Nervous System

Nervous system: The nervous system is a network of specialized cells that communicate information about an organism's surroundings and itself. It processes this information and causes reactions in other parts of the body. It is composed of neurons and other specialized cells called glial cells (plural form glia) that aid in the function of the neurons. The nervous system is responsible for the perception of external and internal conditions and the body's response to them. The human nervous system can be described both by gross anatomy, (which describes the parts that are large enough to be seen with the naked eye,) and by microanatomy, (which describes the system at a cellular level.) In gross anatomy, the nervous system can be divided into two systems: •

The Central Nervous System (CNS)

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The Peripheral Nervous System (PNS

The nervous system is, on a small scale, primarily made up of • •

Neurons Glial cells

Neurons: Neurons are electrically excitable cells in the nervous system that process and transmit information. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves. 1

A number of different types of neurons exist which are : • • •

Sensory neurons Motor neurons Interneurons

Sensory Neurons

Sensory neurons are neurons that are activated by sensory input (vision, touch, hearing, etc.), and send projections into the central nervous system that convey sensory information to the brain or spinal cord. Unlike neurons of the central nervous system, whose inputs come from other neurons, sensory neurons are activated by physical modalities such as light, sound, temperature,

chemical stimulation, etc. In complex organisms, sensory neurons relay their information to the central nervous system or in less complex organisms, such as the hydra, directly to motor neurons and sensory neurons also 2

transmit information (electrical impulses) to the brain, where it can be further processed and acted upon. For example, olfactory sensory neurons make synapses with neurons of the olfactory bulb, where the sense of olfaction (smell) is processed. They respond to touch, sound, light and numerous other stimuli effecting sensory organs and send signals to the spinal cord and brain

Motor Neurons

In vertebrates, the term motor neuron (or motoneuron) classically applies to neurons located in the central nervous system (or CNS) that project their axons outside the CNS and directly or indirectly control muscles. The motor neuron is often associated with efferent neuron, primary neuron, or alpha motor neurons.

They receive signals from the brain and spinal cord and cause muscle contractions and affect glands.

Interneurons 3

An interneuron (also called relay neuron, association neuron or local circuit neuron) is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways. Like motor neurons, interneuron cell bodies are always located in the central nervous system

(CNS). It connect neurons to other neurons within the brain and spinal cord. Examples of interneurons include the sensory neurin and motor neuron also connecting to the brain through the association neurons.

Glial Cells:

Glial cells, commonly called neuroglia or simply glia (Greek for "glue"), are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the human brain, there is roughly one glia for every neuron with a ratio of about two neurons for every three glia in the cerebral gray matter.[1] Glial cells provide support and protection for neurons, the other main type of cell in the nervous system. As the Greek name implies glia are commonly known as the glue of the nervous system, although this is grossly inaccurate; rather, it is more of a partner to neurons. The four main functions of glial cells are to surround neurons and hold them in place, to supply nutrients and 4

oxygen to neurons, to insulate one neuron from another, and to destroy and remove dead neurons. They also modulate neurotransmission

Central Nervous system: The central nervous system (CNS) is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellular organisms. In vertebrates , the central nervous system is enclosed in the meninges. It contains the majority of the nervous system and consists of The brain and The spinal cord Together with the peripheral nervous system it has a fundamental role in the control of behavior. The CNS is contained within the dorsal cavity, with the brain in the cranial cavity and the spinal cord in the spinal cavity. The brain is protected by the skull, while the spinal cord is protected by the vertebrae

Brain: The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, balance, taste, and smell.

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Brains can be extremely complex. The cerebral cortex of the human brain contains roughly 1533 billion neurons depending on gender and age, linked with up to 10,000 synaptic connections each. Each cubic millimeter of cerebral cortex contains roughly one billion synapses. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body and target them to specific recipient cells.

