PHYSIOLOGY - Nervous system

Nervous system

The nervous system provides homeostasis with the sense organs. The nervous system is fast and short-term. Movement, speech and coordinated functioning of the body are related to the nervous system.

a. Nervous system features

1. Arousal: Receptors in the sensory organs detect changes and are stimulated.
2. Conduction: The stimulus received from the receptors is carried to the brain via afferent nerves.
3. Perceiving senses: Senses are perceived by the brain.
4. responding to stimuli: After the sensations are perceived, a response is created. Motor nerves transmit the resulting response to the muscle or gland.

b.  Classification of the nervous system

1. Central nervous system - CNS
2. Peripheral nervous system - CNS

The nervous system cell is the neuron and is functionally divided into 3 parts, these are:

1. Interneuron - intermediate neuron: It is located in the central nervous system and provides the connection between the sensory neuron and the motor neuron. If only the interneuron is damaged, even if the stimulus reaches the person, it is not evaluated and a response cannot be created. Paralysis is an example of this situation.
2. Afferent neuron - sensory neuron: Located between the receptors and the central nervous system and perceives stimuli. Receptors in the skin, muscles, joints and sensory organs transmit stimuli to the central nervous system. A person whose only afferent neuron is damaged cannot perceive the stimuli, so no response occurs. Local anesthesia is an example of this situation.
3. Efferent neuron - motor neuron: It is located between the effectors and the central nervous system and transmits the response. It provides movement and transmits the responses to the muscles or glands. Only the person whose efferent neuron is damaged perceives the stimulus, a response occurs but the response cannot be transmitted. Botox, which is an aesthetic application, is an example of this situation.

c. Nerve cells according to their extensions

1. Unipolar (single pole)
2. Bipolar (two poles)
3. Multipolar

d. Nervous system cells

1. Neuron (main cell of the nervous system)
2. Glia (astrocytes, microglia, oligodendrocytes, schwann and ependymal cells)

The neuron basically consists of three parts:

1. Cell body - soma: It is the part that contains the nucleus and organelles and the compounds that perform cellular functions. Rough endoplasmic reticulum It is called Nissl body because it appears dark under the microscope. Neurofibrils that form the cytoskeleton are found in the cell body.
2. Axon: It is the part that transmits the signals formed in the cell body to other neurons or effector organs. They are long and the area where they branch out is called the axon tip. The neurotransmitter substance is secreted at the axon tip and the signal is transmitted to the effector. The plasma of the axons is called axoplasm and the membrane is called axolemma. The axons may be covered with a myelin sheath, which has an accelerating effect on electrical conduction.
3. Dendrite: Branching protrusions from the cell body. They receive signals from other nerves. They can be found in large numbers and are short in length.

Communication between neurons occurs chemically and this chemical substance is called neurotransmitter. Chemical transmission occurs at synapses. Synapses are the connection areas between neurons.

e.Glia cells and their properties

It surrounds neurons and provides support. It also provides nutrition and protection for neurons. It produces intermediate substances and concentrates ions. 

1. Schwann cell: Found in the peripheral nervous system and forms the myelin sheath.
2. Astrocyte cell: Helps to nourish neurons. It acts as a barrier between the brain and blood and prevents the passage of most substances from the blood to the nervous tissue.
3. Microglia cell: Protects the nervous system against microorganisms. It has the ability to phagocytosis. It is physically small and forms the smallest glial cell type. It constitutes 15% of total nerve cells. It moves quickly in the brain and can multiply rapidly.
4. Ependymal cell: Produces cerebrospinal fluid (CSF). Provides the connection between the choroid plexus and the central nervous system. Covers the cavities in the brain and forms the wall structure. Found only in the central nervous system.
5. Oligodendrocytes: Produce the myelin sheath of neurons in the central nervous system.

Myelin sheath is the part that surrounds the axon, and axons that contain myelin are called myelinated axons. Myelin sheath has an accelerating effect on electrical conduction, and cells with myelin sheaths conduct conduction 10 times faster than cells without.

