Biol 067: Section 13 - Nervous System A. Overview of the nervous system: 1. 2 parts of the Nervous System: Nervous system Central Nervous System (CNS) Peripheral Nervous System (PNS) 2. How CNS and PNS are divided and interconnected: Central nervous system Brain Spinal cord Peripheral Nervous system messages via spinal and cranial nerves Sensory nerves - carry sensory info to brain and spinal cord Motor nerves - from CNS to effectors somatic sensory nerves - FROM skin, muslces, joints, special senses visceral sensory nerves - FROM body organs Somatic motor nerves - (controls movement) TO skin, skeletal muscles, tendons -voluntary Autonomic motor nerves - (controls body function) To smooth muscle, cardiac muscle, organs, glands - involuntary Sympathetic division "Fight or flight" Parasympathetic division "rest and digest" "normal state"
B. Neurons and how they work: Nervous tissue: made up of 2 types of cells 1) Neurons transmit nerve impulse 2) Neuroglia cells support and service neurons 1. Types of neurons: classified by function i. Sensory neuron -takes info from sensory receptor (detects changes in the environment) to CNS ii. Motor neuron takes info away from CNS to an effector (i.e. muscle fiber, gland, etc) iii. Interneuron convey info between neurons sum up info before passing it along to motor neurons inside CNS 2. Structure: 1. dendrites receive signals from sensory receptors or other neurons and send signals towards cell body from axon 2. cell body contains nucleus and organelles 3. axon conducts nerve impuls along its length 3. Myelin sheaths Cover long axons Protective layer contains myelin (a lipid substance) that insulates axon Has gaps nodes of Ranvier used in neuron transmission In Peripheral Nervous system (PNS) act as insulator sheath formed by Schwann cells (a type of neuroglia) In CNS different kind of neuroglia but still contains myelin Myelin sheath in long axons not short In Central nervous system (CNS) white matter white because of myelin/ grey matter no myelin MS (multiple sclerosis) and leukodystrophies (degeneration of white matter in brain) caused by loss of myelin ( generally caused by defect in genes involved in growth and maintenance of myelin, and/or environmental not really understood)
C. Nerve impulse Is the way a neuron transmits info 2 states: 1. resting potential 2. action potential Can be measured with an oscilloscope by measuring voltage difference between inside and outside of axon. 1. Resting potential: =potential energy of neuron Is when an Axon is not conducting an impulse -65 to -70millivolts mv inside of membrane is negative compared to outside of axon Charge difference is related to ion concentration difference across axon membrane. There is relatively more large negatively charged ions on the inside of the nerve cell during resting potential than outside the nerve cell. Even though it is called resting the nerve cell is busy keeping the potassium at a relatively even concentration it drifts out of potassium channel but then gets attracted back in by the negative ions on the inside and the Na+ ions are being pumped to the outside through the Na+potassium pump. Therefore inside more negative than outside of nerve cell at resting potential (due to large negative ions that stay inside the cell) Conctrn of K+ inside higher then outside larger negative ions keep inside relatively more neg. then outside Conctrn of Na+ greater on outside ***Unequal distribution due to sodium potassium pump = membrane protein that actively transports Na+ out and K+ into axon thru separate channels
2. Action Potential (pg 345 of text shows graphic) When axon is conducting an impulse Rapid change in polarity or charge across membrane Nerve impulse consists of electrochemical change across membrane a) Due to message from chemoreceptor, gates of sodium channels open first and Na+ moves into axon Membrane potential changes from -65 mv to +40mV = depolorization Change inside of axon from to + This is because now the inside is more positive due to all the positive Na ions coming in. Almost immediately after depolarization, the Na channels close and the K+ channels open However, before action potential can be reached, must depolarize enough to cross threshold of ~-40 mv once crossed it will continue all the way through action potential - If it doesn t reach threshold impulse will not go anywhere. b) Positive repel positive so the K+ ions now get pushed out the open gates of potassium channel, K+ flows to outside of axon =40mV -65 mv = repolorization Change in action potential Inside of axon resumes negative charge as positive K+ ions exit now the inside is relatively negative again due to the relative number of the larger negative ions relative to the remaining positive ions (which is the Na+ this time)
After the impulse has passed the nerve cell gets busy returning to resting potential because the Na K pump moves K+ back to inside and Na+ to outside - then its ready for another stimulus E. Propogation of action potential As it travels down axon = successive depolarization and repolarization of axon occurs Refractory period, Na+ gates unable to open as soon as repolarization occurs (while Na+/K+ pump is putting it back to original state) Ensures action potential moves forward (not backward) and towards its axon branches Ion exchange occurs at Nodes of Ranvier in myelinated sheaths Causes action potential to travel faster = saltatory conduction Impulse jumps from node to node action potential F. Transmission across a synapse: Axon axon branch axon bulb (axon terminal) on end Axon bulb lies close to dendrite of cell body of next neuron Space=synaptic cleft Region of close proximity = synapse
