Nervous system

Nervous system:

The nervous system is unique in the vast complexity of thought
processes and control actions it can perform. It receives each minute
literally millions of bits of information from the different sensory
nerves and sensory organs and then integrates all these to determine
responses to be made by the body. The nervous system contains more
than 100 billion neurons and consists of the central nervous system
(CNS) and the peripheral nerves.
The central nervous system (the neural axis or neuraxis) consists of the bra
in and the spinal cord

The Brain

The anatomical structures of brain consist of the cerebrum which consists of two cerebral hemispheres, the cerebellum, and the brain stem. The brainstem consists of the midbrain, the pons and the medulla oblongata. The CNS contains the nerve centers which receive and process the nervous signals, then formulate the response to these signals. The peripheral nerves are divided into cranial and spinal nerves. The cranial nerves are twelve pairs of nerves, which arise from the brain and emerge out through foramina in the bones of the cranium (skull).
Spinal cord
Thirty one pairs of nerves that arise from the spinal cord and emerge out through foramina in the vertebral column. Anatomically, the spinal nerves are sorted out into 5 groups according to their site of origin from the spinal cord which are 8 cervical pair, 12 dorsal pairs, 5 lumbar pairs, 5 sacral pairs and one coccygeal pair.
Divisions of the nervous system
The nervous system comprises three major systems; autonomic, somatic and integrative.
1) THE AUTONOMIC NERVOUS SYSTEM:
The autonomic nervous system (ANS) is the part of the nervous system which is concerned with the involuntary control of the visceral activity. It includes sympathetic, parasympathetic and enteric divisions.
2) THE SOMATIC NERVOUS SYSTEM:
The somatic nervous system is the part of the nervous system which is concerned with conscious perception of different sensations, and voluntary control of the muscular activity.
This system is divided into two divisions;
Sensory division: Which is concerned with conscious perception of somatic sensations .It includes the sensory (afferent) nerves, the sensory (ascending) tracts inside the CNS, the sensory reticular formation, the thalamus and the sensory cerebral cortex.
Motor division: Which is concerned with voluntary control of muscular activity. It includes the motor cerebral cortex, the basal ganglia, the cerebellum, the motor reticular formation, the motor (descending) tracts inside the CNS and the motor (efferent) nerves.