The area where the myelin sheath is interrupted is called the node of Ranvier. Nerve transmission occurs by skipping the nodes of Ranvier.

f.Transmission in the nervous system

The electrochemical changes that a stimulus creates in a nerve cell are called impulses. The lowest stimulus intensity required for an impulse to occur is called the threshold value. The electrical change that an impulse creates in a nerve cell is called an action potential. 

Impulse transmission occurs via electrical charge transfer, namely the Na-K pump. 

There is an anion and cation difference between the nerve cell and the external environment, and this situation is expressed by the resting membrane potential. Normally, there is more potassium inside the cell and more sodium outside the cell. The sodium and potassium balance is maintained by the Na-K pump.

The inside of the cell is negative compared to the outside, this value means the resting membrane potential is -70 millivolts.

All human cells have a resting membrane potential, and this value can vary depending on cell type. The resting membrane potential in muscle and heart cells is -90 millivolts.

g. Situations that produce resting membrane potential

1.  Permanently open Potassium (K+) leak channels: Allows potassium to exit the cell. During rest, Na+/K+ is more than the ATPase pump.

2.  Na+/K+ ATPase pump: It expels 3 sodium and takes 2 potassium into the cell. It increases the number of anions in the cell.

Resting membrane potential = polarized cell

When sodium enters the cell, the cell becomes positive and a depolarized cell is formed. When depolarization reaches the threshold point, an action potential occurs. In a depolarized cell, the -70 millivolt value approaches 0.

During repolarization, sodium gates close and potassium gates open. As potassium leaves the cell, the inside of the cell becomes negative again.

h.  Stages that develop when the axon is stimulated

1.  Slow depolarization (-70 mV --> -55mV)

2.  Threshold value

3.  Fast depolarization (-55mV --> +35mV )

 Impulse transmission is constant and does not change. There are situations that increase the number of impulses, these are:

1. Frequency of the stimulus
2. Intensity of warning
3. Duration of warning

i.  Factors affecting impulse conduction speed 

1. Presence of myelin sheath. 
2. Decrease in the number of nodes of Ranvier.
3. Decrease in the number of synapses (since it is a chemical transmission, the slowest transmission occurs here.)
4. Increased axon diameter.
5. The ambient temperature is cold

j. All or nothing principle

The nerve cell does not respond to stimuli below the threshold value, and responds at maximum level to stimuli above the threshold value. In addition, the threshold value may vary from person to person and even over time.

This principle applies to nerve fibers and muscle fibers, and the staircase effect principle applies to nerve bundles and muscle bundles.

k.Staircase effect principle

A large number of nerve fibers come together to form a nerve bundle. Since the threshold value of each nerve fiber may be different, the concept of a threshold value does not exist for a nerve bundle. 

The nerve fiber with a low threshold value creates a response, and as the intensity of the stimulus increases, the number of muscles and nerve bundles participating in the response increases and a stronger response is given. The increase in the nerve bundle response until all nerve fibers are stimulated is called the staircase effect. 

l.Impulse transmission in synapses

1. The synaptic terminal depolarizes and dies, allowing Ca+2 to enter the cell.
2. It fuses with the plasma membrane of the sending neuron via synaptic vesicles.
3. The sacs open and neurotransmitters are released into the synaptic cleft.
4. Neurotransmitters bind to receptors.
5. By binding to receptors, channels in the cell membrane open and Na+ enters the cell, the cell becomes depolarized. The impulse is transferred to the other neuron.
6. At the end of transmission, the channels close. Neurotransmitter residues are broken down by enzymes in the synaptic gap.

If the impulse continues to conduct after connecting to the dendrite of the neighboring cell, it is called a facilitating synapse; if the impulse does not continue and is stopped, it is called an inhibitory synapse.

2.  Central nervous system - CNS

It consists of the cell bodies of motor neurons and interneurons.

The central nervous system consists of two structures.