Biological functions of brain:

The most important biological function of the brain is to generate behaviors that promote the welfare of an animal. Brains control behavior either by activating muscles, or by causing secretion of chemicals such as hormones. Even single-celled organisms may be capable of extracting information from the environment and acting in response to it. Sponges, which lack a central nervous system, are capable of coordinated body contractions and even locomotion. In vertebrates, the spinal cord by itself contains neural circuitry capable of generating reflex responses as well as simple motor patterns such as swimming or walking. However, 6

sophisticated control of behavior on the basis of complex sensory input requires the informationintegrating capabilities of a centralized brain.

Divisions of Brain The brain is traditionally divided into three parts, the hindbrain, the midbrain, and the forebrain. This drawing is roughly what it would look like if you sliced your brain straight down the middle, like a part in your hair. The front of the brain is on the left, the back on the right:

The hindbrain or brain stem consists of three parts. The first is the medulla, which is actually an extension of the spinal cord into the skull. Besides containing tracts up and down to and from the higher portions of the brain, the medulla also contains some of the essential nuclei that govern respiration and heart rate. The upper part of the medulla contains a pinky-sized complex of nuclei called the reticular formation. It is the regulatory system for sleep, waking, and alertness. The second part is the pons, which means bridge in Latin. The pons sits in front of the medulla, and wraps around it to the back. It is primarily the pathways connecting the two halves of the next part, which is called the cerebellum.

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The cerebellum, which means "little brain" in Latin, is in fact shaped like a small brain, and it is primarily responsible for coordinating involuntary movement. It is believed that, when you learn complex motor tasks, the details are recorded in the cerebellum. The midbrain is, in human beings, the smallest part of the brain. It connects the hindbrain to the forebrain, and contains several pathways important to hearing and vision. It is much larger in lower animals and in the human fetus. The largest and, for psychologists, most interesting part of the brain is the forebrain. It starts with the thalamus, which is practically in the center of your head. The thalamus is like a switching station, conducting signals from the body up to the relevant parts of the higher brain, and down from the brain to the lower brain and spinal cord.

The brain is divided into two symmetrical hemispheres: Left (language, the ‘rational’ half of the brain, associated with analytical thinking and logical abilities) Right (more involved with musical and artistic abilities). The brain is also divided into four lobes: o Frontal – (motor cortex) motor behavior, expressive language, higher level cognitive processes, and orientation to person, place, time, and situation o Parietal – (somatosensory Cortex) involved in the processing of touch, pressure, temperature, and pain o Occipital – (visual cortex) interpretation of visual information o Temporal – (auditory cortex) receptive language (understanding language), as well as memory and emotion

Spinal Cord The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain. The brain and spinal cord 8

together make up the central nervous system. Enclosed within, and protected by, the bony vertebral column, the spinal cord functions primarily in the transmission of neural signals between the brain and the rest of the body, but also contains neural circuits that can independently control numerous reflexes and central pattern generators

Spinal cord segments The human spinal cord is divided into 31 different segments. At every segment, right and left pairs of spinal nerves (mixed; sensory and motor) form. 6–8 motor nerve rootlets branch out of right and left ventro lateral sulci in a very orderly manner. Nerve rootlets combine to form nerve roots. Likewise sensory nerve rootlets form off right and left dorsal lateral sulci and form sensory nerve roots. The ventral (motor) and dorsal (sensory) roots combine to form spinal nerves (mixed; motor and sensory), one on each side of the spinal cord. Spinal nerves, with the exception of C1 and C2 form inside intervertebral foramen (IVF). Note that at each spinal segment the border between the central and peripheral nervous system can be observed. Rootlets are a part of the peripheral nervous system. There are 31 (Some EMS text say 26, counting the sacral as one solid piece) spinal cord nerve segments in a human spinal cord: •

8 cervical segments forming 8 pairs of cervical nerves (C1 spinal nerves exit spinal column between occiput and C1 vertebra; C2 nerves exit between posterior arch of C1 vertebra and lamina of C2 vertebra; C3-C8 spinal nerves through IVF above corresponding

