- One nerve cell-includes dendrites, cell body (soma) axon. Functions to conduct nerve impulses. Theses are the cells that perceive sensory stimuli, learn, remember, control muscles and glands. Carry out the functions of the nervous system by conducting nerve impulses. The functional characteristics of neurons are excitability and conductivity
- Excitability: ability to respond to a stimulus
- Conductivity: ability to transmit an impulse from one point to another
- Both of these characteristics are the result of structural features of the cell membrane
4 Events in Neurons
- Graded potentials dendrites and soma
- Action potentials axon, peripheral process (nerve impulse-a propagated AP)
- Synaptic transmission neurotransmitters cross a synapse; may include signal transduction
- Sensory transduction sensory receptors
A bundle of nerve fibers
A collection of neuron cell bodies located outside the PNS
Clusters of neuron cell bodies in CNS
Neuron cell bodies
Myelinated nerve fibers, form tracts in CNS
Do not conduct impulses, are supporting cells-serve to nourish and protect neurons. Some form blood-brain barrier with capillary wall cells
Form myelin sheath around axons in PNS, myelin acts as insulator and speeds nerve impulse conduction, breaks between schwann cells are Nodes of Ranvier where ions can cross membranes in Aps
Connecting, located in CNS
(Electrical potential) a voltage difference across a cell membrane.
Ion channels are gated 3 ways-voltage (electrical) chemical (receptor) Mechanically (stretch)
Resting Membrane Potential
- Neurons are polarized, Na on outside, K and protein on inside. Inside is negative
- All cells have resting membrane potential. Only muscle and nerve cells are excitable: can change
- Membrane potential in response to a stimulus.
organs are brain and spinal cord
organs are nerves and ganglia
Divisions of PNS
- Afferent or sensory division (transmit information from receptors to CNS)
- Efferent or motor division (transmit information from CNS to effector cells)
Divisions of Motor
Somatic-supplies motor impulses to skeletal muscles (voluntary)
- Autonomic-supplies motor impulses to cardiac, smooth muscle, glands
Divisions of Autonomic
Sympathetic and parasympathetic-dual innervation of effectors
Divisions of Sensory
- General (body wide) and special (localized)
- Somatic (body wall, cutaneous) and autonomic (visceral)
- Visceral-pertaining to organs
- Somatic-pertaining to the body’s framework-skin, muscle, tendon,
- Composed of the brain and spinal cord. Both are composed of gray and white matter.
- Gray matter-neuron cell bodies white matter-myelinated axons
- The brain is composed of an enormous number of association neurons, with accompanying neuroglia, arranged in specific regions
- Functions of the brain include: receiving sensory information, directing the activity of motor neurons,
- Higher brain functions: learning, memory, emotions, consciousness
- The complexity of the brain derives from complexity of neural hookups and synaptic biochemistry
- -Divided into 31 segments, each giving rise to a pair of spinal nerves
- -Peripheral white matter surrounds a core of gray matter
- -Gray matter contains cell bodies for efferent neurons, sensory neuron terminals and interneurons
- Afferent (sensory) nerves enter SC via dorsal root; cell bodies are in dorsal root ganglia
- Efferent (motor) fibers leave via ventral root; cell bodies are in ventral horn
- -White matter contains longitudinal bundles of myelinated nerve fibers called nerve tracts- the tracts each contain bundles of axons with similar functions or type of information
- -Spinal cord function: conduction pathway for impulses going to and from brain, reflex center.
- Sensory impulses travel in ascending tracts, motor impulses on descending tracts.
- -Spinal cord integrates many basic reflexes
nerve fibers, both sensory and motor, cross in the medulla or spinal cord means left hemisphere receives information from and controls right side of body and vice versa.
- Consists of cranial nerves (12 pairs), spinal nerves (31 pairs) and ganglia
- Nerves bundles of nerve fibers, can be sensory, motor or mixed (most are mixed)
- Cranial nerves emerge from inferior surface of brain, most innervate head except vagus
- Spinal nerves connected to spinal cord by dorsal root (sensory) and ventral root (motor, both somatic and autonomic)
- Ganglia collections of cell bodies located outside CNS
- each sensory receptor responds most readily to one form of energy, the adequate stimulus
- Law of Specific Nerve Energies: regardless of how receptor is stimulated, any given receptor gives
- Rise to only one sensation.