Transmission of action potential across synapse accomplished by neurotransmitters which are molecules stored in synaptic vesicles Steps in transmission 1) Nerve impulse travels along axon to axon terminal 2) As nervous impulse reaches bulbs, gated channels open and Ca 2+ enters bulb as concentration of Ca 2+ increases, causes synaptic vesicles to merge with pre-synaptic membrane (NB diagram) 3) Neurotransmitters released into cleft, diffuse across and bind with receptor proteins on post synaptic membrane - 4) Cause post synaptic neuron excitation (causes Na+ to diffuse into post synaptic neuron) or inhibition (causes K+ to diffuse out of post synaptic neuron) depending on neurotransmitter once response is initiated, neurotransmitter removed from cleft by a) Post synaptic membrane enzyme that inactivates the NT or/ b) Pre synaptic membrane reabsorbs the NT possibly for recycling This is required so constant stimulation or inhibition of post synaptic membrane doesn t occur
F. Synaptic integration 1000 10,000 synapses/neuron is common therefore must have a method to integrate signal = summing up all the signals received If neuron receives more excitory signals has depolarizing effect axon will transmit a nerve impulse after reaching threshold signal either by 1 axon sending a rapid # of signals or many signals from different neurons Or if receives more inhibitory signals has a hyperpolarizing effect can stop axon from firing takes it further from an action potential +2 0 2 excitatory signal integration inhibitory signal 4 threshold 7 resting potential 8 Time (milli seconds) G. Divisions of the Nervous System 1. Central Nervous System: Definition: lies in midline of body =spinal cord and brain - Sensory info received, voluntary motor impulses initiated here Both protected by bone vertebrae (spinal cord), skull (brain) Both wrapped in protective membrane known as meninges Space filled with cerebrospinal fluid to cushion and protect CNS parts
a) Spinal Cord: 1) Structure of spinal cord fig 13.7 Located at base of brain and into vertebral canal Vertebra joined so spinal cord passes thru middle Spinal nerves pass thru lateral openings between vertebra Spinal cord contains: Central canal -contains cerebrospinal fluid Grey matter central H shaped, contains cell bodies and short non-myelinated fibers contains parts of sensory and motor neurons and interneurons White matter around grey matter, contains myelinated axons in bundles called tracts Tracts - Ascending take info to brain, located dorsally - Descending take info from brain, located ventrally - Both cross just after they enter and exit brain therefore left side controls rt side and vice versa 2) Function of spinal cord Provides a means of communication between brain and peripheral nerves that leave the cord. Reflex arc centre (talked about later) Integration of incoming info from many sensory neurons before motor impulse sent out. b) Brain Structure and Function Handout be able to id main parts of brain Ventricles: brain has 4 ventricles = interconnecting cavities that produce and act as a reservoir for cerebrospinal fluid.(2x lateral, third, fourth,) Main parts of brain: 1) Cerebrum 2) diencephalon 3) Cerebellum 4) Brain stem
1) Cerebrum Largest part of the brain in humans Left and rt cerebral hemispheres Highest centre to receive info- commands voluntary responses Higher thought processes learning, memory, speech Cerebral cortex (grey matter of cerebrum) Thin convoluted outer layer of grey matter Sensation, voluntary movement, consciousness Cerebral hemispheres (handout be able to id main parts) Cerebrum is divided into left and right hemispheres by longitudinal fissure Each hemisphere divided into lobes by sulci (grooves) i. Parietal lobe back, top ii. Temporal lobe lies below frontal and parietal lobe (sides by ears) iii. Occipital lobe very back iv. Frontal lobe front i. Parietal lobe back a) Primary somatosensory area Sensory info from skin and skeletal muscles ii. iii. iv. b) Primary taste area taste sensations c) Somatosensory association area integrates sensory info from skin and muscles Temporal lobe lies below frontal and parietal lobe Primary auditory area receives info from ears Auditory association area integrates sensory info Speech area called Wernicke s area helps us understand written and spoken word and send to Broca s area Occipital lobe dorsal to parietal lobe Primary visual area - receives info from eyes And association area associates new and previously received visual info frontal lobe: front of cerebral cortex a) pre motor area organizes motor functions for skilled motor activities/ sends to primary motor area b) primary motor area - voluntary muscle commands begin here, sends to cerebellum c) Broca s area motor speech area sends to 1 o motor area
