Full Nerv system PDF

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Comprehensive explenation of the nervous system, collected from lecture note, tutorial and verbal explenation from lecture videos....

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Week 9: Nervous system Introduction: ● The nervous system, together with the endocrine system, maintains the homeostasis of the human body. ● Both systems sense changes in normal physiological set points, integrate the information and respond to control physiological variables to keep us healthy and well. “The nervous system works together with the endocrine system to adjust different physiological variables, in order to maintain homeostasis of the body. The endocrine system makes slower changes by releasing hormones, whereas the nervous system detects small changes and acts quickly to keep variables such as blood pressure, respiration rate, heart rate and body temperature within normal limits”

Functions of the Nervous System

● Sensory - detects internal and external stimuli ● Integrative - processes the sensory information and makes decisions for appropriate responses ● Motor - the nervous system elicits a response by activating effectors (glands and muscles) “The three basic functions of the nervous system are Detection of internal and external stimuli - this is the sensory component Processing of incoming sensory information and decision-making, either conscious or sub-conscious, about the action that needs to be taken - the integrative function Activation of glands and muscles, or effectors, to generate the required response - and this is the motor function”

Divisions ● Central Nervous System (CNS) ○ Brain and Spinal Cord ● Peripheral Nervous System (PNS) ○ Everything else! “When we look at the overall structure of the nervous system, we have two broad systems, the CNS or central nervous system, and the PNS, or the peripheral nervous system. The CNS is the brain and spinal cord, and has some cellular and structural differences we will cover in a later lecture. The PNS is all of the other neurones and related supporting cells and tissue, and can be further subdivided. ”

Peripheral Nervous System ● Somatic Nervous System (SNS) ○ Sensory neurons relay information to the CNS ○ Motor neurons conduct signals from the CNS to skeletal muscle “One of the subdivisions is known as the somatic nervous system. The sensory neurons convey information to the CNS involuntarily, which is then processed either subconsciously, such as for the proprioceptive information used to detect when you might be about to fall over, or consciously, such as when our attention is drawn to the kitchen by delicious smelling food! The motor neurones of the somatic nervous system then act based on the sensory information, either making small adjustments to keep us standing, a voluntary but subconscious act, or co-ordinating a whole body response in order to run to the kitchen for dinner. ”

● Autonomic Nervous System (ANS)

○ Sensory receptors in viscera convey signals to the CNS ○ The CNS processes this information and elicits responses via involuntary actions of glands in a sympathetic or a parasympathetic response ● The sympathetic division of the ANS governs the ‘fight or flight’ response ● The parasympathetic division is often termed the ‘rest and digest’ response “Our autonomic nervous system, or ANS, is a bit like our body’s autopilot controls. It detects and governs changes in many dif ferent homeostatic functions, such as blood pressure, heart rate, respiration, temperature control and pupil dilation.

Autonomic effector cells are usually innervated by both parasympathetic and sympathetic nerves, which have opposite effects. Sympathetic reactions tend to prepare us for danger, increasing the blood flow to our muscles by diverting it away from the GI system, opening our pupils to maximise light and vision, and increasing our heart rate and blood pressure to get ready to be physically active. Parasympathetic reactions are the opposite, to relax and calm the body, reduce the heart rate and encourage digestion. ”

A Note on Stress Our body does not make a distinction between physical and mental stress An important exam or tight deadline may trigger the same reaction as facing a physical threat: wide eyes, butterflies in the stomach, heart pounding, sweating and shaking Which system would you suggest is at work here - the sympathetic or parasympathetic? Is this beneficial for our body in the long term? NO! Prolonged stress can weaken our immune system and make us unwell. Make sure you manage the stresses of study by making time to relax and unwind!