- having receptors respond to low levels of input; there are three ways to change sensitivity
- Convergence if many receptors are hooked up to 1 neuron, sensitivity is increased.
- Spontaneous firing: increases sensitivity; requires less input to change. Response can be + or –
- Adaptation a decrease in sensitivity during sustained stimulation; there are 2 kinds of receptors.
- Tonic receptors-fire steadily in response to a constant stimulus adapt slowly (muscle spindle
- Phasic receptors-transient quickly adapting (pressure, touch)
- sharpness of perception, ability to distinguish two close stimuli
- Receptor field size area served by 1 neuron, acuity varies with density of receptors in a region
- (e.g. acuity of fingertips vs. elbows)
- Lateral inhibition the most strongly activated signal pathway blocks transmission from nearby
- Receptors; facilitates localization.
- range of stimulus intensities that a receptor responds to. This can be increased by:
- Range fractionation individual receptor cells respond to only a portion of the total dynamic range.
- With increasing stimulus intensity, new cells are recruited. (e.g. color vision, pitch recognition)
Coding of Sensory information
- Stimulus type (energy modality) is coded by which tract is activated.
- Modality perceived by CNS depends on the anatomical specificity with which sensory neurons connect with high cognitive centers in the brain; ie any stimulus of visual receptor or along visual nerve pathway is interpreted as light (seeing stars with mechanical stimulus of eye)
- Location of the stimulus is also coded by anatomical hookup, the particular nerve tract stimulated provides the CNS with the location and type of stimulus energy.
- Intensity is coded by frequency of AP’s and by number of receptor cells activated and amplitude of GP’s
- Duration coded by duration of AP’s, but some receptors adapt. Phasic receptors show a decline in response despite continued application of stimulus
If threshold is reached, an AP is stimulated, Ap’s are all the same size-all-or-none
Time during which neuron cannot respond to second stimulus. Is due to nature of gated channel which ensures one way conduction of impulses
One AP results in Na entry, which is the trigger for voltage gated channels on adjacent membrane to open: a series of action potentials is initiated down the axon membrane=propagation-the propagated action potential is called a nerve impulse
- The information that is transmitted is in the frequency of AP’s generated.
- AP’s are frequency modulated. GP’s are amplitude modulated
- Along a neuron. Action potentials=brief reversal of membrane polarity in one place; how is message conducted?
- Saltatory conduction
- AP jumps from one node to the next in myelinated fibers, increases speed of conduction
information flow has a specific directionality in reflex arcs. This is ensured by refractory period in axons and postsynaptic receptors in synapses.
- all conscious activity and higher mental functions occur here
- Divided into two cerebral hemispheres connected by corpus callosum
- Cerebral lateralization-there is specialization of function in one hemisphere or the other: left hemisphere usually dominates for language and analytical abilities. Right for visuospatial tasks
- Gray matter = cerebral cortex the outermost portion of the brain and nuclei –concentrations of cell bodies in white matter of cerebrum
- White matter = myelinated tracts connecting different parts, different hemispheres, rest of brain.
personality, intellect, behavior, initiation of voluntary motor activity in primary motor cortex. The motor and sensory gyri have been mapped; areas of cortex devoted to each body part are proportional to level of motor control and sensory acuity rather than body part size
interpretation of cutaneous and muscular sensations in sensory or somatosensory cortex
conscious perception of vision
interpretation of auditory sensation, smell
an internal area-memory, integration of other cerebral activities
subcortical gray matter, involuntary control of skeletal muscle and cognition
centrally located, includes thalamus, hypothalamus
gray matter, relay station for sensory impulses- sends impulses to appropriate region of cortex for discrimination, localization, interpretation
- key role in maintaining homeostasis, links nervous and endocrine systems. Key functions:
- Regulates and integrates autonomic nervous system
- Regulates emotional responses and behavior-rage, aggression, arousal, fear, pleasure
- Regulates pituitary gland
- Regulates homeostatic functions: foot intake, water balance, thirst, temperature
- Regulates sleep-wake cycle (with other centers)
includes parts of cerebrum and diencephalons, involved in emotions, memory
- the oldest, primitive part of the brain. Includes:
- Reticular Formation
has motor and sensory tracts; red nuclei and substantia nigra- subconscious skeletal
Functions of sensory receptors
- Absorption of energy-sensory receptors are highly selesctive for specific stimulus modalities; they contain specific receptor molecules or structures
- Amplification-incoming signal may need amplification to generate AP's.
