Nervous System + Special Senses Flashcards
Function
- receive stimuli from external and internal environment
- analyse and integrate stimuli
- bring about response
- divided into:
- Central Nervous System (CNS)= brain and spinal cord
- Peripheral Nervous System (PNS)= nerves from CNS (cranial and spinal, autonomic - (para)sympathetic)
Neuron Structure
- responsible for nerve impulse transmission
- synapses transport nerve impulses to neurons
- neurons are supported by connective tissue + neuroglial cells in complex nervous tissue (brain and spinal cord), supply nutrients, carry waste away
-neurons made up of cell body with nucleus, dendrons
(from dendrites) carry impulses to cell body + axon carry impulses away from cell body (leaves cell at axon hillock)
- transmission speed is increased in the presence of a myelin sheath (Schwann cells surround axon, interrupted at intervals by Nodes of Ranvier, for nutrients and O2)
- varying sizes of neurons (length depends on destination of nerve fibre) + shapes vary
Synapses
- axons terminate here
- termination on an individual muscle fibre = neuromuscular junction (NMJ) + stimulates contraction
- presynaptic ending = vesicles containing chemical transmitter substances (ACh) but also adrenaline, serotonin + dopamine
- impulse down axon> vesicles move to presynaptic membrane> transmitter released into synaptic cleft> combines with postsynaptic membrane> allows the continuation of the nerve impulse (only occur with Ca2+ ions)> stopped by cholinesterases (enzymes destroy remaining ACh in synaptic cleft)
Nerve Impulse Generation
-impulse changes electrical charge of neuron (alters negative + positive charge relationship, positive=cell membrane, negative=cell contents) > results from change in cell membrane permeability of Na+, K+ ions exchanging ions on either side of the membrane
- short time after impulse = fibre is refractive (another impulse will not generate a response until it has returned to normal) > ensures one directional flow
- nerve impulse = all-or-nothing phenomenon (stimulation does or does NOT occur)
- effect depends on:
- number of nerve impulses (per second)
- number of neurons (activated by a single stimulus)
- type of neuron (stimulated)
Classification of Nerves
- Peripheral > sensory (to CNS), motor (away from CNS) + mixed (sensory + motor)
- Intercalated neuron = between sensory and motor neuron
- Afferent (towards) + Efferent (away from) a structure
- Visceral > heart, digestive, respiratory, urinary + reproductive (blood vessels, MMs, visceral organs via visceral sensory nerves, to CNS) + impulses transmitted from CNS via visceral motor nerves to smooth muscle + glandular tissue = response
- Somatic > skin, muscles, joints + tendons as well as specialised organs (ear + eye) > sensory to CNS, motor to skeletal muscles = response
Central Nervous System
Brain: control and coordinate activities of normal body> hollow and swollen structure within the skull’s cranial cavity (protection from mechanical damage)> 3 areas= forebrain, midbrain + hindbrain.
