BIOL1410 Lecture Notes - Fall 2024 - Unit 19: Nervous System PDF

BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System UNIT 19: NERVOUS SYSTEM A) Nervous System: Overview — 2 divisions: 1) Central Nervous System (CNS) — “command...

BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System UNIT 19: NERVOUS SYSTEM A) Nervous System: Overview — 2 divisions: 1) Central Nervous System (CNS) — “command centre” — brain and spinal cord — processes and integrates info 2) Peripheral Nervous System (PNS) — consists of: a) cranial nerves — to/from brain b) spinal nerves — to/from spinal cord — 2 divisions of PNS: a) sensory/afferent division — has sensory receptors that detect stimuli (change in internal or external environments) b) motor/efferent division — nerves convey impulses away from CNS — innervates (supplies nerves to) effectors = muscles and glands (endocrine or exocrine) — relationship between CNS and PNS: — example: 1  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System — Nervous System Organization: B) Nervous System: Histology — Cell Types: 1) Neurons — conduct impulses — make up CNS and PNS — mostly amitotic (irreplaceable) o exceptions = taste, olfaction, memory — structure: a) cell body — typical organelles — RER called - Nissl Bodies — groups/clusters in CNS = nuclei (gray matter) — groups/clusters in PNS = ganglia — b) processes from cell body: i. dendrites — receive incoming messages and relay to cell body ii. axon — carry impulses away from cell body — axon hillock = where axon meets cell body — axon terminal = typically branched with synaptic end bulbs (enlarged tips) — may be: 1 myelinated - – wrapped in many layers of cell membrane from Schwann cells (PNS) or oligodendrocytes (CNS) — electrical insulation — gaps in myelin sheath are called Nodes of Ranvier 2  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System myelinated axon bundles in: — o CNS = tracts (white matter) o PNS = nerves 2 unmyelinated – no myelin 2) Neuroglia (glial cells) — support neuron cells = can undergo mitosis (prone to cancer – brain tumor) — types: a) CNS neuroglia i. oligodendrocytes — produce myelin around axon ii. microglia — protective – become phagocytic if detect infected, dead, or damaged neurons (because immune cells can’t enter CNS) iii. astrocytes — surround blood capillaries to form part of blood brain barrier (BBB) — help control capillary permeability iv. ependymal – neural epithelia — line brain ventricles and central canal of the spinal cord — secrete cerebrospinal fluid (CSF) and circulate it (cilia) b) PNS neuroglia i. Schwann cells — form myelin around axons in PNS ii. Satellite cells — surround neuron cell bodies in ganglia – protection and support — Neuron Classification: 1) Structural/Anatomical types: — based on # of cell processes off of cell body a) Unipolar — 1 process that divides into two: central and peripheral — peripheral end has dendrites = sensory receptors (pain, touch etc) - remainder is axon — always sensory b) Bipolar — 2 processes: 1 axon, 1 process with dendrites — sensory – retina, nose (olfaction) c) Multipolar — 3 or more processes: 1 axon, many dendrites — all interneurons and motor neurons 2) Functional types: — based on direction of impulse conduction a) Sensory/Afferent Neurons — mostly unipolar — from sensory receptors to CNS 3  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System b) Interneurons — within CNS (between sensory and motor) — 99% of neurons (mostly multipolar) c) Motor/Efferent Neurons — CNS to effectors (all multipolar) — Neuron Junctions (Synapses): 1) Neuronal junction: — neuron to neuron — can be chemical (use neurotransmitters) or electrical (ions) 2) Neuromuscular junction: — motor neuron to skeletal muscle 3) Neuroglandular junction: — motor neuron to gland — Chemical Neuronal Synapses: o most common o structure: 1) Presynaptic Neuron: — neuron bringing impulse 2) Axon Terminal: — within synaptic end bulbs o inside the end bulb (presynaptic membrane) = synaptic vesicles containing neurotransmitter (nt) 3) Synaptic Cleft: — space between neurons 4) Postsynaptic Neuron: — receives the impulse — has postsynaptic membrane o = cell membrane of dendrites or cell body with