1. Brain
2. Spinal cord

The brain consists of four basic parts, these are:

1. Cerebrum - telencephalon
2. Cerebellum - cerebellum
3. Diencephalon (thalamus, hypothalamus and epithalamus)
4. Brain stem (mesencephalon, pons and medulla oblongata)

The brain is the control center of the human body. It consists of billions of neurons and trillions of glial cells. Its mass is 1,300-1,400 grams. 

The brain is located in the skull. The skull protects the brain from impacts. Just under the skull is a membrane layer called meninges that surrounds the brain.

The meninges consists of layers, which are:

• Tough membrane (dura mater): Located just under the skull and consisting of two layers. Protects the brain from injury and damage.
• Spider membrane (arachnoid mater): It resembles a spider web because it has connective tissue fibers. It connects the tough membrane and the thin membrane.
• CSF fluid: It is located between the spider membrane and the thin membrane. It is formed by the blood pressure effect of the capillary blood vessels. It protects the brain against microorganisms and provides material exchange between brain cells and blood. If this fluid becomes infected, meningitis occurs.
• Thin membrane (pia mater): Carries blood vessels and provides nutrition to the brain. It is the thinnest membrane layer among the layers.

a.  Forebrain

It is the largest part of the brain. The cerebrum (forebrain) and the diencephalon (middle brain) form the forebrain.

b.  Cerebrum - telencephalon

It undertakes consciousness, intelligence and high-level tasks. The organization of complex movements, the reception of sensory information and the storage of learned information in memory occur in this part of the brain. It consists of billions of neurons. Neurons combine to form nerve fibers. Nerve fibers create all behaviors in humans.

The cerebrum consists of two hemispheres, which are:

• Right hemisphere:
• Left hemisphere (cerebral hemisphere):

These hemispheres are connected in the middle by the corpus callosum nerve bundle.

Each hemisphere is divided into 4 lobes, these are:

• Frontal lobe: It is the decision-making region of the brain. It is the place where voluntary muscle movements are controlled.
• Parietal lobe (half lobe): It is located in the middle part of the brain and evaluates the data coming from the sensory organs.
• Occipital lobe (back lobe): Located at the back of the brain, above the cerebellum. It is related to the sense of vision.
• Temporal lobe: Located in the lower part of the brain and is the memory center. It evaluates the senses of hearing and smell.

The outer layer of the hemispheres is the cerebral cortex. The inner part of the cerebral cortex is white matter, and the outer part is gray matter.

While gray matter consists of neuron bodies, white matter consists of myelinated fibers. In the white matter, myelinated fibers form the basal ganglia. 

The basal ganglia regulate the motor functions of skeletal muscles and ensure that voluntary movements continue automatically.

c.  Diencephalon

Diencephalon consists of 3 basic structures, these are:

1. Thalamus
2. Hypothalamus
3. Epithalamus

Thalamus  : It is the region where all senses except the sense of smell are collected and distributed to the relevant center. It controls the state of sleep and wakefulness. It acts as a gate for afferent neurons.

Hypothalamus  : Regulates the functioning of the pituitary gland and controls the autonomic nervous system. Controls body temperature, blood pressure, hunger, water balance of the body (ADH hormone), carbohydrate and fat metabolism. 

Epithalamus  : Epithalamus forms the pineal gland. Malatonin hormone is secreted. This hormone creates the body rhythm and biological clock. Light has a reducing effect on the secretion of melatonin hormone. It works together with the thalamus and hypothalamus.

Limbic system

The hypothalamus, hippocampus, and amygdala make up the limbic system. The limbic system affects mood. 

Hippocampus: It is associated with memory. Learning something and remembering forgotten information is related to the hippocampus.

Amygdala: Affects emotional states.

d.  Midbrain

It is the transition area between the forebrain and the hindbrain. It controls visual and auditory reflexes and also adjusts muscle tone. It affects body posture.

e.  Hindbrain and brainstem

The hindbrain consists of the cerebellum, the medulla oblongata, and the pons.