Forebrain cerebrum, thalamus and hypothalamus> cerebrum is right + left cerebral hemispheres= 90% all neurons in the NS, linked by corpus callosum (white matter) found in the roof of the 3rd ventricle (part of the hollow ventricular system in the brain)> hemispheres are deeply folded= large SA within small cranial cavity (nutrients reach cell bodies of neurons)> folds=gyri (upfolds), sulci (shallow depressions) + fissures (deep crevices), longitudinal fissure divides the 2 hemispheres (each hemisphere has 4 lobes=nuclei)> tissue consists of a grey matter outer layer (cerebral cortex) + white matter inner layer (tracts of myelinated nerve fibres)> thalamus = deep in tissue of posterior part of forebrain, process information from sense organs, relay to cerebral cortex> hypothalamus = links endocrine and NS, controls autonomic NS and exerts influence over homeostasis in the body
Ventral Surface = optic chiasma (nerve impulses - optic nerve - right eye - right then left side of brain)
Pituitary Gland = endocrine gland attached below the hypothalamus by a short stalk
Olfactory Bulbs = pair of bulbs forming the rostral part of the brain - sense of smell (olfaction) - size determined by sense of smell in a species (cats larger than dogs)
- Midbrain* short length of brain, between forebrain and hindbrain> over hung by cerebral hemispheres> pathway for fibres (hearing + sight, from HB to FB)
- Hindbrain* cerebellum, pons + medulla oblongata> cerebellum = dorsal surface of hindbrain, globular appearance, deep fissures, grey + white matter (grey is Purkinje cells)> controls balance + coordination, receives information from semicircular canals of inner ear + muscle spindles within skeletal muscles> voluntary movements initiated by the cerebrum, fine adjustments by cerebellum> pons= bridge of nerve fibres between the cerebellar hemispheres (centres for respiration control)> medulla oblongata= extended from pons merges into spinal cord (centres for respiration + blood pressure control)
Protection of the Brain
- Normal brain function = essential for maintenance of life
- Mechanical/Chemical damage = impaired function (death)
- Cranium* Tough outer shell, protect soft brain tissue
- Ventricular System* Lumen of tube = interconnecting canals + cavities or ventricles inside brain + spinal cord, ventricles + central canal filled with cerebrospinal fluid (CSF), CSF also in the subarachnoid space = secreted by networks of blood capillaries (choroid plexuses), clear fluid (example of transcellular fluid), CSF protects CNS from damage + provide nutrients
- Meninges* 3 protective layers> dura mater= tough fibrous layer of connective tissue (continues with periosteum of cranium bones), below this is subdural space (fat, blood capillaries + connective tissue)> arachnoid mater= network of collagen fibres + larger blood vessels (supply nutrients to adjacent nervous tissue), below is subarachnoid space (CSF flows)> poa mater= delicate membrane, follows gyri + sulci
- Blood-Brain Barrier* Neurological tissue modification connecting + supporting all the neurons> neuroglial cells surround blood capillaries = impermeable layer (protect brain from harmful/unnecessary substances - urea, certain proteins, antibiotics)
Spinal Cord
- Runs from medulla oblongata of the brain to 6th/7th lumbar vertebra = splits into terminal nerves (cauda equine)
- Leaves skull through largest foramen (foramen magnum) + through vertebral canal formed by interlinking vertebrae
- Decrease in size (cranial to caudal) EXCEPT at caudal cervical and midlumbar due to nerves supplied to the fore + hind limbs
- Spinal nerves (to the left and right) leaving the vertebral canal through Invertebral foramen
- Consists of:
- central canal* away from brain’s ventricular system - CSF
- grey matter* tissue surrounding central canal - cell bodies of neurons + non-myelinated nerve fibres
- white matter* surrounds grey matter, white due to myelin, organised into fibre tracts which carry information towards (ascending) or away from (descending) the brain
- Space between periosteum of vertebrae + dura mater = epidural space (local anaesthetic produced)
Peripheral Nervous System + Nerves
PNS=
- Cranial nerves (leave brain)
- Spinal nerves (leave spinal cord)
- Autonomic NS (some brain nerve fibres but mostly spinal)
