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,
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?
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:
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
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
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.
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.
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:
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
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.
Types of Muscle Tissue
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.