Biology 30 Fall 2018 The Nervous System Lecture Notes PDF

The Nervous System Chapter 11  Introduction Back in Biology 20, we discussed how the human body always tries to reach a state of stability. Known as homeostasis, it’s controlled by two body systems: the nervous and endocrine systems. The nervous system sends and rec...

The Nervous System Chapter 11  Introduction Back in Biology 20, we discussed how the human body always tries to reach a state of stability. Known as homeostasis, it’s controlled by two body systems: the nervous and endocrine systems. The nervous system sends and receives electrical impulses that regulate body structures and processes to achieve this despite changes in the internal and external environment. Examples that were talked about in Bio 20 include… Body temperature Acid-base balance Fluid volume  Control mechanisms within an organism Cells within the body are controlled by two mechanisms: 1. The nervous system 2. Hormones (Endocrine System)  Nervous system vs. Hormones In general the nervous system is responsible for coordinating rapid, precise responses and is especially important in mediating interactions with the external environment the endocrine system primarily controls activities that require duration rather than speed including: – Regulating organic metabolism and H2O and electrolyte balance – Inducing adaptive changes to help the body cope with stress – Controlling reproduction – Regulating red blood cell production – Along with the autonomic nervous system, controlling and integrating both circulation and digestion of food  Nervous system vs. Hormones Hormones and the nervous system have two major differences. 1.Speed of action nervous system 100 m/s hormones: seconds, minutes, longer 2.Size of effect nerve: single muscle, or single cell hormones: all cells they reach with the correct receptor  Homeostasis Relatively stable chemical and physical conditions in the internal fluid environment are maintained by the highly coordinated, regulated actions of the body’s systems Body conditions must be maintained in a stable steady state (despite changes in internal or external environment). (i.e. Internal environment kept in a stable/constant condition) Examples: – Body temperature: regulated close to 37ºC – Blood pH: kept at 7.4 – Arterial blood pressure: maintained around 120/80 mm Hg for men 110/70 mm Hg for women  The nervous system uses receptors to collect information about the internal/external environment. Photoreceptors are sensitive to light (ie. sense of vision). Chemoreceptors are sensitive to chemicals (ie. sense of taste and smell). Mechanoreceptors are sensitive to vibrations (ie. sense of sound, balance, body position, touch, pressure & pain). Thermoreceptors are sensitive to temperature (ie. sense of hot and cold). It’s the nervous system that receives and processes the info provided by these receptors and then sends the commands necessary to adjust the body as a whole.  Divisions of the Nervous System The human nervous is arguably the most complex system in our body and is divided into two main sections. Central Nervous System (CNS) - Consists of your brain and spinal cord. It integrates and processes electrochemical info sent by nerves. Now just what can our body do without a brain?  Peripheral Nervous System (PNS) - Includes nerves that carry sensory messages to and from the CNS. It is further divided into two sections. Somatic Nervous System (SNS) - This is the voluntary portion of the PNS, which controls skeletal muscles. Autonomic Nervous System (ANS) - Provides involuntary control of glandular secretions and smooth/cardiac muscle. The sympathetic and parasympathetic divisions of this system often work in opposition to each other to regulate involuntary processes of the body (ie. vasoconstriction vs vasodilation in blood vessels to maintain body temperature).  Cells of the Nervous System Neurons - Functional units of the nervous system. They respond to physical/chemical stimuli, conduct electrochemical signals and release chemicals that regulate various body processes. Neurons are grouped into bundles of tissues known as nerves. These bundles are similar to the way muscle are arranged (see next slide). Glial Cells - Outnumber neurons 10 to 1 and function to support them. They provide nourishment & repairs, remove wastes and defend against infection.  Nerve Muscle  Types of Neurons Sensory (Afferent) Neurons - Part of the PNS, they transmit impulses from sensory receptors in the body to the CNS. Most are unipolar (ie. they have only a single projection extending from their cell body).  Motor (Efferent) Neurons - Part of the PNS, they transmit information from the CNS to muscles, glands and other organs (ie. effectors). Most are multipolar (ie. they have multiple projections extending from their cell body - 99% of neurons in your body are multipolar).  Interneurons (Association Neurons) - These are found entirely within the CNS and act as a link between the sensory and motor neurons. Most are multipolar.  