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cervica vertebra, with the exception of C8 pair which exit via IVF between C7 and T1

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vertebra) 12 thoracic segments forming 12 pairs of thoracic nerves (exit spinal column through IVF below corresponding vertebra T1-T12) 5 lumbar segments forming 5 pairs of lumbar nerves (exit spinal column through IVF, below corresponding vertebra L1-L5) 5 (or 1) sacral segments forming 5 pairs of sacral nerves (exit spinal column through IVF, below corresponding vertebra S1-S5) 1 coccygeal segment forming 1 pair of coccygeal nerves (exit spinal column through the sacral hiatus)

The vertebral column grows longer than the spinal cord, spinal cord segments do not correspond to vertebral segments in adults, especially in the lower spinal cord. In the fetus, vertebral segments do correspond with spinal cord segments. In the adult, however, the spinal cord ends around the L1/L2 vertebral level, forming a structure known as the conus medullaris. For example, lumbar and sacral spinal cord segments are found between vertebral levels T9 and L2. Although the spinal cord cell bodies end around the L1/L2 vertebral level, the spinal nerves for each segment exit at the level of the corresponding vertebra. For the nerves of the lower spinal cord, this means that they exit the vertebral column much lower (more caudally) than their roots. 10

As these nerves travel from their respective roots to their point of exit from the vertebral column, the nerves of the lower spinal segments form a bundle called the cauda equina. There are two regions where the spinal cord enlarges: •

Cervical enlargement - corresponds roughly to the brachial plexus nerves, which innervate the upper limb. It includes spinal cord segments from about C4 to T1. The vertebral levels of the enlargement are roughly the same (C4 to T1).

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Lumbosacral enlargement - corresponds to the lumbosacral plexus nerves, which innervate the lower limb. It comprises the spinal cord segments from L2 to S3, and is found about the vertebral levels of T9 to T12.

Functions and control system of central nervous system(brain and spinal cord): Typically the brain and spinal cord act together, but there are some actions, such as those associated with pain, where the spinal cord acts even before the information enters the brain for processing. The spinal cord consists of the Brainstem which is involved in life sustaining functions. Damage to the brainstem is very often fatal. Other parts of the brainstem include the Medulla Oblongata, which controls heartbeat, breathing, blood pressure, digestion; Reticular Activating System (Reticular Formation), involved in arousal and attention, sleep and wakefulness, and control of reflexes; Pons – regulates states of arousal, including sleep and dreaming.  Cerebellum – balance, smooth movement, and posture  Thalamus – "central switching station" – relays incoming sensory information (except olfactory) to the brain  Hypothalamus – controls the autonomic nervous system, and therefore maintains the body’s homeostasis, which we will discuss later (controls body temperature, metabolism, and appetite. Translates extreme emotions into physical responses.  Limbic System – emotional expression, particularly the emotional component of behavior, memory, and motivation 11

 Amygdala – attaches emotional significance to information and mediates both defensive and aggressive behavior  Hippocampus – involved more in memory, and the transfer of information from shortterm to long-term memory

Peripheral nervous system: The peripheral nervous system (PNS) resides or extends outside the central nervous system (CNS), which consists of the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs. Unlike the central nervous system, the PNS is not protected by bone or by the blood-brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system, the autonomic nervous system, and the sensory system

Division of Peripheral Nervous system: 12

The peripheral nerves include the 12 cranial nerves, the spinal nerves and roots, and what are called the autonomic nerves. The autonomic nerves are concerned with automatic functions of the body. Specifically, autonomic nerves are involved with the regulation of the heart muscle, the tiny muscles lining the walls of blood vessels, and glands. The divison of peripheral nerves are as follows: Sural nerve Branch of the tibial nerve ensuring nerve sensation especially to the outer part of the calf, the ankle and the heel.  Tibial nerve