- Transduction-change incoming form of energy into electrical energy; mechanism for this involves ion channel gating and flow of ions; a receptor potential is generated
- Each sensory receptor responds most readily to one form of energy, the adequate stimulus.
- Law of specific Nerve Energies: regardless of how receptor is stimulated, any given receptor gives rise to only one sensation.
- Having receptors respond to low levels of input; there are ways to change sensitivity
- -Convergence-if many receptors are hooked up to 1 neuron, sensitivity is increased spontaneous firing; increases sesnitivity; requires less input to change, response can be positive or negative
- -Adaptation-a decrease in sensitivity during sustained stimulation, there are 2 kinds of receptors
- -tonic receptors: fire steadily in response to a constant stimulus, adapt slowly
- -phasic receptors: transient, quickly adpating eg:pressure, touch
Acuity or discrimination
- sharpness of perception, ability to distinguish two close stimuli
- -Receptor field size: area served by 1 neuron, acuity varies with density of receptors in a region eg acuity or fingertips vs. elbow
- -Lateral Inhibition: The most strongly activated signal pathway blocks transmission from nearby receptors; facilitates localization
- Range of stimulus intensities that a receptor responds to. This can be increased by:
- -Range fractionation: individual receptor cells respond to only a portion of the total dynamic range. With increasing stimulus intensity, new cells are recruited.
Special Properties of Sensory Receptors
- Acuity or discrimination
Coding of Sensory Information
- Stimulus Type
Is coded by which tract is activated. Modality perceived by CNS depends on the anatomical specificity with which sensory neurons connect with higher cognitive centers in brain
Of the stimulus is also coded by anatomical hookup, the particular nerve tract stimulated provides the CNS with the location and type of stimulus energy
is coded by frequency of APs and by number of receptor cells activated and amplitude of GPs
Coded by duration of APs but some receptors adapt phasic receptors show a decline in response despite continued application of stimulus
- Includes sensation from skin, body wall, bone, muscle, tendon, joint
- Temperature, Touch and Pressure, Pain, Proprioception
There are specific thermoreceptors for cold and heat in skin and internal organs
Touch and Pressure
Mechanoreceptors found in dermis, tendons, ligaments, hollow viscera
Receptors are called nociceptors-respond to tissue damage. Intense mechanical or thermal input, noxious chemicals. Pain can be blocked by electrical stimulation of inhibitory pathways
- The sense of position, orientation and posture
- Examples: muscle spindle organ and Golgi tendon organ.
Taste and Smell
Chemoreceptors-transduce chemical energy
Equilibrium and Hearing
- Mechanoreceptors-transduce mechanical energy.
- The organs involved are the vestibular apparatus and the cochlea, both are located within a connecting membranous labyrinth which is located in a bony cavity in skull
The specialized and very sensitive receptors for equilibrium and hearing.
is a wave of disturbance of air molecules.
The sense organ, function is amplification and transmission of sound to receptor cells
- Vibrates at same frequency as sound waves.
- Middle ear cavity-hole in temporal bone of skull-connects to pharynx via Eustachian tube, contains ossicles
3 bones which amplify signal so that it can move the more resistive fluid of the inner ear. Malleus, Incus, Stapes
A bony labyrinth (vestibule, cochlea, semicircular canals) and membranous labyrinth
Snail shaped tube filled with endolymph, connects to middle ear by oval and round windows
Organ of Corti
Contains the hair cells, sit on a basilar membrane, are overlaid by tectorial membrane
How we Hear
- Sounds Waves travel through air-move eardrum (tympanic membrane) ossicles and oval window
- Travel through cochlear fluids, cause bending of cilia, energy disipates at round window.
How we hear...transduction mechanism
- As cochlear fluid moves,it moves basilar membrane upwards. This causes hair cell cilia to be deflected by tectorial membrane.
- The deflection of cilia gates mechanically gated channels and K enters the cilia and cell.