Structure of a Neuron Dendrites - Receive nerve impulses from other neurons and relay it to the cell body. Cell Body (Soma) - Contains major cell organelles, cytoplasm and nucleus. If the input received from the dendrites is large enough, it will relay it to the axon.  Axon - Conducts nerve impulses away from the cell body. The axons of some neurons are enclosed in a fatty, insulating protein layer called the myelin sheath, which is formed by Schwann cells (a type of glial cell). Gaps between the myelin sheath are known as nodes of Ranvier. Axons with this sheath are called myelinated while axons without it are called unmyelinated. Axon/Synaptic Terminal - This is the end of the neuron and site of the release of neurotransmitters in space between it and the dendrites of the next neuron (known as the synaptic cleft).  Schwann cells Located in the PNS – Each schwann cell provides myelin for one neuron – A single axon will have many schwann cells on the axon Entire schwann cell surrounds axon Functions: – Digest dead and dying neurons – Arrange as cylinders for paths of growth for new cells – Stumps of severed axons may regenerate if they find a cylinder; otherwise they will die  Reflex Arc When a change in the environment is detected by one of the various sensory receptors in your body (thermoreceptors, mechanoreceptors, etc.), we call it a stimulus. An example could someone pinching your arm. If the body reacts to this stimulus, we call that a response. If the response is rapid and unconscious, it is known as a reflex. An example of this would be jerking your hand away from a hot object you just touched. These reflexes are facilitated by a simple connection of very few neurons called reflex arcs. This generates very rapid signal transmission (ie. reflexes often occur in about 50 ms).  To remember how a reflex arc works remember ASIME: A sensory receptor/affector (such as the ones in your fingertips) receives a stimulus. Travelling through the peripheral nervous system (PNS), sensory (afferent) neurons relay information from the receptor(s) to the central nervous system (CNS). Once in the CNS, interneurons (association neurons) processes and integrates the incoming sensory information and relays the outgoing commands to the motor neurons in the PNS. The motor (efferent) neurons then relay the information from the CNS to a muscle/effector, which then completes the reflex response to the initial stimulus.  Nerve Impulse So how do these sensory, inter and motor neurons generate electrical impulses and how do these signals get transmitted along their length? Neurons create an electrical difference similar to a battery in the form of a membrane potential. This is a charge separation across a membrane to create a form of potential energy can be used to form an impulse later. In a resting neuron, the intracellular fluid (ICF) has a negative charge relative to the extracellular fluid (ECF) surrounding it. This difference is known as the resting membrane potential and is roughly -70 mV.  This potential achieved by a high concentration of positive ions outside the neuron and a lower concentration of them inside. The neuron also contains many negative substances like (PO4)3- ions found in DNA, proteins and Cl- ions all of which cannot leave the cell. A neuron that exhibits this resting potential is said to be polarized. The most important contributor to polarization of a neuron are the sodium-potassium exchange pumps found in its cell membrane.  These pumps use ATP energy to transport three Na+ ions out of the neuron for every two K+ ions brought in. K+ ions can diffuse out of the neuron more easily than Na+ ions can diffuse into it (in both cases through channel proteins). As a result, an excess positive charge forms in the ECF.  After a neuron has established a resting potential, it has the capacity to undergo depolarization when it receives a stimulus. During this process, the charge in the neuron rapidly shifts from negative to positive. This change is called an action potential and is an “all-or- nothing” event. A depolarization of between -70 mV and -55 mV has no effect, while any change higher than -55 mV will result in the propagation of the signal. Due to this, the potential difference of -55 mV is known as the threshold potential. It’s worth noting that as a stimulus becomes stronger, the frequency of action potentials increases (the brain reads this as pain) and/or more neurons “fire” simultaneously. IT WILL NOT GENERATE A LARGER ACTION POTENTIAL.  Depolarization is achieved when sodium channels open along the cell membrane and allow Na+ ions to travel from the outside to the inside of the axon along their concentration gradient. This causes a change in the membrane potential to +35 mV. As a result of the change in membrane potential, the sodium channels close and potassium gates open. K+ ions travel along their concentration gradient to the outside of the neuron. This is called repolarization. The membrane potential overshoots to -90 mV, causing the membrane to become briefly hyperpolarized. At this point, the potassium channels close. The sodium- potassium pumps quickly bring the membrane back to its resting potential of -70 mV. This brief period is called the refractory period.  