Peripheral Nervous System ● Enteric Nervous System (ENS) ○ Termed ‘the brain of the gut’ ○ Plexuses of neurons monitor chemical and physical changes in the gastrointestinal tract ○ These neurons govern smooth muscle contraction to propel food through the GIT, as well as enzyme secretion and hormones “The Enteric Nervous system, or the brain of the gut, is a complex net, or plexus, of neurons connecting to the gastro intestinal tract. It monitors chemical changes in the GIT, as well as stretching of the intestinal walls. Using this information the ENS then controls the contraction of the smooth muscle to push the partially digested food through the intestines, secretes enzymes to aid digestion and also releases various hormones. “

So here we can see how the central nervous system, the brain and the spinal cord, connect to the peripheral nervous system. The cranial nerves coming out of the base of the brain have mixed somatic and autonomic actions. The ganglia are the cell bodies of the autonomic nerves in the trunk, which balance our sympathetic and parasympathetic responses. We can see the enteric plexuses of the gastrointestinal tract coming out of the lower sections of the spinal cord, which are a mix of sensory and effector nerves to control the digestion of our food. And finally we can see the sensory receptors in the skin and subcutaneous tissue.

Cells of the Nervous System Neurons Form the networks which convey sensory information to the CNS, process it and send afferent signals to all regions of the body. Neurons form the networks which convey sensory information to the CNS, process it and send afferent, or outbound, signals to all regions of the body They can do this because they are electrically excitable, or can generate action potentials, which we will discuss in the next lecture

Identify Cell body Dendrites Axon Axon hillock Axon collateral Axon terminal Myelin sheath (schwann cell) Node of Ranvier Synaptical end bulb This is a typical motor neuron. It has three parts, the cell body (show) The dendrites (show) And the axon (show) The cell body contains the nucleus (show) and other organelles. A neuron has special rough endoplasmic reticulum, called nissl bodies, which produce protein required by the neuron. The dendrites are named after ‘little trees’ and have many branches. They are the receiving or ‘input’ portion of the neuron Here we have the axon. It joins the cell body at the axon hillock (show) and propagates the nerve impulses towards other neurons or effector glands or muscles, via the axon terminals (show). The axon terminals (show) end in synaptic end bulbs (show) which is where a chemical or electrical signal is transmitted to another cell. The axon collaterals (show) branches off and may connect with another nerve, gland or muscle fibre. We can see that this neuron is insulated by myelin sheaths, which we will discuss later this lecture.

Structural Classification of Neurons When we look at the structure of a cell to classify it, this is termed structural classification. There are three main types of neurons in the CNS and PNS 1. These are multipolar neurons: with several dendrites and one axon. These are the most common type of neuron in the brain and spinal cord, or CNS 2. Bipolar neurons: these neurons have one main dendrite, and one axon. These are a more specialised type of neuron and mostly found in the retina of the eye, the inner ear and the olfactory area. 3. Unipolar neuron: These have multiple dendrites which are fused straight to the axon. The dendrites act as sensory receptors to touch, pressure, pain or heat. The cell bodies of unipolar sensory neurons are located in the ganglia of spinal and cranial nerves. A ganglion is simply a group of cell bodies outside of the nervous system.

Multipolar neurons

Have several dendrites and one axon. Most common type of neuron in the CNS.

Bipolar neurons

Have one main dendrite and one axon. More specialised and found in the retina, the inner ear and the olfactory area.

Unipolar neuron

Multiple dendrites fused straight to the axon. The dendrites act as sensory receptors to touch, pressure, pain or heat. The cell bodies of unipolar sensory neurons are located in the ganglia of spinal and cranial nerves.

Functional Classification of Neurons This diagram demonstrates the functional classification system that we use for neurons. We can see here the sensory information being conveyed along the afferent, or sensory neurons, towards the CNS. These black lines represent the interneurons, which are located in the CNS between the sensory and motor neurons. The efferent, or motor, neurons elicit a response from either muscles or glands. All but the most simple reflexes and responses require interneurons between the sensory and motor neurons to process the information and generate an appropriate response. Sensory neurons

Conduct action potentials from the PNS to the CNS (afferent)

Motor neurons

Conduct action potentials from the CNS to the PNS (efferent)

Interneurons

Conduct action potentials from sensory neurons to motor neurons to elicit a motor response (reflex arc)