Cranial Nerves -12 pairs leaving the cranial cavity via the foramina -Some relatively short but others supply distant structures so are much longer (vagus nerve) -May carry = only sensory (organs to brain), only motor (brain to organs) or both (mixed nerves) I = Olfactory II = Optic III = Oculomotor IV = Trochlear V = Trigeminal VI = Abducens VII = Facial VIII = Vestibulocochlear IX = Glossopharyngeal X = Vagus XI = Accessory XII = Hypoglossal
Spinal Nerves
- Spinal cord passes down vertebral canal = segments give off spinal nerves (left + right)
- Nerves numbered according to number of spinal vertebra in front
- 8 cervical nerves, only 7 cervical vertebrae (1st= cervical nerve Ce1 leaves in front of C1, Ce7 leaves in front of C7 and Ce8 leaves behind it)
- 2 components:
- Dorsal root* sensory fibres towards the spinal cord = small swelling containing cell bodies = dorsal root ganglion
- Ventral root* motor fibres away from spinal cord, no ganglia outside (only in grey matter)
- Some pathways = one or more intercalated neurons in the grey matter
Examples of Nerves
Pectoral Limb
- formed of spinal nerves C6, C7, C8, T1, T2
- radial nerve = largest brachial plexus nerve, follow musculospiral groove of humerus (triceps), branches into superficial + deep but supplies the extensors of the elbow beforehand and then supplies the extensors of the carpus + digits afterward (superficial branch carries sensation from forepaw and lower forelimb)
- median nerve = caudal to brachial artery + vein on the forelimb, supplies flexors of carpus and digits, carrying sensations from the elbow joint capsule
- ulnar nerve = shares some fibres with median nerve, supplies flexors of carpus and digits but carries sensation from lower forelimb + paw
Pelvic Limb
- formed of spinal nerves L3, L4, L5, L6, L7, S1, S2
- femoral nerve = main hindlimb nerve (equivalent: radial), runs deep within quadriceps femoral muscle tissue, supplying extensors of stifle joint
- saphenous nerve = more superficial (from femoral), supplies sarotius muscle + carries sensation from thigh, stifle + lower limb
- sciatic nerve = largest nerve of lumbosacral plexus, deep within biceps femoris, supplies hamstring muscle group (extensors of hip, flexors of stifle) + carries sensation from regions of thigh
- tibial nerve = from sciatic, supplying flexor muscles on caudal surface of lower limb + plantar surface of the hind foot
Reflex Arcs
-Fixed voluntary response to certain stimuli > rapid, automatic, only involves nerve pathways in the spinal cord (correct spinal nerve)
-Means of protection = prompt response
-Common reflexes (dog + cat):
*withdrawal/pedal = patellar
*anal = panniculus = palpebral
[Patellar = monosynaptic reflex (one set of synapses)]
-Quadriceps femoris muscle tendon passes over patella, insert on tibial tuberosity
-Nerve Pathway:
1) tendon lightly tapped, muscle fibres stretch
2) muscle spindles respond (nerve impulse across sensory to spinal cord)
3) grey matter = sensory synapses with motor, impulse passes to muscle fibres of quadriceps femoris = contraction
4) extends or kicks out leg
-Polysynaptic reflex (withdrawal) = 1+ intercalated neurons, several grey matter synapses
-Unconscious = will occur with severed spinal cord
-Conscious reponse = intact spinal cord (dog bites = pian)
>Unconditional reflexes<
-Unconscious + automatic, cannot be overcome
>Conditional reflexes<
-Overcome by conscious thought (ie, Pavlov’s dogs)
-Basis of toilet training + obedience training
Autonomic Nervous System
- Visceral motor nerves = supply cardiac + smooth muscle, glandular tissue of all organs + blood vessels
- Unconscious control
- 2 parts = parasympathetic + sympathetic systems
- Control involves both
Midbrain> eye
Pons/Medulla> tear + salivary glands, heart, lungs + bronchioles, abdominal viscera
T1/T2/T3/T4: Heart, Lungs + Bronchioles
T5/T6/T7/T8/T9: Coeliac ganglion (Liver, Kidney + Stomach)
T10/T11/T12: Cranial mesenteric ganglion (Large + Small Intestine)
L1/L2: Caudal mesenteric ganglion (Bladder)
S2/S3/S4: Urogenital organs + large intestine (in part)
Taste Buds: Taste
- Sensation = gustation
- Organs = taste buds, epiglottis + soft palate (moist MM)
- Taste buds = chemoreceptors
- Each taste bud consists of receptor/gustatory cells surrounded by sustentacular/supporting cells
- Hair-like processes project from taste bud
- Nerve fibres (facial VII), glossopharyngeal (IX), vagus (X)
Nasal Mucosa: Smell
- Sensation= Olfaction
- Receptor cells = chemoreceptors (works with gustation)