The action potential lasts only a few milliseconds. Once an action potential occurs at one spot, it excites the adjacent portion of the membrane into an action potential. Sections of the neuron undergoing refractory period cannot create a new action potential, therefore a wave of potential sweeps all the way down the neuron in only one direction.  In myelinated neurons, action potentials occur only at node of Ranvier because the myelin sheath insulates the axonal membrane that it encircles. Because action potentials are forced to jump from one node to the next, the conduction of impulse is called saltatory conduction. This causes myelinated axons to send potentials at much faster speeds than in unmyelinated axons (ie. 400 km/h as opposed to 1.8 km/h.  Saltatory Conduction Results in a very rapid transmission of the nerve impulse down the axon – The wave of depolarization physically skips the sections of the neuron covered in myelin (i.e. the impulse “jumps” from node to node) much faster than in unmyelinated neurons White matter refers to myelinated neurons Gray matter refers to unmyelinated neurons  One disorder that greatly affects the ability of neutrons to carry electrochemical signals is multiple sclerosis. It causes the breakdown of the myelin sheath surrounding the axons in the CNS. It is thought that MS is an autoimmune diseases. Symptoms include blurred vision, loss of balance, muscle weakness, fatigue and slurred speech. MS sufferers will experience periods of remission and progression of the condition. It usually strikes between the ages of 20 to 40 and affects more women than man and there is no cure. Over 20 000 people a year die from the condition.  Synapse A neuronal axon may terminate on: – A muscle, a gland, or another neuron. Synapse is the junction between a neuron and another cell – Neuron-to-neuron synapse involves a junction between an axon terminal of one neuron (the presynaptic neuron) and the dendrites or cell body of a second neuron (the postsynaptic neuron) Two types of synapses: 1. Electrical 2. Chemical  Electrical Synapse Cell membranes of the presynaptic and postsynaptic cells are connected – Action potentials in the presynaptic cell can be passed directly to the postsynaptic cell – Facilitates rapid transmission of action potentials (faster than chemical synapses) Electrical Synapse  Action at a Chemical Synapse 1. The action potential reaches a terminal button causing the opening of voltage-gated Ca2+ channels 2. Ca2+ ions move down a concentration gradient and move into the terminal button (from ECF) 3. Ca2+ then initiates the release of neurotransmitters secreted by exocytosis. Neurotransmitters are contained in synaptic vesicles Neurotransmitters are released from the pre-synaptic neuron (before the synapse)  Chemical Synapse (cont’d) 4. Neurotransmitters diffuse across the synapse – Bind to receptors on the post-synaptic neuron (after the synapse) 5. The receptors signal a change in the membrane potential in the post-synaptic neuron. – The change may be: Excitatory (a positive voltage change; -70mV to - 50mV) Inhibitory (a negative voltage change; -70mV to - 90mV) Neurotransmitters must then be removed  Removal of neurotransmitters Neurotransmitters must be removed or they will continuously stimulate the post-synaptic neuron Neurotransmitters can be removed by: 1. Diffusion away from synapse 2. Destruction by an enzyme (e.g. acetylcholine destroyed by an enzyme: cholinesterase) 3. Re-absorption by the pre-synaptic membrane (re-uptake)  When an action potential arrives at the terminal button in the axon terminal, calcium ions diffuse into the end of the neuron. This causes synaptic vesicles, which produce and store neurotransmitter chemicals, to fuse with the presynaptic membrane. The fusion of the vesicles to the membrane is facilitated by proteins known as SNAREs. These proteins are the target of bacterial neurotoxins responsible for botulism and tetanus. This causes the release neurotransmitter molecules into the synaptic cleft. These neurotransmitters diffuse across the synaptic cleft from the presynaptic neuron to the postsynaptic neuron. Once there, they bind with receptor proteins.  Depending on whether the neurotransmitters released have an excitatory (increasing the probability of producing a membrane potential) or inhibitory (decreasing the probability of producing a membrane potential) effect on the postsynaptic membrane, one of two things will occur. Excitatory Neurotransmitters: Ion channels on the postsynaptic membrane will open causing sodium ions to diffuse through them causing depolarization of the membrane. This initiates an action potential that begins moving down the postsynaptic neuron. Inhibitory Neurotransmitters: Ions channels will open on the postsynaptic membrane causing potassium ions to flow out and chloride ions to flow in. This will result in a more negative membrane potential resulting in hyperpolarization, effectively stopping the signal from travelling further.  