Neuroglia There are 4 types of Neuroglia in the CNS: Astrocytes Oligodendrocytes Microglia Ependymal cells And 2 in the PNS: Schwann cells Satellite cells “Neuroglia were previously thought to be the ‘glue’ that holds the nervous system together, but now we know they have a much more active role in nerve cell activity and health. They are much more numerous than neurons and can divide by mitosis. They do not transmit any electrical signals, called action potentials. There are 4 types of neuroglia in the CNS - the astrocytes, oligodendrocytes, microglia and ependymal cells, and two in the PNS - schwann cells and satellite cells”

Astrocytes Adhere to capillaries, neurons and pia mater and Form the blood-brain barrier Maintain appropriate chemical environment for action potential generation by: regulating the concentration of ions

Provide neurons with nutrients Remove excess neurotransmitter “Astrocytes are the largest and most numerous of the neuroglia. They cling to and support neurons physically as well as maintaining an optimal environment for the generation of action potentials. They do this by ensuring neuron has sufficient nutrients to support cell function, regulating the concentration of ions and removing excess neurotransmitter (both of which we will talk about this in the next lecture this week). They also form the blood-brain barrier, an important role that prevents the movement of potentially harmful substances in the blood into the neurons, which we will talk about next week. “

Oligodendrocytes Oligodendrocytes form and maintain the myelin sheath around the central nervous system axons “Oligodendrocytes wrap around several neurons within the CNS neurons and insulate the axons with myelin, which we will talk about more in a minute”

Microglia Small cells with slender processes Phagocytose microbes and damaged nervous tissue Microglia, or microglial cells, clean up microbes and damaged nervous tissue in the central nervous system by phagocytosis, which is absorbtion of the foreign or damaged material into the cell itself to be removed.

Ependymal Cells A single layer of cuboidal or columnar cells with microvilli and cilia Line the ventricles in the brain and the central canal of the spinal cord Produce and circulate cerebrospinal fluid (CSF) Involved with the blood-cerebrospinal fluid barrier Ependymal Cells line the ventricles of the brain and the central canal of the spinal cord, and produce and circulate the cerebrospinal fluid, which bathes the brain and spinal cord in a clear fluid similar to blood plasma. The CSF provides physical protection to the brain as well as immunological protection via the blood-CSF barrier.

Schwann Cells Form myelin sheaths around PNS axons When PNS neurons are damaged Schwann cells assist in the regenerative process Schwann cells can either form the myelin sheath around a single axon, or around several at once. The spaces between the Schwann cells are called the nodes of ranvier, which we will discuss in the next lecture this week. They also form the tube and secrete neurotransmitters which help to regenerate damaged neurons, a process which happens much more readily in the PNS than the CNS.

Satellite Cells Surround the cell bodies of PNS ganglia Provide structural support Regulate the exchange of materials between the cell bodies and the interstitial fluid Satellite cells are flat cells which surround the ganglia in the PNS. As well as providing structural support they control the exchange of materials between the cell bodies and the interstitial fluid which bathes them

Myelination The myelin sheath is a multilayered lipid and protein produced in neuroglia to insulate the axons of neurons Schwann cells in the PNS either wrap many layers of the neurolemma around a single axon to form the myelin sheath, or extend the cytoplasm around several unmyelinated axons Oligodendrocytes in CNS wrap around part of several different neuron axons Myelination increases the conduction velocity of the nerves “The myelin sheath is a made up of lipid and protein, and is produced in neuroglia to insulate the axons of neurons. The CNS and PNS have different neuroglia to myelinate the axons, the schwann cells in the PNS and the oligodendrocytes in the CNS. Schwann cells either wrap many layers of the neurolemma around a single axon to form the myelin sheath (like this one just here) or for unmyelinated neurons they extend the cytoplasm around several axons (as in the picture on the right) Oligodendrocytes wrap around and insulate several different neuron axons”

Peripheral Myelination The neurolemma spirals around the axon up to 100 times to form the myelin sheath The Nodes of Ranvier are between each Schwann cell and help propagate action potentials

The neurolemma of Schwann cells assist in nerve regneration “During embryonic development and infancy the schwann cell neurolemma spirals around the axon up to 100 times to form the myelin sheath. In between each schwann cell is a node of ranvier. This is a segment of the axon where the action potential is propagated, which we will discuss further next lecture. Regeneration of nervous tissue in the PNS is more prevalent due to the continuous nature of the schwann cell neurolemma”