- Receptor cells = rod-shaped bipolar neurons, distributed throughout mucosa of nasal cavities + turbinate bones
- Axon leaving combines with others = olfactory nerve fibres (I)
- Chemicals responsible dissolve in mucous of nasal cavity + stimulate nerve impulse production
- Along olfactory nerve fibres, through cribriform plate of ethmoid bone and into olfactory bulbs in brain (forebrain)
The Eye: Sight
- Eye = organ of sight
- Photoreceptor cells = respond to stimulus of light, found in retina
- Orbit = deep bony cavity in which the eye lies
- Dog + Cat = forward-facing eyes (predator)
- Prey Species = eyes at the side
- 3 main parts: eyeball, extrinsic muscles + eyelid
Eyeball
-Globe-shaped structure, 3 layers (sclera, uvea + retina)
Sclera
-fibrous outer covering, conjunction with cornea
-cornea = transparent anterior + bulges slightly outwards of the orbit, poor blood supply but lots of sensory nerve fibres, outer layer of squamous epithelium (conjunctiva), first to be hit by rays of light
-dull white, dense fibrous connective tissue + elastic fibres (extrinsic muscles insert) > protects delicate internal structures + maintains eye shape
-junction between the two (limbus) - drainage point for aqueous humour
Uvea
-vascular pigmented layer, firmly attached to sclera at exit of optic nerve
- choroid = darkly pigmented vascular lining (2/3 of uvea contains blood vessels, pigmented cells prevent light rays escaping)
-tapetum lucidum = triangular area of yellow-green iridescent light-reflecting cells (function is to reflect light back to photoreceptor cells of retina)
-ciliary body = thickened structure, smooth muscle fibres (ciliary muscle) controlling thickness and shape of lens
-suspensory ligament = continuation of ciliary body, circular support around the lens perimeter
-iris = anterior continuation of ciliary body (radial + circular smooth muscle fibres - free edge forms the pupil, regulates amount of light entering the eye
Retina
-innermost layer of eye, light focused on photoreceptor cells of retina (via lens) and the optic nerve (I0 transmits this
-pigmented layer = prevent light leaking out
-photoreceptor cells = rods (B+W/Night), cones (colour)
-bipolar neurons = info from rods + cones for next layer
-ganglion cells = axons to optic disc, forming the optic nerve (II), no rods/cones = blind spot
-iris divides into:
>anterior chamber< iris + cornea - aqueous humour
>posterior chamber< iris + lens - aqueous humour
-posterior eye (lens + retina) = vitreous chamber - vitreous humour
-both humours provide nutrients + maintain eyes shape
Extrinsic Muscles
- originate on connective tissue of sclera + insert on periosteum in the orbit
- essential for fine movements of the eyeballs = coordination for accurate binocular/3D vision
- nerve supply = oculomotor (III), trochlear (IV), abducens (VI) nerves
Eyelids
- orbit protects majority of the eyeball
- anterior third = soft = risk of mechanical damage
- upper and lower eyelids joined at medial canthus and lateral canthi
- upper eyelid>lower eyelid (mobility)
- covered in hairy skin and lined with MM on inner surface
- reflects back (fornix)
- Meibomian glands protect and lubricate eyelid inner surface
- outer edge of upper eyelids=cilia=associated with sebaceous + rudimentary sweat glands
- tissue of eyelids>palpebral muscles (movement) > protects eye from damage (blink reflex) = useful during anaesthesia, how deep
- third eyelid/nictitating membrane = located in medial canthus (Dog: hyaline cartilage, Cat: elastic cartilage, Both: smooth muscle) > covered by a MM, supplied with glandular + lymphoid tissue (protect + moisten eye) and is via sympathetic NS
- lacrimal gland (moisten and protect cornea) > modified cutaneous glands, beneath the upper eyelid, produces secretions (tears) flowing over the anterior eye’s surface, draining into the nasolacrimal duct (nostril)
Eye > Formation of an Image
Eye function > conduct rays of light from an object + form an image via the retina
1 - light rays pass cornea direct to retina
2 - light rays through pupil
3 - light rays strike lens
4 - light rays strike retina and pass through layers of cells until they hit photoreceptor cells
5 - some light reflected back by tapetum lucidum
Ability to accommodate (change lens shape) develops shortly after birth but increases as the animal ages (less flexibility) > animals learn to adapt