Once neurotransmitter molecules are used, they must first release from the receptors on the postsynaptic membrane and then are either: Taken back across the synaptic cleft intact to the presynaptic neuron button terminal by a process called reuptake. Or broken up by enzymes in the synaptic cleft. The fragments then diffuse back into the presynaptic terminal button where they are recycled.  Synapses between a motor neuron and a muscle cell, are known as neuromuscular junctions. A neurotransmitter that crosses these junctions is acetylcholine (Ach). Acetylcholine excites the muscle cell membrane (ie. sarcolemma), causing depolarization and contraction of the muscle fibre.  The enzyme acetylcholinesterase/ cholinesterase is released into the neuromuscular junction by glial cells to break down acetylcholine so it can be “deactivated” to prevent continuous stimulation. Here’s how it works… A nerve gas called sarin blocks the release of cholinesterase. This allows acetylcholine to build up to critical levels, causing the heart and diaphragm to enter a state of constant contraction or paralysis.  Lethal injection involves a cocktail of three drugs which interfere with neurotransmitters. Sodium thiopentol is an anesthetic used to induce a rapid medical coma. It’s also known as sodium pentothal, which in smaller doses is used as a truth serum. Pancuronium bromide (pavulon) blocks acetylcholine receptors in skeletal muscles which stops breathing. Potassium chloride hyper polarizes the neurons of the heart, causing cardiac arrest.  There are more than 50 neurotransmitters in the human body. The following are the most common ones. Dopamine: Usually inhibitory, it affects brain synapses in the control of body movements. When released by the brain reward system, it produces feelings of pleasure (ie. when eating good food). Inadequate amounts are linked to an increased risk of Parkinson’s disease. Serotonin: It’s involved in mood, appetite and sensory perception. In the spinal cord, it inhibits pain pathways. Inadequate amounts are linked to depression. Norepinephrine (Noadrenaline): Acts as a neurotransmitter and a hormone. In the PNS, it’s part of the fight-or-flight response. In the brain, it’s usually excitatory in regulating normal brain function. Overproduction can lead to high blood pressure. Endorphins: These act as natural painkillers in brain synapses and also affects emotional areas of the brain. Inadequate amounts are linked to an increased risk of alcoholism. GABA (Gamma-AminoButyric Acid): The major inhibitory neurotransmitter in the brain. Glutamate: The major excitatory neurotransmitter in the brain.  Most drugs (legal or illegal) have some affect on neurons by either promoting or decreasing the action of neurotransmitters. They do this by binding to the receptors on the postsynaptic membrane and by doing so can alter an individual’s mood or emotional state. For example, nicotine (derived from tobacco plants) rapidly stimulates the reward centre of the brain to release dopamine. It also stimulates areas of the body by mimicking the actions of acetylcholine, causing increased heart rate and blood pressure.  Summation A single neuron may have synapses with hundreds of other neurons. The initiation of an action potential in a postsynaptic neuron may require more than one signal from a pre-synaptic neuron(s) This is called summation. Two types of summation: 1. Temporal summation - rapid, repetitive excitation from a single persistent input. 2. Spatial summation - simultaneous activation of several excitatory inputs. Neuron Questions Use this package, your notes and pages 368 to 382 in Inquiry into Biology to answer questions 1-12 in your “Neurons” question booklet. Central Nervous System To protect itself, the spinal cord and brain are surrounded by bone (ie. vertebrae and the skull) and are wrapped in meninges [three layers of tough, elastic tissue called the dura mater (outermost), arachnoid (middle) and pia mater (innermost)]. The space between the meninges is filled with cerebrospinal fluid (CSF), which absorbs shock, nourishes and eliminates waste.  Oligodendrocytes only located in the CNS Function: – Produce the Myelin sheath – One oligodendrocyte may provide myelin sheath for many axons. In the CNS regenerating axons encounter scar tissue produced by astrocytes and cannot grow through – this stops the neurons from regenerating  Astrocytes Are large glial cells (support cells) found throughout the CNS. Function: – Hold neurons in place (the “glue” of the CNS) – Important in the formation and function of synapses – Influence blood flow in the brain – Store glycogen – Help establish the blood-brain barrier induce small blood vessels (capillaries) of the brain to change to establish the blood-brain barrier  The meninges also create a blood-brain barrier that protect cells in the CNS from substances that could harm them. Unlike capillaries in the rest of the body that are relative leaky to a variety of molecules, the capillaries in the brain are tightly fused. Some substances like glucose & oxygen can get through but many toxins and infectious agents are blocked from entry. Most drugs do not get into the brain. Only ones that are fat soluble can penetrate this barrier.  