produced by a stimulus. A semipermeable membrane, ion
channels and the concentration of ions and proteins on either side of the
membrane of excitable cells determine the resting and active state of nerve
cells.
Resting Membrane Potential:
Is approximately -70mV for a nerve, with the inside
(intracellular) being negative to the outside (extracellular). This is due to
concentration and electrical gradients produced across the semi permeable membranes.
The inside of the neurone contains high concentration of K+ ions and proteins with negative charges.
The outside contains high Na+ and Cl- ions. Thus the diffusion for K+ is from
inside to outside and that for Na+ is from Outside to inside.There is also a
Na+/K+ pump in the membrane wall which throws out 3 Na+ for every 2 K+ that
enter and uses 1 ATP. This pump is important in keeping the resting membrane
constant. Any decrease in Oxygen or
blockage of this pump will damage the cells. An electrical potential opposes
the concentration gradient of K+ leaving the cell, also maintaining the resting
potential.
Action Potential:
Since the membrane becomes permeable to Na+ ions, then
further and further entry of Na+ causes an action potential (+40mV), with the
inside now positive and the outside negative. You have to cross a threshold
potential to produce an action potential i.e all or none principal.
Depolarisation is making the axon more unstable and is caused by Na+ entry into
the inside of the cell. Repolarisation is making the axon more stable by entry
of K+ ions to re-establish the resting membrane. Hyperpolarisation is making
the axon very stable (inside more negative than normal).
Nerve Cell Action Potentials:
The nerve cell synapses with other nerves and dendrites.
These can produce both excitatory and inhibitory nerve action potentials. Such
as Excitatory post synaptic potentials (EPSP) and inhibitory post synaptic
potentials (IPSP). The mechanism of action potential generation is the same as
in the axons. This action potential can be modified by presynaptic inhibition
and postsynaptic inhibition. Also excitation can be graded and the final output
depends on size and number of impulses.
Transport of the nerve impulse:
The nerve impulse can be transmitted both electrically and
chemically. A third type of nerve transport is Axosplasmic flow which is
transport of substances such as hormones down the nerve (egs neuropeptides
synthesized in the cell bodies of neurones in hypothalamus and then transported
along the axon to the posterior pituitary).
Electrical transport:
Making a part of the nerve membrane unstable by entry of Na+
ions leads to depolarization (starting an action potential) or instability of
the membrane. Returning the membrane to its resting state by return of K+
ions is called repolarization. The
refractory period is the period in which no stimulus can start an action
potential.
The conduction velocity is proportional to the size of the
nerve. Faster transport in bigger nerves. Also myelinated nerves conduct
faster, due to jumps at Nodes of Ranvier, without the leakage found in
non-myelinated nerves.
Synaptic
Transmission:Can be chemical or electrical.
Electrical Synapses such as gap junctions are low resistance
pathways which connect the cytoplasm of adjacent cells. They exchange ions and
small molecules across a tiny gap of only 3nanometres, so transport is
extremely fast. These gaps are found in cardiac cells, smooth muscle and liver
cells.
Chemical synapses have larger gaps (30 nanometres)
than electrical synapses. They consist of a presynaptic terminal, a synaptic
cleft and a postsynaptic membrane and receptors. Synapses are found between
nerves (axoaxonal), between nerves and dendrites (axodendritic), between axons
and nerve cell body (axosomatic) and
between nerves and muscle end plates (on muscle fibres). Each neurone in the
CNS is in contact with 100,000 presynaptic axon terminals.
The synaptic stimulation can be modified into 2 types:
Spatial summation- inputs from several axons arrive
simultaneously at same postsynaptic membrane, these are then added to get the
final signal.
Temporal summation- successive stimulation by signals coming
one after the other leading to final signal which starts action potential. The
chemical transmitter crossing the synapse varies. They can be excitatory such
as Glutamate and Aspartate in brain and spinal cord or they can be inhibitory
such as Gabba Amino Butyric Acid (GABA) and Glycine in brain and spinal cord. Other neurotransmitters
include dopamine, Noradrenaline and Adrenaline in the sympathetic and CNS.
Serotonin in the midbrain. Acetylcholine in the CNS and all motor neurones, and
all preganglionic neurones of autonomic
nervous system and postganglionic parasympathetic nerves of the autonomic nervous
system.
Chemical neurotransmitter crosses the synapse and then
starts another action potential. These action potentials can be excitatory
(EPSP= excitatory postsynaptic potential) or inhibitory (IPSP=inhibitory
postsynaptic potential).
Neuromuscular transmission occurs as follows:
When a motor neurone comes in contact with muscle fibres, a
neuromuscular junction is present.Nerve impulse travels along the motor nerve
and causes calcium to be released into the axon ending, this causes vesicles
containing the neurotransmitter Acetylcholine (Ach) to fuse with the membrane
of the axon ending and release the Ach (process called exocytosis) into the
synapse, which then travels across and stimulates the muscle end plate to
depolarize and start a muscle action
potential, this leads to calcium release which causes the muscle fibres to
contract.
Factors which affect the above mechanisms include,
Not producing enough Ach, blocking or breaking down of Ach,
neuromuscular blocking agents such as Curare (blocks Ach at muscle end plate-
so muscle can’t contract), Hexamethonium (which blocks Ach at autonomic
ganglia) and Atropine (blocks the Ach at heart muscle, thus heart rate goes up,
since Ach is inhibitory).