receptor sites for nt C) Central Nervous System: Protective Features 1) Bone — skull and vertebral column 2) Meninges — connective tissue around brain and spinal cord — layers: a) dura mater (outer) — brain = 2 fused layers o separated in some places to form spaces = venous (dural) sinuses – contain blood — spinal cord = 1 layer — deep to dura mater = subdural space 4  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System o filled with ISF — superficial to dura mater = epidural space (spinal cord only) o filled with fat, blood vessels, CT, etc. b) arachnoid mater (middle) — avascular — subarachnoid space o contains cerebrospinal fluid o web-like strands of CT secure it to pia mater below — has arachnoid granulations (brain only) o project into dural sinuses o CSF enters granulations to return to blood c) pia mater (inner) — on surface of CNS (brain and spinal cord) — vascular — meningitis = inflammation of meninges 3) Cerebrospinal Fluid (CSF): — surrounds brain and spinal cord — in brain ventricles and central canal of spinal cord o ventricles: spaces inside brain (filled with CSF) 2 lateral ventricles (1st and 2nd) – in cerebrum 3rd ventricle = in diencephalon 4th ventricle = surrounded by pons, medulla oblongata, cerebrum o central canal: space in spinal cord — cushions CNS – brain buoyant — formed from blood plasma (similar composition) — produced by choroid plexuses (blood capillaries) found in each ventricle — circulation: 5  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System 4) Blood Brain Barrier (BBB): — 2 cell types: a) endothelial cells (of capillaries) with tight junctions b) astrocytes – foot processes wrap around endothelial cells — selectively permeable o allows: glucose, fat soluble material o not toxins, antibiotics, etc. 5) Cerebral Arterial Circle: — cerebral arteries form circle at base of forebrain o circles pituitary gland and optic chiasma (where optic nerves cross) o unites the two major blood supplies to the brain (anterior and posterior) o provides an alternate route for blood if vessels blocked — Blood Flow to Brain: (*Cerebral Arterial Circle = red boxes) o Posteriorly: 6  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System o Anteriorly: — Blood Return from the Brain: o Posteriorly: D) Central Nervous System: Brain 1) Forebrain: a) cerebrum (= right and left cerebral hemispheres) — lobes (named after overlying bones): i. frontal ii. temporal iii. parietal iv. occipital v. insula – deep to temporal lobe — surface features: i. fissures = deep grooves 1 longitudinal fissure — separates right and left cerebral hemispheres 7  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System 2 transverse fissure — separates cerebellum and cerebrum 3 lateral fissure — separates temporal lobe from rest of cerebrum ii. gyri = ridges — examples: o postcentral gyrus in parietal lobe o precentral gyrus in frontal lobe iii. sulci = shallow grooves (separate gyri) — example: o central sulcus – between frontal and parietal lobes — 3 layers: i. cerebral cortex — 2 – 4 mm thick, gray matter — has functional areas: 1 motor areas — all in frontal lobe — control skeletal muscle movement — 3 regions: ⓐ primary motor area (precentral gyrus) ⓑ premotor area ⓒ Broca’s area (motor speech) 2 sensory areas ⓐ general sensory area (pain, touch, temp, pressure) — postcentral gyrus in parietal lobe ⓑ vision — occipital lobe ⓒ auditory and olfaction — temporal lobe ⓓ taste and visceral sensation (e.g. full bladder) — insula 3 association areas — recognize info from memories o parietal, occipital, temporal lobes 4 others: — memory o temporal lobes — conscious intellect (personality, learning, ideas, judgement, etc.) 8  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System o prefrontal cortex ii. tracts (white matter) — types: 1 association tracts — from gyrus to gyrus in same hemisphere 1 commissural tracts — from gyrus to gyrus in opposite hemispheres — e.g. corpus callosum 3 projection tracts — run vertically (brain to spinal cord / spinal cord to brain) iii. basal nuclei — paired masses of gray matter (within white matter) — control skeletal muscle movement 2) Diencephalon — all gray matter a) thalamus — 2 lobes connected by intermediate