Blood-Brain Barrier The blood-brain barrier is set up by closing the holes or pores in the capillaries – Nothing can pass through the modified capillaries by passing between the cells – blood-brain barrier is a highly selective barricade between the blood and brain – protects the brain and spinal cord from fluctuations in the blood and body – minimizes the possibility that potentially harmful blood- borne substances might reach the central neural tissue and cause damage  Microglial Cells Microglial cells are phagocytic glial cells located in the brain and spinal cord. – Microglia remove plaques, damaged neurons, and infectious microbes.  The CNS is comprised of two types of nervous tissue. Grey Matter: Contains neurons with unmyelinated axons. It is found around the outside areas of the brain and the core of the spinal cord. White Matter: Contains neurons with myelinated axons. It forms the inner region of the brain and the outer area of the spinal cord.  The spinal cord is a column of nerve tissue that extends from the brain through a canal within the vertebrae (backbones). It serves as the primary reflex centre, coordinating rapidly incoming and outgoing neural Sensory (afferent) neurons enter information (ie. reflex through the dorsal root ganglion (ie. a arcs). cluster of lots of neurons) on the posterior side. Motor (efferent) neurons leave through the ventral root ganglion on the anterior side Interneurons (association neurons) in the spinal cord initiate reflexes as well as relay information to the brain for processing.  The gray matter of the spinal cord is located centrally, forming a “H” shape and is composed of interneurons. The white matter of the spinal cord surrounds the grey matter and delivers information to and from the brain.  The brain is the centre for intelligence, consciousness and emotion. Despite its small size, scientists estimate there are more neurons in the human brain than stars in the Milky Way. It is subdivided into three general regions: the hindbrain, midbrain & forebrain.  Meninges Meninges are 3 protective and nourishing layers of connective tissue: 1. Dura mater - outermost layer; tough, inelastic covering 2. Arachnoid mater - delicate middle layer; the space between this layer and underlying pia mater is the subarachnoid space (filled with cerebrospinal fluid (CSF)) 3. Pia mater - innermost layer; most fragile; closeladheres to surface of brain and spinal cord; highly vascularized (brings blood to deep brain tissues)  Cerebrospinal Fluid (CSF) CSF - produced by specialized tissue of the pia mater called choroid plexuses. CSF circulates through the brain ventricles (inner hollow chambers) and into the spinal cord. CSF also leaves ventricles and enters subarachnoid space gets reabsorbed into blood. Total volume = 125 -150ml; replaced more than 3 times a day (formed - circulated - reabsorbed)  The hindbrain is divided into three main areas. The medulla oblongata is located at the base of the brainstem. It controls involuntary responses such as heart & breathing rates and reflex actions (ie. vomiting, sneezing, coughing & swallowing). The cerebellum is located at the back of the skull below the cerebrum. It’s involved in balance and coordination of posture, as well as fine voluntary motor skills (ie. riding a bike, writing). It can be programmed to remember movements. The pons is located above and in front of the medulla. It’s a relay station that sends messages between the cerebellum and the cerebral cortex.  The midbrain is found just above the pons and relays visual and auditory information between areas of the hindbrain and forebrain. As well, it plays an important role in eye movement and control of skeletal muscles.  The forebrain is divided into three main areas. The thalamus is at the base of the forebrain. It screens sensory information so that it can direct attention to stimuli of importance (ie. it acts as a filter for information brought to conscious thought). It’s “the great relay station of the brain”, connecting various parts with one another and areas of the sensory system (except smell) with the cerebellum. The hypothalamus lies just below the thalamus and helps to regulate the body’s internal environment including heart rate, blood pressure and temperature. It receives information from internal organs and co- ordinates the nervous and endocrine systems. It also controls basic drives (ie. thirst, hunger & sex) and emotions. It coordinates the actions of the adjoining pituitary gland, by producing and regulating the release of certain hormones. The cerebrum is the largest part of the brain and accounts for 80% of it’s weight. It contains the centres for intellect, memory, consciousness and language. It interprets and controls the response to