3) THE INTEGRATIVE NERVOUS SYSTEM:
The integrative nervous system is the part of the nervous system which is concerned with the sophisticated functions of the brain. These functions include memory, thinking, learning, language, speech, emotions and general behavior. The main parts of the integrative division are the cortical association areas and the limbic system.
The basic functional unit in the nervous system is the reflex action.
A reflex action: is an involuntary action in response to a stimulus e.g. a painful stimulus applied to the hand leads to reflex withdrawal of the arm (the withdrawal reflex).
The basic structural unit of the nervous system which is capable of conducting a reflex action is the reflex arc.
A reflex arc; consists of 5 components:
Receptor: A sensor which is excited by the stimulus.
Afferent nerve: Which conveys input signals to the CNS, The afferent nerve is also called the sensory nerve.
Center: A collection of neurons that receive the sensory information and issue the order for proper response.
Efferent nerve: A nerve that conveys output signals from the CNS to the effector organ. The efferent nerve is either a motor nerve to a muscle or a secretary nerve to a gland.
Effector organ: A muscular or glandular structure which receives the final order and executes the reflex response.
Cranial Nerves :
There are 12 pairs of nerves that originate from the brain itself. These nerves are responsible for very specific activities and are named and numbered as follows:
Olfactory: Smell.
Optic: Visual fields and ability to see.
Oculomotor: Eye movements; eyelid opening.
Trochlear: Eye movements.
Trigeminal: Facial sensation.
Abducens: Eye movements.
Facial: Eyelid closing; facial expression; taste sensation.
Auditory/vestibular: Hearing; sense of balance.
Glossopharyngeal: Taste sensation; swallowing.
Vagus: Swallowing; taste sensation.
Accessory: Control of neck and shoulder muscles.
Hypoglossal: Tongue movement.
Lobes of the brain
The cerebral hemispheres have distinct fissures, which divide the brain into lobes. Each hemisphere has 4 lobes: frontal, temporal, parietal, and occipital. Each lobe may be divided, once again, into areas that serve very specific functions. It’s important to understand that each lobe of the brain does not function alone. There are very complex relationships between the lobes of the brain and between the right and left hemispheres.
Frontal lobe
Personality, behavior, emotions.
Judgment, planning, problem solving.
Speech: speaking and writing (Broca’s area).
Body movement (motor strip).
Intelligence, concentration, self-awareness.
Parietal lobe
Interprets language, words.
Sense of touch, pain, temperature (sensory strip).
Interprets signals from vision, hearing, motor, sensory and memory.
Spatial and visual perception.
Occipital lobe
Interprets vision (color, light, movement).
Temporal lobe
Understanding language (Wernicke’s area).
Memory.
Hearing.
Sequencing and organization.
Physiological divisions of nervous system:
I. THE AUTONOMIC NERVOUS SYSTEM: Is the part of the nervous
system which is concerned with the involuntary control of the visceral
activity. It includes sympathetic, parasympathetic and enteric
divisions.
II. THE SOMATIC NERVOUS SYSTEM: Is the part of the nervous
system which is concerned with conscious perception of different
sensations, and voluntary control of the muscular activity.
This system is divided into two divisions;
(i) SENSORY DIVISION: Which is concerned with conscious
perception of somatic sensations? It includes the sensory (afferent)
nerves, the sensory (ascending) tracts inside the CNS, the sensory
reticular formation, the thalamus and the sensory cerebral cortex.
(ii) MOTOR DIVISION: Which is concerned with voluntary control of
muscular activity? It includes the motor cerebral cortex, the basal
ganglia, the cerebellum, the motor reticular formation, the motor
(descending) tracts inside the CNS and the motor (efferent) nerves.
(iii). THE INTEGRATIVE NERVOUS SYSTEM: Is the part of the
nervous system which is concerned with the sophisticated functions of
the brain. These functions include memory, thinking, learning,
language, speech, emotions and general behavior. The main parts of the
integrative division are the cortical association areas and the limbic
system.
The reflex action is a basic functional unit in the nervous system.
A reflex action is an involuntary action in response to a stimulus e.g. a.
painful stimulus applied to the hand leads to reflex withdrawal of the
arm (the withdrawal reflex).
The basic structural unit of the nervous system which is capable of
conducting a reflex action is the reflex arc (fig. 1-2). A reflex arc;
consists of 5 components:
Figure 1-2: The five basic components of a reflex arc
1. Receptor: A sensor which is excited by the stimulus.
2. Afferent nerve: Which conveys input signals to the CNS? The
afferent nerve is also called the sensory nerve,
3. Center: A collection of neurons that receive the sensory
information and issue the order for proper response.
4. Efferent nerve: A nerve that conveys output signals from the CNS
to the effector organ. The efferent nerve is either a motor nerve to a
muscle or a secretary nerve to a gland.
5. Effector organ: A muscular or glandular structure which receives
the final order and executes the reflex response.
NEURAL SYNAPSES:
A synapse is the junctional area between a nerve terminal and another
cell. If the second cell is a neuron the synapse is then called a "neural
or neuronal synapse".
The axon of a neuron conducts impulses away from the cell body to
relay onto another cell at the synapses (fig. 1.3). The axon branches
extensively near its end, giving off 1000 branches on the average. Each
branch ends in a nerve terminal. This terminal is a disc-like expansion
called the synaptic knob (the terminal button or the end foot). There
is a gap between the nerve terminal and the adjacent neuron 30-50 nm
wide called the synaptic cleft. So, at the neural synapse there is
contiguity but no continuity of the two adjacent neurons.
The neuron which conducts impulses to the synapse is called the
"presynaptic neuron'' or "input neuron" and that which conducts
impulses away from the synapse is called the "postsynaptic neuron"
or "output neuron". The synaptic knobs of the presynaptic neuron
contain vesicles called synaptic or transmitter vesicles which contain
the chemical transmitter of the neuron. A polypeptide called synapsin
is found in the walls of the vesicles which bind the transmitter vesicles
to the cytoskeleton keeping them in the cytoplasm away from the release
sites on the presynaptic membrane.

CLASSIFICATION OF SYNAPSES

Synapses could be classified according to either their location between
the pre and postsynaptic neurons (histological classification), or the
mechanism of transmission of impulses across them (physiological
classification),
HISTOLOGICAL CLASSIFICATION
According to this classification, synapses are classified into three types
(fig.1-4):
1. Axodendritic synapses: These are synapses between the axon
terminals of the presynaptic neuron and the dendrites of the
postsynaptic neuron.
2. Axosomatic synapses: These are synapses between the axon
terminals of the presynaptic neuron and the soma of the postsynaptic
neuron
Figure 1-4: histological classification of synapses
3. Axoaxonic synapses: These are synapses between the axon
terminals of the presynaptic neuron and the axon of the postsynaptic
neuron.

PHYSIOLOGICAL CLASSIFICATION:

According to this classification, synapses are classified into three
types:
1. CHEMICAL SYNAPSES: In these synapses, transmission of signals
occurs by releasing a ''chemical transmitter" from the presynaptic
terminal into the synaptic cleft. The transmitter then acts on specific
receptors on the postsynaptic membrane to generate postsynaptic
potential. There are more than 40 different synaptic transmitters in the
CNS which are either small molecule rapidly acting (acetylcholine) or
large molecule slowly acting (substance P).
Chemical synapses are the only type of synapses found in the
human nervous system.
2. ELECTRICAL SYNAPSES: In these synapses, there are gap
junctions between the pre and postsynaptic membranes which allow
the transmission of the depolarization wave directly from the pre to the
postsynaptic membrane.
3. CONJOINT SYNAPSES (ELECTROCHEMICAL): In these synapses,
transmission of impulses occurs by both mechanisms electrical and
chemical. They are found in some fish and invertebrates.