- K gates voltage gated channels and Ca enters the cell
- Ca causes exocytosis of NT which crosses synapse and gates chemically gated channels.
- This initiates GPs then APs in auditory nerve
Sense organ, focuses light on receptor cells.
change in lens shape for far vs. near vision
Thin layer of neural tissue lining back of eyeball, contains photoreceptor cells, neuron cell bodies, and pigment epithelial cells
Rods and Cones
- These are the receptor cells
- Contain photoexcitable pigment molecules associated with receptor membranes inside the cells
- Rods are more sensitive-many converge on single sensory fiber
- Cones specialize in color vision and have greater acuity 1:1 hookup with next neuron
- Study of hormones.
- One of two control/communication systems in the body. Other is NS
- Regulates and coordinates slower functions, those not requiring immediate responses. Homeostatic regulation of water, salt, glucose, growth, reproduction, part of response to stress
A chemical synthesized and secreted by an endocrine tissue, released into the blood stream, which influences the activity of a target cell.
- Or tissue
- The distant cells on which hormones act. These have receptors.
Molecules that specifically recognize and bind a hormone.
Organs that secrete hormones, these include organs that are exclusively endocrine (thyroid, pituitary) or can be organs with other functions (CNS, gut, pancreas)
Secrete into ducts (salivary, pancreas) vs. ductless endocrines which secrete into blood
- Protein and catecholamines
- Act at cell surface
Steroids and thyroxine (lipid soluble) act inside cells
- Steroid diffuses through membrane of target cell.
- Binds to specific receptor in cytosol or nucleus. This complex initiates DNA transcription and new protein synthesis.
- New proteins can be structural or functional proteins
- Signal transduction and second messenger systems
- Involves binding of hormone (first messenger) with cell surface/membrane bound receptors
- Receptor is linked to second messenger which acts in cytosol for these non-penetrating hormones
- Second messenger often activate (phosphorylate) proteins which carry out the hormone's message
What determines a cell's response to a given hormone?
- 1 hormone can have different effects in different tissues
- 1 cell can respond to 2 different hormones by having 2 types of receptors
- 1 cell can have 2 responses to 1 hormone
Hormones act via the blood and blood levels of hormones are controlled. Hormone secretion (proteins) or synthesis (steroids) can be stimulated by a variety of mechanisms
Direct Feedback of regulated substance
- When pancreatic beat cells are exposed to increased levels of glucose, they release insulin
- Insulin promotes transfer of glucose to glycogen in the liver
- This action removes glucose from blood and insulin is no longer released.
- Beat=insulin Alpha=glucagon
Regulatory control by other hormones
All anterior pituitary hormones are under excitatory or inhibitory stimulation by hypothalamic releasing hormones, and in turn often control secretion of other hormones in their target tissues.
- Direct Innervation: some endocrine tissues (pancreas) are innervated, nerve impulses can initiate or inhibit secretion
- Adrenal Medulla: is homologous to a sympathetic ganglion. Cells are innervated, release epi in response to APs
- Milk ejection: Baby suckles->sensory nerve APs->hypothalamus->release of oxytocin->stimulates release (not synthesis) of milk
Receptor Regulation (Endocrine)
Hormone receptor numbers can also be controlled. One way hormones can exert their effect is via induction of receptors for another hormone (permissive effect) in a target tissue. Thyroxine induces epi. receptors in adipose tissue and therefore "permits" epi. stimulation of fatty acid release. Estrogen induces progesterone receptors in uterine lining.
Stem for hyposecretion, hypersecretion, lack of receptors
- Consists of 2 parts:
- Anterior (glandular)
- Posterior (neural)
- Neuroendocrine cells are in hypothalamus, axons extend to the posterior pituitary, end on capillaries and release hormones into the blood.
- Hormones are ADH and oxytocin.
- ADH stimulates collecting duct of kidney, causes water retention and vascoconstriction
- Oxytocin stimulates uterine contractions and milk let down
- Adjoins posterior pituitary but of somatic origin, an endocrine gland.
- Hormones are LH, FSH, GH, PRL, TSH, ACTH.