Central Myelination In the CNS the oligodendrocytes myelinate central neurons Axons in the CNS demonstrate little regrowth, possibly due to the lack of a neurolemma and also due to the inhibitory influence of oligodendrocytes The oligodendrocytes extend about 15 broad, flat processes that wrap around the axons of several neurons to form myelin sheaths. The oligodendrocytes lack a neurolemma, and this is thought to be one of the reasons why the CNS neurons display very limited regenerative properties. It is also thought that the oligodendrocytes have an inhibitory effect on nerve regeneration. The lack of myelin in newborn babies accounts for their slow reflexes compared to children and adults, as the process of myelination continues through infancy

Grey and White Matter In a freshly dissected spinal cord or brain, some regions are white and some regions appear grey in colour. White matter contains a high density of myelinated nerve fibres, which give the white matter its colour, whereas grey matter has a higher proportion of unmyelinated fibres and cell bodies. The Nissl bodies are grey in colour, which colours the grey matter sections of the brain.

Electrical Signalling in the Nervous System Neuron Communication Neurons generate two types of electrical signals Graded potentials - useful only over short distances Action potentials - can communicate across large distances Neurons have the capacity to generate an electrical signal because of charged ions Specialised ‘gates’ called ion channels control the movement of specific ions in and out of the cells This leads to there being a difference in the electrical voltage of the intra-cellular fluid and the extra-cellular fluid, called the resting membrane potential

There are two types of electrical signals which transmit a message from the dendrites of a neuron to the synapses at the other end: a graded potential for short distances, and action potentials for long range messages. The action potentials of the sensory and motor neurons of the feet easily and quickly traverse the distance of the whole body. We will discuss what leads to the generation and maintenance of the resting membrane potential, graded potential and action potentials in the following slides

Ion Channels ● Leak channels ○ Randomly open and close ○ Ligand-gated channels ○ Specific chemical (ligand) binding to receptor opens or closes channel Firstly lets talk about how the charged ions move in and out of the cell. There are 4 types of ion channels which allow ions to pass through. The first and most simple is the leak channels, which randomly open and close. The leak channels which allow sodium and potassium to leak in and out of the cell are important in maintaining an electrical difference between the inside and the outside of the cell. The second is a ligand-gated channel. You can think of the ligand channels as needing a chemical ‘key’ to open it and the ‘locks’ or receptors are located on the channel itself. Ligand channels that bind acetylcholine allow sodium and calcium to diffuse inwards and potassium to flow outwards, and this is also an important channel in maintaining an the electrical charge of a cell.

● Mechanically gated channels ○ Mechanical stimulation changes position to open or close channel ○ Voltage-gated channels ○ Change in membrane potential opens channel ○ Participate in action potentials Another type of ion channel is the mechanically gated channel. This is physically distorted by sound waves, touch, pressure or tissue stretching and opens to allow calcium and potassium influx into the cell. The final type of ion channel is a voltage-gated channel. This channel opens in response to a change in voltage, or membrane potential. These are important in generation and propagation of action potentials.

Resting Membrane Potential There are usually more positive ions on the outside of a cell membrane and more negative ions inside the cell membrane The separation of charges forms potential energy, called the resting membrane potential

When measured with microelectrodes this is usually -70mV Resting membrane potential is what allows the neuron to generate an action potential. There is an overall negative charge inside the cell, and an overall positive charge on the outside of the cell membrane. This is maintained by three main mechanisms:

Membrane Potential 1.

The cell membrane has more potassium leak channels than sodium leak channels. This means that there is more potassium leaking out of the cell than sodium leaking in, which leads to more positive ions outside of the cell.

2.

Most of the anio ns inside the cell cannot leave. We can see here we have phosphate ions and protein, which carry a negative charge. These cannot diffuse across the cell membrane and contribute to the negative solution inside the cell membrane.

3.

Sodium-potassium pumps move more sodium ions out of the cell than potassium ions into the cell

So we have potassium leaking OUT through lots of leak channels, and being brought slowly back IN by the sodium...