mass (bridge through 3rd ventricle) — relay station for impulses coming to cortex (from spinal cord) b) hypothalamus — inferior to thalamus, above pituitary — major regulator of the internal environment (visceral control) o e.g. blood pressure, heart rate 3) Midbrain: — connects pons and diencephalon — contains cerebral aqueduct — anterior portion = cerebral peduncles (projection tracts) — posterior portion = 4 nuclei = corpora quadrigemina o 2 superior colliculi visual reflexes o 2 inferior colliculi hearing 4) Hindbrain a) Pons — anterior to cerebellum — tracts between brain and spinal cord, and tracts to/from cerebellum — pontine respiratory centres b) Medulla — inferior to pons — ends at foramen magnum — 2 bulges (called pyramids) = large motor tracts (= part of corticospinal tracts) o just above spinal cord = decussation (crossover) of pyramids — 3 vital centres in medulla: i. cardiac ii. vasomotor (blood vessels) 9  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System iii. respiratory — several non-vital centres for swallowing, sneezing, vomiting **NOTE: Brain Stem = midbrain, pons, medulla c) Cerebellum — posterior to pons/medulla — has folds similar to gyri = folia — cortex = gray matter — arbor vitae (deep to cortex) = white matter — coordinates skeletal muscle contraction o balance, posture 5) Functional Systems: a) Limbic System — nuclei in cerebrum and diencephalon — regulates emotions (laughing, crying, etc.) — contains areas involved in memory (memories evoke emotional responses) b) Reticular Formation — nuclei in brain stem — cortex, thalamus, and hypothalamus involved o (together form RAS = Reticular Activating System) — regulates alertness and attention o filters stimuli and only sends new/unusual signals to other brain areas — sleep results when inhibited — if damaged = coma E) Central Nervous System: Spinal Cord 1) Structure Overview: — foramen magnum to L1/L2 (conus medullaris) — nerves continue down from spinal cord through vertebral foramina as cauda equina (horse’s tail) o exit at intervertebral foramina — filum terminale o is a connective tissue extension of pia mater that anchors conus medullaris of spinal cord to coccyx o location where CSF samples taken 2) Cross-Sectional Structure: a) anterior median fissure and posterior median sulcus — separate cord into right and left halves b) central canal 10  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System — contains CSF c) Gray Matter — cell bodies and dendrites of motor neurons and interneurons — H-shaped: o cross bar = gray commissures o horns i. dorsal horn = sensory ii. lateral horn = motor iii. ventral horn = motor d) White Matter — myelinated axons containing ascending (sensory) or descending (motor) tracts — forms columns: i. dorsal column ii. lateral column iii. ventral column 3) Functions: — sensory and motor impulses — produces reflexes o fast, predictable, automatic responses to changes in environment § e.g. withdrawal reflex F) Peripheral Nervous System: Structures 1) Cranial Nerves — 12 pairs total — brain regions associated with cranial nerves I-XII: 11  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System — 2 pairs = only sensory neurons o I and II — 1 pair = mainly sensory neurons o VIII — 9 pairs = mixed nerves o carry both sensory and motor neurons § motor neurons have cell bodies in brainstem nuclei § sensory neurons have cell bodies in ganglia of PNS o e.g. trigeminal nerve (V) § motor function = chewing § sensory function = conveys general sensations (touch, pressure, pain, temperature) from face to CNS 2) Spinal nerves — 31 pairs = all mixed nerves o 8 cervical o 12 thoracic o 5 lumbar o 5 sacral o 1 coccygeal — exit via intervertebral foramina (except 1st – between atlas and occipital) — each has 2 points of attachment to spinal cord: a) Dorsal Root — sensory neurons; cell bodies in dorsal root ganglion b) Ventral Root — autonomic and somatic motor neurons; cell bodies in ventral or lateral horn — spinal nerve = joining of dorsal and ventral roots o immediately divide into branches = rami: a) dorsal ramus — innervate skin and muscles of the back b) ventral ramus — forms thoracic nerves (T2 – T12) ) OR further branch and join up forming