Biology

neurones, which convey information as nerve impulses (electrochemical signals)

a cell body, containing a nucles, and nerve fibres, long extensions that transmit nerve impulses rapidly from one part of the body to another.

axons

dendrons

small dendrons (dendrites) extending from the cell body

In mammals, sensory neurones carry messages from peripherial sense organs to a central nervous system (CNS) consisiting of the brain and spinal cord. The CNS acts as an integration centre and processes information from many sources.

Motor neurones convey instructions from the CNS to effector organs (mainly muscles and glands). can convey information rapidly and over considerable distances

These fast-conducting neurones are enclosed along most of their lengt by a thick insulating material called the mylein sheat.

The mylein sheat is produced by special supporting cells called Schwann cells. The sheath is essentially a series of cell membranes, each produced by a Schwann cell and wrapped many times around the axon.

Gaps between the membranes of each Schwann cell are called the nodes of Ranvier (key to fast transmission of nerve impulses).

conveys information rapidly between specific locations so that quick responses can be made.

the endocrine system uses cheimical to convey information from one source to many destinations to bring about long-lasting responses.

The endocrine system consists of a number of glands that secrete hormones ( the chemical signals, usually proteins or steroids).

They secrete their hormones directly into the bloodstream. Once inside a blood vessle, a hormone is carried in the bloodstream so that it can reach almost every cell in the body.

each hormones has its own taret cells on which  acts. --> Each hormone has its own specific effect on the body.

In some cases, a target cell has specific repector molecules on its cell surface membrane which bind the hormone molecule.

hormones regulate a wide range of activities --> blood sugar

is a three chabered, spirally wound, fluid-filled tube resembling a snail's shell.

communicate with each other via a small hole called the helicotrema at the tip of the cochlea.

This fluid in the upper and lower chamber is perilymph, rich in sodium ions.

The central chamber (the cochlear canal) contains the organ of Corti.

Reissner's membrane (vestibular membrane) separates the cochlear canal from the vestibular canal.

This has hair cells which are mechanoreceptors that transduce sound waves into nerve impulses.

The fluid surrounding the organ of Corti in the middle chamber is endolymph, rich in potassium ions.

The hair cells are modified ciliated cells. Their bases are embedded in the baislar membrane which forms the floor of the middle chamber and their hair like heads are embedded in a rigid membranous flap, the tectorial membrane.

pressure waves pass along the upper chamber and cause the Reissner's membrane to vibrate. These vibrations are transmitted to the endolymph, causing the baislar membrane in the organ of Corti to vibrate. As the baislar membrane moves, the hair cells are bent against the rigid tectorial membrane. Thsi produces a generation potential which, when it reaches a treshold level is transmitted as a nere impulse along the auditory nerve to the midbrain.

ear can discriminate between different soundwaves because of different responsiveness of hair cells along the length of the cochlea.

base of the cochlea --> basilar membrae = narrow, thin and rigid --> high frequencies

apex of the cochlea --Y baislar membrane = wider and less rigid --> low frequency sounds.

depend on the number of neurones that are activated an the frequency of their impulses.

come form the vestibular apparatus in the inner ear.

The vestibular appartus consists of the semicircular canals, containing organs called cristae, and sacs called the saccule and utricle, containing organs called maculae.

The inner ear has three semicircular canals, each lying in a different plane at right angles to the other two. --> changes of movement (accleration / deccleration) in any direction can be detected.

Each canal is filed with fluid (endolymph)and has a small swelling (ampulla) containing a sense organ.

The sense organ called crista, has hair cells embedded in a gelatinous structure called a cupula. The cupula moves in response to movements of the endolymph canal, bending the cilia that project from the hair cells.

they produce generator potentials --> treshold level

two sacs in the vestibular aparatus contain sense organs called maculae , which are lined with hair cells sourrounded by a jelly-like fluid. Fine granules ove in response to changes in body positiion and bend the cilia on the hair cells. If the cilia bend in one direction, the frequency of nerve impulses increases, bending the cilia in the other direction decreases the frequency.

The information from the vestibular appartus is transmitted to several parts of the cnetral nervous system: spinal cord --> body position, cerebellum --> smooth coordinated movements, higher centres of the brain --> control of eye movements

nerve impulses

A resting neurone is so called because it does not convey a nerve immpulse, not because it is inactive. a resting neurone expends much eergy in maintaining a potenntial difference across its membrane.

measured: about - 70 milivolts.

During the resting potential, the inside of a neurone is negative relative to the outside because of an unequel distribution of charged ions.

On the outside, sodium ions Na+, chloride ions Cl- and calcium IOns Ca2+ are present in higher concentrations than inside the cell. By contras the inside of the cell has a higher concentration of potassium ions K+ and organic anions (negative ions).

This unequela distribution of ions results from a combination of active transport and diffusion of sodium and potassium ions across the cell membrane, and the inability of large organic anions to pass out of the cell. A sodium-potassium pump actively transports sodium ions out of the neurone and potassium ions in.

This wouldn't be enough. --> membrane is about 50 times more permeable to potassium ions than to sodium ions. potassium ions --> able to diffuse back out of the cell down their concentration gradient. sodium ions diffuse back ionto the cell only very slowly. --> negative electrical charge inside

the axon becomes depolarised (the inside becomes temporarily less negative). If the stimulus is strong enought (exceeds the threshold level) --> action potential occurrs.

--> complete change of the charge across the nerve cell:

• interior becomes positively charged relative to the outside
• +35 milivlts
• potential difference then back down, undershoots the resting potential and finally returns to it.

entire action potential --> 7 miliseconds