Autonomic Nervous System
The nervous system can be
divided into the central (CNS) consisting of the brain and spinal cord and the
peripheral consisting of the nerves that are outside the CNS. The peripheral
can be further subdivided into the
efferent nerves which carry signals to
the peripheral tissues and the afferant nerves which carry signals back to the
CNS. The nerves can be further
subdivided into the somatic nerves (which control voluntary activity such as
skeletal muscle contraction and also send sensory information from the
periphery such as muscles and skin to the brain) and the autonomic nerves which
are involuntary and control the everyday needs of the body.
Autonomic nervous system:
The
autonomic nervous system (ANS) coordinates the regulation of a variety of functions ( egs digestion, movement of food,
urination, exercise, blood pressure and blood flow control) together
with the endocrine system.It is a
major controller of smooth muscle, cardiac muscle and the exocrine glands.
Structure:
It
can be split into the 2 divisions of the sympathetic and parasympathetic
nervous systems. The efferent motor fibres travel via the spinal cord and end in organs or ganglia. The afferant
fibres travel either directly via the cranial nerves or via the dorsal roots of
the spinal cord to the central nervous system.Both divisions are bilateral, and
both have 2 neurones between the spinal cord and their effector organs and one
synaptic junction before the target organs. Between the 2 neurones is a
ganglion (a collection of nerve cell bodies ) . The ganglionic neurotransmitter
is Acetylcholine (Ach). Most organs of the body are innervated by both the parasympathetic and sympathetic but to
different degrees. Some organs however only receive innervation from the
sympathetic nerves eg adrenal medulla, kidney, pilomotor muscles and sweat
glands. The parasympathetic system is normally dominant at rest and when
digesting food, whilst the sympathetic is more dominant in activity.
Sympathetic nerves originate
from the cell bodies in the thoracic and lumbar spinal cord (T1-L2), then short
preganglionic and long postganglionic fibres to finally the organs. They are
the most widely distributed. At the organs the sympathetic nerves release the
chemical transmitter Noradrenaline.
The parasympathetic nerves arise
from the sacral region (S2-S4) of the spinal cord and the hindbrain and midbrain (cranial
nerves), they have a long preganglionic fibre and a short postganglionic fibre.
The sacral nerves innervate the blood vessels of the genetalia, bladder and
large bowel, whilst the cranial supply the head and viscera. At the organs the
parasympathetic nerves release the chemical transmitter Ach.
Control
of the heart:
The heart is innervated by the parasympathetic and sympathetic.
Parasympathetic nerves are distributed to the SA and AV nodes and to the atria.
Stimulation of these nerves causes a release of Ach which decreases the heart
rate and atrial contraction. Sympathetic fibres innervate SA and AV nodes and
all the heart muscle. Stimulation causes release of noradrenaline which increases heart rate and force of contraction of atria
and ventricles.
Nervous
control of blood vessels:
Sympathetic Control:
Is brought about predominantly
by sympathetic innervation which is of 2 types, sympathetic adrenergic and
sympathetic cholinergic. Sympathetic adrenergic nerves are the majority and are
found in most areas of the body, they release Noradrenalin at their nerve
endings which stimulate special membrane receptors to cause vasoconstriction. Their degree of
innervation varies e.gs the brain, large arteries and veins and precapillary
sphincters are poorly innervated whilst the skin, small arterioles and gut
areas are highly innervated.
There are very few sympathetic
cholinergic fibres in man, more in other species such as dogs and cats. These
fibres innervate precapillary resistance vessels of skeletal muscles.
Stimulation of these releases Acetylcholine which causes dilation. Sympathetic
cholinergic fibres also innervate the adrenal medulla and stimulation releases adrenaline and
noradrenaline (ratio of 4:1 respectively), and also sweat glands.
Receptors of the sympathetic
nervous system-
Are called Adrenergic
receptors which are called alpha and beta receptors. These receptors
respond differently depending on their location. In general activation of the
SNS occurs in activity, stress, trauma, shock etc. and leads to the production
of increased blood flow to organs (increased cardiac output) particularly to
muscles, increased energy (mainly glucose) and increased release of
Noradrenaline and Adrenaline (from the adrenal medulla). Other effects include:
1. vasoconstriction of blood
vessels (eg gut and skin), increased peripheral resistance, increased blood
pressure and dilation of the pupil.
2. The heart is activated equally by adrenaline or
noradrenaline and leads to tachycardia, and increased force of contraction.
3. vasodilation of skeletal and
coronary vessels, increased muscle and
liver glycogenolysis, increased release of glucagon, relaxation of the uterine
smooth muscle and bronchodilation.
Parasympathetic control:
Parasympathetic fibres are not
very important to control of blood flow except in the following cases:
i) direct vasodilation, is only
really important in genital erectile tissue
ii) Indirect vasodilation, can
however be more important for example during exercise. Stimulation of
parasympathetic fibres to the salivary, sweat and gastrointestinal glands leads
to , which leads to the production of vasodilator substances called Kinnins
which bring about vasodilation. e.g. Bradykinin - a powerful vasodilator.
iii) dilates the coronary
arteries and face blood vessels.
Receptors of the parasympathetic
nerves:
There are 2 types of cholinergic
receptors which respond to Ach.
1. Muscurinic receptors- are mainly
found on the organs the heart, smooth muscle and exocrine glands.
2. Nicotinic receptors- are found
in the CNS, adrenal medulla, autonomic ganglia and the neuromuscular junction.
Functions:
Ach causes a decrease in heart
rate and cardiac output, decrease in blood pressure, stimulates movement and
secretions in the gut, increased secretions and constriction in the bronchioles, stimulates secretions from
the salivary glands and constriction of
the pupil.
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