nerve plexuses (= nerve network) 12  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System — Spinal Nerve Plexuses: c) rami communicantes — connect ventral ramus (spinal nerve) to sympathetic trunk — contain autonomic nerve fibres (neurons) 3) Cross section of a nerve: — CT wrappings: a) Epineurium — around whole nerve b) Perineurium — around fascicles c) Endoneurium — around axon and myelin G) Peripheral Nervous System: Sensory (Afferent) Division — stimulus g receptor g Central Nervous System (CNS) — consists of: 1) Receptors — detect changes in environment — classified by: a) Location i. exteroceptors — stimulus in external environment = receptors at body surface (e.g. touch, special senses) ii. interoceptors — stimulus in internal environment (in viscera, blood vessels, etc.) iii. proprioceptors — located in joints, skeletal muscles, etc. — monitor body position (stretch receptors, muscle spindles ) – balance and movement 13  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System b) Type of stimulus received i. mechanoreceptors — mechanical stimuli (e.g. pressure, touch, hearing) ii. thermoreceptors — temperature iii. chemoreceptors — chemical iv. photoreceptors — light v. nociceptors — pain c) Structure of Receptor (general senses only) i. free nerve endings — terminal dendrites of unipolar sensory neurons — e.g. pain, root hair plexus ii. encapsulated nerve endings — terminal dendrites enclosed in CT — e.g. Meissner’s and Pacinian corpuscles 2) First Order Neurons — unipolar neuron attached to or associated with receptor (may be encapsulated) — axon travels in cranial/spinal nerves to CNS — cell bodies in: a) sensory ganglia of cranial nerves b) dorsal root ganglia of spinal cord — axon terminals in: a) brain b) dorsal horn of spinal cord H) Peripheral Nervous System: Motor (Efferent) Division — Central Nervous System (CNS) g Effector — 2 subdivisions: 1) Somatic — effector = skeletal muscle — consists of lower motor neurons: o = single multipolar neuron a) cell bodies in ventral horn of spinal cord to effector b) cell bodies in motor nuclei of brainstem to effector (e.g. facial motor nucleus for CN VII) 2) Autonomic — effector = smooth muscle, cardiac muscle, glands — 2 successive multipolar neurons from CNS to effector: a) preganglionic neuron — myelinated — cell body in brain stem or lateral horn of spinal cord b) postganglionic neuron — unmyelinated 14  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System — cell body in autonomic ganglion (outside CNS) — 2 subdivisions: a) Sympathetic (SNS) b) Parasympathetic (PSNS) — Comparison of SNS and PSNS: I) Nervous System Pathways 1) Ascending Pathway (Sensory): — conducts impulses from general sense receptors into brain — 3 successive neurons from receptor to cortex: a) First Order Neuron — receptor to spinal cord (PNS) b) Second Order Neuron — interneuron, multipolar (CNS) — cell body in dorsal horn of spinal cord or medulla (CNS) o decussates (crosses over) in medulla c) Third Order Neuron — interneuron, multipolar — cell body in thalamus – impulses to postcentral gyrus (sensory cortex) (CNS) — axons of 1st and 2nd order neurons form ascending spinal tracts: a) Dorsal (Posterior) Column Pathway/Tract — sensations can be precisely located (touch) — receptors = free nerve endings, Meissner’s etc. 15  BIOL1410 Lecture Notes – Fall 2024 Unit 19: Nercous System b) Spinothalamic Pathway/Tracts — non-specific, difficult to localize — receptors = temperature, pain c) Spinocerebellar Pathway/Tracts — ascending tracts from spinal cord to cerebellum — receptors = proprioceptors — 2nd order neuron directly to cerebellum (no 3rd order) o no conscious perception o no decussation 2) Descending Pathway (Motor): — conduct impulses from brain to effector — 2 neuron pathway: a) Upper Motor Neuron (CNS): — multipolar interneurons — cell bodies in cortex or brainstem nuclei b) Lower Motor Neuron (cell bodies in CNS and axons in PNS) — axons of upper motor neurons form descending spinal tracts: a) Corticospinal Tracts: — cell bodies in cerebral cortex — tracts mainly decussate in medulla b) Indirect Tracts — cell bodies in brainstem nuclei — receive impulses from motor cortex, basal nuclei, cerebellum 16