sensory information. Voluntary movement. Cerebrum  Each of the cerebrum consists of an internal mass of white matter and a thin outer covering of grey matter known as the cerebral cortex. It’s responsible for language, memory, personality, vision, conscious thought and other activities associated with thinking and feeling. The cerebral cortex is folded into ridges (gyri) and depressions (sulci) to increase the amount of surface area for the brain to fit inside of the skull. The cerebrum is divided into a left and right hemisphere, with each side being more dominant for certain functions. The right brain is associated with intuitive thinking, visual-spatial skills and artistic abilities. The left brain is associated with sequential and logical ways of thinkings, as well as linguistic and mathematical skills. The right and the left hemispheres of the cerebrum are connected by a bundle of white matter called the corpus callosum. This allows each half of the brain to tell what the other half is doing. But are people really left-brained or right-brained? As well, can a person live with only one hemisphere of their cerebrum?  Cerebral Cortex Four regions of the cerebral cortex: 1. Frontal lobe: located at the front of the cortex 2. Parietal lobe: located posterior to the central sulcus 3. Temporal lobe: located below the parietal lobe 4. Occipital lobe: located at the back of the head (posteriorly)  Frontal Lobe Located at the front of the cortex – Separated from the parietal lobe by the central sulcus – Functions: voluntary motor activity elaboration of thought speaking ability memory Responses related to personality  Hemispheres Brain divided into two halves called hemispheres. Left hemisphere: – performance of logical, analytical, sequential and verbal tasks ex: math, language forms and philosophy – Sends and receives signals from the right side of the body Right hemisphere: – nonlanguage skills, especially spatial perception and artistic and musical endeavors – Sends and receives signals from the left side of the body Corpus callosum: – allows for communication between the hemispheres  Each hemisphere of the cerebrum is divided into four lobes. Frontal Lobe: Associated with personality, intelligence/problem solving, emotional reactions & smell recognition (overlaps with temporal lobe). As well it controls voluntary movement via the motor cortex. Broca’s area is located here and allows us to speak. Parietal Lobe: This region contains the sensory cortex which receives sensory information from the skin (ie. touch and temperature). As well, it processes information about taste and body position/awareness. Temporal Lobe: Interprets auditory information (ie. hearing). Wernicke’s area is located here and allows us to comprehend speech. As well, it’s involved in facial & smell recognition, musical rhythm and memory. Occipital Lobe: Interprets visual information.  Scientists first learned about brain functions by studying people with brain disorders or injuries. For example, in 1848, railway construction foreman Phineas Gage (1823-1860) blasted a 13 pound tamping rod straight through his left frontal lobe. Although he survived, he experienced significant changes to his personality. Peripheral Nervous System The PNS consists of nerves that link the CNS to the rest of the body, including sensory organs, muscles, glands and internal organs. The two main divisions of the PNS are the somatic system and the autonomic system. The somatic nervous system (SNS) is largely under voluntary and controls the skeletal muscles throughout the body. It includes 12 pairs of cranial nerves and 31 pairs of spinal nerves, all of which are myelinated. Each nerve contains both sensory and motor neurons which services the area of the body where they are found.  Cranial nerves originate directly from the brain and bypass the spinal column all together. They generally control the head, neck and face functions. The exception is the vagus nerve which connects to many internal organs, including the heart, lungs, digestive tract, liver and pancreas.  The autonomic nervous system (ANS) is under the control of your hypothalamus and medulla oblongata. Its neurons are bundled together with neurons from the SNS. However, unlike the SNS, the ANS is involuntary, controlling glands and non-skeletal muscle (ie. smooth & cardiac). It’s divided into two main branches: The sympathetic nervous system prepares the body for “emergencies”. It’s often referred to as the fight-or-flight response. Sympathetic neurons release the neurotransmitter norepinephrine, which has an excitatory effect on target muscles. The adrenal glands are stimulated to produce epinephrine (adrenaline). Skeletal muscles are given an energy boost by increasing heart rate and blood pressure. Some of these changes can be picked up by devices such as polygraphs (lie detectors). The parasympathetic nervous system is activated when the body is at rest. It’s often called the rest-and-digest response. It slows down heart rate, reduces blood pressure and promotes digestion of food.