The brainstem consists of the medulla oblongata, pons, and mesencephalon.

Cerebellum: It is called the tree of life and contains many nerve cells. It is the 2nd largest part of the brain. It provides muscle movements and balance. It has gray matter on the outside and white matter on the inside.

Spinal cord bulb = medulla oblongata (last brain): It allows systems to work together, reflexes and blood sugar regulation. Motor nerves cross in the spinal cord bulb and the right brain controls the left side of the body, the left brain controls the right side of the body. It contains white matter on the outside and gray matter on the inside.

Pons (Bridge of Varol): It is a nerve bundle located between the midbrain and the spinal cord. It connects the hemispheres and carries out impulse transmission. It is found in vertebrates and mammals.

Mesencephalon: Carries impulses from the brainstem to the hemispheres. Secretes dopamine, and dopamine deficiency causes Parkinson's disease, while excess causes schizophrenia.

3.  Spinal cord - spinal cord 

It is approximately 45 cm long and consists of 33 overlapping vertebrae, which are:

1. Cervical (neck vertebrae): 7 pairs C1-C7
2. Thoracic (back vertebrae): 12 pairs T1-T12
3. Lumbar (lumbar vertebra): 5 pairs L1-L5
4. Sacral (tailbone): 5 pairs S1-S5
5. Coccygeal (lower part, immovable): 4 pairs 

The spinal cord is surrounded by membranes, just like the brain, and there is also CSF ​​fluid. There is white matter on the outside and gray matter on the inside.

The dorsal root (dorsal root) comes out of the arm coming out of the back part of the spinal cord, and the ventral root (abdominal root) comes out of the part coming out of the front part.

The spinal cord transmits signals from the brain to the relevant center and the responses from the brain to the relevant center. It controls acquired reflexes and habitual movements. Sensory neurons cross in the spinal cord before reaching the brain.

a.  Reflex arc - nerve pathway

The brain controls the operation of the spinal cord. In the reflex arc, impulses are transmitted from the spinal cord to the brain.

Peripheral nervous system - Peripheral nervous system

It consists of nerve cells and nerve fibers outside the brain and spinal cord. It contains afferent (sensory neuron, afferent neuron) and efferent neurons (motor neuron, efferent neuron). It transmits the stimuli it receives from the outside to the center, and the stimuli it receives from the center to the effector organ.

There are 43 pairs of nerves in the peripheral nervous system.

There are 12 pairs of nerves coming out of the brain and the 10th nerve is called vagus. The vagus nerve has a parasympathetic effect.

There are 31 pairs of nerves coming out of the spinal cord. These are sensory and motor neurons. The largest is the sciatic nerve that goes to the legs.

a.  Cell types found in the peripheral nervous system

1. Glial cell (schwann cell): Produces the myelin sheath.
2. Satellite cell: Supports the cell body. They are small cells and are found only in the peripheral nervous system. They surround the nerve cells in the sympathetic and parasympathetic nervous system.

b.  Somatic nervous system (voluntary)

Nerve bundles originating from the brain and spinal cord form the somatic nervous system. It is controlled by the cerebrum and cerebellum. It contains sensory and motor neurons. Nerve cells contain myelin sheaths and impulse propagation is fast. Axon diameters are wide. 

Voluntary muscle movements occur through the somatic nervous system.

c.  Autonomic nervous system (involuntary)

It consists of motor neurons originating from the brain and spinal cord. It is generally composed of unmyelinated cells and conduction is slow. Axon diameters are narrow.

The functioning of my internal organs is regulated by the autonomic nervous system.

The sympathetic and parasympathetic nerves and the enteric nerves, which work in opposition to each other, constitute the autonomic nervous system.

d.  Autonomic nervous system 2 important functions

1. Flight or fight: In emergency situations [ Sympathetic nervous system ]
2. Rest or digest: In non-emergency situations [ Parasympathetic nervous system ]

e.  Vegetal life

A person whose brain is damaged continues to live because the autonomic nervous system is working, and this state is called a vegetative state.