- All but GH and PRL are tropic to other endocrine glands
- GH stimulates growth; prolactin stimulates milk synthesis
- Ant. Pit. is under the control of a 2nd neurosecretory system: cell bodies are in the hypothalamus, end on capillaries in median eminence. These join to form the hypothalamo-pituitary portal vessel which re-opens into capillaries in the ant. pit.
- The hypothalamic releasing hormones have specific releasing or inhibiting actions on ant pit hormones.
Functions of Muscles
- Body movement and movement in internal organs
- Energy transducers, changing chemical to mechanical
- Skeletal muscle contraction provides movement, posture, joint, stability, heat production
- Muscles can only contract; are often arranged as antagonists so that opposing movements can occur at joints
Skeletal Muscle Characteristics
- Excitability-can conduct APs
- Contractility-muscle contracts or shortens in response to stimulus
- Extensibility-muscle can be stretched or extended, by load or antagonistic muscle
- Elasticity-Muscles contain elastic fibers, return to original shape after contraction or extension.
Muscle (organ), Fassicle, Muscle Fiber (cell), myofibril, myofilament
Myofibrils are composed of...
- Thick filaments made of myosin molecules and thin filaments made of actin=the contractile proteins
- The thin filament also contains 2 other proteins tropomyosin and troponin
- From z disk to z disk
- Functional unit of a muscle
Sliding Filament Mechanism
Muscle proteins do not contract-thick and thin filaments slide past each other and the sarcomere shortens. This sliding is effected by repetitive cycle of events.
Basic Ideas of Muscles
- Muscles are organs, composed of muscle fibers (cells), held together by connective tissue and attached to bones by tendons.
- Muscles cross joints, only contract, work in pairs (antagonist and agonist)
Force exerted by a muscle
Force exerted on muscle by external load
- Response of a muscle fiber to a single action potential.
- AP lasts 1-2 ms, contraction lasts 100 ms.
- Minimal response
Kinds of Muscle Contractions
Isotonic muscle contraction
Muscle changes length during contraction, maintains constant tension
Isometric Muscle Contraction
Muscle cannot change length, but still produces tension. Muscle can also be lengthened by heavy load, cross bridge cycle still occurs if muscle resists.
Increase in muscle fiber tension with increase in AP frequency
Maintained maximal contraction due to elevated cytosolic (Ca)
How are graded muscle contractions acheived?
- 1. Vary amount of tension developed by each fiber (Summation, tetanus)
- 2. Vary number of fibers contracting at one time (motor unit, recruitment)
1 motor neuron + all innervated fibers
- The process of increasing the number of active motor units; this is the primary means of varying tension in a whole muscle.
- Asynchronous recruitment delays fatigue
How are sustained contractions achieved?
Continuous signal and fibers contract and relax asynchronously.
How is maximal contraction achieved?
- Tetanus + full recruitment + having muscle at optimum length to contract maximally.
- Length-tension relation: maximal tension is generated at resting length
Muscle Sensory Systems
- Muscle spindle organ
- Golgi tendon organ
Muscle Spindle Organ
- Muscle length monitor
- These sensory receptors are muscle stretch receptors, detect passive stretch of muscle spindle organ.
- Consists of intrafusal fibers and mechanosensitive afferent neurons.
- Afferent sensory neurons detect stretch in the middle non-contractile portion of spindle
- Output: infor used in spinal reflexes and sent to brain: maintains posture, position, prevents muscle damageExcitatory
Golgi Tendon organ
- Muscle tension monitor
- Information: how much tension is being exerted on or by a muscle? (from contraction or external stretch)
- Receptor is Golgi tendon organ: afferent nerve endings wrapped around collagen bundles in tendon, fire when tendon is pulled.
- Output: informs CNS; if tension is excessive tendon organ over rides excitatory commands, inhibits contracting muscle and activates antagonist
- Prevents tendon damage
Muscle Fiber Types
- Vary by differences in ATP supply (glucolysis/oxidative phosphorylation) and speed up contraction (high or low myosin ATPase activity)
- Slow oxidative
- Fast oxidative
- Fast glycolytic
Slow Oxidative Muscle Fiber
Lots of mitochondria and myoglobin,red, resist fatigue, posture muscles.
Fast oxidative muscle fiber
Mitochondria and myoglobin, red
Fast glycolytic muscle fiber
High glycogen stores (glycolysis glucose supply) white, good for short intense efforts.