d) Prefrontal area reasoning and plan actions, receives info from other association areas (Association area = any part of the cerebral cortex involved in the integration of information and where memory is stored) White matter of cerebrum Rest of cerebrum composed of white matter Consists of long myelinated fibers organized into tracts Ascending from lower brain centres, sends info to primary somatosensory area Descending communicates with lower brain centres Tracts inside cerebrum take info between different sensory, motor, and other association areas Corpus callosum contains tracts that join 2 cerebral hemispheres like a bridge between 2 hemispheres 2) Diencephalon Region that encircles 3 rd ventricle Contains thalamus and hypothalamus and pineal gland Thalamus = integrates sensory info (except smell) and relays to cerebrum involved in memory and emotion Hypothalamus = homeostasis - integration centre for autonomic system regulates hunger, sleep, thirst, temperature, water balance and controls pituitary gland (serves as link btwn nervous system and endocrine system) Pineal gland = secretes hormone (not sure of its role) gland pokes out from under hypothalamus 3) Cerebellum Posterior to brainstem, separated by 4 th ventricle Has 2 portions surface = grey matter inside = white matter Integrates sensory and motor information i.e. posture, balance, skeletal muscles and coordination
receives info from sensory (eyes, ears, joints etc) as to where body is presently positioned then receives motor output from cerebral cortex about where it should be then sends message to skeletal muscles to correct learning new motor skills 4) Brain Stem Contains: a) Midbrain also has reflex centre for visual, auditory and tactile senses -Relay stn for tracts going to and from spinal cord and cerebrum or spinal cord and cerebellum b) Pons (bridge) -This contains bundles of axons travelling between cerebellum and CNS -Helps with breathing rate and head movement c) Medulla oblongata lies between spinal cord and pons -Contains vital centres which regulate heart beat, breathing, blood pressure -Contains reflex centres for vomiting, coughing, sneezing hiccupping and swallowing etc. Reticular formation Network of nuclei which are masses of grey matter and fibers which extend length of brainstem Receives sensory signals pass up to higher centres Receives motor signals passes down to spinal cord 2. Peripheral Nervous System a) Location and Structure: PNS lies outside the CNS (brain and spinal cord) Contains nerves (bundles of axons) Nerve=bundle of nerve fibers (axons)/single nerve fiber=axon 2 types of nerves: cranial - arise from the brain/ spinal arise from spinal cord Sensory fibers/nerves send info to CNS Motor nerves info from CNS Cell bodies are in CNS or in ganglia (ganglia=collection of cell bodies in PNS
b) Types of Nerves: 1) Cranial nerves 12 pairs, attached to brain Some sensory, Some motor (don t need to know function of each one) some mixed contain both sensory and motor fibers Most cranial nerves inervate head, neck, and face - but vagus nerve (which begins in medulla oblongata) goes to most organs as well as pharynx and larynx 2) Spinal nerves 31 paired nerves Emerge from spinal cord by 2 roots or branches
Both roots join to form spinal nerve that leaves CNS becomes a mixed nerve = both sensory and motor bundled together in one nerve. Conduct impulse away from cord to effectors c) Divisions of the PNS: The peripheral nervous system has 2 divisions: 1) Somatic nervous system 2) Autonomic nervous system 1) Somatic Nervous system Nerves that take sensory impulses to CNS from sensory receptors and motor commands away. Serve the skeletal muscles, skin, and tendons Voluntary response = brain Involuntary response = reflex = spinal cord or brain Reflex Arc short circuit response thru sensoryinterneuronsmotor nerves in spinal cord integrate info from sensory neurons and relay signals to motor neurons allows response rapidly to sensory stimulus Some info also reaches brain (say Ow later) 2) Autonomic nervous system Fig 13.17? good review of sympathetic and parasympathetic structure and function see contrary effects give example Regulates cardiac and smooth muscle, organs, and glands Has 2 divisions: Sympathetic and parasympathetic Both: Act automatically and are usually involuntary Innervate all internal organ They use 2 neurons and 1 ganglion for each impulse 1 st neuron cell body in CNS and preganglionic fiber that enters the ganglion 2 nd neuron cell body in ganglion and postganglionic fiber that leaves the ganglion Reflex actions of the autonomic system regulate things such as blood pressure and breathing rate and are important to maintain homeostasis
Autonomic motor nerves vs somatic motor nerves 1. Sympathetic division fight or flight Used in flight or fight situations Preganglionic fibers from middle of spinal cord (thoracic-lumbar region) Pre-ganglionic fiber is short, terminate in ganglion close to spinal cord Postganglionic fiber is long (contacts organ) Causes increased heartbeat, increased ventilation, decreased digestion Usually neurotransmitter is Norepinephrine (acts like adrenaline) 2. Parasympathetic division rest and digest Includes some cranial nerves (i.e., vagus nerve etc.) and sacral nerves (arise from the sacral {bottom} part of the spinal cord) therefore this division may be referred to as craniosacral portion of the autonomic nervous system preganglionic fiber is long ganglia lies near or in organ postganglionic fiber is short this division promotes normal steady state relaxed promotes digestion, decreased heartbeat uses neurotransmitter called acetylcholine
Summary of somatic and autonomic motor nerves: Autonomic motor pathways Somatic motor pathway Sympathetic motor Parasympathetic motor Control Vol/invol invol involuntary # of neurons/message 1 2 (preganglionic shorter than post) 2 (preganglionic longer than post) Loctn of motor fiber Most cranial nerves and all spinal nerves Thoracic-lumbar spinal nerves Cranial and sacral spinal nerves Neurotransmitter acetylcholine Norepinephrine (NE) Acetylcholine (Ach) effectors Skeletal muscle, skin, tendons Smooth & cardiac muscle, glands, & organs Smooth & cardiac muscle, glands, & organs