Branch of the sciatic nerve extending through the posterior tibial nerve and providing nerve sensation to certain muscles of the leg and the sole of the foot. Posterior cutaneous nerve of thigh Branch of the sacral plexus ensuring movement in part of the buttock (greatest gluteal muscle) and sensation in the posterior part of the thigh. Gluteal nerve The lower gluteal nerve (originating in the posterior cutaneous nerve of thigh) and the upper gluteal nerve (branch of the sacral plexus) provide nerve sensation to the greatest, medium and small gluteal muscles. Digital nerve Nerve originating in the brachial plexus ensuring nerve sensation in the fingers of the hand. Sacral plexus Network formed of several nerves whose branches ensure movement and sensation in the buttock and part of the thigh. Lumbar plexus Network formed of the first four lumbar nerves whose six branches ensure movement and sensation in the lower limb. Intercostal nerve Nerve ensuring motor function and sensation in the muscles between the ribs, as well as in a portion of the diaphragm and the abdominal wall.

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Radial nerve Branch of the brachial plexus providing nerve sensation especially in the extensor muscles of the upper limb and fingers. Axillary nerve Branch of the brachial plexus providing nerve sensation especially in the deltoid and small round muscles; it also ensures sensitivity in the shoulder joint.

Cranial nerves Each of 12 pairs of nerves connected to the brain providing nerve sensation to the head and neck; they serve a motor or sensory function. Deep peroneal nerve Branch of the common peroneal nerve ensuring nerve sensation mainly in the muscles of the anterior part of the leg and the back of the foot. Superficial peroneal nerve Branch of the common peroneal nerve ensuring nerve sensation mainly in the lateral peroneal muscles of the outer leg and the back of the foot.

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Common peroneal nerve Branch of the sciatic nerve ensuring nerve sensation especially in the muscles of the anterior and external parts of the leg. Saphenous nerve Branch of the femoral nerve ensuring nerve sensation in the inner leg and knee. Sciatic nerve The organism’s largest nerve, originating in the sacral plexus, ensuring nerve and motor sensation in a large portion of the lower limb. Femoral nerve Large branch of the lumbar plexus ensuring nerve sensation especially in the flexor muscles of the thigh and the extensor muscles of the leg. Lateral cutaneous nerve of thigh Branch of the lumbar plexus ensuring nerve sensation mainly to the buttock and the outer thigh. Ilioinguinal nerve Branch of the lumbar plexus that, along with the iliohypogastric nerve and the lateral cutaneous nerve of the thigh, provides nerve sensation to a portion of the abdomen, the genital organs and the thigh. Iliohypogastric nerve Branch of the lumbar plexus ensuring nerve sensation in one section of the abdominal wall and in the genital organs. Obturator nerve Branch of the lumbar plexus providing nerve sensation especially to the abductor muscles of the inner thigh. Ulnar nerve Branch of the brachial plexus providing nerve sensation, with the median nerve, especially to the flexor muscles of the hand and toes. Median nerve Branch of the brachial plexus providing nerve sensation to various muscles in the lower part of the forearm and part of the hand, where it divides into five branches.

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Brachial plexus Network formed of the last four cervical nerves and the first dorsal nerve whose branches ensure motion and feeling in the upper limb.

Functions of peripheral nervous system: By function, the peripheral nervous system is divided into the somatic nervous system, autonomic nervous system and the enteric nervous system. The somatic nervous system is responsible for coordinating the body movements, and also for receiving external stimuli. It is the system that regulates activities that are under conscious control. The autonomic nervous system is then split into the sympathetic division, parasympathetic division, and enteric division. The sympathetic nervous system responds to impending danger or stress, and is responsible for the increase of one's heartbeat and blood pressure, among other physiological changes, along with the sense of excitement one feels due to the increase of adrenaline in the system. The parasympathetic nervous system, on the other hand, is evident when a person is resting and feels relaxed, and is responsible for such things as the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, and the stimulation of the digestive and genitourinary systems. The role of the enteric nervous system is to manage every aspect of digestion, from the esophagus to the 1stomach, small intestine and colon.2

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